A critical reevaluation of palaeoclimate proxy records from loess in the Carpathian Basin

Earth-Science Reviews 190 (2019) 498–520

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A critical reevaluation of palaeoclimate proxy records from loess in the Carpathian Basin

T

Igor Obrehta, , Christian Zeedenb,c, Ulrich Hambachd, Daniel Verese, Slobodan B. Markovićf, Frank Lehmkuhlg ⁎

a

Organic Geochemistry Group, MARUM-Center for Marine Environmental Sciences and Department of Geosciences, University of Bremen, Leobener Str. 8, 28359 Bremen, Germany b IMCCE, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Univ Paris 06, Univ Lille, 75014 Paris, France c LIAG, Leibniz Institute for Applied Geophysics, Stilleweg 2, 30655 Hannover, Germany d BayCEER & Chair of Geomorphology, University of Bayreuth, 95440 Bayreuth, Germany e Romanian Academy, Institute of Speleology, 400006 Cluj-Napoca, Romania f Physical Geography, Faculty of Sciences, University of Novi Sad, Trg Dositeja Obradovića 3, 21000 Novi Sad, Serbia g Department of Geography, RWTH Aachen University, Templergraben 55, 52056 Aachen, Germany

ARTICLE INFO

ABSTRACT

Keywords: Loess Dust Sedimentology Rock magnetism Geochemistry Biomarkers

In the Carpathian Basin, loess is the most important archive of Quaternary palaeoclimate evolution, but only in the past two decades systematic and high-resolution investigations were conducted. Those studies remarkably improved our knowledge of the regional past environmental change; palaeoclimate inferences based on the magnetic susceptibility and grain-size distribution, as the most commonly used palaeoenvironmental proxies for the Carpathian Basin loess, indicate colder and drier climatic conditions during glacials when compared to interglacials. With an increasing number of studies using novel proxies in loess research, such a traditional understanding of dry and cold glacials and humid and warm interglacials in the Carpathian Basin has been questioned. As an illustrative example, mollusc-based climate reconstructions suggest generally warm and very dry summer conditions with mean July temperatures up to 21 °C for the southern Carpathian Basin during the last glacial. Results based on stable carbon isotopes strongly oppose such high summer temperatures, but studies based on n-alkanes are in general agreement with the mollusc data when it comes to the vegetation reconstruction indicating mostly steppic conditions. However, n-alkanes studies contradict warm and dry glacial conditions as indicated by mollusc-based reconstructions, pointing instead to cold and relatively humid glacials. In addition, there is an ongoing debate whether or not millennial-scale climatic oscillations can be observed in the Carpathian Basin loess, as well as whether this area was an important Northern Hemisphere dust source or rather a sink of far distance dust transport. Consequently, the current state of the art of the palaeoclimate reconstructions from loess in the Carpathian Basin is rather inconsistent. In order to “make sense” of the existing palaeoclimate data from the Carpathian Basin loess, we have reevaluated and reinterpreted the available data. We discuss and propose a coherent interpretation of rock magnetic, grain-size, malacological, stable carbon and nitrogen isotope, n-alkane and bacterial membrane lipid data for the last glacial cycle loess archives from the Carpathian Basin. We show that glacial conditions in the Carpathian Basin led to a notable increasing North-South gradient in temperature and an even stronger expressed decreasing trend in humidity, and that most of the biomarker proxy data conducted in loess for the very dry southern part of the Carpathian Basin show a strong bias towards arid conditions. In particular, palaeotemperature reconstructions seem to be misleading. Glacial conditions were drier and colder than previously proposed (summer temperatures likely under 15 °C during glacials), but notably warmer than in other parts of Western, Central, and Eastern Europe. The vegetation consisted mostly of steppic environments during both, glacials and interglacials. We indicate that the Carpathian Basin has a potential to be a major dust source of the Northern Hemisphere during glacials, although it was at the same time exposed to the deposition of fine far distance travelled dust. Moreover, the main issues in the regional and continental correlation of loess are highlighted, as well as the sensitivity of the Carpathian Basin loess to the millennial-scale climate variability recorded in other Northern Hemisphere records. Finally, we suggest that the onset of loess formation in the

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Corresponding author. E-mail address: [email protected] (I. Obreht).

https://doi.org/10.1016/j.earscirev.2019.01.020 Received 8 June 2018; Received in revised form 13 December 2018; Accepted 23 January 2019 Available online 25 January 2019 0012-8252/ © 2019 Elsevier B.V. All rights reserved.

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investigated area occurred during the Middle Pleistocene Transition, and it was probably related to intensive silt production related to glacier dynamics in the Alpine ice cap.

1. Introduction

modern loess research is considered to have begun with the first magnetostratigraphic investigation on the Red Hill loess sequence in Czech Republic (Bucha et al., 1969) followed by correlation with palaeoclimate oscillations identified in deep-sea sediments (Kukla, 1970, 1975, 1977). Since then, advances in the field of loess research led to a better understanding of the Northern Hemisphere palaeoclimatic evolution (e.g. Buggle et al., 2013; Guo et al., 2002; Hao et al., 2012, 2015; Heller and Liu, 1984; Nie et al., 2015; Schaetzl et al., 2018). In particular, the most prominent improvements were conducted in understanding records from the Chinese Loess Plateau, where large-scale atmospheric dynamics were successfully reconstructed from loess and the primary mechanisms of past monsoon variations over Eastern Asia understood (e.g. Guo et al., 2002; Hao et al., 2012; Nie et al., 2014; Stevens et al., 2008, 2018; Sun et al., 2006; Yang et al., 2015). Loess covers vast areas of Eurasia, but a detailed understanding of continental-wide co-evolution (and comparison) of past climate variability is yet missing. To enable for such continent-wide comparisons, the climatic signal from loess sequences of different regions has to be fully understood. Yet, this is not always the case, being one of the most important limitations for a better understanding of the Eurasian Quaternary climatic evolution. The Carpathian Basin (Fig. 1) is the beststudied loess region in Europe and in some parts preserves a continuous record of dust accumulation for the past ~1 Ma (Marković et al., 2011; Újvári et al., 2014a). Although more often loess sequences in this region cover the Middle (e.g. Buggle et al., 2013; Marković et al., 2009, 2014a; Zeeden et al., 2016) and Late Pleistocene (e.g. Galović et al., 2011; Marković et al., 2014b; Stevens et al., 2011; Timar-Gabor et al., 2015), loess from the Carpathian Basin is still considered the longest and most

Loess is aeolian sediment ubiquitous in the Quaternary sedimentological and palaeoclimate record, particularly of the Northern Hemisphere. It is estimated that loess covers more than 10% of the world’s continents, and it is especially widespread in the mid-latitudes of Eurasia (Pécsi, 1990; Sprafke and Obreht, 2016). Since loess is mostly formed from silts transported by wind, it sensitively records temporal dynamics of past near-surface wind systems that have been active during its deposition. Consequently, it represents a valuable palaeoclimate archive, especially important for reliable reconstructions of palaeowind and past atmospheric systems evolution (Hao et al., 2012; Obreht et al., 2016, 2017; Stevens and Lu, 2009; Stevens et al., 2011). Moreover, loess deposits preserve important information on past atmospheric dust dynamics (Derbyshire, 2003; Muhs et al., 2014; Újvári et al., 2015, 2017), and apparently react to global climate forcing. After deposition, loess undergoes a unique process called loessification, a still poorly understood process that is associated with simultaneous weak influences of diagenesis and pedogenesis (Sprafke and Obreht, 2016). Since loessification is related to syn- and post-depositional processes, loess also provides an insight into the environmental conditions during and after dust deposition (Smalley et al., 2011). Consequently, loess is a valuable archive for palaeoclimate reconstructions, particularly for the Eurasian mid-latitudes that usually lack long-term climate records. Although research on loess has a relatively long tradition (e.g. Berg, 1916, 1964; Leonhard, 1824; Ložek, 1965; Lyell, 1834; Marsigli, 1726; Marković et al., 2015, 2016; Pécsi, 1990; Richthofen, 1882; Russell, 1944; Smalley et al., 2001, 2010),

Fig. 1. Loess distribution in the Carpathian Basin (after Lehmkuhl et al., 2018) and here often mentioned sections: (1) Tokaj, (2) Bina, (3) Ságvár, (4) Paks, (5) Dunaszekcső, (6) Semlac, (7) Crvenka, (8) Mošorin (Titel loess plateau), (9) Irig, (10) Orlovat, (11) Stari Slankamen, (12) Surduk, (13) Batajnica. In the legend, loess refers to primary loess, while loess derivates include also loess-like sediments (see Lehmkuhl et al., 2018). 499

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continuous archive of dust accumulation in Europe. Moreover, the loess areas surrounding the Carpathian Basin were the favourable habitat and ecozone for the dispersal of anatomically modern humans, especially during the time of the Aurignacian culture (Staubwasser et al., 2018). It likely represents one of the first corridors for anatomically modern human dispersal into Europe (Hauck et al., 2018). Consequently, loess is one of the most important archives of Quaternary palaeoclimate in the Carpathian Basin, where systematic and high-resolution investigations were conducted over the past decade (Antoine et al., 2009a; Buggle et al., 2008, 2009, 2013; Lukić et al., 2014; Marković et al., 2006a, 2007, 2008, 2011, 2015; Újvári et al., 2016, 2017; Stevens et al., 2011; Obreht et al., 2015; Varga et al., 2012; Zeeden et al., 2016; Zech et al., 2013). Recent research on the Carpathian Basin loess has fundamentally improved our knowledge of the past climatic and environmental evolution of this region (Buggle et al., 2013; Fuchs et al., 2008; Marković et al., 2008; Zeeden et al., 2016), and a general understanding of past large-scale atmospheric circulations was established based on loess proxy data (Marković et al., 2015; Obreht et al., 2016; Stevens et al., 2011; Újvári et al., 2017). Nevertheless, an increasing number of studies using novel proxies also induced a rising number of different interpretations of palaeoclimatic and palaeoenvironmental conditions. Recent advances in biomarkers and organic geochemical analyses enabled application of novel proxies in the Carpathian Basin loess research, providing new insights into past environmental conditions (Hatté et al., 2013; Obreht et al., 2014; Schatz et al., 2011; Schreuder et al., 2016; Zech et al., 2009, 2013; Marković et al., 2018). However, the interpretation of results from various proxy data is often conflicting and contradictory. For example, traditional and most commonly used proxies such as grain-size and rock magnetic properties of loess indicate clearly colder climatic and drier environmental conditions with increased wind dynamics during glacials, when compared to interglacials (Bokhorst et al., 2009; Buggle et al., 2009; Marković et al., 2008, 2011, 2015; Sartori et al., 1999; Vandenberghe et al., 2014; Zeeden et al., 2016). However, the mollusc analyses suggest relatively warm glacial summer conditions (Bösken et al., 2018; Hupuczi and Sümegi, 2010; Marković et al., 2004, 2005, 2006a; Osipova et al., 2013; Sümegi and Krolopp, 2002; Sümegi et al., 2013, 2016), with mean July temperatures up to 21 °C in the southern Carpathian Basin even during full glacial conditions (Marković et al., 2007). In contrast, results based on stable carbon isotopes strongly oppose such high summer temperatures (e.g. Hatté et al., 2013; Obreht et al., 2014), whereas n-alkane studies

suggest cold but rather humid conditions during glacials (Zech et al., 2013). Moreover, studies on soil bacterial membrane lipids indicate a cold and dry Marine Isotope Stages (MIS) 4, but high air temperature and soil moisture during the MIS 2, even when compared to interstadial and interglacial conditions (Schreuder et al., 2016; Zech et al., 2012). Further, the question whether or not millennial-scale climatic oscillations are recorded in the Danube loess is also debated (Stevens et al., 2011; Újvári et al., 2017; Obreht et al., 2017; Zeeden et al., 2018a), as well as if the Carpathian Basin has acted as an important source or sink area of the Northern Hemisphere dust (Újvári et al., 2015). These examples highlight the fact that current knowledge of the palaeoclimate conditions in the Carpathian Basin is limited. Therefore, it is essential to reevaluate the existing palaeoclimatic and palaeoenvironmental data from loess records in the Carpathian Basin and to establish a reliable and coherent picture of the past climates in this region. This review aims at briefly introducing the principles of the most widely used palaeoclimate proxies in the Carpathian Basin (Table 1), reevaluate and when necessary reinterpret conclusions from previous studies, and finally, establish a consistent picture of loess proxy data interpretation by providing a new palaeoclimate synthesis for the Carpathian Basin. This review contributes to the knowledge of European palaeoclimate for the Quaternary, improves the understanding of associated individual methods in general loess research, and draws attention to the need for a more careful proxy data interpretation. It also raises attention to the importance of proper proxy and data interpretation in the frame of a multi-proxy approach, not only in loess research but palaeoclimate reconstruction in general. We demonstrate that oversimplification of proxy data interpretations, consideration of proxies in isolation from other data, and disregarding the possible local topographic or microclimatic influence may lead to erroneous and conflicting interpretations. 2. Study area The Carpathian Basin extends over southeastern part of Central Europe and northwestern part of Southeastern Europe. It is a climatically sensitive region under the influence of Atlantic, Continental and Mediterranean climates. Carpathian Basin is surrounded by the Alps in the West, the Carpathians in the North and East, and the Dinarides, Rhodopes and Balkan mountains to the South (Fig. 1). Tectonic movements in central-eastern Europe were the determining precondition for loess formation in this region. Miocene to Pliocene orogeny of

Table 1 Commonly used proxies in loess in the Carpathian Basin, how they are interpreted, the relevant references, and indications where these proxies are discussed/ reevaluated in the paper. Proxy

Mainly interpreted as

Literature (relevant selection)

Discussed in chapters (this paper)

Grain-size distribution

Wind intensity, wind direction, pedogenesis intensity

3.1., 4.2., 4.3.1.

Rock magnetic properties (χ and χfd) X-ray fluorescence (XRF)

Soil/sediment humidity, the intensity of pedogenesis and weathering Provenance and weathering intensity

Molluscs

Vegetation, humidity, summer temperature

Stable carbon isotopes (δ13C)

C3/C4 vegetation domination, indications on temperature and precipitation Indications of openness and closedness of Ncycle, mean annual temperatures and precipitation Vegetation reconstruction Temperature reconstruction

i.a. Antoine et al., 2009a; Bokhorst et al., 2009, 2011; Bösken et al., in press; Obreht et al., 2015; Újvári et al., 2016; Vandenberghe et al., 2014; Zeeden et al., 2016 i.a. Bösken et al., in press; Buggle et al., 2009, 2014; Hošek et al., 2017; Marković et al., 2009, 2011, 2014; Újvári et al., 2016; Zeeden et al., 2016 Bokhorst et al., 2009; Bösken et al., 2018, in press; Buggle et al., 2008, 2011, 2013; Galović et al., 2011; Hošek et al., 2017; Obreht et al., 2014; Újvári et al., 2008, 2014a; Varga et al., 2011; Zech et al., 2013 i.a. Bösken et al., 2018; Marković et al., 2004, 2005, 2006a, 2006b, 2007; Sümegi, 1989; Sümegi and Krolopp, 1995, 2002; Sümegi et al., 1998, 2011, 2013, 2016. Hatté et al., 2013; Obreht et al., 2014; Schatz et al., 2011; Zech et al., 2013 Obreht et al., 2014; Schatz et al., 2011; Zech et al., 2013

3.3.2.2., 4.3.3.

Schatz et al., 2011; Zech et al., 2009, 2013 Schreuder et al., 2016; Zech et al., 2012

3.3.2.3., 4.3.3. 3.3.2.4., 4.3.4.

Stable nitrogen isotopes (δ15N) n-alkanes brGDGT

500

3.2., 4.2., 4.3.1. 3.3.1., 4.1., 4.3.1., 4.4. 3.4., 4.3.2. 3.3.2.1., 4.3.2.

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the Alps, Carpathians, and Dinarides led to the formation of the basins along the Danube hosting large river networks, particularly since the Lower/Middle Pleistocene. These acted as essential dust source areas in Southeastern Europe (Buggle et al., 2008). The Pliocene and Pleistocene uplift of the surrounding mountains resulted in enhanced erosion, coinciding with increased production of silt particles. The Carpathian Basin (also called the Pannonian or Middle Danube Basin; Fig. 1) is a large lowland basin (Fig. 1) that during the Miocene and Pliocene was covered by the Pannonian Sea/Lake (e.g. Harzhauser and Piller, 2007). The Pannonian Sea was part of the Paratethys, and became isolated during the Miocene uplift of the Carpathian Mountains. The Pannonian Sea existed for about 9 million years (e.g. Ivanov et al., 2011). Eventually, the sea lost its connection to the Paratethys and became a lake (the Pannonian Lake; Magyar et al., 1999). Its last remnant, the Slavonian Lake, dried up during the Pleistocene (e.g. Leever et al., 2011). Consequently, tectonic processes actively controlled the Quaternary landscape evolution of the Carpathian Basin. The basin inversion with NW–SE, N–S and NE-SW compression was the dominant endogenic triggering mechanism, causing uplift of the mountains at the basin margin and accelerated subsidence of basins (Bada et al., 2007). In more recent times, the Carpathian Basin has been covered mainly with Quaternary sediments, out of which loess comprises a major part beside aeolian sands and fluvial deposits. Large river systems play an important role for loess formation (e.g. Badura et al., 2013; Smalley et al., 2009). For example, it was demonstrated that the formation of the Chinese Loess Plateau (the largest loess area in the world) was directly linked to the presence of Yellow River (Nie et al., 2015). Large lowland rivers, such as the Danube and its tributaries, carry a considerable amount of fine sediment that is deposited in the river floodplains during water retreat following flooding events. During low water stands, the sediment deposited in that way represents an important pool that can easily be remobilised and transported by wind. Therefore, hydrological conditions play a major role in dust availability. This also means that changes in river discharge and sediment supply, changes in river courses and local/regional environmental conditions can be expected to have played an essential role in dust availability, dust deposition and loess formation. Consequently, for a proper understanding of loess records, it is important to understand the hydrological conditions of river basins that acted in the past as important source areas. The Danube is the second longest river in Europe and exceeds 2800 km in length. The present watershed divide between the Danube, Rhone and Rhine Rivers lies in the border region between France, Germany and Switzerland. The stream gradient in the Danube River headwaters as far as Bratislava on the western edge of the Carpathian Basin is relatively steep. In the vicinity of the Alps, large moraine systems and glaciofluvial terraces formed in response to Quaternary glaciation events (e.g. Marković et al., 2015). In comparison, the lowland part of the Danube River in the Carpathian Basin further downstream has a gentler stream gradient, because this basin is filled with thick polycyclic sedimentary deposits of Pliocene and Quaternary age (e.g. Jipa, 2014). Beside Neogene tectonic dynamics, glaciofluvial and fluvial activities in the Danube headwaters have therefore determined the timing and nature of sediment transport downstream. Pekarova and Pekar (2006) provide the long-term trends of yearly discharge time series and runoff variability at seven stations along the River Danube during the period 1901–2000. The major tributaries (Drava, Tisa (Tisza), Sava and Tamiš (Timiș)) join the Danube River in the sector between the Drava and the Velika Morava confluences, and they influence the Danube with an almost double increase in mean water discharge. When reaching the Carpathians in the southeastern Carpathian Basin, the Danube flow represents 70% of its overall drainage area, with nearly 90% of its total discharge. Consequently, the Danube tributary rivers in the Carpathian Basin are critical in the process of water and sediment transport and supply.

3. Commonly used proxy data in Southeastern Europe loess research 3.1. Grain-size and textural analyses Grain-size measurements are a fundamental technique used in aeolian sediment research (Nottebaum et al., 2014, 2015; Újvári et al., 2016; Schaetzl and Attig, 2013; Schulte et al., 2016; Stauch et al., 2012; Vandenberghe, 2013; Vandenberghe et al., 2018). The analysis of grainsize distribution is a reliable technique for determining the characteristics of aeolian transport processes. Wind transport of particles is traditionally divided into three basic categories: rolling, jumping over short distances near the surface (or saltation) and suspension (Vandenberghe, 2013). The rate and distance of such transport strongly depend on the availability of particles, grain properties (size and shape), relief and wind strength. A comprehensive overview of the characteristics of particular wind transports can be found in Kok et al. (2012), Shao (2008), Újvári et al. (2016) and Vandenberghe et al. (2013). In loess research, detailed reviews on the physical background of particle transport and deposition mechanisms were presented by, e.g. Pye (1987), Tsoar and Pye (1987) and recently by Újvári et al. (2016). However, numerous grain-size studies based on loess sediment tend to oversimplify the processes involved. Especially in loess research, additional problems for understanding the transport mechanisms arise from postdepositional weathering of particles and the formation of secondary clay minerals. Therefore, it is essential to know the primary environmental conditions under which loess forms. According to Lehmkuhl (1997), Quaternary loess was mostly accumulated in areas with steppe vegetation, which was constrained to an annual precipitation amount of 250–600 mm. In drier regions, vegetation was generally too sparse to trap dust, while in wetter areas soil development was generally more extensive and vegetation too dense. However, the Negev desert biocrusts (dominated by cyanobacteria and/or mosses) are reported as effective dust traps (Danin and Ganor, 1991; Zaady and Offer, 2010). According to examples of dust trapping under very dry conditions, recent studies have suggested that loess may also form under drier conditions than previously proposed due to the presence of biological crusted surfaces and their abilities to retain dust particles (Dulić et al., 2017; Smalley et al., 2011; Svirčev et al., 2013, 2016). The differences in the grain-size determination methodology (as multivariate sedimentological loess proxy) also make the comparison between different studies challenging. Traditional methods, such as a combination of sieving and pipette analyses, are straightforward methods. These are, however, very time-consuming, and the obtained results exhibit rather a low sample resolution and limited possibilities for establishing useful grain-size ratios. On the contrary, laser diffraction analyses allow for insight into a more extensive resolution and fraction size range (Pye and Blott, 2004). Those methods are also less time-consuming (Özer et al., 2010). However, the laser diffraction analyses show limitations in establishing reliable proportions of fine fractions, making the comparability of results by different methods at least challenging (Konert and Vandenberghe, 1997; Schulte and Lehmkuhl, 2018). One main issue lies in the non-spherical shape of fine particles, especially clays. Fine fractions with non-spherical shape are classified into larger particle size groups if illuminated orthogonally, while classified to smaller size groups if the particles were parallel to the light beam. Therefore, laser diffraction analyses lead to an overall overestimation of clay (< 2 μm particles) contributions (Eshel et al., 2004; Konert and Vandenberghe, 1997; Schulte and Lehmkuhl, 2018). In addition, different device parameters, as well as different optical models used for laser diffraction analyses may yield different results (Schulte and Lehmkuhl, 2018; Varge et al., 2018). In the Carpathian Basin both, sieving and pipette analyses (Marković et al., 2004, 2005, 2006a, 2007, 2008), and laser diffraction analyses (Antoine et al., 2009a; Bokhorst et al., 2009, 2011; Obreht et al., 2014; Vandenberghe 501

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et al., 2014; Table 1) were used in deriving grain-size data. Therefore, care has to be taken when comparing different datasets, especially regarding the finer silt and clay fractions.

diamagnetic carbonate, quartz, and feldspar content. The use of frequency dependent magnetic susceptibility (χfd) overcomes such limitations, although there are also several other (more complicated) ways to get a better insight into the pure pedogenic signals, (see, e.g. Liu et al., 2004). χfd refers to the difference of low to high-frequency susceptibility values (calculated as χfd = (χlf – χhf)[kg−1m3] or χfd = (χlf – χhf) / χlf * 100 [%], where χlf and χhf are low-frequency and high-frequency susceptibility, respectively). It is based on the principle where the threshold interval between superparamagnetic and stable single domain particles (20–40 nm) decreases with increasing frequency of the magnetic field, the relative amount of newly formed ultra-fine particles related to pedogenesis can be determined by the dependence of magnetic susceptibility on the applied frequency. Consequently, since χfd is controlled by superparamagnetic particles that are almost exclusively products of (organic) soil formation and weathering, it provides a highly sensitive proxy for soil/sediment humidity during and after dust accumulation (e.g. Buggle et al., 2014; Hambach et al., 2018; Zeeden et al., 2018a). Moreover, χfd relies on the ratio from low and high frequencies and thus represents a qualitative proxy. As a qualitative proxy, its properties are minimally influenced by a change in the grain-size distribution during deposition (e.g. possible detrital influence of multidomain particles) and changes in source areas (e.g. changes in the magnetic mineral content). Nevertheless, a possible enhancement in superparamagnetic particles at source areas from previously eroded loess (recycled loess; e.g. Licht et al., 2016) cannot be fully ruled out. Although the influence of such fine particles is rather negligible in practice, their possible impact on this proxy should not be ignored.

3.2. Rock magnetic analyses The magnetic susceptibility and other rock magnetic properties are rather common proxies in loess research. The low-frequency magnetic susceptibility (χ) is often used as a proxy for increased weathering/ pedogenesis and soil/sediment moisture (e.g. Buggle et al., 2009, 2014; Hao et al., 2008; Marković et al., 2009; Necula et al., 2013, 2015; Table 1). The principles behind this proxy in loess are based on the mineralogical homogeneity of the original unweathered loess and the neoformation of ferrimagnetic minerals in the course of silicate weathering and pedogenesis (e.g. Hambach et al., 2018; Zeeden et al., 2018a). Recent studies (Buggle et al., 2014; Újvári et al., 2016; Zeeden et al., 2016) suggest homogenous magnetic grain-size distributions and mineralogically homogenous signature of parent dust in the Carpathian Basin. In such homogeneous loess, the intensity of χ is determined by the amount and mineralogy of iron-bearing paramagnetic and ferromagnetic minerals. Especially important are magnetite and maghemite (ferrimagnetic minerals), since these have significant higher χ than hematite and goethite (antiferromagnetic minerals; e.g. Buggle et al., 2009; Thompson and Oldfield, 1986). Although those ferrimagnetic minerals may be inherited and not formed in-situ, in the areas with (rather) homogenous magnetic grain-size distributions of unweathered loess, as the Carpathian Basin, such ferrimagnetic minerals are predominantly formed during loessification and pedogenesis. Consequently, even weak weathering and pedogenesis can significantly increase the χ signal compared with unweathered loess. Another important factor controlling χ is the grain-size of magnetic particles. Magnetic grains of superparamagnetic size particles (< ~30 nm) have a significantly higher χ than stable single-domain particles (> ~30 nm particles with only one region with parallel coupled atomic magnetic moments) or multidomain particles (> ~10 μm particles with several regions with parallel coupled atomic magnetic moments; Thompson and Oldfield, 1986). In some parts of Eurasia, where the loess has accumulated as plateaus (area such as the Chinese Loess Plateau or loess plateaus in the Carpathian and Lower Danube Basins), it has been demonstrated that superparamagnetic particles form during pedogenesis precipitating from weathering solutions in the processes of soil formation (e.g. Buggle et al., 2009; Evans and Heller, 2003; Heller et al., 1991; Heller and Liu, 1984; Maher, 2011). Therefore, in these regions, χ represents an established proxy for the strength of pedogenesis. The most important factors for the formation of ferromagnetics are soil moisture, pH value, soil temperature, and the content of organic matter (Evans and Heller, 2001). Nonetheless, a reduction of χ in palaeosols, when compared with intercalated loess units, has been reported from high latitude Alaskan and Siberian loess deposits. This phenomenon has been explained by an increased wind strength during glacial periods, which more efficiently transports dense iron oxide particles (e.g. Evans, 2001). Soils developed during warmer intervals when aeolian transport of dense magnetic particles from the parent sources did not play an important role. However, such processes are generally negligible in the mid-latitude Eurasian loess belt (Buggle et al., 2014; Hambach et al., 2018; Zeeden et al., 2018a). Although χ is a widely used proxy in loess research including the mid-latitude Eurasian loess belt, it may be slightly biased by magnetic grain-size distribution, and rapid shifts in the source area and thus the parent material. This is because multidomain particles still play an important role in the general enhancement of χ. However, multidomain particles (albeit rarely) may be associated with detrital grains originating from far distant transport, and therefore may be more influenced by a change in the source area. Moreover, since χ is also a mass/volume dependent proxy, it is controlled by the change in concentration of

3.3. Geochemistry 3.3.1. Inorganic geochemistry Geochemical analysis of loess constitutes a powerful tool for tracing the provenance, source area, and the degree of sediment weathering in general. Several geochemical studies, mostly using X-ray fluorescence (XRF), were conducted for establishing the general provenance or the source area of the sediment material in the Carpathian Basin (Buggle et al., 2008; Újvári et al., 2008, 2012; Obreht et al., 2015; Table 1). However, detailed analyses of elemental geochemistry in Southeastern European loess showed that bulk geochemistry results cannot always determine the provenance of loess, but rather can rule out some hinterlands as potential sources (Buggle et al., 2008; Újvári et al., 2008). Despite general limitations in establishing the provenance of the particles, elemental geochemistry has turned out useful in demonstrating the local source area of loess, confirming previously proposed source area of the Danube River and its confluence valleys (Smalley and Leach, 1978). However, the specific provenance of the dust remained unknown. A detailed zircon U–Pb dating of the loess sediments in the Chinese Loess Plateau yielded very detailed and precise results regarding the provenance of the sediment (Nie et al., 2014, 2015; Stevens et al., 2010, 2013). Újvári et al. (2012) applied Sr-Nd isotopic data, detrital zircon U–Pb ages, and bulk and clay mineralogy to provide new insight into the provenance of the Carpathian Basin loess. It was shown that besides Danube alluvial fans and late Pleistocene fluvial sands accumulated by the palaeo-Danube, eroded uplands and local rocks throughout the basin were also important source areas (Újvári et al., 2012). Especially promising approaches are the U–Pb geochronology of detrital rutile grains (Újvári et al., 2013) and Hf isotopic composition of zircons (Újvári and Klötzli, 2015) since these provide more detailed information on loess sources. However, such analyses are limited to the north-easternmost parts of the Carpathian Basin, and the full potential of these methods has still to be explored. Beside provenance, several studies used chemical indices for investigating the degree of weathering in loess records from the Carpathian Basin (Buggle et al., 2008, 2011; Bösken et al., 2018, 2019; Obreht et al., 2015; Újvári et al., 2014a; Varga et al., 2011). Generally, 502

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chemical weathering indices are relying on the concept of mineral alteration, where the selective removal of soluble and mobile elements from a profile section is compared to a relative enrichment of immobile and non-soluble elements (e.g. Buggle et al., 2011; Fedo et al., 1995; Harnois, 1988; Kronberg and Nesbitt, 1981; Nesbitt and Young, 1982; Yang et al., 2004). Numerous elemental ratios have been used for the reconstruction of palaeoenvironmental conditions of loess sequences (e.g. Bokhorst et al., 2009; Buggle et al., 2011; Krauß et al., 2016; Meyer-Heintze et al., 2018; Muhs et al., 2008; Obreht et al., 2014; Varga et al., 2011). However, a robust understanding of the weathering inferred from such ratios is still missing. During the last years, some studies attempted to establish more appropriate weathering indices that are not biased by other proxies or change in the source area, e.g. grainsize distribution, carbonate content (Buggle et al., 2011; Schatz et al., 2015; Yang et al., 2006). An extensive overview of the evaluation of weathering indices is presented by Buggle et al. (2011). The concept of weathering indices is establishing a most appropriate ratio between soluble and mobile elements, and immobile non-soluble elements. Elements such as Al, Si, Ti, K, Ba, Rb and Zr are most frequently used as non-soluble elements in weathering indices because these form insoluble hydrolyzates (Buggle et al., 2011). However, Ti, Zr, and Si are relatively sensitive to changes in parent material composition, while K, Ba, Rb are not always recommended since under intense weathering conditions significant loss of these elements can occur during the transformation of micas, feldspars and other host minerals into secondary clay minerals (Gallet et al., 1996; Muhs et al., 2001). Elements Ca, Mg and Sr are soluble elements common in silicate minerals such as plagioclase, pyroxene, amphibole and biotite, and are susceptible to weathering (Nesbitt et al., 1980). However, in a parent material containing carbonate, the mobility of these elements is predominantly controlled by the behaviour of calcite and dolomite, which make these elements difficult to interpret in terms of weathering in loess. Probably the most suitable selection of mobile elements are Na and K since these elements are hosted in the same mineral group as Al (e.g. feldspars). Therefore, the widely used Chemical Index of Alteration (CIA) = Al2O3 / (Al2O3+Na2O+CaO*+K2O)) * 100 (where CaO* is silicatic CaO; Nesbitt and Young, 1982) is probably one of the most appropriate weathering indices. However, Buggle et al. (2011) specified ratios relying only on Na as the only soluble element, and established the Chemical Proxy of Alteration (CPA) = (Al2O3/ (Al2O3+Na2O)) * 100 (also known as CIW´ (Cullers, 2000)) as the most suitable geochemical proxy of silicate weathering for loess-palaeosol sequences, excluding potential effects arising from the stronger weathering resistance of some K phases such as K-feldspar and K fixed on clay (Buggle et al., 2011; Nesbitt and Young, 1984; Yang et al., 2004). Nevertheless, the ratio between Al2O3, Na2O+CaO* and K2O (usually presented as A-CN-K diagram; Nesbitt and Young, 1984), represents an important ratio informing about weathering and sorting effects of aluminosilicates, as well as the initial composition of the unweathered material (e.g. Nesbitt and Young, 1989; Nesbitt et al., 1996).

the degree of physiological water stress. Probably the most obvious changes in δ13C for organic carbon from loess samples would appear with a shift in the photosynthetic pathway. The C3 photosynthetic pathway is a dominant way of photosynthesis in higher plants, where plants use the Calvin cycle during CO2 fixation. The net acquisition of carbon by C3 photosynthetic organisms is catalysed by ribulose-1.5-bisphosphate carboxylase/oxygenase (Rubisco). However, Rubisco evolved early in the history of life (more than 3 billion years ago; Hayes, 1994) when the CO2 content of the atmosphere was orders of magnitude greater than today (Holland, 1994). Therefore, Rubisco is less efficient in low atmospheric CO2 environments. C4 grasses developed another carbon-fixing enzyme in addition to Rubisco, such as phosphoenolpyruvate carboxylase. This evolutionary progress enabled C4 plants to use atmospheric CO2 more efficiently. C4 plants are more selective when using heavier carbon isotopes than C3 plants. The average values of δ13C from organic carbon for C3 plants range between −27 and −25‰, while the average values of organic carbon δ13C in C4 plants are between −15 and −12‰ (O’Leary, 1981; von Caemmerer et al., 2014). Thus, major changes from a C3 to a C4 plant community generally induce large amplitude changes in δ13C values, in the order of more than 9 ‰ enrichment of δ13C in the organic material (Boutton et al., 1998). Since C4 plants use carbon from CO2 more efficiently than C3 plants, lower atmospheric CO2 levels would favour C4 plants. Moreover, species using the C4 photosynthetic pathway have higher water use efficiency and a higher temperature optimum for net primary production (Collatz et al., 1998; Ehleringer et al., 1997; O’Leary, 1981; Hatté et al., 1999, 2001). Therefore, higher atmospheric CO2 levels, lower temperature, and higher rainfall favour C3 plants over C4 plants (Sage et al., 1999), where temperature effects generally dominate over changes in precipitation (Hall et al., 2012). Consequently, δ13C values in soils, palaeosols and terrestrial sediments can reflect palaeoenvironmental conditions. Care has to be taken for possible changes in the isotopic composition due to degradation effects (Wynn et al., 2006) and/or selective mineralization of organic matter (Veres et al., 2009). However, in the Carpathian Basin, the absence of any systematic enrichment in the palaeosols as compared to the loess units suggests that degradation effects did not significantly alter the carbon isotope values (Hatté et al., 2013; Schatz et al., 2011; Zech et al., 2013). 3.3.2.2. Stable nitrogen isotopes (δ15N) composition of organic matter. The isotopic composition of soil nitrogen (δ15N) from organic matter has been widely used to study soil biogeochemical cycles (Ross and Hales, 2003; Houlton et al., 2007; Wang et al., 2014). These studies focused on the nitrogen biogeochemical cycle in modern ecosystems. However, due to the complexity of the N-cycle, only a few studies have used δ15N values for palaeoenvironmental reconstructions (Liu and Liu, 2017; Obreht et al., 2014; Schatz et al., 2011; Zech et al., 2013; Table 1). In modern environments, changes in the soil δ15N values are usually positively related to mean annual temperatures and negatively related to mean annual precipitation (Martinelli et al., 1999; Houlton et al., 2007; Zhou et al., 2014). A positive correlation of δ15N values with increasing temperatures is likely due to the decomposition rates of soil organic matter with increasing temperatures, causing the loss of soil organic matter and the enrichment of 15N in soils (Zhou et al., 2014). The origin of the observed negative relation between δ15N and higher precipitation is not clear, although it is suggested that soil microbial activity is restrained by increasing precipitation and soil moisture, which leads to depletion in δ15N values (Liu and Liu, 2017). However, changes in the nitrogen isotope enrichment in soils are most likely an imprint of changes in the N-cycle, more specifically the openness and closedness of the N-cycle. In open N-cycle environments, isotopically depleted inorganic nitrogen is preferentially released by nitrification or denitrification reactions and subsequently leached or degassed from the pedosphere (Krull and Skjemstad, 2003; Obreht et al., 2014; Zech et al., 2011). Hence, isotopic enrichment of the

3.3.2. Organic geochemistry 3.3.2.1. Stable carbon isotopes (δ13C) composition of organic matter. Using stable carbon isotopes (δ13C) of organic matter in reconstructing Quaternary climate variability based on loess is a relatively widely used method (Hatté et al., 1998, 1999, 2001; Obreht et al., 2014; Yang and Ding, 2006; Yang et al., 2015; Zech et al., 2013). However, it has been only recently applied in the Carpathian Basin (Hatté et al., 2013; Schatz et al., 2011; Zech et al., 2013; Table 1). The most important factors controlling the isotopic signal in soils and sediments are changes in the photosynthetic pathway (shifts from C3 to C4 vegetation), physiological water stress, and changes in the atmospheric CO2 concentration (O’Leary, 1988; Hatté et al., 1999). Therefore, with already known past CO2 concentration, δ13C is a useful proxy for inferring past photosynthetic pathways and 503

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remaining soil nitrogen can be taken as an indicator for open N-cycles. In closed N-cycles there is only a small loss of isotopically depleted inorganic nitrogen due to low input rates or high uptake rates of N. Therefore, δ15N values have the potential to provide information on the relation between precipitation and temperatures, but more importantly about the openness or closedness of the N-cycle in the ecosystem.

pH were reconstructed by using the methylation of branched tetraethers (MBT) and the cyclisation of branched tetraethers (CBT) indices, based on the amount of methyl branches (4–6) and cyclopentyl moieties (0–2) of the brGDGTs (Weijers et al., 2007). Furthermore, also the relative abundance of brGDGTs compared to that of crenarchaeol (an isoprenoid GDGT that is produced in soils by ammonia oxidising archaea) has been shown to relate to soil moisture availability, and can be quantified in the Branched and Isoprenoid Tetraether (BIT) index (Hopmans et al., 2004).

3.3.2.3. N-alkanes. N-alkanes are organic compounds resistant to biochemical degradation and diagenesis, and consequently of high importance in palaeoclimatic studies (Cranwell, 1981). In terrestrial environments, n-alkanes are important constituents of natural waxes. Cuticular plant leaf waxes are characterised by long-chain (25 to 33 carbon atoms; nC25 – nC33) n-alkanes, with a strong odd-over-even predominance (OEP = (C27+C29+C31+C33) / (C26+C28+C30+C32)) (Bush and McInerney, 2015; Kolattukudy, 1976; Schreuder et al., 2018; Zech et al., 2009, 2013). Short-chain n-alkanes (nC15–nC19) are usually related to microbes and/or indicate aquatic sources (Li et al., 2016; Zech et al., 2009). With leaf litter, long-chain n-alkanes are deposited and stored in soils and sediments. Since the nC31 and nC33 are mainly found in grasses and herbs, and nC27 and nC29 are reported to dominate in trees and shrubs, the ratio between these n-alkanes has the potential to indicate the ratio between trees and shrubs versus grasses and herbs. Therefore, n-alkanes represent a valuable proxy in palaeovegetation reconstruction. In the Carpathian Basin, vegetation reconstructions based on n-alkanes were conducted at Crvenka (Zech et al., 2009, 2013) and Tokaj (Schatz et al., 2011; Table 1). However, it has been demonstrated that the (C31+C33) / (C27+C29) ratio decreases during decomposition and formation of soil organic matter as a result of organic matter degradation. Nevertheless, the OEP seems to be a useful proxy for assessing leaf wax degradation (Zech et al., 2009). Taking this into consideration, Zech et al. (2009) conducted an end member model considering the different degree of organic matter degradation in loess and soil. Employing modelling results allowed for a more reliable palaeovegetation reconstruction in the Carpathian Basin. However, this model has shown several limitations, e.g. the most accurate results can be expected for samples with high OEP values, whereas accuracy decreases with decreasing OEP values due to the converging degradation lines. Accuracy is furthermore limited by the scattering of the modern dataset and as a result, modelling can yield negative percentage values for tree- or grass-derived alkanes, respectively).

3.4. Malacology Continuous, relatively fast and undisturbed sedimentation of loess, as well as its relatively high carbonate content, are suitable preconditions for good preservation of Quaternary calcareous fossils (e.g. Ložek (1990)). Thus, fossil remains preserved in loess provide valuable records for palaeoenvironmental studies. Among these, mollusc shells represent the most wide-spread and best studied fossil remains (e.g. Marković et al., 2004, 2007; Moine, 2014; Moine et al., 2005, 2008; Osipova et al., 2013; Sümegi and Krolopp, 2002; Sümegi et al., 2013, 2016; Wu et al., 2018). Mollusc shells are usually well preserved only in pure (calcareous) loess, while in the (fossil) soils and aeolian loams the shells are rapidly corroded and dissolved (Ložek, 1990). Therefore, the presence of shells is a good indicator of generally dry conditions predominantly related to glacials, while very humid (usually interglacial) conditions lead to the dissolution of shells in mature palaeosols. Molluscs are highly sensitive to environmental changes, and consequently, their shell remains represent an excellent palaeoenvironmental proxy. According to Ložek (1990), the three main groups of malacofauna found in European loess are Pupilla, Columella and Helicopsis Striata fauna. The Columella fauna is representative of cold stadial conditions and relatively hygrophile facies. In contrast, the Helicopsis Striata fauna prefers warmer and driest (xerothermic) environments and is an important element of steppic environments. The Pupilla fauna is assumed to reflect mainly intermediate environmental conditions. In the Carpathian Basin, malacological investigations highly improved our understanding of the Quaternary environments, especially over the last glacial cycle (e.g. Bösken et al., 2018; Marković et al., 2004, 2007; Sümegi and Krolopp, 1995, 2002; Sümegi et al., 1998, 2011, 2012, 2013; Table 1). Moreover, based on current mollusc distribution and its relation to temperature, Sümegi (1989) proposed a mollusc-based proxy for inferring palaeotemperatures. A malaco-thermometer method (Sümegi, 1989; Sümegi and Krolopp, 2002) is based on the patterns of 16 dominant gastropod species from a composite malacofauna (Sümegi, 2005). For selected gastropod species, the optimal climatic condition could be determined along with the minimum and maximum temperate values of tolerance (activity range of gastropods), with the help of data from meteorological stations. The estimated palaeotemperature values are valid only for the vegetation (growing) period, because the studied gastropod species are active only during certain times of the year. The malaco-thermometer method is based on the following equation (Sümegi, 1989, 2005):

3.3.2.4. Branched glycerol dialkyl glycerol tetraethers. Archaea and other bacteria are abundant in marine and terrestrial aquatic environments, sediments, and soils (Hinrichs et al., 1999; Pearson and Ingalls, 2013; Sinninghe Damsté et al., 2000). Many Archaea and soil bacteria produce membrane lipids called glycerol dialkyl glycerol tetraethers (GDGTs). These organisms are able to adjust their membrane permeability and fluidity to changing environmental conditions, for example pH and temperature, by varying the exact molecular composition of the membrane lipids. After their deposition, these signatures are preserved in soils and sediments (Weijers et al., 2007, 2010). Consequently, analysis of GDGTs in sedimentary archives may offer interesting new approaches for quantitatively reconstructing past climate and environmental variability. For marine and lake sediments it has been shown that the degree of cyclicity of isoprenoid GDGTs correlates with temperature, and as a result the TEX86 index (tetraether index of 86 carbon atoms) used for sea/lake surface temperature reconstruction was established (Schouten et al., 2002). A group of membrane lipids constitute the branched GDGTs (brGDGTs), which have been detected in peat bogs and soils (Sinninghe Damsté et al., 2000). Therefore, brGDGD may be used in the temperature and pH reconstruction in terrestrial environments, especially soils and loess (e.g. Jia et al., 2013; Peterse et al., 2011, 2014). In the Carpathian Basin, brGDGTs have been used for temperature and pH reconstruction at the Crvenka (Zech et al., 2012) and Surduk (Schreuder et al., 2016) sections (Table 1). Past changes in mean annual temperature and soil

T=

n AT i=1 i i n A i=1 i

T = Estimated July palaeotemperature [ °C ] Ai = The abundance of a given species i in the sample Ti = The optimum temperature of a given species i in the sample n = The number of species used for the estimation In addition to malacological investigations on the species distribution, Újvári et al. (2017) used the stable isotope composition of mollusc shells to reconstruct the palaeoenvironment, showing the potential of this method. Beside palaeoenvironmental reconstructions, mollusc 504

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shells are useful material for radiocarbon dating. Recent advances in dating techniques and mollusc shells selection yielded very promising results in dating of loess profiles from the Carpathian Basin for the past 40 ka (Újvári et al., 2014b, 2017).

fine dust was likely present (Zeeden et al., 2016). During interglacials, far distance transported fine background dust had a notable contribution in fine dust (Albani et al., 2015), but still representing a minor component in silt-dominated bulk loess (Varga et al., 2013). However, our knowledge is still limited in detailed understanding of dust deposition in the Carpathian Basin. We point to open questions regarding the uncertainty on the degree to which the dust deposits of the Carpathian Basin were derived from local (Danube and its tributary) or distant sources, and whether the finest dust fractions from the Carpathian Basin have a wider, hemispheric influence.

4. Discussion 4.1. Provenance and source areas of dust in the Carpathian Basin and surrounding regions In the Carpathian Basin, loess deposits reach their thickest and most complete distribution in Europe, with several sequences potentially extending to the Early Pleistocene (Marković et al., 2011, 2015). Especially thick loess deposits are found close to the Danube River, pointing to the Danube and its tributaries as the main proximal source areas for dust particles (Smalley and Leach, 1978). More recent studies based on provenance related XRF geochemical investigations of loess (Buggle et al., 2008; Újvári et al., 2008, 2012; Obreht et al., 2015) confirmed the Danube catchment as the dominant source area, although those studies could not rule out contributions of other regional dust sources, or clearly determine the precise sediment provenance. Thus, to fully understand the principles of dust formation, shifts in sources, and depositional patterns in this region, it is crucial to determine whether the Carpathian Basin received high dust input from other distal dust pools, or if it was acting as a dust source area for a wider region. Based on clay mineralogical and Sr, Nd and Hf isotopic data, Újvári et al. (2015) proposed that besides Northern Mongolia and the Chinese Loess Plateau deposits that appear to be the most likely Northern Hemisphere dust sources, the Carpathian Basin cannot be excluded as a potential contributor to the Greenland dust as well (especially during the Last Glacial Maximum). Besides a potential contribution of the Carpathian Basin fine dust to the Northern Hemisphere, it still remain unclear to which extent coarser particles influence surrounding areas. Geophysical (environmental magnetic) and geochemical (XRF based element composition) analyses from loess sections distant (~120 km south) from the Danube River (Basarin et al., 2011; Obreht et al., 2014, 2016), located in the Central Balkans area, indicate a clear domination of the local rivers and their alluvium as source material for loess. Almost no indication of the influence related to the Danube or its catchment alluvium (Fig. 2) could be detected, but this has not been investigated in detail and therefore may have been overlooked. This suggests that river plains and loess from the Carpathian Basin were not important contributors to the loess deposits in the Central Balkans. Thus, it is still debatable weather the Danube and its tributaries acting as the main dust pools for proximal loess records the Carpathian Basin could be regarded as an important dust source for the wider region. Accordingly, it is reasonable to question if the Danube catchment area were the major source for the loess within the Carpathian Basin that is distant from the lowland rivers. Zeeden et al. (2016) reported that loess around Semlac (western Romania) shows a remarkably fine grain-size distribution. This suggests that the source may not have been related to large lowlands rivers as generally suggested for the loess deposits in the Carpathian Basin, but rather related to the continuous accumulation of far distance travelled dust particles. Low sand and high fine particle contents in these sections further support this suggestion. However, although the Semlac section indicates different source area than lowland rivers, it does not indicate that alternative source areas are not necessarily related to other dust pools in the Carpathian Basin. Additionally, for the fine dust input an important distal source may be the Sahara (Stuut et al., 2009). Supply of Saharan dust have had more importance especially during interglacial periods (Longman et al., 2017), contributing to the fine dust deposition during soil formation processes with 20–30% (Varga et al., 2013, 2016). Consequently, dust deposition was a continuous process in the Carpathian Basin; during glacials more dust originated from local sources, although background

4.2. Stratigraphy, regional to continental correlation, and millennial-scale climatic oscillations in loess Establishing a common stratigraphy for loess in the Carpathian Basin, and therefore enabling a regional environmental and climatic reconstruction is a very challenging task. The size of the Danube loess belt, and a large number of local stratigraphies reflecting nomenclatures within the countries sharing this region present major limiting factors in developing a unified approach that shall enable a coherent regional correlation of loess deposits. However, Marković et al. (2015) proposed a common loess stratigraphy for the whole region of the Danube loess belt, aiming for a common pan-European loess scheme on the glacial-interglacial time scale. In general, loess in the Carpathian Basin shows a good pedostratigraphical correlation to the other Danube loess regions (Buggle et al., 2009, 2013; Fitzsimmons et al., 2012; Fig. 3), as well as to the Chinese Loess Plateau (Marković et al., 2012a, 2015; Zeeden et al., 2018b; Fig. 4). Nevertheless, the simple stratigraphical correlation among sections can sometimes be misleading. Illustrative examples are the

Fig. 2. Magnetic susceptibility versus frequency dependent magnetic susceptibility from Semlac (Zeeden et al., 2016, 2018d) and Stalać (Obreht et al., 2016). Note that at Semlac, as an example from homogeneous loess in the Carpathian Basin, magnetic susceptibility and frequency dependent magnetic susceptibility both increase with enhanced weathering and pedogenesis, while the environmental magnetism of inhomogeneous loess from the Balkans, like at Stalać, is order of magnitude higher and solely related to the changes in the source area. 505

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Fig. 3. Correlation of the magnetic susceptibility records from the longest measured loess profiles in the Danube Basin region, along with the global Plio-Pleistocene stack of Lisiecki and Raymo (2005) (modified after Marković et al. (2015)). BMM stands for detected Brunhes-Matuyama boundary.

famous Stari Slankamen (Marković et al., 2011) and Paks (Sartori et al., 1999) sections that were exposed to erosional events resulting in the erosion of certain palaeosols. While such erosional events are clearly notable in the Stari Slankamen section, the gap in the accumulation at the Paks section was detected only after numerical dating was applied (Frechen et al., 1997; Thiel et al., 2014). Moreover, Kostić and Protić (2000) attempted to correlate the Batajnica and Stalać sections based on palaeosol layers and concluded that palaeoclimatic evolutions of the Central Balkans and the southern Carpathian Basin were similar. However, recent investigations reevaluated the stratigraphy of the Stalać section based on numerical dating (Bösken et al., 2017) suggesting instead remarkably different palaeoclimatic conditions between the Central Balkans region and the south of the Carpathian Basin (Obreht et al., 2016). With this in mind, establishing a common chronostratigraphy for the mid-latitude Eurasian loess belt cannot rely on observed similarities in lithostratigraphy, but has to be further tested and confirmed by comparison of multi-proxy data and numerical dating. Although numerous methods and an increasing number of multi-proxy studies were conducted in the Carpathian Basin (i.a. Antoine et al., 2009a; Obreht et al., 2015; Újvári et al., 2015, 2017; Schatz et al., 2011; Stevens et al., 2011; Zech et al., 2013), these studies were limited to the last glacial-interglacial cycle. Studies for longer time intervals were usually conducted in considerably lower resolution, partly insufficient for intercontinental comparison (e.g. Buggle et al., 2013, 2014), or more often limited only to studies of χ as palaeoclimatic proxy (Basarin et al., 2014; Marković et al., 2009, 2011, 2012b; Sartori et al., 1999). Consequently, at this point it is only possible to test proposed similarities in climate patterns over the mid-latitude Eurasian

loess belt by comparing fluctuations in χ. Fig. 4 shows a similar stratigraphy and χ variations among mid-latitude loess from the Carpathian Basin and China, indicating the rather common climatic teleconnection over Eurasia as defined from loess-palaeosol records on orbital timescale. However, χ fluctuations in these records on glacial-interglacial time scales are not always in agreement, e.g. at ~200 ka and ~300 ka (Zeeden et al., 2018b; Fig. 4). Observed inconsistencies in specific time intervals between the Carpathian Basin and Chinese Loess Plateau should not be surprising since even the continental correlation and the origin of similarity is still not understood in detail. Soil development is clearly dependent on the local source of humidity, where higher humidity intensifies soil formation. Therefore, the formation of interglacial palaeosols does not necessarily have to be entirely synchronous among sections spanning different geomorphological conditions, different precipitation regimes and/or soil humidity, and asynchronous sedimentation rates. A good example is the assumption of a uniform MIS 3 soil formation. It has been shown that climatic conditions over Southeastern Europe during MIS 3 were different and sometimes even opposing between the Carpathian Basin, Lower Danube Basin and Balkans (Obreht et al., 2016, 2017). Nevertheless, already within the Carpathian Basin, the MIS 3 soil formation shows clearly different patterns. The southern part of the Carpathian Basin is characterized by intensive MIS 3 soil formation after ~40 ka (Bokhorst et al., 2009, 2011; Stevens et al., 2011), albeit south-west sections also preserved older weaker palaeosols during MIS 3 (e.g. Antoine et al., 2009a; Wacha et al., 2013). In contrast, the northern part of the Carpathian Basin shows more diverse soil formation patterns during MIS 3, where the northwestern part 506

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Fig. 4. Comparison of a marine benthic δ18O isotope stack (Lisiecki and Raymo, 2005; top, black), an ice volume model (Imbrie and Imbrie, 1980; green), June insolation for 65 °N (Lourens et al., 1996; orange), a magnetic susceptibility record from Yimaguan in China (Hao et al., 2012; red), and the standardized Southeastern European magnetic susceptibility records from Titel by Marković et al. (2012; black) and Basarin et al. (2014; black). Magnetic susceptibility is standardised, where the values are scaled to a mean of zero and a standard deviation of 1. ‘T’ denotes tephra layers. Modified after Zeeden et al. (2018a). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

experienced the strongest pedogenesis in early MIS 3 (Hošek et al., 2017; Novothny et al., 2011; Meyer-Heintze et al., 2018). Pedogenesis in the northeastern part was the strongest in late MIS 3 (Bösken et al., in press; Schatz et al., 2011). Therefore, labelling the palaeosols with any proposed scheme (e.g. L1SS1; Marković et al., 2015) has to be treated with caution since such weakly-expressed palaeosols are not necessarily coeval. This points to a need of independent chronological frameworks for each investigated site before firm regional correlations of loess records could be achieved within, for example, MIS 3. One of the potential approaches to bridge this problem is the use of correlative age-models supported by both numerical dating and tephrochronology (e.g. Obreht et al., 2017; Veres et al., 2013; Zeeden et al., 2018a). However, it is still debated whether the Carpathian Basin loess recorded climate pacing related to Greenland Stadials (GS) and Interstadials (GI) (e.g. Stevens et al., 2011), and thus, use of any correlative age model needs proper statistical testing. Although GS and GI are recorded in many Northern Hemisphere records (e.g. Fleitmann et al., 2009; NorthGRIP Community Members, 2004; Sirocko et al., 2016; Veres et al., 2009; Wang et al., 2001; Yang and Ding, 2014), a clear pacing related to all GI and GS events has not been unequivocally recognized in the Carpathian Basin loess proxy data so far. Recently, Veres et al. (2018) demonstrated that palaeosol development in Eastern Europe loess is not only related to GI, indicating complex hydroclimate variability over the mid-latitudes loess in Europe. In contrast to conventional proxy data, Újvári et al. (2017) recently demonstrated that dust mass accumulation rates from the Carpathian Basin loess match

fluctuations in Greenland dust record (Rasmussen et al., 2014), indicating strong correlation with GS and GI. Therefore, in order to securely detect the pacing of GI and GS in the proxy data in the Carpathian Basin, appropriate proxies able to reflect such short-term environmental changes in loess shall be targeted (Újvári et al., 2017). Újvári et al. (2016) observed that dust mass accumulation rates do not always covary with the grain-size data. It has to be noted that the grain-size distribution can be influenced by numerous factors beyond a straightforward relation to climate changes (e.g. proximity to source area, sediment availability, environmental conditions at the source, trapping mechanisms, and even on the basic physical background of dust particle transport and deposition mechanisms (Újvári et al., 2016; Vandenberge et al., 2013)). Thus, this proxy needs to be considered with care for palaeoenvironmental climatic reconstruction when related to GS and GI variability, although on the orbital time scales the grain-size as a proxy may be more representative of the general palaeoclimate variability. Magnetic susceptibility, as an indicator of soil moisture, and thus less influenced by non-climatic processes, may be a more reliable proxy. However, magnetic grains influencing χ may also partly be detrital, consequently, not necessarily representing a pure climatic signal. Nevertheless, χfd holds the potential that in the great extent overcome such limitations. For example, χfd is a sensitive record for humidity changes in sediment, but at the same time it is minimally related to changes in provenance since it represents a ratio from low and high frequencies of magnetic susceptibility. Especially useful is using a χ vs. χfd scatter diagram, which indicates a humidity-induced 507

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weathering pattern in homogeneous loess (Fig. 2). If χ and χfd data are plotted on a line which reflects the trend of magnetic enhancement via the formation of super-paramagnetic particles (e.g. Zeeden et al., 2016), it may be concluded that the increase in χ is related to an increase in precipitation and/or humidity, as well as subsequent weathering (Fig. 2). When χ and χfd data are not in line and show relatively high or low χ in relation to χfd, and thus plotting outside the moisture-induced enhancement trend, predominance of inhomogeneous dust pools and/ or strong contribution of the detrital grains is very likely. However, care has to be taken since χfd can also be affected by more local sources of soil moisture rather than a regionally representative signal (Obreht et al., 2017). Moreover, especially short events may not be recorded in full extent by (frequency dependent) magnetic susceptibility, because the (microbial) formation of small magnetic active particles is non-instantaneous and may require some time. Therefore, the expression of high-frequency signals (as Greenland Interstadials) may be expected to be non-linear and suppressed. In such cases, the neoformation of magnetic particles may be approximated by a smoothing or low-pass data filtering (Zeeden et al., 2018c). Consequently, especially for the last glacial climatic evolution, understanding the soil formation patterns and relation to GS/GI in the Carpathian Basin is crucial for improving our knowledge in regional palaeoclimate and environment interactions. Answering this question demands improved resolution in correlative and numerical dating. For the younger part of the Upper Pleistocene, significant advances have been recently made using highresolution optically stimulated luminescence (OSL) dating on quartz at Veliki Surduk section on Titel loess plateau (Perić et al., in press) and radiocarbon dating of molluscs at Dunaszekcső (Újvári et al., 2014b, 2017). Unfortunately, reliable dating from these sections is still limited to the past 40 ka, what is the limit of radiocarbon dating and beyond this time interval OSL ages often provided contrasting results. New approaches relying on high-resolution feldspar post-IR Infra-Red Stimulated Luminescence dating (e.g. Buylaert et al., 2012; Stevens et al., 2018) and tephrochronology combined with correlative techniques (e.g. Obreht et al., 2017; Zeeden et al., 2018b) might help in closing this gap. Moreover, establishing a regionally representative tephrostratigraphic framework for loess records (Horváth, 2001; Veres et al., 2013; Marković et al., 2015) would open new perspectives in comparing the palaeoclimatic signals archived in loess, as well as their relationship with lake and marine records in southeastern Europe (ie., Govin et al., 2015).

scale (Bokhorst et al., 2009; Buggle et al., 2011; Újvári et al., 2014a; Varga et al., 2011). Nevertheless, high-resolution geochemical investigations suggest that application of weathering indices for reconstruction of short-scale climatic fluctuations may be challenging, since their values are influenced by several other factors besides weathering, such as initial grain-size and carbonate distribution, and may not represent a pure climatic signal (Bösken et al., in press; Obreht et al., 2015; Schatz et al., 2015). Consequently, the use of weathering indices for detecting short-term climatic changes is challenging, suggesting that weathering acts on longer timescales and is not necessarily sensitive to short-scale climatic oscillations. Grain-size distributions, a widely used proxy for palaeoclimate reconstruction in the Carpathian Basin loess (Antoine et al., 2009a; Bokhorst et al., 2009, 2011; Bösken et al., in press; Marković et al., 2007, 2008; Újvári et al., 2016; Vandenberghe et al., 2014; Varga et al., 2012; Zeeden et al., 2016), allow for similar conclusions with these derived from environmental magnetics, pointing to much colder climatic conditions and stronger wind dynamics during glacials. The grain-size record often documents increased amounts of clay and fine particles in palaeosols, suggesting weaker wind dynamics and stronger clay formation due to pedogenesis in those periods (e.g. Antoine et al., 2009a; Marković et al., 2008; Vandenberghe et al., 2014; Újvári et al., 2016). Moreover, grain-size data allows for insights into past atmospheric circulations and palaeowind directions. The dominant wind systems of the Carpathian Basin had a northerly and/or northwesterly direction (Bokhorst et al., 2011; Sebe et al., 2011), although the southeastern area was likely exposed to local southeasterly winds (Gavrilov et al., 2018; Obreht et al., 2015). Surrounding areas of the Carpathian Basin strongly support such climatic evolution on a multimillennial scale (where the coherence between those records on the millennial scale still has to be evaluated in more details; see Chapter 4.2.). Cold glacial conditions are inferred from lacustrine records in the Carpathians (Magyari et al., 2014) and Black Sea records (Ménot and Bard, 2012; Wegwerth et al., 2015), as well as speleothem records from Romanian Carpathians (Staubwasser et al., 2018) and Turkey (Fleitmann et al., 2009; Ünal-İmer et al., 2015). In general, there is a lack of MIS 5 climate archives for surrounding areas, but generally warm MIS 5 conditions are inferred from the records from Ohrid (Sadori et al., 2016) and Tenaghi Philippon (Tzedakis et al., 2006), as well as marine palaeotemperature records from the Mediterranean Sea (Wang et al., 2010). In the southern part of the Carpathian Basin, an unusual trend is observed where coarser grain-size particles are present in L1LL2 loess layers (mostly related to MIS 4) when compared to L1LL1 (mostly related to MIS 2) (e.g. Bokhorst et al., 2009, 2011; Marković et al., 2008, 2015). Although this cannot be explained by the general Northern Hemisphere climatic evolution, where the most severe conditions occurred during MIS 2, a colder MIS 4 in the Carpathian Basin is also supported by χfd, which also shows (although less clearly expressed) a similar trend (e.g. Basarin et al., 2014). We speculate that a colder MIS 4 than MIS 2 in the southern Carpathian Basin may be explained by a southward shift in the Westerlies during late MIS 3 and MIS 2 due to a larger extent of the Fennoscandian ice-sheets and a possible intensification of the Siberian High (Obreht et al., 2017; Ünal-İmer et al., 2015). Especially for the MIS 2, southwards shift in the jet stream is supported by modelling studies (Laîné et al., 2008; Riviére et al., 2010; Pausata et al., 2011; Merz et al., 2015). During MIS 4, westerlies likely followed a north-westerly route as suggested by an oxygen isotope record from Dim cave (Ünal-İmer et al., 2015) bringing colder air masses from North Atlantic area deeply into the basin. MIS 3 was characterised by more dynamic chages in westerlies, where during interstadials (stadials) these had a more northerly (southerly/zonal) track (for more details see Újvári et al., 2017). A general shift towards a more southern track after ~40 ka is suggested by several studies (Obreht et al., 2017; Ünal-İmer et al., 2015). This trend continued into MIS 2, where the southward track in the westerlies induced wetter and warmer air

4.3. Climatic and environmental reconstruction of the last glacial 4.3.1. Climate reconstruction based on rock magnetics and grain-size proxies The majority of high-resolution multi-proxy studies in the Carpathian Basin are limited to the last glacial cycle. Consequently, more attention is given to the last glacial climate and environmental reconstruction than to older time periods recorded in the Carpathian Basin loess sections. Magnetic susceptibility and its frequency dependence were the most often used proxies for inferring palaeoclimate conditions in the Carpathian Basin loess (e.g. Buggle et al., 2009, 2014; Marković et al., 2008, 2009, 2011; Újvári et al., 2016; Zeeden et al., 2016, 2018a). Studies clearly show enhanced χ and χfd values in palaeosols (associated with odd-numbered interglacial/interstadial MIS), while loess has clearly lower χ and χfd values (Figs. 3, 4), and is associated with even-numbered MIS and glacial periods. A slight increase in χ and χfd is also observed in weaker interstadial soils (e.g. MIS 3 soils). This strongly suggests much warmer and wetter conditions during palaeosols formation, where especially increased values of χfd indicate intensive neo-formation of superparamagnetic particles due to high soil moisture and pedogenesis. Enhanced weathering during interglacials, and hence higher precipitation and soil humidity, is supported by XRF based weathering indices on the glacial/interglacial 508

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masses from the warmer and wetter areas of the Mediterranean and the Balkans (Obreht et al., 2016; Tzedakis et al., 2002, 2006) penetrating into the basin’s southern parts. Thus, due to the late last glacial shift in the Westerlies, the southern parts of the basin experienced milder conditions during MIS 2 than MIS 4, but the Westerlies did not strongly influence the northern part of the basin during this period. In addition, intense moisture advection from the south is known to be related to pronounced Rossby-wave breaking caused by southwards migration of Westerlies, which induces enhanced meridional flow of moist and warm air masses south and south-east of Alps (Luetscher et al., 2015). Consequently, the Carpathian Basin in general may have experienced weaker storminess and warmer conditions when compared to other parts of Europe during MIS 2. This suggests that the Carpathian Basin acted at times as a climate and consequently biological boundary zone, and the related palaeoenvironmental imprint might be recorded in other archives as well. Nevertheless, this hypothesis needs further testing.

mean summer temperatures (Fig. 5). Such a high summer temperature during full glacial conditions is also supported by studies based on soil bacterial lipid signatures preserved in loess at the Crvenka (Zech et al., 2012) and Surduk (Schreuder et al., 2016) sections, albeit the reliability and validity of these data are critically discussed by the authors. New malacological results from the Crvenka section (Bačka loess plateau) suggest slightly colder summer temperatures when compared to the Irig section (between 15 and 20 °C for most of the last glacial, with lower temperatures ~12 °C only during the deglaciation, Fig. 5; Sümegi et al., 2016), but still higher than expected for the summer temperatures over full glacial conditions. Most studies dealing with climatic reconstructions of the last glacial cycle in the Carpathian Basin refer to the rather high summer temperatures during the glacial period (> 15 °C) indicated by molluscs as a widely accepted fact, and consequently many studies rely on these temperatures when interpreting palaeoclimate conditions inferred for other proxy data (e.g. Bösken et al., 2018; Marković et al., 2007, 2018; Obreht et al., 2014; Schreuder et al., 2016; Zech et al., 2012, 2013). Unfortunately, mollusc-based temperatures were not very critically discussed in those studies, but more readily accepted as reliable temperature reconstructions. However, warm conditions indicated by such high glacial temperatures are not in good agreement with information derived from other palaeoclimatic proxies, such as grain-size, environmental magnetics, n-alkanes and stable carbon isotopes. Textural data cannot be directly related to temperature, but grain-size distributions can suggest stronger storminess and wind dynamics and weaker weathering during glacials, opposing warm conditions. Environmental magnetic data points to drier and colder glacial conditions as well. Especially stable carbon isotopes (δ13C) strongly contradict warm summer temperatures. The δ13C values (Figs. 6, 7) in loess can be used as a proxy for precipitation and temperature reconstruction, but most importantly as an indicator of C3 or C4 vegetation abundance (Hatté

4.3.2. Temperature and vegetation reconstruction based on molluscs and stable carbon isotopes The grain-size and environmental magnetic proxy data indicate dry and cold climatic and environmental conditions during glacials, while warm and humid conditions persisted during interglacials. Such a traditional understanding of dry and cold glacials and humid and warm interglacials in the Carpathian Basin has been questioned by several biomarker-based studies (limited only to the last glacial; Marković et al., 2007, 2018; Obreht et al., 2014; Schreuder et al., 2016; Zech et al., 2013). Malacological investigations in the southern Carpathian Basin showed an absence of any cold-resistant or cryophiluos species at the Irig section (Marković et al., 2007), indicating warm and dry summer conditions over the full glacial conditions, with the mean July temperatures between 17 and 21 °C, almost comparable to present-day

Fig. 5. Comparison of mollusc based temperatures from the Crvenka and Irig sections. Note a difference in the temperature scale. 509

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which is at the limit of the present highest average monthly temperature in the Carpathian Basin (highest average monthly temperatures are ~21–22 °C in August). According to Collins and Jones (1985), the C4 vegetation contributes less than 2% of the modern flora in the region. However, at atmospheric CO2 levels of ~180 ppm, as during the Last Glacial Maximum, or ~220–240 ppm, as during interstadials of the last 40 ka (Jouzel et al., 1993), crossover temperatures would be about 10 °C and 12–13 °C, respectively. The δ13C signal from organic carbon in the Carpathian Basin demonstrates clear domination of C3 during the whole last glacial (Fig. 7), with only a few excursions of slightly enhanced C4 species abundance (Hatté et al., 2013; Obreht et al., 2014; Schatz et al., 2011). In addition, this is supported by δ13C data from fossil dental remains, indicating that the large herbivores analyzed were primarily C3 grazers (Kovács et al., 2012). This demonstrates that the summer temperatures in the Carpathian Basin were significantly lower than 20 °C as indicated by mollusc data, and probably even below 10 °C during the Last Glacial Maximum. However, Újvári et al. (2017) demonstrated that warmer periods associated with GIs were characterised by mixed C3/C4 vegetation. Relying on those findings, it may be assumed that the summer temperatures of GI phases were close to the C4 crossover temperature range, and therefore notably higher than in Western, Central and Eastern Europe. Another aspect that demonstrates the inconsistency in malacologicaly based summer temperatures in the Carpathian Basin is a hightemperature difference between sections in the northern and southern Carpathian Basin, in a range of more than 10 °C for some time intervals. Although the North-South gradient of increasing temperature is expected for the Carpathian Basin, such a high gradient of more than 10 °C in summer temperature is unlikely. In addition, stable oxygen isotopes of dental remains from large mammal fossils indicate 2–9 °C lower temperatures during past ~33–12 ka when compared to presentday conditions, where the North-South temperature gradient was present but in a range of only a few °C (Kovács et al., 2012). Further, a

Fig. 6. Comparison of n-alkanes based modelled vegetation (light green according to Zech et al. (2009) and olive-green modified by Zech et al. (2013)), stable carbon (δ13C) and nitrogen (δ15N) isotopes (Zech et al., 2013) from the Crvenka section.

et al., 1998, 2001, 2013, Obreht et al., 2014; Schatz et al., 2011; Zech et al., 2013). With a known past atmospheric CO2 level (EPICA Community Members, 2004), it is possible to assess the C4 crossover temperatures of the main growing season of the past. Today’s crossover temperature is about 22 °C (Ehleringer et al., 1997; Collatz et al., 1998),

Fig. 7. Values of δ13C presented on the North-South gradient from the Tokaj (Schatz et al., 2011), Crvenka (Zech et al., 2013), Surduk (Hatté et al., 2013) and Belotinac (in the Balkans; Obreht et al., 2014) sections. Grey rectangles present the average values of C3 plants. 510

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notably different mollusc species distribution among close by sections indicates a strong influence of local conditions (Marković et al., 2007; Molnár et al., in press; Sümegi et al., 2016). A large difference in mollusc-based reconstructions of palaeoenvironmental conditions and summer temperatures between the south and north-facing slopes of neighbouring sections (e.g. the Irig and Ruma sections (Marković et al., 2006a, 2007) with the absence of cold-resisting species on south-facing slopes, and Mišeluk, Susek and Petrovaradin (Marković et al., 2004, 2005, 2006b) with more than 20% of cold-resisting species on northfacing slopes of the Fruška Gora Mountain) is another convincing indication that temperature was not the main determining factor controlling the mollusc species distribution. As a matter of fact, such distribution of mollusc species can be much better explained by differences in humidity. A good example of the impact of humidity on the mollusc species can be observed at the Crvenka section. Fig. 8 clearly shows that most of the warmth-loving mollusc species in the Crvenka section are also the xerophilous species that can survive under very dry conditions. Since these species were most abundant during most of the last glacial, it is likely that very dry conditions prevailed. Only from the upper ~2 m to the modern soil of the Crvenka section is characterised by an increased number of cold-resistant species (Fig. 8a). It is demonstrated that this layer represents the deglaciation period (Marković et al., 2018; Stevens et al., 2011), and it is very unlikely that the temperatures in the Carpathian Basin significantly decreased during deglaciation when compared to the rest of the last glacial, but rather increased. Nonetheless, the most of cold-resistant species are also characterised as hygrophilous species (preferring humid conditions; Fig. 8), rather suggesting a remarkable increase in humidity and precipitation during this period (Fig. 8b). Therefore, we argue that in the semi-arid environments in the southern part of the Carpathian Basin, very dry environmental conditions were the limiting factor for many species during the last glacial, and only a limited biodiversity of xerophilous species (which are only present-day related to warm-loving species) survive very dry glacial conditions, despite low temperatures. Moreover, the glacial mollusc species were smaller in size than the recent species in order to survive cold and severe glacial climatic conditions, which enabled them to adapt to the lower glacial temperatures, when compared to the recent species and their survival under modern temperatures. Therefore, using environments with modern mean summer temperatures as the analog to glacial environments with mean summer temperatures might be in this case misleading, especially considering differences in shorter vegetative periods, the smaller size of molluscs, and absence of competitive species

due to the prevalence of limiting factors such as drought. However, with an increase in humidity, as during the deglaciation period, the biodiversity at, e.g. the Crvenka section became more diverse. The number of the mollusc species that were able to cope with such environments became higher, leading to increasing numbers of mesophilous, cryophilous and cold-resistant species (Fig. 8) despite the increase in temperature. Consequently, for molluscs humidity was a more important limiting factor than temperature during glacials, indicating that only xerophilous species could adapt to those conditions. According to the strong dependence of the mollusc species on humidity, a higher diversity in the molluscs in the northern part of the Carpathian Basin, when compared to the south, indicates higher humidity in the north of the basin. Such a gradient in humidity has also been suggested independently by Hošek et al. (2017), based on information derived from sedimentological and geochemical investigations of loess profiles at the northernmost edge of the Carpathian Basin. This is additionally supported by the occurrence of the permafrost in the northern part of the basin, while the central and southern parts lack any evidence of permafrost, suggesting warmer and/or drier climatic conditions (Fábián et al., 2014; Kovács et al., 2007; Lehmkuhl et al., 2016; Maier et al., 2016; Vandenberghe et al., 2012). Therefore, average mollusc-based summer temperatures for the northern part of the Carpathian Basin may be more representative due to humidity levels high enough to support the survival of cryophilous and cold-resistant species. Nevertheless, the interpretation of mollusc based summer temperature reconstructions must be evaluated critically. 4.3.3. Vegetation reconstruction based on n-alkanes Palaeovegetation interpretations derived from n-alkanes from the Carpathian Basin sections are generally in agreement with past vegetation reconstructions based on mollusc assemblages, indicating the domination of steppic environments during the last glacial/interglacial period (Marković et al., 2018). However, the signal of n-alkanes has been interpreted as an indication of “arid and warm” interglacials without trees and “humid and cold” glacials with trees (Zech et al., 2013). According to Zech et al. (2013), despite higher precipitation during interglacials compared to glacials, low soil humidity was assumed due to a high evapotranspiration-precipitation ratio, which inhibited the growth of trees during interglacials in this region; during glacials vice versa. Nonetheless, this clearly contradicts the very dry climatic conditions during glacials indicated by χ, grain-size, and mollusc studies. This contradiction in the interpretation of such data was not discussed in depth in previous studies on the Crvenka section

Fig. 8. The dominance changes (in percentage) of the mollusc-based palaeoecological fauna from the Crvenka loess profile (Sümegi et al., 2016) according to (a) humidity, (b) temperature and (c) vegetation type. 511

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(Marković et al., 2018; Sümegi et al., 2016; Zech et al., 2013), as well as in other studies focusing on the Carpathian Basin. The issue for the interpretation of n-alkane data may reside in the fact that the ratio of C31 and C33 (dominant origin from grasses and herbs), and C27 and C29 (dominant origin for trees and shrubs) n-alkanes is interpreted as abundance of absolute number of grasses and herbs versus trees and shrubs, rather than domination in their biomass. Good indicators of biomass production are δ15N isotope values (Fig. 6), with an increase in values usually indicating the domination of open N cycle characteristics for not N-limited ecosystems. All the sections with δ15N isotopes data in the Carpathian Basin and surrounding areas (the Crvenka (Zech et al., 2013), Tokaj (Schatz et al., 2011) and Belotinac (Obreht et al., 2014) sections) show similar patterns of an increase in δ15N values during the warmer periods. In glacial environments, the overall biomass has been remarkably reduced due to the cold and dry environments, leading to a closed N-cycle and an N-limited ecosystem. Fig. 6 presents the reconstruction of palaeoenvironments based on nalkanes and δ15N isotope values at the Crvenka section, showing a strong correlation between the pattern of n-alkanes and δ15N isotopes data fluctuations during MIS 5 and 3. Based on this correlation between grassland abundance according to n-alkanes and high δ15N, we speculate that the increase in biomass production is mostly related to grasses and herbs, rather than trees and shrubs. In fact, in the case the amount of shrubs and trees remained similar over the whole glacialinterglacial period, such a scenario may be expected since the increase in biomass production would likely influence more the grass and herbs biomass than biomass from trees and shrubs. Although the volume of trees would greatly increase during the warmer intervals, this increase would be limited to the particular crown, and contribution of biomass would be limited to annual tree litter-fall. In contrast, the biomass of grass and herbs would become remarkably denser over wider areas due to a longer growing season and consequent higher growth rate. Therefore, the biomass ratio between grasses and herbs versus trees and

shrubs would be remarkably high during warmer periods when compared to colder periods, even if the number of trees and shrubs remained the same. Consequently, the modelled decrease in grass domination during colder stages probably indicates a decrease in grass and herb biomass due to a shorter growing season, rather than a remarkable increase in trees and shrubs. Moreover, mollusc data from the Crvenka section (Sümegi et al., 2016) suggest forest-steppe environments only during the deglaciation period, while during full glacial conditions mollusc data indicate mostly open environmental conditions (Fig. 8c). Therefore, we argue the n-alkanes are good indicators for the environmental biomass production, as well as the duration of the main growing season, rather than the absolute amount of grasses and herbs versus trees and shrubs. In addition, it is demonstrated that the present-day n-alkane chain length shows a clear positive relation to the temperature (Bush and McInerney, 2015). Consequently, it may be expected that during glacial conditions a shortening of the n-alkane chain length may be to a certain extent due to a decrease in temperature, rather than a change in vegetation communities. However, n-alkane based indication of a higher contribution of trees and shrubs at northern part of the basin (e.g. Tokaj), when compared to the southern part (e.g. Crvenka), clearly points to more forest-steppe dominated conditions in the north of the Carpathian Basin, while in the south the open steppe was the dominant environment. 4.3.4. Temperature reconstruction based on soil bacterial membrane lipids The interpretation of soil bacterial membrane lipids (brGDGT; Schreuder et al., 2016; Zech et al., 2012) is probably most challenging for palaeoclimatic reconstructions based on loess records from the Carpathian Basin. Research on past climate change in the Chinese Loess Plateau based on brGDGT turned out to be useful for precipitation and temperature reconstruction (e.g. Jia et al., 2013; Peterse et al., 2011, 2014). In contrast, the results based on brGDGT from the Crvenka (Zech

Fig. 9. brGDGT derived temperature records from Surduk (Schreuder et al., 2016, on depth scale), Yuanbao (Jia et al., 2013; on age scale) and Mangshan (Peterse et al., 2014; on age scale). Dashed lines indicate a tentative correlation. 512

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et al., 2012) and Surduk sections (Schreuder et al., 2016) in the Carpathian Basin yield surprising but consistent results of higher temperatures during full glacial conditions when compared to interglacial/ interstadial conditions (Fig. 9). Zech et al. (2012) attributed the unexpected trend in temperature to incomplete peak separation of coeluting brGDGT isomers during analysis using high-performance liquid chromatography. Schreuder et al. (2016) resolved this potential issue following the protocol of De Jonge et al. (2014), adding to the reliability of the generated proxy records. Based on similar results between the Crvenka and Surduk sections, Schreuder et al. (2016) suggested that the timing of atmospheric warming in the Carpathian Basin did not directly correlate with insolation or established climate records for the Northern Hemisphere. They argued that although their record may reflect summer temperatures instead of mean annual temperatures, regional factors played a more important role, possibly because the Carpathian Basin is surrounded by mountains to all sides and therefore somewhat isolated. However, such an interpretation is in disagreement with sedimentological data from the Surduk section (Antoine et al., 2009a), which shows similarity to other records in the Carpathian Basin (e.g. Marković et al., 2008; Zeeden et al., 2016) and beyond (e.g. Antoine et al., 2009b). This demonstrates that the (palaeo)climate evolution in the Carpathian Basin was firmly connected to the general Northern Hemisphere trends. Therefore, an alternative explanation for brGDGT data is needed. It is well known that in marine environments, the GDGT production is influenced by seasonality (e.g. Pearson and Ingalls, 2013; Wörmer et al., 2014). However, in terrestrial environments soil bacteria are mostly active during the growth period. Therefore, brGDGT would likely record a mean growing season temperature when bacteria were active. A brGDGT study in sections from the eastern Chinese Loess Plateau (Peterse et al., 2011, 2014) suggested that this proxy records a temperature range encompassing variability during the summer months (Fig. 9), although the brGDGT record from the western Chinese Loess Plateau shows lower temperature than expected for summer months (between 1.1 and 20.8 °C; Jia et al., 2013). This probably indicates a longer bacterial activity in the western part of the Chinese Loess Plateau when compared to the eastern part; due to a longer bacterial activity period lipid biomarkers may be used as a proxy for longer productive periods over the year, pointing to temperature values more indicative for the annual temperature ranges instead. However, the data from the whole Chinese Loess Plateau consistently indicate warmer temperatures during the interglacials than during glacial conditions. In contrast, brGDGT based temperatures from the Carpathian Basin show low temperatures during Holocene and MIS 3, where MIS 2 conditions are characterised by quite higher temperatures (between 18 and 20 °C even during the Last Glacial Maximum; Schreuder et al., 2016; Zech et al., 2012), while brGDGT were not detected at the Surduk section during MIS 4 (Fig. 9; Schreuder et al., 2016). The most likely explanation is that the bacterial activity during MIS 2 was reduced only to very short periods during summer with sufficient moisture availability. On the other hand, the higher humidity during MIS 3 and the Holocene (as indicated by many other loess climate proxies) enabled notably longer bacterial activity, resulting in the brGDGT record averaging a temperature of several months, and therefore suggesting a lower temperature range. The reason for a different bacterial activity pattern between the Carpathian Basin and the Chinese Loess Plateau (Fig. 9) is likely related to the precipitation regime. In Eastern Asia, rainfall is mostly connected to the Asian Summer Monsoon, and thus most of the precipitation occurs during summer, enabling bacterial activity over this period. In contrast, currently the precipitation maximum in the Carpathian Basin occurs in the late spring, followed by a less pronounced peak in autumn. However, the precipitation regime might have been different during glacials. As discussed above, it is plausible that in the Carpathian Basin bacterial activity during glacials was possible only during the short periods with enough humidity during summers, while during interglacials/interstadials it also

encompassed the spring and autumn seasons. Therefore, mean annual temperature calculations based on brGDGT in the Carpathian Basin might be biased for the duration of the growing season and seasonal precipitation, where warmer (colder) periods with longer (shorter) growing seasons would tend to record lower (warmer) temperatures due to longer (shorter) bacterial activities over the year. Observed higher temperatures based on brGDGT during MIS 2 are biased by arid conditions and rather indicate a very short period of bacterial activity, supporting severe, cold and especially dry conditions. In contrast, the brGDGT distribution during MIS 3 indicates generally milder (warmer and wetter) conditions. BrGDGT data for the modern soil show lower temperatures as well, probably integrating information from several months of bacterial activity. Nevertheless, this interpretation should be considered with care, since the similar trend of decreasing temperatures during the Holocene is also observed in the Chinese Loess Plateau (e.g. Jia et al., 2013; Peterse et al., 2011, 2014). For the Chinese Loess Plateau, this is explained by possible changes in the microbial community (i.e., archaea production exceeding production of brGDGT producing bacteria) or preferential degradation of isoprenoid GDGTs or upward increase in living archaea relative to bacteria in the palaeosol (Jia et al., 2013). Therefore, also in the Carpathian Basin the origin of signals may be complex. Severe (summer) conditions characterised by drought would disable bacterial activity, and brGDGT may not be detected. Such a scenario is very likely for MIS 4 in the Carpathian Basin, which supports the absence of brGDGT in MIS 4 loess at the Surduk section (Schreuder et al., 2016). As discussed above, the glacial climatic conditions during MIS 4 were likely more severe than during MIS 2 in the southern Carpathian Basin, which is in agreement with the absence of brGDGT in this time interval. Thus, brGDGT have a great potential for being an indicator of the growing period and the precipitation maximum over summer in the Carpathian Basin. However, more studies including several palaeoclimate proxies are necessary to better understand this proxy and its relation to palaeoenvironmnets. Finally, it has to be stated that all biomarker studies in loess may need to be reevaluated in light of adjusted biomass contributions of cyanobacteria. Svirčev et al. (2013, 2016) indicated that biological loess crusts dominated by cyanobacteria might have had an important role in loess formation. Cyanobacterial secreted metabolites, such as primarily exopolysaccharides, might explain trapping and preservation of dust, as well as the formation of loess during very severe glacial conditions. Svirčev et al. (2013) demonstrated that scytonemine, a pigment characteristic only for cyanobacteria, was found in loess from the Carpathian Basin, indicating the presence of cyanobacteria. Since the cyanobacteria have different photosynthesis pathways from higher plants, as well as the different contributions of dominant n-alkanes, their presence may influence vegetation reconstructions based on higher plants only. However, this hypothesis needs further development, and should be considered in future palaeovegetation studies. The possibility to use cyanobacteria pigments, such as scytonemine and mycosporine amino acids, as biomarkers in palaeoclimatic studies of loess remain to be evaluated. 4.4. Longer-term palaeoclimate evolution in the Carpathian Basin The main focus of this study was on the last glacial-interglacial cycle since very most of the multi-proxy data is limited for this time span. It is observed that the last glacial in the Carpathian Basin experienced a notable increasing North-South gradient in temperature and strong decreasing North-South gradient in humidity. We propose that MIS 2 in the south of the basin experienced milder conditions with weaker storminess than MIS 4 (however, still very dry in comparison to other regions) due to a southern track of westerlies (see Chapter 4.3.1.). We demonstrate that most of the biomarker-based proxy data in loess from the dry southern part of the Carpathian Basin show a strong bias towards arid conditions. We suggest that malacological results cannot be 513

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used in palaeotemperature reconstructions in this region, while n-alkanes and GDGTs should be interpreted with great care considering the length of the growing season, and the overall biomass production. Thus, we suggest that glacial conditions were drier and colder than previously proposed (summer temperatures likely under 15 °C during glacials), but notably warmer than in other parts of Western, Central and Eastern Europe. Vegetation was dominated by steppic environments during both, glacials and interglacials, although in the northern part of the basin more mosaic (forest-steppe) environments persisted. The main growing season was notably shorter during glacials than interglacials, consequently having remarkably less biomass production during glacials. Yet, we cannot evaluate if the previous glacials and interglacials experienced similar conditions. Unfortunately, investigations of longer climatic loess records in the Carpathian Basin are limited to only a few studies, mostly relying on low-resolution datasets of few proxies (Buggle et al., 2009, 2013; Marković et al., 2011; Újvári et al., 2014a; Varga et al., 2011). It is suggested that deposition of loess in the Carpathian Basin was preceded by the formation of a red clay basal complex (Kovács et al., 2011; Marković et al., 2011; Újvári et al., 2014a). This basal pedocomplex should not be confused with or correlated to the Mio-/Pliocene Chinese Red Clay Formation at the base of thick Quaternary loess of the Central Chinese Loess Plateau (Ding et al., 1999). The formation of the red clay in the Carpathian Basin occurred from 3.5 to ~0.5 Ma years, with strong local and regional differences of the upper and lower boundary age (Kovács et al., 2011). The deposition of the longest loess records in the basin started ~1 Ma ago (Fig. 3; Marković et al., 2011; Sartori et al., 1999; Újvári et al., 2014a). Loess formation in the area of interest may have begun during the Middle Pleistocene Transition, coinciding with a southward shift of the polar front during glacials displacing the source region of thermocline waters within the Subtropical Gyre from high to mid-latitudes (Bahr et al., 2018), and the consequent formation of significant ice caps in the Alps (Muttoni et al., 2003). Thus, we argue that intensive silt production and deposition, as recorded in the Carpathian Basin, may be directly associated with glacial activity in the Alps. This implies that the Danube and its tributaries, as main carriers of Alpine silt particles, have an important role in dust and loess accumulation in the basin, at least in terms of a major transport mechanism. According to geochemical (XRF-based weathering indices), magnetic and (low-resolution) grain-size data, most of loess records indicate a trend of progressive aridization in the Carpathian Basin over time (Buggle et al., 2009, 2013; Marković et al., 2011). Újvári et al. (2014a) questioned the intensity and extent of the progressive aridization, suggesting that increasing physical erosion indicated by clay mineralogy can partly explain this trend found in weathering indices. Nevertheless, progressively increasing dust sedimentation points to larger dust availability with time, generally supporting a progressive aridization (Újvári et al., 2014a). Alternatively, progressive cooling instead of aridization during glacials may explain the increase in sedimentation rates since aeolian surfaces are more active during cold periods, and consequently aeolian sediments are plausibly more actively eroded during colder conditions (Újvári et al., 2016). However, the progressive aridization trend is clearly observed during interglacials, where the shift from Cambisols to forest-steppe soils (mostly Chernozem) occurred from the S3 unit (MIS 9) to present (Bronger, 2003; Buggle et al., 2013). This indicates a progressive decrease in precipitation during interglacials (Buggle et al., 2009, 2013) and likely a notable weakening of the influence of Mediterranean climate in the Carpathian Basin after MIS 9 (Obreht et al., 2016). A similar trend in aridization has not been observed in global and many other European palaeoclimatic records. Buggle et al. (2013) suggested that the Quaternary uplift of surrounding mountain ranges was responsible for the reduced precipitation, related to westerly winds. However, recent investigations on loess from the interior of the Balkan Peninsula indicated a more complex relation between Mediterranean climate and

progressive aridization over Southeastern Europe through time (Obreht et al., 2016). Moreover, a similar trend in the aridization has been observed in loess sections from the Azov Sea region (Liang et al., 2016). Although the influence of the Quaternary uplift of Central-eastern European mountains ranges (Dinarides, Alps and Carpathians) on the observed aridization trend cannot be fully ruled out, it seems that this process of aridization is more complex and presents a supra-regional trend influencing larger area than the Carpathian and Lower Danube Basins as previously proposed. Understanding of mechanisms responsible for the progressive aridization holds the potential to answer questions regarding the interaction of the Quaternary uplift of Eurasian orogens, Northern Hemisphere ice sheets extent, the evolution of largescale atmospheric systems, and global climate forcing. This is even more important from the perspective of future climate predictions which propose an increase of aridity and more frequent manifestation of hydroclimatic and environmental extreme conditions in the investigated region (Mihailović et al., 2016). 5. Conclusions This review focuses on the reevaluation and reinterpretation of palaeoclimate proxy data from loess in the Carpathian Basin, and points to the challenge of interpreting past climatic and environmental reconstructions in this region based on loess records. A more consistent interpretation of the most widely used proxies is proposed. Loess formation in the Carpathian Basin started close to 1 Ma ago, likely due to increased silt production in the Alps and possibly Carpathians by glacier expansion. The Carpathian Basin was exposed to a progressive aridization trend, leading to the evolution from Mediterranean-like to steppic interglacial palaeosols, culminating during MIS 9, when a shift from Cambisols to Chernozems and other forest-steppic soils occurred. The observed drying trend was likely supra-regional, and although the Quaternary uplift of the surrounding mountain ranges may partly explain such a climatic evolution, a full explanation is yet missing. Since most of the high-resolution multi-proxy investigations were conducted on last glacial loess, a detailed interpretation of the data was performed for this period. In contrast to conclusions from some malacological, nalkanes and bacterial membrane lipids studies, we argue that the last interglacial experienced remarkably warmer and wetter conditions when compared to the last glacial. Glacial climatic conditions in the Carpathian Basin were characterised by a notable decreasing NorthSouth gradient in temperature and a stronger gradient in humidity over time. Especially in the southern part of the basin, biomarkers were strongly biased by very arid conditions, and thus their use for past air temperature reconstructions is challenging. Even though the summer air temperature was not as high as indicated by biomarker data, it was still elevated compared to glacial conditions in Western, Central and Northern Europe, likely close to the crossover summer temperatures for C4 plant. The main growing period was longer and the biomass production was notably higher during interglacials. Vegetation was mostly represented by steppic environments during both, glacials and interglacials. The appearance of forest-steppe environments was possible in the north of the Carpathian Basin, while in the south open steppe environments may have prevailed. This review paves the way for new (re)interpretations and discussions on existing and future data that will aim at improving our understanding of proxy interpretation in loess and climate evolution of the continental climate during the Quaternary. Especially the interpretation of the novel organic geochemical proxies in loess research has to be considered with caution, and needs careful cross-evaluation with better-understood proxies. Only with a consistent interpretation of proxies, new conclusions may be reached. For example, integrating nalkanes, brGDGT, stable carbon and nitrogen isotopes and evaluating them with better-understood χ and grain-size data might improve our understanding of seasonality, duration of the growing season, and biomass production. A better understanding of mollusc-based climate 514

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reconstruction, as the most often used biomarkers in loess, especially in semi-arid and cold regions, is needed. Moreover, multi-proxy and highresolution investigations on Middle and Early Quaternary loess and other deposits are desirable. This review demonstrates the importance of careful proxy and data interpretation, especially pointing to the danger of oversimplied and isolated interpretation that may lead to erroneous and conflicting interpretations. Finally, one of the crucial issues for the integration of multi-proxy loess data for the Carpathian Basin is the yet limited data availability in open data repositories. Unavailability of data makes comparison among different sections challenging and often impossible. Consequently, we urge the loess research community towards supporting the practice of making their data available in open repositories as a step forward in achieving better visibility of loess research.

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