Insights into continental temperatures in the northwestern Black Sea area during the Last Glacial period using branched tetraether lipids

Quaternary Science Reviews 84 (2014) 98e108

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Insights into continental temperatures in the northwestern Black Sea area during the Last Glacial period using branched tetraether lipids Lise Sanchi*, Guillemette Ménot, Edouard Bard Aix-Marseille Université, CNRS, IRD, Collège de France, CEREGE UM34, 13545 Aix en Provence, France

a r t i c l e i n f o

a b s t r a c t

Article history: Received 26 June 2013 Received in revised form 14 November 2013 Accepted 15 November 2013 Available online 15 December 2013

In the continental realm, continuous quantitative temperature reconstructions spanning the Last Glacial are rare, especially in central and eastern Europe. Here, we provide a reconstruction from the northwestern Black Sea catchment, spanning 40 to 9 ka BP, from a study of the relative distribution of branched glycerol dialkyl glycerol tetraethers (brGDGTs) in Black Sea lacustrine sediments. First, the origins of brGDGTs are discussed. The comparison of geochemical proxies from the same core supports a dominant terrestrial origin for brGDGTs during the Last Glacial, and a strong decrease in the soil derived brGDGT proportion toward the Holocene. Since the lowering of soil vs. lacustrine derived brGDGTs is prone to bias the temperature signal that is reconstructed using a soil calibration, a correction for in situproduction is applied. The corrected signal is compared to independent discrete temperature records from the study area. The brGDGT-temperature relative evolution reconstructed in this work provides additional insight regarding millennial-scale climate variability in central and eastern Europe. Notably, the imprints of Heinrich event cold spells and Lateglacial climatic oscillations are consistent with other regional paleorecords from the Northern Hemisphere. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: brGDGTs Black Sea Continental paleotemperatures MBT/CBT Last Glacial Abrupt climate changes

1. Introduction The Last Glacial period was marked by millennial-scale climate oscillations that were first detected in Greenland and the North Atlantic. Such climate oscillations coincided with environmental and climate changes in many regions around the globe (see Clement and Peterson, 2008, for a review). The continental climate response to these major climatic events is still poorly understood, in part due to the scarcity of well-dated continuous terrestrial records with a sufficiently high resolution (e.g. Voelker, 2002), especially those that make quantitative temperature reconstructions useful for model-data comparisons (e.g. Van Meerbeeck et al., 2011). Groundwater recharge temperatures, obtained from dissolved atmospheric noble gases (the noble gas temperature, NGT), are often employed for estimating multicentennial temperature variations on an aquifer scale (e.g. Beyerle et al., 1998; Aeschbach-Hertig et al., 2002). While this method provides major temperature shifts, dispersion and dating difficulties hamper abrupt climate variability reconstructions. Pollen, coleoptera, chironomids, and plant macrofossil remains are frequently used to infer paleotemperatures (Atkinson et al., 1987;

* Corresponding author. Tel.: þ33 4 42 97 15 98. E-mail address: [email protected] (L. Sanchi). 0277-3791/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.quascirev.2013.11.013

Bartlein et al., 2011). However, such records remain particularly scarce in certain areas, as for central and eastern Europe. To our knowledge, thus far, no continuous temperature reconstruction spanning more than the last deglaciation has been attempted for this portion of Europe. New paleorecords for this transition zone, that potentially undergoes Atlantic, Mediterranean, and continental influences, could help to evaluate the accuracy of climate models (Braconnot et al., 2012) and contribute to understand abrupt past and future climate change. Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are membrane lipids produced by bacteria that thrive in soils (Weijers et al., 2006a) and are transported by rivers to sediments. As a result, brGDGT based proxies measured within the sediment archives recovered at the mouth of major river systems can record changes in river drainage basin conditions in response to climate changes. Parallel measurements of the amount of crenarchaeol, an isoprenoid GDGT synthetized by aquatic Archaea, enable the tracing of soil organic matter inputs through the BIT index (Hopmans et al., 2004). Additionally, brGDGTs show promise for reconstructing high-resolution continental past temperature and soil pH. Indeed, variations in the relative distribution of brGDGT compounds (associated with structural changes) have been shown to be linked to temperature and to soil pH changes (Weijers et al., 2007b). To quantify these relationships, Weijers et al. (2007b) have defined the methylation index of branched tetraethers (MBT), and the

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cyclization ratio of branched tetraethers (CBT), and they have developed the first calibrations of the MBT and CBT that enable mean annual air temperature (MAT) and soil pH reconstructions, from a set of globally distributed soils. Since the time of the successful use of the MBT/CBT proxy in marine sediment cores to reconstruct past MATs (e.g. Weijers et al., 2007a; Schouten et al., 2008), the calibration of the MBT/CBT proxy has been revised and based on an extended set of soils (Peterse et al., 2012). In addition to global calibrations, regional calibrations have also been proposed (e.g. Bendle et al., 2010). The application of MBT and CBT indexes to lacustrine environments has also been investigated (e.g. Tierney and Russell, 2009; Bechtel et al., 2010; Blaga et al., 2010; Tierney et al., 2010; Tyler et al., 2010; Zink et al., 2010; Sun et al., 2011; Tierney et al., 2012). Most of these studies have suggested that brGDGT production in lake water columns and/or sediments can occur, and when it does, it complicates the application of calibrations that are based on soil datasets. As such, new calibrations of brGDGT proxies in lake sediments have been established (e.g. Tierney et al., 2010; Sun et al., 2011). Hence, the application of brGDGT proxies in ancient lake sediments is promising for reconstructing MATs, especially if a specific lacustrine calibration is used (e.g. Sinninghe Damsté et al., 2012), or if constraints on brGDGT origin are provided (e.g. Niemann et al., 2012). Here, we present a high-resolution record of brGDGT distributions in the lacustrine sediments of core MD04-2790, recovered from the northwestern Black Sea. The presented record spans the Last Glacial and the last deglaciation, from 40 to 9 cal ka BP. First, we assess the dominant origin of brGDGTs (in situ production vs. soil production) along the core using a comparison of geochemical proxies that trace the terrestrial inputs in sediments, the sedimentation rates, and evidence of lacustrine productivity. The suggested dominant soil origin of brGDGTs during the full glacial period enables us to reconstruct a realistic MAT signal using the recent global soil calibration of brGDGT proxies. However, the decrease in the soil derived brGDGT proportion that is suggested in the early Holocene sediments points to the need for a correction of the soil calibrated MAT. We, therefore, propose a correction method and test its sensitivity. The corrected MAT record is consistent with existing punctual records from central and eastern Europe. As a result, we provide further insights regarding European millennial scale climate variability.

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2. Materials and methods 2.1. Study area: the Black Sea drainage basin The Black Sea drainage basin (BSDB) spans a large portion of central and eastern Europe (from 8 to 48 in longitude and 37 to 57 in latitude), and is composed of extensive lowlands (in the Hungarian and Eurasian Plains) and several steep mountains, including the Alps and the Carpathians Mountains (Fig. 1). Major rivers draining the northern and western portions of the BSDB are the Don and Kuban (through the Sea of Azov), and the Dniepr, southern Bug, Dniestr, and Danube. These river drainage basins span nearly 1.9  106 km2 and currently provide approximately 47% of the Black Sea annual sediment load (Panin and Jipa, 2002). Currently, the main river inputs to the Black Sea originate from the Danube. The Dniestr and the Dniepr are less significant sources (Panin and Jipa, 2002). However, their inputs were larger during the last deglacial period (Bahr et al., 2005; Soulet et al., 2013). Additionally, Black Sea Rim currents transport sediments cyclonically from north to west and favor the deposition of sediment on the slope and the deep western basin (Özsoy and Ünlüata, 1997; Oguz and Besiktepe, 1999). Therefore, the northwestern continental slope of the Black Sea is a suitable place for studying the terrestrial organic matter transported by northern and western Black Sea rivers and investigating organic proxies such as the MBT/CBT indexes. From north to south, modern MATs in the BSDB vary from 6 to 12  C, with the exception of mountainous regions (the Alps, the Carpathians, and the Balkans) where MATs are lower than 6  C. Within the Danube Basin, modern MATs are between 10 and 12  C. Within the Dniestr and the lower Dniepr Basins they are between 8 and 10  C (Fig. 1). 2.2. MD04-2790 core sediments Core MD04-2790 was recovered from the upper slope of the northwestern Black Sea in the direct axis of the mouth of the Danube River (44130 N, 30 600 E) at a water depth of 352 m during the ASSEMBLAGE 1 cruise, aboard Marion Dufresne (Fig. 1). The typical marine Units I and II (Ross and Degens, 1974) were found within the core from the top to a core depth of 1.24 m (Soulet et al.,

Fig. 1. Geographic setting of the study: the Black Sea Drainage Basin. The star symbol indicates the position of the MD04-2790 core and the black line outlines the modern Black Sea drainage basin limits (inferred using the CCM River and Catchment DatabaseÓ of the European Commission, JRC 2007, Vogt et al., 2007). Colored lines indicate modern MAT isotherms inferred using the 1961e2000 CRU CL 2.0 100 global climatology (New et al., 2002). The locations of the main archives providing the quantitative temperature reconstruction mentioned in the text are also provided. The white dots and the associated values are sites with the LGM MAT anomalies provided by Bartlein et al. (2011); the dashed line contains sites with noble gas temperature reconstructions (Stute and Deak, 1989; Varsányi et al., 2011); the black squares indicate sites with Lateglacial temperature reconstructions (1. Feurdean et al., 2008a; 2. Bordon et al., 2009; 3. Kotthoff et al., 2011; 4. Tóth et al., 2012; these reconstructions are based on pollen, with the exception of the one obtained from Tóth et al. (2012) study that is based on chironomids). The green diamond indicates a Serbian soil with known amounts of brGDGTs and crenarchaeol (Zech et al., 2012, see Fig. 5 for additional details). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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2011a). The lowermost limnic Unit III (Ross and Degens, 1974) was found at a core depth of 1.24 m to the base of the core (w30 m, Soulet et al., 2011a). The age model employed is based on 14C measurements, tuning, and varve counting (Soulet et al., 2011a, 2011b). The core spans the Last Glacial and the deglacial periods (from 40 ka to 9 ka BP) when the Black Sea was a giant lake isolated from the Mediterranean Sea due to the oceans low-stand (e.g. Stoffers et al., 1978; Schrader, 1979). The last reconnection of the Black Sea “Lake” to the global ocean occurred at ca 9 ka BP (Bahr et al., 2008; Soulet et al., 2011b). The uppermost sediments (Units I and II) were deposited following reconnection to the Mediterranean Sea, and represent the current

MBT ¼

and the isoprenoid GDGT crenarchaeol (m/z ¼ 1292), which enabled the BIT index to be measured (Hopmans et al., 2004). Samples were run, at least, in duplicate. Our BIT index signal was similar to the record published by Ménot and Bard (2012). The absolute mean difference of 265 samples was 0.05, less than the reproducibility of 0.4 for the S1 sample within the round robin study of Schouten et al. (2009) and on the same order of repeatability (0.029). The small difference between the two BIT records may arise from the difference between LCeMS methods. MBT and CBT were calculated according to the definition of Weijers et al. (2007b) (roman numerals refer to the compounds of their study), as follows:

GDGT  I þ GDGT  Ib þ GDGT  Ic ½GDGT  I þ GDGT  Ib þ GDGT  Ic þ GDGT  II þ GDGT  IIb þ GDGT  IIc þ GDGT  III þ GDGT  IIIb þ GDGT  IIIc

interglacial Black Sea marine stage. For this work, we focus on Black Sea “Lake” sediments (i.e. the last lacustrine period recorded, from 40 to 9 cal ka BP). 2.3. Spatial origin of terrestrial material in northwestern Black Sea sediments over time From 25 to 18 ka BP, the εNd values of the sediments (Soulet et al., 2013) suggest a mix of western Black Sea major river inputs for sediments deposited within the northwestern Black Sea. From 18 to 15.5 ka BP, the work of Soulet et al. (2013) has enabled precise insights regarding the origin of four particular sediments layers (the so called Red Layers) found in MD04-2790 (and adjacent cores). These specific layers, associated with melt water from a proglacial lake (Lake Disna) arose from the upper Dniepr catchment. From approximately 15.5 ka BP to the early Holocene, the MD04-2790 sediment provenance may have progressively turned toward the Danube Basin, as suggested by decreasing εNd values (Soulet et al., 2013). Therefore, the reconstructed signal presented in this study is compared, for this time window, to independent records from the Danube catchment.

CBT ¼ logð½GDGT  Ib þ GDGT  IIb=½GDGT  I þ GDGT  II Þ We also calculated the MBT0 proposed by Peterse et al. (2012) for a revised global calibration on soils. This index is similar to the MBT index but does not include compounds IIIb and IIIc. The average standard deviations for replicate mean analyses of 274 lacustrine samples were 0.003 for the MBT and MBT0, and 0.009 for the CBT. Since no specific calibration has yet been established for central and eastern Europe, MATs were calculated with the last global calibration on the soils’ dataset (Peterse et al., 2012), as follows:

MAT ¼ 0:81  5:67  CBT þ 31  MBT’

(1)

Since our study focus on the sediments of the Black Sea “Lake”, we also tested the calibration based on global lake sediments (Sun et al., 2011), as follows:

MAT ¼ 6:803  7:062  CBT þ 37:090  MBT

(2)

3. Results 2.4. Branched GDGTs analyses Polar fractions from the sediment lipid extracts purified previously (Ménot and Bard, 2012) were re-dissolved in a hexane:isopropanol (99:1, v:v) prior to HPLC-MS (high performance liquid chromatographyemass spectrometry) analysis with an Agilent/(HP) 1100 series LC-MS. As a control, a few sediment samples were re-extracted and purified. Compounds were separated using a prevail cyano column maintained at 30  C, with a 0.2 mL/min hexane:isopropanol gradient flow rate. GDGTs were first eluted isocratically with 99% hexane and 1% isopropanol for 5 min, followed by a linear gradient to 1.8% isopropanol for 45 min. GDGTs were detected using atmospheric pressure chemical ionization mass spectrometry (APCI-MS), according to Hopmans et al. (2004); the nebulizer pressure was 60 psi, the vaporizer temperature was 400  C, the drying gas was N2 with a flow rate of 6 L/min, the temperature was 200  C, the capillary voltage was 3 kV, and the corona was 5 mA. Single ion monitoring (SIM) was used to generate a positive ion spectrum. SIM parameters were set in order to detect the branched GDGTs used for the MBT and CBT index calculation (m/z ¼ 1022, 1020, 1018, 1036, 1034, 1032, 1050, 1048, and 1046),

The relative fractional abundances for the nine brGDGTs in MD04-2790 sediments are provided in Fig. 2. Throughout the entire record, branched GDGTs with one or two cyclopentyl moieties (Ib-IIIb and Ic-IIIc) are less abundant than brGDGTs without cyclopentyl moieties (IeIII), referred to as “main” brGDGTs (Fig. 2A), as observed in modern soils and lakes (Weijers et al., 2007b; Blaga et al., 2010). GDGT III is predominant and represents 34e56% of the nine brGDGTs, while GDGT I and II account for less than 16% and 31%, respectively. Furthermore, as shown in Fig. 2, GDGT III reaches maximum abundance between 27 and 19 ka BP, during and surrounding the Last Glacial Maximum (LGM, Mix et al., 2001). Such a finding is in line with the predominance of GDGT III in cold regions, revealed by global surveys in soils and lakes (Weijers et al., 2007b; Pearson et al., 2011), and confirms the potential of brGDGT distributions for reconstructing temperature in MD04-2790 sediments. The MBT and MBT0 indices display similar patterns and range between 0.1 and 0.25 units. The CBT index is located between 0.09 and 0.67, with a minimum during the early Holocene (Fig 3A). The MBT0 /CBT-derived temperature signal reconstructed using

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Main brGDGT relative abundance (%)

A.

101

60 GDGT III (1050) 50

40

GDGT II (1036)

30

20 GDGT I (1022) 10 (1)

(2)

(3)

(4)

(5)

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GDGT III (1050) GDGT IIIb (1048) GDGT IIIc (1046)

GDGT II (1036) GDGT IIb (1034) GDGT IIc (1032)

GDGT I (1022) GDGT Ib (1020) GDGT Ic (1018)

40

30

20

10

0 (1) early Holocene 9-10 ka (n=8)

(2) Younger Dryas 11.7-13 ka (n=6)

(3) Late Glacial (BA) 13.5-14.5 ka (n=12)

(4) LGM 19.9-21.2 ka (n=10)

(5) MIS 3 34-36 ka (n=19)

Fig. 2. MD04-2790 brGDGT relative abundance: A. Main brGDGT signals from 40 to 9 ka BP (the error bars are two mean standard deviations of the replicate analyses) , B. brGDGT distribution at various time periods (the error bars indicate the standard deviation calculated for n samples and numbers refer to the compound’s m/z).

Equation (1) (the global soil calibration) ranges from 0.5 to 6  C (with an average analytical standard deviation for replicate means of sd ¼ 0.1 ; n ¼ 274). The MBT/CBT-derived temperature signal reconstructed using Equation (2) (the global surface lake sediment calibration) ranges from 6 to 12  C (sd ¼ 0.1, n ¼ 274). The temperature differences between the two reconstructions are between 5.5 and 6.5  C. However, the temperature signals calculated using both calibrations display similar trends. From 40 to 20 ka BP, the MAT signal is stable but punctuated by cold spells with minima reached at approximately 30, 27, 24, and 21e20 cal ka BP. The MATs then slightly increase toward Holocene temperatures, with two maxima reached at ca 14 and 11 ka BP (Fig. 3B). 4. Discussion 4.1. Soil vs. lacustrine origins of brGDGTs and implication for continental temperature reconstruction BrGDGTs extracted from surface lake sediments derive from the surroundings soils. However, they could also be produced in situ (i.e. within the water column and/or within lake sediments, e.g. Sinninghe Damsté et al., 2009; Tierney and Russell, 2009; Blaga et al., 2010; Tierney et al., 2010). In a similar manner and despite the position of core MD04-2790 close to the mouth of major rivers, in situ production of brGDGTs in the paleo Black Sea cannot be

excluded, and the relative proportion of terrestrial/aquatic brGDGTs should be examined prior to any discussion regarding temperature reconstructions. During the Last Glacial, at least from 25 ka BP and until ca 15 ka BP, low calcium carbonate (CaCO3) and the total organic carbon (TOC) content from northwestern and southern Black Sea sediments (Major et al., 2002; Bahr et al., 2008; Kwiecien et al., 2009; Soulet et al., 2011a) suggest reduced lacustrine productivity. Additionally, during this time window, the sediment’s carbonate fraction is quasi-exclusively detrital (i.e. terrestrial) in origin (Major et al., 2002; Bahr et al., 2005; Kwiecien et al., 2009). In core MD04-2790, strong co-variation of the three independent proxies tracing terrestrial inputs between 18 and 15 ka, namely the BIT index (Ménot and Bard, 2012; Soulet et al., 2013, this study, Fig. 4), that traces the relative proportion of the dominant brGDGTs (I, II, III) on crenarchaeol (i.e. a mainly aquatic isoprenoid GDGT), C25-alkane/TOC (Soulet et al., 2013), and the XRF-Ti/Ca (Soulet et al., 2011a, 2013), as well as a high sedimentation rate (Soulet et al., 2011a), strengthen the hypothesis for organic matter, including brGDGTs, being predominantly derived from the surrounding land due to fluvial transport to the Black Sea. Therefore, prior to 15 ka BP, most brGDGTs likely originated from BSDB soils. As a result, brGDGT-based temperature, reconstructed from 40 to 15 ka BP, is expected to reflect the MAT of the paleo BSDB northwestern area.

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A.

0.24 0.2 0.16 0.12

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0.08

0.4

CBT

MBT

and MBT'

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MBT(')/CBT derived temperature (°C)

0.2 16

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corrected

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reconstructed with lake sediment calibration (Sun et al. 2011)

12

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4 reconstructed with soil calibration (Peterse et al. 2012)

0 10000

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Fig. 3. BrGDGT proxies and associated reconstructions in core MD04-2790. A. BrGDGT proxies: MBT, MBT0, and the CBT index. B. BrGDGT derived mean annual temperature (MAT) reconstructions. The green line corresponds to the MAT reconstructed with MBT0 /CBT calibrated on global soils (Peterse et al., 2012); and the blue line to the MAT reconstructed with the MBT/CBT calibrated on global lake sediments (Sun et al., 2011). The gray line indicates the corrected temperature based on a mixing model with a modern Serbian soil as the soil end member (see Fig. 5 for further details). The vertical black bar indicates the modern MAT in the Danube catchment. For each sample, the error bar is two mean standard deviations of the replicate analyses. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

In core MD04-2790, proxies of terrestrial input indicate a simultaneous decrease from 15 ka BP (i.e. the BøllingeAllerød) to the early Holocene (Soulet et al., 2013) and the sedimentation rate reaches the lowest values (Soulet et al., 2011a), suggesting reduced terrestrial inputs to the Black Sea “Lake” (Fig. 4). This is in-line with the strong concomitant decrease in runoff of northwestern Black Sea rivers (Sidorchuk et al., 2011). On the other hand, the high TOC and authigenic calcite content of northwestern Black Sea sediments suggest high lacustrine productivity conditions (Bahr et al., 2008; Soulet et al., 2011a). In this context, the soil brGDGTs likely decreased as compared to lacustrine brGDGTs. Therefore, from 15 to 9 ka BP, the brGDGT MAT reconstructed using the global soil calibration may be biased and should be corrected for in situ produced brGDGTs. In conclusion, the application of soil-based calibration to MD042790 brGDGT record likely yields a proper range of values during the Last Glacial. However, for the more recent portion of the record, a correction for the rising influence of lacustrine derived brGDGTs relative to soil-derived brGDGT is required. 4.2. Correction of brGDGT temperature and comparison to independent signals Since they constitute a first order estimation of the relative proportion of soil derived vs. lacustrine GDGTs, the correction model is based on BIT values. Similar to Weijers et al. (2006b) and Ménot and Bard (2012) who estimated the bias induced by soil GDGTs on sea and lake surface temperatures, we calculate the bias induced by lacustrine derived GDGTs on soil GDGT based temperatures using a binary mixing model. Specifically, binary mixes of the GDGT distributions (crenarchaeol and the nine brGDGTs relative amount) from a soil and a lacustrine end-member are established, as follows:

½GDGT  Xmix ¼ a  ½GDGT  Xsoil þ ð1  aÞ  ½GDGT  Xlake (3) with a varying between 1 and 0. [GDGT-X] indicates the GDGT-X fraction (i.e. the ratio of crenarchaeol or one of the nine brGDGTs (compounds I, Ib, Ic, II, IIb, IIc, III, IIIb, and IIIc mentioned above) on the sum of crenarchaeol and the nine brGDGTs. Therefore, [GDGT  X] takes ten values. [GDGT  X]soil and [GDGT  X]lake correspond to the GDGT distribution of soil and lake end-members, respectively. End-members are chosen to best represent Black Sea catchment soils and lake sediments, particularly when the correction is mainly required (i.e. for the early Holocene). The GDGT distribution of MD04-2790 early Holocene sediments (the mean distribution of eight samples with an age between 9 and 10 ka BP) represents the lake end-member. GDGT data are only available for one site within the northwestern Black Sea catchment, a modern soil sample from Serbia (Zech et al., 2012). The GDGT distribution from this soil represents a single domain, and a single period over the entire catchment area and the Holocene period. Since the binary mixing model may be sensitive to the choice of soil end-member, we simulated a series of virtual soil GDGT data, generated randomly with a Monte Carlo method. A total of 5994 virtual soil GDGT distributions that realistically fit brGDGT proportions and temperature conditions (Table 1) were simulated in order to integrate the heterogeneity of Black Sea watershed soils and climate conditions. These distributions, in addition to the one for the modern Serbian soil mentioned above, represent the soil end-members. The binary mixing model (Equation (3)) is computed for each soil end-member and the lake end-member described above (i.e. 5995 times). The MAT, reconstructed using the soil calibration (Peterse et al., 2012), and the BIT indices are calculated for each GDGT distribution mix between the two end-members. The

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Fig. 4. Evolution of terrestrial inputs in MD04-2790 sediments. A. Sedimentation rate inferred using the age model based on Soulet et al. (2011a) and (2011b) studies. B. XRF-Ti/Ca (Soulet et al., 2011a; Soulet et al., 2013). C. BIT index (this study). D. C25-alkane/TOC (Soulet et al., 2013).

modeled temperature bias is the difference between the MAT of the soil end-member and the MAT derived from the GDGT distribution mix. The modeled temperature bias is, therefore, at a minimum when the GDGT distribution is equal to 100% of the soil endmember (i.e. when a is equal to 1, Fig. 5). To apply the correction to the MD04-2790 record (from 40 to 9 ka BP), the temperature bias is determined for each sample using a comparison of the sample to the modeled BIT values (as illustrated by the arrows in Fig. 5B) and added to the original soil calibrated temperature (Fig 3B, green line). Since 5995 soil endmembers (and associated binary mixing models) are employed, we obtain just as many corrected temperature signals. The median

Table 1 Conditions applied to generate realistic random soil end-members. 1) A random function generates 10 equiprobable values between 0 and the max. GDGTs fraction (see below) which sum is equal to 1; the 10 resulting values are allocated to the 10 GDGTs (Crenarchaeol þ 9 brGDGTs) 2) Limit conditions to approach realistic soils (of the study area). Random distributions generated in 1) have to fulfill these criteria: min. GDGTs fractions (10 values) for each GDGT, min. relative proportion inferred from a compilation of published data (39 samples) from modern mid latitudes soils max. GDGTs fractions (10 values) for each GDGT, max. relative proportion inferred from a compilation of published data (39 samples) from modern mid latitudes soils BIT min 0.2 (in the above mentioned compilation BIT min ¼ 0.276) BIT max 1 (maximum, frequently approached in soils) MBT and MBT0 min. inferred from a compilation of published data (551 samples) 0 MBT and MBT max. inferred from a compilation of published data (551 samples) CBT min: corresponds to pH ¼ 9 (with Peterse et al., 2012 calibration) CBT max: corresponds to pH ¼ 4 (with Peterse et al., 2012 calibration) MAT min: 5  C MAT max: 25  C

corrected temperature signal is shown in Fig. 6B (black line) with its 90% confidence interval (i.e. the 5th and 95th percentiles, as dashed lines in Fig. 6B). Since it integrates BSDB soil variability and climatic conditions more than a single soil end-member correction, the median signal is used as a reference for the following discussion. We chose such a correction approach rather than the simpler binary mixing between the temperature signal reconstructed with a calibration for soils and a temperature signal reconstructed with a calibration for lake sediments for several reasons. One of the main reasons is that in order to apply a binary mixing model, we have to assume that the end-members are almost pure. Samples that have been used for establishing lake sediment calibrations (i.e. the Sun et al., 2011 calibration) are likely to contain a mix of brGDGTs arising from lakes and surrounding soils. Therefore, if we chose to use a mix of signals reconstructed using the calibration for soils and lake sediments, we would have mixed a signal inferred with a “pure soil brGDGTs” calibration with a signal inferred using a calibration for brGDGTs of mixed origin (lake and soils). Another reason is that the Black Sea “Lake” was a very peculiar lake and had distinct properties as compared to the lakes associated with the dataset used for the lacustrine calibrations (i.e. greater depth and likely higher salinity than most lakes). Additionally, the value of early Holocene Black Sea salinity is still a matter of debate, with estimates ranging from 1-2 to 7-13 psu (Soulet et al., 2010; Bradley et al., 2012), and past Black Sea alkalinity is unknown. Further investigations regarding saline and alkaline lakes are required in order to determine how changes in the distribution of brGDGTs respond to environmental variations (Wang et al., 2012). Attempts to include data from saline and alkaline lakes introduce scatter into the lacustrine calibration (Tierney et al., 2010; Sun et al., 2011). Therefore, we established a temperature correction method independent from the existing calibration of the brGDGT proxy for lake sediments. Independent and discrete temperatures for the Holocene and the LGM are benchmarks for assessing the robustness of brGDGT MAT reconstructions. The corrected brGDGT MAT of the early Holocene is in the range of the Noble Gas Temperature (NGT) for

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Fig. 5. Binary mixing model illustrating the influence of terrigenous vs. lacustrine GDGT inputs on MBT0 /CBT derived temperatures. A. Soil and lake sediment GDGT distribution used as end-members. The soil end-member data displayed here are based on Zech et al. (2012) study and correspond to the mean of the GDGT composition of two samples at a depth of 10 and 30 cm. B. Mix between the two end-members applied in order to evaluate the bias associated with the temperature reconstruction for each BIT value (thick gray line). Arrows provide an example of the assessment of a temperature bias with this binary mixing model. Note that the binary mixing models based on the virtual soil end-members mentioned in the text are of the same kind.

groundwater in Hungary (Stute and Deak, 1989; Varsányi et al., 2011, Figs. 1 and 6B). Moreover, at ca 9 ka BP, brGDGT MAT are in the range of modern temperatures within the Danube catchment, and similar to continental and regional annual temperature reconstructions based on pollen data that show early Holocene temperatures roughly comparable to modern temperatures within the Black Sea catchment (Davis et al., 2003; Feurdean et al., 2008b). For the LGM, pollen-derived temperature anomalies within the Dniestr watershed (Fig. 1, Bartlein et al., 2011) and the Hungarian NGT result in absolute temperatures ranging between 1 and 3  C, consistent with the original and corrected MBT0 /CBT reconstructed temperature (Fig. 6B). Although not as precisely dated as previous values, a few glacial noble gas-based temperatures have been reconstructed (Stute and Deak, 1989) and are generally within the same range as the brGDGT MAT (except three being slightly higher, Fig. 6B). 4.3. Millennial-scale temperature variability in a regional context The Black Sea brGDGT record provides a unique continuous record of past MAT over 40 ka BP. During the Last Glacial and early deglacial times (between 40 and 15 ka BP), brGDGT MATs are relatively stable. However, successive cold spells of 1e1.5  C at 30 and 27 ka BP and of larger amplitudes (2e3  C) at 24 ka BP imprint the Last Glacial signal (Fig 6B). Nevertheless, the maximal amplitude of the 24 ka BP cold spell is recorded on the basis of a single sample and may not be significant. If not, the cooling amplitude of this event is similar to the amplitude of the previous cold episodes (ca 1e1.5  C). The cold spells seem concomitant to Heinrich Events (HE), and/ or Stadials 3 and 2 (Hemming, 2004; Sanchez Goñi and Harrison, 2010) detected in the North Atlantic Ocean (Heinrich, 1988; Hemming, 2004) and also in the Northern Hemisphere continental climate record of reference (e.g. Hulu Cave, Wang et al., 2001). HE temperature shifts are clearly expressed within the North Atlantic Ocean and the Mediterranean Sea (e.g. Cacho et al., 1999; Bard et al., 2000; Pailler and Bard, 2002). On the contrary, in European continental settings, few quantitative temperature reconstructions have been attempted, and HE have mainly been inferred from changes in pollen assemblages and modeling

experiments (e.g. Fletcher et al., 2010; Huntley et al., 2013). In the northwestern BSDB, substantial environmental changes have been recorded during the HE discussed here. Loess coarse grain pulses were deposited synchronously with HE 2 and 3 within the Dniepr drainage basin and possibly in the Danube drainage basin (Rousseau et al., 2011; Stevens et al., 2011). The proportion of faunal and vegetal cryoxerophilous species (as well as microcharcoal concentrations) increased during HE 2 within the Carpathian Basin (Sümegi and Krolopp, 2002; Sümegi et al., 2013), supporting a significant temperature (and moisture) decrease. However, the absence of a quantitative temperature reconstruction spanning periods including HE 3 and 2 within the BSDB prevents a further comparison with the brGDGT MAT signal. The decreasing trend toward the LGM, as well as the distinct brGDGT MAT minimum between 20 and 21 ka, is compatible with permafrost reaching the northern portion of the BSDB (e.g. Velichko and Zelikson, 2005). Furthermore, a LGM continental temperature decrease was also seen within the pollen and malacological records of the Carpathian Basin (Sümegi and Krolopp, 2002; Sümegi et al., 2013). The brGDGT MAT reconstruction is notably characterized by the absence of rapid warming events associated with Dansgaarde Oeschger (DO) interstadials, as depicted in the climate reference sequence from Greenland (Fig. 6). In the BSDB and in areas nearby, ecosystem responses to the DO have been evidenced and are expressed by woodland expansion and enhanced soil productivity (Fleitmann et al., 2009; Müller et al., 2011; Sümegi et al., 2013). The contrast between the brGDGT MAT signal and the above cited DO signatures can possibly be explained by the various climate factors that influence terrestrial archives and derived proxies, including seasonality. Indeed, both temperature and precipitation/soil moisture variations are recorded in pollen and stalagmite oxygen and carbon isotopes records; and these factors are difficult to disentangle in the proxy responses (Fleitmann et al., 2009). Precipitation and soil moisture could be the driving factors for stalagmite and pollen-derived proxy responses within the Black Sea region during the glacial. A pollen record from Greece also suggests that tree growth was mainly facilitated by an increase in rainfall (Müller et al., 2011). More precisely, precipitation seasonality, mainly summer precipitation (associated with other seasonal factors such

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Fig. 6. Millennial scale climate variations in Greenland and central eastern Europe. A. Greenland ice core d18O signal (Andersen et al., 2006; Rasmussen et al., 2006; Svensson et al., 2006; GICC05 modified for BP ages and a 40 year moving average) as a reference for Last Glacial millennial scale climate variability. B. MD04-2790 brGDGT derived temperatures corrected using a binary mixing model with a single end-member (modern Serbian soil, gray line) and the median corrected temperature inferred from binary mixing model with 5995 soil end-members (black line). Dashed lines correspond to the 5th and 95th percentiles, and delimit the 90% confidence interval. The red vertical bar indicates the modern temperature range within the northwestern Black Sea watershed. Horizontal blue lines indicate noble gas temperature (NGT) data from Hungary (Stute and Deak, 1989). C. The green line indicates the total tree percentage from the pollen record of Tenaghi Philippon (NE Greece, Müller et al., 2011, with linear interpolation between the calendar and tephra ages provided in the corresponding study). D. The blue line is the TEX86 derived sea surface temperature (BIT-corrected, Ménot and Bard, 2012). Vertical gray bars indicate cold periods in the Northern Hemisphere - YD indicates the Younger Dryas. HS and HE indicate Heinrich Stadial and Heinrich Events (according to the age model of Soulet et al., 2011a), respectively. DO indicates DansgaardeOeschger events (or Greenland interstadials). BA indicates the BøllingeAllerød, and LGM indicates the Last Glacial Maximum. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

as killing frost and growing degree day with baseline 5  C limitation) is important for reconciling vegetation data and the simulation experiment of MIS 3 millennial variability (Van Meerbeeck et al., 2011). Models depict a decreasing amplitude for the mean annual atmospheric temperature difference between stadials and interstadials from the NW to the SE of Europe (Ganopolski and Rahmstorf, 2001; Sima et al., 2009). A difference of only 0.5e1  C is expected for the BSDB (Sima et al., 2009, as shown in Rousseau et al., 2011), clearly in line with the absence of DO cycles on the brGDGT MAT (Fig. 6). The Lateglacial/deglaciation brGDGT MAT is punctuated by an abrupt warming of 3.5  C at 14.7 ka associated to the Bøllinge Allerød (BA) episode recorded elsewhere in Europe (Fig. 6). For this time window, since terrestrial matter in MD04-2790 sediments may have arisen from this area (see Section 2.3), the brGDGT MAT is preferentially compared to records from the southern portion (the Danube catchment) of the northwestern BSDB. The onset of the BA interstadial is characterized by a July temperature shift of 2.8 in the southern Carpathians (Tóth et al., 2012), while no clear signal is found for the pollen sequences of NW Romania (northeastern Carpathians) either for the warmest month or for the annual reconstructed temperature (Feurdean et al., 2008a, Fig. 1). However, for the southerly direction the temperature increase is larger: more than 5  C for the annual mean in the Balkans and Aegean Sea catchment (Bordon et al., 2009; Kotthoff et al., 2011, Fig. 1) and of approximately 4.5  C for the Black Sea “lake” surface temperature (Ménot and Bard, 2012). The continental brGDGT MAT amplitude of this event is, therefore, intermediate and tentatively reflects an average signal between the NW and SE MAT of the western portion of the Black Sea catchment.

The brGDGT MAT increase toward the Holocene is interrupted by an abrupt 1e2  C decrease with a minimum that is reached at approximately 12 ka associated with the Younger Dryas (YD). A similar weak cooling is determined for the Carpathians (<1  C for the chironomid derived July temperature decrease (Tóth et al., 2012) and approximately 2 and 2e5 for the pollen derived warmest month and annual decrease, respectively (Feurdean et al., 2008a)). Southerly, however, the pollen based records provide larger YD annual temperature variations (9  C for the Balkans (Bordon et al., 2009), and 6  C (Kotthoff et al., 2011) for the Aegean Sea, Fig. 1). Therefore, the brGDGT MAT may integrate the YD temperature conditions of the Danube watershed and the Carpathians, and may differ from the southern region’s condition. Furthermore, the weak YD brGDGT MAT decrease can potentially be explained by increasing seasonality, with a shortened growing season and a mainly winter temperature decrease in the Carpathians (Feurdean et al., 2008a; Buczkó et al., 2012; Magyari et al., 2012; Tóth et al., 2012). For the southern Balkans, both summer and winter temperature decreases are depicted and contribute to a substantial MAT decrease (Bordon et al., 2009). As compared to the Black Sea lake temperature reconstruction provided by Ménot and Bard (2012), the continental brGDGT MAT overall trend is similar, as follows: all cold events (HE, YD and LGM) are depicted whereas DO warm events are not (Fig. 6). Moreover, absolute temperatures of both of the corrected signals are within the same range. Such similarities suggest generally close continental and maritime/lacustrine air mass influences within the northwestern Black Sea area at the end of the Last Glacial period (40e9 ka BP). Nevertheless, if compared in a more rigid manner, the lake and continental temperature signals present differences for the duration of HE 2 event and the LGM, and

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for the amplitudes of all cold events (which are smaller for the continental signal). The difference in HE 2 event timing, and notably earlier and shorter events on the continent, could tentatively be explained by punctual competing influence of continental and maritime air masses. Similarly, in regards to the LGM, the Black Sea’s lake surface temperature (Ménot and Bard, 2012) decrease is longer (it lasts until approximately 17.8 ka BP) and does not show an abrupt increase between 20 and 19 ka, as recorded for the brGDGT MAT. The difference may be explained by the Fennoscandian Ice sheet’s fluctuations. Indeed, the Fennoscandian Ice sheet’s maximal advance and retreat may have had a more direct and rapid influence on the atmospheric circulation patterns, and thus, on the continental air temperature (influencing brGDGT in soils), than on the more distant lacustrine area. Interestingly, the beginning of the abrupt brGDGT MAT increase at ca 20 ka is synchronous with the European ice sheet recession (Toucanne et al., 2009). Finally, besides discrepancies due to calibrations of GDGTs proxies, the amplitude of cold events recorded within the continental brGDGT derived signal, smaller than the cold event recorded in the Black Sea Lake signal, could possibly be due to more buffered temperature events within soils seasonally covered by snow. Indeed, snow cover partially disconnects the soil surface temperature from the air temperature (e.g. Smerdon et al., 2004), and brGDGTs are likely to record soil rather than air temperature (Weijers et al., 2007b; Peterse et al., 2012). Therefore, if cold events were associated with seasonal shifts toward much colder winters (as suggested by Denton et al., 2005), the soil brGDGT derived temperature shifts may have been further reduced as compared to the lake temperature shifts. 5. Conclusion In this work, we provide the first continuous quantitative reconstruction for continental temperatures based on the brGDGTs within the Black Sea area from 40 to 9 ka BP. Since in lacustrine settings the in situ production of brGDGTs is likely, special attention has been paid regarding the origin of these compounds. A comparison between the proxies of terrestrial inputs and lacustrine productivity in sediments from the Black Sea allowed us to propose a correction of the bias induced by in situ lacustrine production. Our results confirm that brGDGT based proxies have the potential to reconstruct relative continental paleotemperature evolution and millennial-scale variability. Cold spells related to Heinrich Events, the LGM, and the Younger Dryas are quantified for the northwestern BSDB. Their systematic smaller amplitudes as compared to those recorded for lake temperature (Ménot and Bard, 2012) may be the result of seasonal shifts toward longer winters and snow cover. Moreover, DansgaardeOeschger variability that is evidenced in other terrestrial records within the area of study, is not detected in the brGDGT-derived signal. This finding is interpreted as seasonal and/or precipitation (rather than annual temperature) shifts associated with DansgaardeOeschger events in central Europe. Acknowledgments We would like to thank two anonymous reviewers for their constructive comments. We thank G. Soulet for useful discussions regarding Black Sea hydrology. We are grateful to X. Giraud and N. Cherpeau for their help with implementation of the Monte Carlo method. Core MD04-2790 was retrieved as part of the ASSEMBLAGE project conducted by G. Lericolais and funded by the European Commission (EVK3-CT-2002-00090) during a Marion Dufresne cruise in 2004. Paleoclimate work at CEREGE is supported by the European Community (Project Past4Future) and the Collège de France.

Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.quascirev.2013.11.013.

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