An Arctic perspective on dating Mid-Late Pleistocene environmental history

Quaternary Science Reviews xxx (2013) 1e23

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Invited review

An Arctic perspective on dating Mid-Late Pleistocene environmental history Helena Alexanderson a, *, Jan Backman b, Thomas M. Cronin c, Svend Funder d, Ólafur Ingólfsson e, k, Martin Jakobsson b, Jon Y. Landvik f, Ludvig Löwemark g, Jan Mangerud h, Christian März i, Per Möller a, Matt O’Regan e, Robert F. Spielhagen j a

Department of Geology, Lund University, Sölvegatan 12, SE-223 62 Lund, Sweden Department of Geological Sciences, Stockholm University, SE-106 91 Stockholm, Sweden US Geological Survey, Mail Stop 926A, 12201 Sunrise Valley Drive, Reston, VA 20192, USA d Geological Museum, University of Copenhagen, Øster Voldgade 5-7, DK-1350 Copenhagen K, Denmark e Faculty of Earth Sciences, University of Iceland, Askja, IS-101 Reykjavik, Iceland f Department of Plant and Environmental Sciences, Norwegian University of Life Sciences, P.O. Box 5003, NO-1432 Ås, Norway g Department of Geosciences, National Taiwan University, No 1. Sec. 4 Roosevelt Road, P.O. Box 13-318, 106 Taipei, Taiwan h Department of Earth Science, University of Bergen, P.O. Box 7803, N-5020 Bergen, Norway i Institute for Chemistry and Biology of the Marine Environment (ICBM), Carl-von-Ossietzky-Universität Oldenburg, Carl-von-Ossietzky-Strasse 9-11, 26129 Oldenburg, Germany j Academy of Sciences, Humanities, and Literature Mainz, c/o GEOMAR, Helmholtz-Zentrum für Ozeanforschung, Wischhofstr. 1-3, D-24148 Kiel, Germany k University Centre in Svalbard e UNIS, P.O. Box 156, NO-9171 Longyearbyen, Norway b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 15 February 2013 Received in revised form 9 September 2013 Accepted 23 September 2013 Available online xxx

To better understand Pleistocene climatic changes in the Arctic, integrated palaeoenvironmental and palaeoclimatic signals from a variety of marine and terrestrial geological records as well as geochronologic age control are required, not least for correlation to extra-Arctic records. In this paper we discuss, from an Arctic perspective, methods and correlation tools that are commonly used to date Arctic Pleistocene marine and terrestrial events. We review the state of the art of Arctic geochronology, with focus on factors that affect the possibility and quality of dating, and support this overview by examples of application of modern dating methods to Arctic terrestrial and marine sequences. Event stratigraphy and numerical ages are important tools used in the Arctic to correlate fragmented terrestrial records and to establish regional stratigraphic schemes. Age control is commonly provided by radiocarbon, luminescence or cosmogenic exposure ages. Arctic Ocean deep-sea sediment successions can be correlated over large distances based on geochemical and physical property proxies for sediment composition, patterns in palaeomagnetic records and, increasingly, biostratigraphic data. Many of these proxies reveal cyclical patterns that provide a basis for astronomical tuning. Recent advances in dating technology, calibration and age modelling allow for measuring smaller quantities of material and to more precisely date previously undatable material (i.e. foraminifera for 14C, and single-grain luminescence). However, for much of the Pleistocene there are still limits to the resolution of most dating methods. Consequently improving the accuracy and precision (analytical and geological uncertainty) of dating methods through technological advances and better understanding of processes are important tasks for the future. Another challenge is to better integrate marine and terrestrial records, which could be aided by targeting continental shelf and lake records, exploring proxies that occur in both settings, and by creating joint research networks that promote collaboration between marine and terrestrial geologists and modellers. Ó 2013 Published by Elsevier Ltd.

Keywords: Arctic Chronology Dating methods Pleistocene Stratigraphy

1. Introduction

* Corresponding author. Tel.: þ46 46 222 4483. E-mail address: [email protected] (H. Alexanderson).

Time is a critical concern when reconstructing past environments, determining the true age of geological records, correlating between different sites or records, and defining rates of change.

0277-3791/$ e see front matter Ó 2013 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.quascirev.2013.09.023

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2. Absolute dating methods 2.1. Radiocarbon (14C)

Fig. 1. Absolute dating methods applied in recent studies. The compilation is based on 140 primary research papers from terrestrial (non-ice core) and/or marine studies in the Arctic and which employed at least one absolute dating method and were published 2007e2012, according to the Science Citation Index Expanded database. The overview does not claim to be complete but gives an impression of which absolute dating methods are the most frequently used. Dating methods in the ‘Other’ category include e.g. U-series dating, Ar/Ar and dendrochronology and other methods with less than three publications each.

Within the Arctic Palaeoclimate and its Extremes (APEX) network (http://www.apex.geo.su.se) and other efforts to reconstruct the Pleistocene history of the Arctic, a number of dating methods have been used to provide a chronological framework (Fig. 1), see also e.g. White et al. (2010). These include relative methods (e.g. lithostratigraphy, morphostratigraphy), absolute methods (e.g. radiocarbon dating, varve chronology), methods of age equivalence (e.g. tephrochronology, palaeomagnetism) and astronomical tuning. Within APEX we have worked with terrestrial and marine records that differ in resolution, continuity and chronologic precision. Correlating across the physical e and sometimes also sciencecultural e land-sea boundaries remain a challenge, but linking such records and their environmental signals is essential for understanding the integrated Earth system development through the Pleistocene. Additionally, the characteristics of the Arctic environment present special limitations and opportunities to dating, both in the marine and the terrestrial realm. The aim of our paper is to give a user’s perspective overview of dating Pleistocene events in the Arctic based on the expertise and experience of the author group. Therefore we focus on the Eurasian Arctic, Svalbard and Greenland e the main study area of APEX (cf. Fig. 2), and, temporally, on the Middle to Upper Pleistocene (i.e. the last 781 ka; Pillans and Gibbard, 2012). Also, we do not aim to provide an exhaustive overview of all dating methods used in the Arctic, but have selected the most commonly used ones. In the following sections, we will thus provide a review from an Arctic point of view of common dating methods used for marine and terrestrial events of the middle to late Pleistocene (excluding ice cores). We examine how they are used together with stratigraphy and proxy data to correlate between records and give examples of applications from sites across the Arctic (Fig. 2). Section 2 deals with absolute dating methods and Section 3 with relative dating methods and correlation tools. Here, we want to emphasise the Arctic perspective on dating by focussing on how Arctic conditions influence the possibility and quality of dating, i.e. the geological rather than the analytical uncertainty of ages. For many of these methods detailed reviews already exist and the reader is referred to these for explanation of basic principles. In Section 4, Arctic marine and terrestrial stratigraphic approaches are reviewed and ways to link them are discussed. Finally, in Section 5, we identify challenges and possibilities for future chronological work in the Arctic.

Radiocarbon is the most common method used for dating organic material less than ca 45,000 years old (Fig. 1), which is the effective limit of standard applications (Walker, 2005). It has been applied to all types of environments where organic material can be found. Good overviews of radiocarbon are, e.g. Björck and Wohlfarth (2001; lakes), Walker (2005; general), Hajdas et al. (2007; terrestrial) and Hughen (2007; marine) and useful reviews with an Arctic perspective include Wolfe et al. (2004) and Oswald et al. (2005). In some terrestrial parts of the Arctic, organic matter accumulations have been common over geologic time, for example as widespread peat covers or more or less abundant tundra vegetation, and these provide material for radiocarbon dating of sediment successions. However, in large areas, especially in coastal areas and in polar deserts, organic production can be very low. In such areas, terrestrial chronology is more dependent on dating fossils from glacio-isostatically raised marine sediments than at lower latitudes. Especially molluscs and whale bones have been available for collection and analyses, and in addition to date sediment successions, these have been extensively used for sea-level and glacial history reconstructions (Fig. 3). The preservation, especially of bones, is also favoured by commonly occurring permafrost. The main disadvantage of using marine fossils for dating purposes is the marine reservoir age, which can be poorly constrained (see below). Most radiocarbon dates from the deep Arctic Ocean have been obtained on the planktonic foraminifera Neogloboquadrina pachyderma, whereas a few are on mixed benthic foraminiferal species. However, for many marine sites a limiting factor for radiocarbon dating is the absence or limited quantity of calcareous microfossils available for dating (Backman et al., 2004; Stein, 2008). In addition to increasing uncertainty of pre-Holocene age corrections (Hanslik et al., 2010; Poirier et al., 2012), variable sedimentation rates through time result in changing chronological resolution downcore. For example, low sedimentation rates, particularly during glacial Marine Isotope Stage (MIS) 2, result in low numbers of datable samples per time interval (Poirier et al., 2012), see discussion in Jakobsson et al. (in this volume). Estimates of present day marine reservoir ages, here presented as DR values (Stuiver et al., 1986), are fairly well constrained in coastal areas, but only limited data are available for the open marine areas (Butzin et al., 2005), see Fig. 4. For areas in the Arctic affected by Atlantic water near the surface, i.e. the Barents Sea and the area around Svalbard, DR is given as 20  30 14C years or alternatively 105  24 14C years (Mangerud and Bondevik/Gulliksen, respectively, in Mangerud et al., 2006). These values probably apply also further to the east and north-east. The DR is of the same order (w85 14C years) along the east coast of Greenland (McNeely et al., 2006), but considerably higher in the Canadian Arctic, where it is decreasing from 335  85 14C years in the north-western part to 65  60 14C years in the southeast (McNeely et al., 2006; Coulthard et al., 2010). A DR of 300 14C years has been suggested for the central Lomonosov Ridge and deep and intermediate regions of the Arctic Ocean (Hanslik et al., 2010; Poirier et al., 2012). Comparisons between ages of driftwood and whale bones in eastern Svalbard (Bondevik et al., 1995; Mangerud, unpublished) suggest that DR has been relatively constant throughout the Holocene. Before the Holocene, however, DR values were significantly higher around the Arctic Ocean due to slower exchange of water masses, and, probably, reduced exchange between the atmosphere and a reduced ice-free ocean surface. For example, DR was much higher during the Younger Dryas along the coast of western Norway

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Fig. 2. Location of areas and sites that are discussed in this paper. The focus is on the Eurasian Arctic, as shown by the distribution of red dots representing mentioned sites. The base map is IBCAO version 3 (Jakobsson et al., 2012). See the Online kml-file for detailed locations.

(up to w300 14C years; Bondevik et al., 2006) and in Arctic Canada (w600e1000 14C years; Vickers et al., 2010; Coulthard, 2012). In the Arctic Ocean, higher DR values (1000 14C years) are suggested during and prior to the Lateglacial (Hanslik et al., 2010; Poirier et al., 2012). However, off southeast Greenland a Younger Dryas reservoir age not larger than that of today is suggested by comparison of tephrochronology and radiocarbon dating (Jennings et al., 2002, 2006). Most available reservoir age information concerns surface waters, and may thus not be directly applicable to e.g. deep-sea

benthic foraminifera. Comparisons of radiocarbon ages from benthic-planktonic foraminifera pairs show large differences that also vary with time (Hanslik et al., 2010). An additional problem in the high Arctic is dating sedimenteating bivalves such as Portlandia, Nuculana, Yoldia and Nucula. Especially Portlandia and Nuculana may totally dominate some environments, such as glacier-proximal areas, arctic river mouths or offshore mud, and it is therefore tempting to use them for dating. However, they take parts of their carbon intake from ‘old food’ and/or from sediment pore waters, leading to a bias to

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In terrestrial environments, permafrost and associated low temperatures, frequent water logging and ensuing oxygen depletion cause a lowered rate of decomposition of organic matter (Zech et al., 2011). This may allow for excellent preservation of organic materials, such as microorganisms, plant macrofossils and mega-fauna remains. However, the slow decomposition together with low production rates of organic matter and frequent reworking through cryoturbation or solifluction lead to a high risk of contamination of the organic materials used for dating (Wolfe et al., 2004). This may result in age overestimation (e.g. reworked macrofossils or contamination by old carbon; Oswald et al., 2005) or age underestimation (e.g. introduction of fresh bacterial biomass; Sulerzhitsky, 1998; Wohlfarth et al., 1998). The latter, which can yield finite radiocarbon ages of, e.g. material from the Last Interglacial, has been a complicating factor in separating interstadial from interglacial deposits based on absolute ages (see 3.4 below). To prevent the effect of reworking, the best preserved fossils or specific fractions of organic material are chosen for dating (e.g. Brock et al., 2010; Lachniet et al., 2012; Long et al., 2012) (Fig. 3). Large series of radiocarbon age determinations in a sediment succession can also indicate the occurrence of reworking and help to exclude erroneous sediment ages due to incorporation of reworked organic material (e.g. Jørgensen et al., 2012). 2.2. Luminescence dating (OSL, IRSL, TL)

Fig. 3. Relative sea-level change on Svalbard, as elsewhere in the Arctic, is commonly dated by radiocarbon on marine fossils and driftwood found on raised beaches. The marine fossils generally suffer from larger uncertainties than the drift wood due to less well known reservoir ages and unclear relationship with sea level, but by targeting juvenile Astarte borealis shells, Long et al. (2012) reduce some of these problems. Modified from Bondevik et al. (1995) and Long et al. (2012). Age uncertainty within symbol size, unless marked with black lines.

older 14C ages, ranging unsystematically from a few hundred years to several millennia, especially in areas with carbonate bedrock (Mangerud et al., 2006; Funder et al., 2011; England et al., 2013).

Luminescence dating has been successfully used in the Arctic to provide chronological frameworks for Late Quaternary glacial history reconstructions (e.g. Funder et al., 1998; Mangerud et al., 1998; Svendsen et al., 2004). Terrestrial and emerged littoral sediments have been the main target in Eurasia (e.g. Berger et al., 2004; Möller et al., 2007), Greenland (e.g. Mejdahl and Funder, 1994), Svalbard (e.g. Alexanderson et al., 2011b, 2013, Fig. 5) and Canada (e.g. Demuro et al., 2008), but deep-sea sediments from the Arctic Ocean have also been dated (Jakobsson et al., 2003; Berger and Polyak, 2012). There are several recent reviews of luminescence dating in various environments relevant for the Arctic (Wolfe et al., 2004; Bateman, 2008; Fuchs and Owen, 2008; Jacobs, 2008) as well as

Fig. 4. Modelled (colour range) and measured (at red squares) radiocarbon reservoir ages in part of the Arctic. The map illustrates both the variable reservoir ages in different parts of the Arctic and the lack of data points from the central Arctic Ocean. Modified from http://radiocarbon.ldeo.columbia.edu/ (Butzin et al., 2005).

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Fig. 5. The chronology of the Leinstranda site on western Svalbard is, like for many other similar sites elsewhere in the Arctic, based on a combination of absolute ages from several methods and relative age information from lithostratigraphy and amino acid data. The sediment succession is here presented as an event stratigraphy consisting of seven high sea level events, the ages of which constrain the timing of glacial advances. Modified from Alexanderson et al. (2011b).

general overviews of the method (Lian and Roberts, 2006; Rhodes, 2011). Thorough technical descriptions of luminescence dating are available in Aitken (1985, 1998), Preusser et al. (2009; quartz) and Buylaert et al. (2012; feldspar).

A key factor is the resetting of the luminescence clock during or immediately before the event to be dated, i.e. whether the quartz or feldspar grains were exposed to sufficient daylight (or heat) at the time of deposition. Resetting (also called zeroing or bleaching) is

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faster for optically stimulated luminescence (OSL; used mainly on quartz) than for infrared stimulated luminescence (IRSL; used mainly on feldspar), which in turn is reset more quickly than thermoluminescence (TL; used mainly on feldspar) (Godfrey-Smith et al., 1988; Buylaert et al., 2012). If the exposure is not sufficient for complete resetting, the result will be incomplete bleaching and apparent age overestimation. In the Arctic, good conditions for bleaching occur during summer when sediment processes are most active, while no or only little light for bleaching is available during winter (Berger, 2006; Bateman, 2008). Light availability to sediment in transport may be further limited by high sediment content in glacial meltwater or by sea ice. The extensive reworking within periglacial environments imply many but short exposures to light, which may or may not be sufficient for complete zeroing. Interestingly, in some cases careful selection of grain sizes or sediment for dating seems to yield information on past depositional processes (Johnson and Ståhl, 2010; Berger and Polyak, 2012) and provenance (Berger, 2011; Sawakuchi et al., 2011). Bleaching of the luminescence signal in Arctic settings has been studied by luminescence dating of modern or independently dated deposits. In general it seems that distal glacifluvial, shallow marine, littoral and aeolian sediments are generally fairly well bleached, and any age overestimation due to incomplete bleaching is less than a few thousand years (Hansen et al., 1999; Bateman and Murton, 2006; Alexanderson, 2007; Alexanderson and Murray, 2012a), see Fig. 6. Age overestimation of even a few thousand years is often insignificant for dating Pleistocene, particularly preLGM, sediments, for which the error may be of the same or a larger amount, but it would be significant for younger (Lateglacial e Holocene) deposits. Proximal glacifluvial and polar deep marine sediments, on the other hand, may have large age overestimations, in the order of 7e25 ka for quartz OSL and even larger for feldspar IRSL (Berger, 2006; Berger et al., 2010; Alexanderson and Murray, 2012a,b; Alexanderson and Håkansson, 2013) (Fig. 6), since feldspar bleaches more slowly than quartz (see above). However, even in such challenging environments good results have been achieved through dating of single grains or small aliquots (Berger and Polyak, 2012; Alexanderson and Håkansson, 2013). The environmental dose rate required for age calculation is determined by the amount of radioactive elements in the ground

and the cosmic radiation (Aitken, 1985). Water or ice in the sediment will absorb some of the radiation, thereby decreasing the dose rate for the sediment grains. Water content can vary considerably over the often tens of thousand years that luminescence samples have been buried, especially because of major environmental change during glacialseinterglacialseinterstadials. Sediments are often postulated to have been continuously water saturated in the Arctic (e.g. Mangerud et al., 2004), but this may give an overestimation of the age of about 1% for 1% too high water content. Simple models of water content variation through time may provide more accurate ages (Alexanderson et al., 2010; Lukas et al., 2012), and can be used to estimate uncertainties in age related to likely hydrogeological scenarios. In this respect permafrozen sediments may provide a less variable radiation environment through time than unfrozen sediments. However, massive ground ice bodies in permafrost will significantly change the dose rate in their vicinity and cause too young luminescence ages if not accounted for. Sedimentological evidence of past, or present, ice wedges or other ground ice should therefore be noted (cf. e.g. Andreev et al., 2004). The main uncertainty arises if ground ice has not been present throughout the burial time, but has formed and/or disappeared at some point in time after deposition. As this time is usually unknown it may be difficult to estimate the average dose rate back in time. On the other hand, if ground ice has been present since deposition the dose rate will not change through time, assuming otherwise constant conditions. Likewise, if ice formed just prior to sampling it will not affect the average dose rate received by the sampled sediment. The regional geology and the history of the mineral grains influence the luminescence characteristics of the dated minerals (cf. Pietsch et al., 2008), which in turn has implications for luminescence ages. Experience shows that the luminescence characteristics of quartz vary across the Arctic. They seem to be very good in Russia and Siberia, with bright and clean quartz with a strong fast signal component (Thomas et al., 2006; Arnold and Roberts, 2011). In Scandinavia and Greenland, quartz is in general dimmer and may suffer from feldspar contamination or a minor fast signal component (Alexanderson and Murray, 2012b; Alexanderson and Håkansson, 2013), which may lead to less precise ages. Similar irregular luminescence characteristics are known from Canada (Demuro et al., 2008, 2013), although also good quartz

Fig. 6. OSL-analysis on quartz and two types of IRSL-analysis (IRSL 50 and post-IR IRSL 290 ) on feldspar from present-day deposits in the Engelskbukta Bay near Leinstranda on western Svalbard show the effects of incomplete bleaching in a glaciated setting (see Alexanderson and Murray, 2012a for details). The highest doses (given in Gray, Gy), which come from the most poorly bleached sediments, are found close to the glacier, while distal sediments have lower values, particularly for quartz. The ages given in parenthesis are the apparent age overestimations the measured doses would correspond to in similar but Pleistocene-aged sediments nearby (dose rates from Alexanderson et al., 2011a). The distance along the beach from the foot of the mountains in the upper left corner of the photo to the glacier margin on the right is w4 km. Photo: Monica Sund.

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characteristics occur there (Bateman and Murton, 2006; Murton et al., 2010). Svalbard quartz shows much variation, as expected from the varied geology of the archipelago (Alexanderson et al., 2011b, 2013). Feldspar, on the other hand, usually has good luminescence characteristics, but will suffer from fading to various degrees (Spooner, 1994), and in some areas (e.g. Svalbard) a low feldspar content in the bedrock may cause limitations (Mangerud et al., 1998). New analytical protocols, such as the pIRIR290 protocol (cf. Fig. 6), have been developed that reduce the effect of fading and they are promising for future work with feldspar in environments with sufficient daylight exposure (Buylaert et al., 2012). It should, however, be noted that exceptions occur in all areas, but can be identified and, if necessary, corrected for in most cases (Arnold and Roberts, 2011). 2.3. Cosmogenic exposure dating Cosmogenic nuclides, formed by the interaction of cosmic radiation with the atmosphere and/or the earth surface, can be used for dating in different ways; as exposure dating (Gosse and Phillips, 2001; Ivy-Ochs and Kober, 2008; Dunai, 2010), burial dating (Balco and Shuster, 2009) and as a stratigraphic tool (Frank et al., 2008; Sellén et al., 2009). Here we will focus on exposure dating, while 10 Be stratigraphy is discussed below (3.5). The most commonly used isotopes are 10Be and 26Al, but depending on lithology and age span, other nuclides such as 14C, 21Ne or 36Cl are also used (Kurz and Brook, 1994; Walker, 2005). Cosmogenic exposure dating has emerged as an important tool particularly for reconstructing glacial histories (Balco, 2011; Briner, 2011), including past ice thickness. Erratic boulders and glacially eroded bedrock have been dated to determine the deglaciation ages and the timing of different ice extents in most Arctic land areas, e.g. Arctic Canada (Briner et al., 2005), Greenland (Håkansson et al., 2007, 2009; Kelly et al., 2008; Young et al., 2013b), Svalbard (Landvik et al., 2003, 2013; Hormes et al., 2011; Henriksen et al., in this volume) and northern Russia and Siberia (Gualtieri et al., 2005; Mangerud et al., 2008b). The ‘global’ production rates of 10Be and 26Al (most recent: Balco et al., 2008) used in most cosmogenic dating studies are based on data sets from low- and mid-latitude areas. However, recent observations indicate that the production rates in the Arctic are lower than the global ones (Fenton et al., 2011; Goehring et al., 2012, Norway; Briner et al., 2012, SW Greenland) and based largely on these observations an Arctic 10Be production rate has just been published (Young et al., 2013a). The lower production rates yield comparatively higher ages, the validity of which is supported by agreement with independent dating (Henriksen et al., in this volume; Young et al., 2013b). The frequently unstable glacial and periglacial landscapes in the Arctic, including dead-ice or permafrost melting, solifluction etc., may cause boulders to topple or emerge after deposition of the landform that is to be dated. This will result in too young apparent ages. Such post-depositional shielding is an important process that

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leads to scatter in cosmogenic exposure ages and means that, generally, the maximum age in a boulder group should be considered the minimum age of the landform (Putkonen and Swanson, 2003; Heyman et al., 2011). The age underestimation due to post-depositional shielding can be significant; Houmark-Nielsen et al. (2012) show up to 6 ka delay in boulder exposure in Lateglacial dead-ice landscapes and, in extreme cases, melting of buried dead ice may be delayed by up to 80 ka (Henriksen et al., 2003). An example of this is shown in Table 1 and in Fig. 7, where large boulders on moraines known to be of early and middle Weichselian age are dated to 31e2.5 ka (Mangerud, unpublished data) and thus suffer from age underestimation in the order of tens of thousands of years. Sampling multiple sides of single boulders may overcome such issues (cf. Abbühl et al., 2009). However, boulders resting directly on bedrock or on thin soil cover are less likely to suffer from these problems, and most cosmogenic ages on erratic boulders from glacially scoured landscapes provide representative ages of the last deglaciation (e.g. Briner et al., 2005). In contrast, in landscapes with limited erosion, nuclide inheritance and age overestimation is a common problem. From such settings, several studies show younger boulders sitting on bedrock with considerably higher exposure ages (Briner et al., 2006; Hormes et al., 2011; Landvik et al., 2013). This causes scattered ages, and large numbers of samples are required to analyse local or regional age distributions (Briner et al., 2005; Håkansson et al., 2009). To unravel a complex exposure history, analyses of more than one nuclide per sample may be helpful (e.g. Bierman et al., 1999; Miller et al., 2006), and it has recently been shown that paired datings on boulders and nearby exposed bedrock surface in cases can resolve this issue (Larsen et al., 2013). 2.4. Electron spin resonance (ESR) Electron spin resonance (ESR) dating is in its principles similar to luminescence dating; it builds on measurement of the amount of radiation-induced paramagnetic centres that have been created in the dated material due to natural radiation from its geologic background. The dated material is preferentially tooth enamel, coral, and molluscs, but also sediments can be targeted, and the age span that can be dated covers most of the Pleistocene. Technical descriptions of ESR dating are available in, e.g. Grün (1997, 2001), Rink (1997) and Molodkov et al. (1998). The main application of ESR in the Arctic is dating of molluscs e and thus marine sediment sequences in which these are embedded e beyond the reach of radiocarbon dating. ESR dating of glacioisostatically raised sediments has been performed over a broad zone from Svalbard and the Kola Peninsula to the Taymyr Peninsula (see map in Molodkov (2012), Fig. 1). On Severnaya Zemlya (Möller et al., 2007) and on Cape Chelyuskin (Möller et al., 2008), ESR dating has been a helpful chronological tool in constraining the age of marine sequences intercalated with Kara Sea ice-sheet tills and, thus, for the palaeoenvironmental reconstruction of the area. Likewise,

Table 1 Previously unpublished 10Be ages from 1 to 2 m high boulders on the Halmer and Usa moraines near the Polar Urals (Fig. 2.) show severe underestimation of the timing of initial moraine formation due to post-depositional shielding. The Halmer moraine (Fig. 7) represents an early Weichselian advance of a Kara Sea ice sheet (Svendsen et al., 2004), while the Usa moraine represents a MIS 4 glacial advance from the Polar Ural Mountains (Mangerud et al., 2008b). 10

Field id

Pechora Pechora Pechora Pechora Pechora

1999-2038 1999-2016 1999-2018 1999-2019A 1999-2017A

Be

2s precision

Age ka

Uncertainty ka

18.2 30.5 17.9 17.0 2.5

1.2 1.7 1.3 1.1 1.2

Location

Timing of ice advance

Halmer moraine ridge Usa moraine Usa moraine Usa moraine Usa moraine

90-80 ka (Svendsen et al., 2004) >55 ka (Mangerud et al., 2008a) >55 ka (Mangerud et al., 2008a) >55 ka (Mangerud et al., 2008a) >55 ka (Mangerud et al., 2008a)

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Fig. 7. The Halmer moraine lobe on the left is characterised by a multitude of hummocks and lakes. It was deposited in the early Weichselian, but as shown by its fresh look and young 10Be ages on boulders (Table 1) the landscape has been active fairly recently. The road that stretches through the middle of the photo is approximately 3 m wide and follows the outer part of the moraine. Photo: Mona Henriksen.

ESR dating of planktonic foraminifera from deep-sea sediments in the Fram strait (Hoffmann et al., 2001) has yielded results that support the age model obtained by oxygen isotope stratigraphy for the last c. 200 ka and reveal the general potential of the method. Error sources are similar to those in luminescence dating, e.g. residual ESR signal in the dated material, leakage from electron traps and temporal variations of pore-water content (see 2.2). Another problem that can cause erroneous ages is postdepositional uranium uptake (Grün, 2001). Late uranium uptake from groundwater is believed to be the cause of ESR age scatter and ESR age underestimation compared to luminescence ages from a sediment succession on Svalbard (Alexanderson et al., 2011b, Fig. 5). These error sources should be taken seriously, and precision of the ‘true’ depositional age might be much less than indicated by the technical lab ages (Walker, 2005). Better constraints on the movement of uranium can be achieved by coupling U-series (see 2.5) and ESR dating (US-ESR; Grün and McDermott, 1994), which would improve the accuracy of the age. 2.5. Uranium-series dating U-series dating includes a range of methods, including the common 230Th/234U, which can be applied to a variety of materials and time ranges from a few hundred years to more than 500 ka (Schwarcz, 1989; Walker, 2005). Arctic applications include dating of marine mollusc shells (Arslanov et al., 2002), peat (Schirrmeister et al., 2002), corals (López Correa et al., 2012) and speleothems (Lauritzen, 2006). U-series nuclides are also useful stratigraphic tools in marine sediments (see 3.5 below). A requirement for accurate U-series dating is that the dated material acts as a geochemically closed system, but Arctic samples are generally considered unpredictable in this respect (e.g. wood, peat, molluscs, calcareous algae; Kaufman et al., 1971; Schwarcz, 1989; Walker, 2005; Linge et al., 2008). However, permafrost may provide a closed system (Schirrmeister et al., 2002) with stable conditions and limited exchange with water and air. If open-system behaviour is recognised, nuclide migration such as uranium uptake can still be modelled and ages calculated, yet with a larger uncertainty.

alteration is determined (Miller and Brigham-Grette, 1989; Wehmiller and Miller, 2000; Walker, 2005; Kaufman, 2006). The strength of the method is that it can be used well beyond the range of 14C dating. However, the rate is both species- and temperature-dependant (Miller and Mangerud, 1985; Miller and Brigham-Grette, 1989) so the method is today more often used for correlation of deposits that have experienced similar postdepositional temperature histories (aminostratigraphy; Miller and Brigham-Grette, 1989) than for direct age estimates. Here, the deep sea provides very suitable conditions with its relatively stable temperature (Kaufman et al., 2008), compared to the variable conditions on land. Aminostratigraphy has successfully been based on mollusc shells from raised marine deposits in many Arctic coastal areas (e.g. Miller, 1982; Möller et al., 2007), and on foraminifera, particularly N. pachyderma, in the deep Arctic Ocean (e.g. Kaufman et al., 2008). Being a temperature dependant process, an interesting application of the amino acid method in the high Arctic is that it can be used to estimate the duration of glaciations across a site. If the annual mean temperature is well below zero, diagenesis will almost stop, whereas under warm-based ice or sea water with temperatures about 0  C, the rate is considerably faster. However, this approach will generally not be able to differentiate between periglacial conditions and cold-based ice cover, both of which result in sub-zero temperatures and very limited diagenesis. The epimerization of isoleucine to allo-isoleucine has, e.g. been used on Svalbard to estimate the duration of LGM glaciation (Mangerud et al., 1992; Andersson et al., 1999, 2000), and to calculate the integrated period of glacial cover since the Last Interglacial (Mangerud and Svendsen, 1992). However, this will only be minimum estimates of duration as recent studies show that parts of the ice sheet experienced periodically cold based conditions (Landvik et al., 2005, in this volume). Other amino acids undergo faster diagenesis and therefore provide higher resolution in cold climate. For example, analysis of aspartic acid and glutamic acid in molluscs from Novaya Zemlya indicate that the LGM glaciation lasted 3000 or 10,000 years at most, assuming basal temperatures of 0  C and 5  C, respectively (Mangerud et al., 2008a).

3. Correlation tools 3.2. Tephrochronology and pumice marker horizons 3.1. Amino acid geochronology The diagenesis of amino acids in, e.g. marine molluscs is a process that can be used as a dating method if the rate of chemical

Tephrochronology uses widespread volcanic ash horizons as marker beds to correlate and date records (Lowe, 2011). In the Arctic, volcanoes on Iceland, Jan Mayen and in Alaska are the most

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Fig. 8. The Svalbard glacial event stratigraphy, here presented as a glaciation curve (on the right), is a composite based on records from several sites; the black dots show which events are recorded at each site (see Fig. 1 and online kml-file for locations). The glacial event stratigraphy is compared to stacked IRD- and d18O-records from marine cores on the continental slope off Svalbard. The glaciation curve mainly represents fjord/ice-stream settings and may not be representative for inter-fjord areas on Svalbard (cf. Landvik et al., in this volume). Modified from Mangerud et al. (1998).

important sources and tephras from these sources have been found in terrestrial, marine and ice-core records (e.g. Wastegård and Davies, 2009; Jensen et al., 2011; Abbott and Davies, 2012; Zamelczyk et al., 2012; Ponomareva et al., 2013). Tephrochronology can thus provide an important synchronisation link between different types of records and, particularly with the identification and dating of tephras in the Greenland ice cores, a useful framework for much of the Last Glacial cycle, at least in the North Atlantic sector of the Arctic (Abbott and Davies, 2012; Davies et al., 2012). However, so far there are very few reports of tephra from Arctic lakes, except for Alaska, or the Arctic Ocean. Difficulties to find tephra could be due to rapid sedimentation rates and/or relatively coarse material that ‘drown’ cryptotephras. Another complicating factor in Arctic environments is temporary storage after initial deposition on long-lasting snow cover, glaciers or sea ice (Bond et al., 2001; Jennings et al., 2002; Bergman et al., 2004). This may result in delayed final sedimentation, mixing or dilution of the tephra, which reduces its value and ease of use as a time-synchronous marker. Pumice is another volcanic product that can be used for chronological work. It is composed of highly vesicular volcanic glass foam that has low-enough density for it to float (Fisher and Schmincke, 1984). Pumice has been described from numerous locations in Arctic Canada (Blake, 1970, 1975), Greenland (Blake, 1970), Svalbard (e.g. Donner and West, 1957; Boulton and Rhodes, 1974; Salvigsen, 1978, 1984a,b) and northern Norway (Binns, 1972). The pumice generally occurs as spread pieces in distinct zones on raised beach sequences. The zones have at most sites been dated by 14C on driftwood or whalebone, and are used for correlating relative sea level records. There is a lack of high-quality geochemical data on Arctic pumice, and better geochemical and chronological constraints can significantly improve its usefulness as marker horizons for regional correlations.

3.3. Glacial event stratigraphy In an event stratigraphy, sedimentary units are correlated based on the palaeoenvironmental interpretation of the event that caused their deposition, rather than directly on any sediment properties (Whittaker et al., 1991). In glacial event stratigraphy, the events are glaciations or glacial advances that are distinguished based on icemovement direction (cf. kineto-stratigraphy, Berthelsen, 1978), pattern of glaciation (cf. e.g. glacial inversion, Kleman and Borgström, 1996), glacio-isostasy (see below) or chronostratigraphy (dating sediment below and above a till bed; e.g. Mangerud and Svendsen, 1992). Kineto-stratigraphic types of glacial event stratigraphy are less applicable in high-relief and fjord areas, but work well in extensive low-relief areas such as northern Russia (Kjær et al., 2003). Glacial inversion principles (see overview in Kleman et al., 2006) have been used on a large scale to reconstruct both the Fennoscandian and the Laurentide ice sheets. Glacio-isostasy is an important factor in glaciated areas, and high sea levels with subsequent regressions occur at times of deglaciations of ice sheets. During these times, coarsening-upward successions of marine-to-littoral sediments are typically deposited, and are presently found raised above sea level (Mangerud et al., 1998; Alexanderson et al., 2011a), cf. Fig. 5. The sediments commonly contain fossils that can be dated, or are themselves suitable for e.g. luminescence dating, and such high-stand deposits have therefore been targeted to constrain the timing of glacial events. It is, however, a disadvantage that non-glacial deposits are relied upon to date glacial events, including the common use of beds interpreted to be interglacial as marker beds for correlation in glacio-isostatic event stratigraphies (cf. 3.4). The assumption is that if a considerable glacio-isostatic adjustment took place after each deglaciation, different high stands have to be separated by several thousand years, probably at least 10,000

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years e the time needed for new glacial growth, associated isostatic depression and following deglaciation. Thus, low-precision dating methods as luminescence, ESR and amino acids, can be used to resolve which high-sea-level events can be correlated. For pre-LGM glaciations, the age asynchronicity related to progressively younger deposits towards the past ice centre (cf. the timing of formation of the marine limit during the last deglaciation) is small compared to the precision of other correlation/dating methods and can be disregarded. The glacio-isostatic approach has been used with success in Svalbard (Mangerud et al., 1998) and on Severnaya Zemlya (Möller et al., 2007). Similar approaches could be used for large icedammed lakes, for example in northern Russia. When an ice sheet blocks major drainages, such as the north-flowing Russian rivers, ice-dammed lakes are formed and their shorelines represent synchronous events (Mangerud et al., 2004). Glacial event stratigraphies have been correlated with event deposits in marine cores, especially IRD layers and intervals of drastically changing sedimentation rates (e.g. Mangerud et al., 1998, Fig. 8), but such correlations have generally assumed a rather uniform waxing and waning ice sheet. However, ice-sheet behaviour on a regional scale, as seen from the Antarctic ice sheet (Rignot et al., 2011) and from palaeo-reconstructions from the Canadian Arctic and Svalbard (e.g. Briner et al., 2008; Ingólfsson and Landvik, 2013; Landvik et al., in this volume) and elsewhere, suggests that past ice sheets were spatially and temporally much more dynamic than previously believed, which has implications for e.g. the rapidity of glacial advances and the correlation with marine proxies and thus for the use of glacial event stratigraphy. 3.4. Interglacial deposits as marker beds Non-glacial deposits such as marine sediments or organic deposits that are found beneath, between or above tills and represent ice-free phases are important for correlation and for reconstructing glacial histories. A key event is the Last Interglacial, and its deposits are commonly used as a marker horizon in Arctic areas, also outside glaciated areas. Deposits are generally interpreted to be of Last Interglacial age if they indicate temperatures higher than or similar to today, occur in the expected stratigraphic position and have absolute ages in agreement with established ages. Commonly, however, not all of these criteria can be fulfilled for a single bed or site, and there are other relatively warm and ice-free phases, both older and younger, during which the studied intervals could have been deposited. Henriksen et al. (2008) for example conclude that the Odderade interstadial (MIS 5a) was warmer than at present in northern Russia and, similarly, Helmens et al. (2012) show warmer than present summer temperatures during MIS 5c in northern Fennoscandia. Common causes of misidentification include fragmentary records, apparently finite radiocarbon ages from old deposits (see 2.1 above and e.g. Briant and Bateman, 2009) and possible underestimation of OSL ages from Last Interglacial sediments (Murray et al., 2007). The latter may lead to a w10% age underestimation, which seems to be at least partly due to changes in luminescence characteristics during measurement and could be overcome by using other analytical protocols for quartz (Murray et al., 2007) or by analysing feldspar instead (Buylaert et al., 2012). These problems are well exemplified by the two widespread marine formations (Kazantsevo and Karginsky horizons) in northwestern Siberia with boreal faunas indicating warmer than present climate. According to the much used stratigraphic scheme of Sachs (1953), the Kazantsevo Horizon represents the Last Interglacial (wMIS 5e), while the Karginsky Horizon for long has been correlated to MIS 3, mainly based on finite, although close to the

Fig. 9. Shell fragment of Cyrtodaria angusta from a site at Bolshaya Balaknya River on the southern Taymyr Peninsula, Siberia. The stratigraphic sequence in which it occurs is ESR dated to 430  41 ka (Möller, unpublished data). Photo: Svend Funder.

limit, radiocarbon ages (Kind, 1974; Kind and Leonov, 1982; Arkhipov, 1989). However, deposits defined as Kazantsevo sediments at places host the extinct bivalve Cyrtodaria angusta (Sachs, 1953; see below) (Fig. 9), suggesting that classic Kazantsevo sediments actually contain at least two marine horizons with boreal fauna, one of which must be older than MIS 5e. In addition, a comprehensive OSL dating effort at the type locality for the Karginsky horizon (where previous 14C ages were finite) gave a clustered age group with a mean age of 111 ka (Astakhov and Nazarov, 2010). This, taken together with an ESR age of c. 122 ka at the same site, indicates that the Karginsky horizon should be correlated to the European Eemian (wMIS 5e), and that the Kazantsevo should be considered Middle Pleistocene (MIS 7) in age (cf. Astakhov and Mangerud, 2005; Astakhov, 2013). The ‘Cyrtodaria beds’, characterised by the presence of C. angusta (Fig. 9), is a principal marker horizon for the Pleistocene of northern Russia. C. angusta was an important component of shallow water faunas in northern seas especially during the Pliocene, it is accompanied by boreal fauna elements and signifies interglacial conditions. In European Russia the Cyrtodaria beds were considered to date to the PlioceneePleistocene transition (Zharkidze, 1983), but in western Siberia they were traditionally associated with the last e Kazantsevo e interglacial, corresponding to the Eemian in Western Europe. However, as discussed above, modern dating methods have pushed the Cyrtodaria beds back in time. The Russian Cyrtodaria has changed name several times as noticed recently by Gusev et al. (2012). It was erected to species level and named Cyrtodaria jenisseae by Sachs (1953), but later reduced to subspecies rank by Merklin et al. (1979). However, all the specimens we found e both east and west of the Urals, as well as those inspected in Russian museums e fall within the wide range of variability in C. angusta (Nyst and Westendorp 1839) seen in museum collections from Western Europe, and described by Strauch (1972). The date of extinction of C. angusta in different areas is, however, still poorly known. In Western Europe it disappeared at the Plio-Pleistocene transition (Strauch, 1972), while the youngest well dated occurrence is in the Tjörnes beds of northern Iceland, where it survived until ca 1 Ma (Eiriksson, 1981; Eiriksson et al., 1990). 3.5.

230

Th and

10

Be stratigraphy

Records of radiogenic isotopes measured in bulk sediments have been used to establish age models for Arctic Ocean sediment cores (Eisenhauer et al., 1994; Aldahan et al., 1997; Spielhagen et al., 1997, 2004; Frank et al., 2008; Sellén et al., 2010). The thorium isotope 230 Th is produced in the water column from radioactive decay of uranium (cf. 2.5), which is supplied to the ocean by terrestrial erosion. The isotope has a residence time of only 5e40 yr and is either exported by currents or deposited through adsorption to carriers like organic carbon (e.g. in fecal pellets) and other fine-

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Fig. 10. Correlation of cores from the Makarov Basin and the nearby Lomonosov Ridge (Fig. 2) by coarse fraction and

grained particles. After a correction for the 230Th component that is produced in the sediment itself by uranium decay, profiles of excess 230 Th (230Thex) in Arctic sediment cores typically show a pattern of downcore decreasing 230Th contents which reach background values at w300 ka. Any 230Thex concentrations above the background are thus indicative of late Quaternary deposits. The beryllium isotope 10Be (cf. 2.3) has also been used as a stratigraphic tool, e.g. in the ACEX core age model (see 4.3). It reaches the ocean surface by adsorption to aerosols and its residence time in the ocean is 400e1000 yr. Short-term events are therefore less likely recorded than with 230Thex. Besides using the radioactive decay of 230Th and 10Be as a measure of age, peak concentrations of these elements in the sediments have been applied as indicators of times with reduced ice coverage in the Arctic Ocean (interglacials and interstadials). This conclusion is based on the relation of 230Th and 10Be deposition to processes like bioproduction and sea ice transport of finegrained particles. Arctic shelves, which are the main areas for modern sea ice production and fine-grained sediment entrainment, were dry due to a lowered sea level during glacials and stadials (Jakobsson, 2002). Extremely low microfossil contents in sediments from glacial periods (Spielhagen et al., 1997, 2004; NørgaardPedersen et al., 1998, 2003, 2007; Jakobsson et al., 2000; Backman et al., 2004) point to a limited primary production, caused by the limited penetration of light through the denser sea ice cover during glacials and stadials. The low particle flux likely led to a horizontal oceanic export of 230Th and 10Be and deposition in areas of sea ice melt outside the Arctic Ocean (Eisenhauer et al., 1990, 1994). Accordingly, concentrations of these isotopes are usually low in glacial time sediments in the Arctic Ocean. In contrast, sediments from warmer periods usually hold relatively high 230Thex and 10Be concentrations. This finding has been used to determine interglacial and interstadial sediments in deposits that do not preserve biogenic carbonate. For example, deposits at site PS2178 in the Makarov Basin (Fig. 2) are barren of biogenic carbonate while nearby core PS2185 from the crest of the Lomonosov Ridge holds several layers with relatively high amounts of planktonic foraminifers and coccoliths, indicating the presence of interglacial and interstadial sediments (Spielhagen et al., 2004). While grain size and palaeomagnetic data provide only a rough

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10

Be records.

correlation between these two cores, the 10Be records show a remarkable similarity attributed to comparable vertical fluxes of 10 Be at both sites (Fig. 10). This permits identification of specific time intervals (e.g. MIS 5e and 3) with enhanced deposition of 10Berich fine-grained sediments both on the ridge and in the basin. 3.6. Manganese stratigraphy Late Pleistocene sediments from the central Arctic Ocean are typically characterised by a near cyclical occurrence of dark brown layers intercalated with yellowish or olive grey sediment (e.g. Stein, 2008). These brownish manganese-rich layers are normally intensely bioturbated and host high numbers of calcareous microfossils and have been interpreted as being deposited under interglacial/interstadial conditions (Clark et al., 1980; Poore et al., 1993). This led Jakobsson et al. (2000) to propose a direct peakby-peak correlation between the Arctic manganese cycles and a stacked low-latitude oxygen isotope record of orbital (Milankovitch) glacial-interglacial climatic cycles (Fig. 11). One advantage of the Mn-stratigraphy as a cyclostratigraphic tool is that it may have a basin-wide coverage and can be applied where foraminifera and other tools are missing (Löwemark et al., 2013). However, its use as a stratigraphic tool has been hampered by a lack in our understanding of the physical and chemical mechanisms controlling the formation of the Mn-rich layers, something that recent studies have resolved. It is now, for example, clear that the major sources of Mn to the Arctic Ocean are rivers and coastal erosion (Middag et al., 2011; Macdonald and Gobeil, 2012), although the Mn transport pathways to the deep sea are complex (cf. Martin and Knauer, 1980; Macdonald and Gobeil, 2012). In general, the Mn input and transport pathway is strongly climatecontrolled and is largely responsible for the large glacialinterglacial variations in sedimentary Mn contents (Fig. 12). The input of Mn to the Arctic Ocean during glacial intervals is drastically reduced compared to interglacials due to rivers being blocked by continental ice sheets and the shelves being either covered by ice sheets or emerged above the ocean (März et al., 2011; Macdonald and Gobeil, 2012). Burrows produced by benthic organisms suggest that enhanced Mn deposition was simultaneous to increased biological activity at

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Fig. 11. Correlation between central Arctic manganese cycles and the global benthic oxygen isotope curve. A. Coarse fraction content (% wt. > 63 mm) and manganese cycles (raw counts from XRF-scanning) in the upper 7 m of core 96/12-1 PC from the Central Lomonosov Ridge (Fig. 2). Independent age control was used to identify MIS 5 in this core (Jakobsson et al., 2000, 2001, 2003) B. Raw counts of Mn from the XRF core scanner (Red) are shown against the benthic marine d18O record of Lisiecki and Raymo (2005) (Blue) using the correlation proposed by Jakobsson et al. (2000). Black lines between panels A and B illustrate the position of prominent tie points between the depth and age domains.

Fig. 12. Conceptual model showing the interglacial and glacial manganese input scenarios (after Löwemark et al., 2012).

the sea floor under interglacial conditions; this supports the interglacial nature of Mn-rich layers (März et al., 2011; Löwemark et al., 2012). Mn enrichment seems to be directly related to increased primary productivity and organic matter export, and can in principle be used for chronostratigraphic purposes. In contrast, there is no support for the hypothesis that decreased bottom water oxygenation during glacial intervals would be responsible for low Mn levels (Jakobsson et al., 2000; Löwemark et al., 2008). No significant loss of Mn in glacial sediments under anoxic conditions is indicated (März et al., 2010, 2011), and ostracode (Poirier et al., 2012) and trace fossil (Löwemark, 2012; Löwemark et al., 2012) studies document well-mixed oxic bottom waters even during peak glacials (Cronin et al., 2012). However, Mn-stratigraphy is complicated by diagenetic processes in the sediment (Li et al., 1969; März et al., 2011). Enhanced deposition of organic material may lead to formation of a redox front that dissolves Mn-oxides as it migrates into the sediment. Furthermore, thin, closely spaced horizontal bands of dense material observed in radiographs (Fig. 13) showed pronounced Mn enrichments (Löwemark, 2012; Löwemark et al., 2012). These bands are post-depositional, as they do not show any relation to primary sedimentary structures, and their formation is likely related to a fluctuating redox boundary. Since both types of diagenetic Mn remobilization can be readily identified when X-ray radiographs are combined with XRF-scanning of the sediment, it is possible that climate-controlled Mn layers can be separated from diagenetic ones, thus allowing Mn layers to be used for stratigraphic correlations over large distances.

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cores. As a result, d18O measurements on Arctic foraminifera are not typically used in deep Arctic sediments as a standard means of correlation. Nonetheless, other proxy methods discussed in Section 4.3 are often used for cyclostratigraphy in the deep Arctic Ocean and have allowed some cores to be correlated to standard marine Oxygen Isotope Stages back as far as MIS 21 (w850 ka; Polyak et al., 2013). Even in studies where oxygen isotopes have not been analysed, such as for some terrestrial sediment successions, marine oxygen isotope (MIS) stages (Martinson et al., 1987) or Greenland ice-core chronology (Svensson et al., 2008) are frequently used as a chronological framework. The correlation is then made indirectly via other proxies such as ice-rafted debris (IRD) or vegetation indicators, which may introduce additional uncertainties due to, e.g. glacial dynamics (cf. Landvik et al., in this volume) or variable climate gradients (e.g. Hättestrand, 2008). 4. Arctic stratigraphy 4.1. Marine and terrestrial approaches to dating While similar methods can be used both for marine records, such as deep-sea or shelf cores, and for terrestrial records, such as coastal cliff sections, the overall approach to dating in a marine setting differs from that in a terrestrial setting. This largely depends on the nature of the records, which lends itself better or worse to different dating methods. Terrestrial records of fluvial, lacustrine and raised shallow-marine sedimentation are mostly fragmented, both in time and space, and may vary greatly over short distances due to heterogeneous depositional environments and patchy preservation; lake basins partly being an exception. Lateral correlations are therefore often difficult, and thus rely heavily on the combination of stratigraphy and the interpretation and absolute dating of depositional/erosional events (e.g. Gibbard and West, 2000), see 4.2. In comparison, marine records are largely continuous, although hiatuses may occur, and since they are relatively uniform over large areas (e.g. Backman et al., 2004) correlation of various sediment properties to well-dated key cores is a fundamental tool (see 4.3). Fig. 13. X-ray radiograph showing post-sedimentary bands of Mn likely formed as the result of diagenetic processes in the sediment.

3.7. Oxygen isotope stratigraphy Foraminiferal oxygen isotope stratigraphy (d18O) is a standard chronostratigraphic method used to correlate marine sediments at orbital timescales because d18O variations reflect mainly global ice volume and, to a lesser extent, water temperature and regional hydrology. Correlating Arctic sediment using the standard Marine Isotope Stage (MIS) chronostratigraphy is a key to a better understanding of the relationships between Arctic palaeoceanographic history and processes in the Nordic Seas and the North Atlantic and Pacific Oceans during key climatic events. In the Arctic Ocean, d18O on the planktonic species N. pachyderma and various benthic foraminiferal species do show cyclic variations but the amplitude is larger than in extra-Arctic oceans (Polyak et al., 2004; Spielhagen et al., 2004). This is attributed to the influence of surface hydrological processes that are specific to the Arctic Ocean and vary strongly on both seasonal and glacial-interglacial timescales (sea ice formation, brine rejection, riverine fresh water input) (Polyak et al., 2003; Brennan et al., 2013) and foraminiferal size dependence among other factors on the isotopic signal (Hillaire-Marcel et al., 2004). In addition, calcitic microfossils are typically absent during glacial maxima in most

4.2. Arctic terrestrial stratigraphy Due to the fragmented and heterogeneous nature of terrestrial records, where each site commonly contains deposits from only one or two depositional events, a longer palaeoenvironmental history usually has to be pieced together from evidence from several sites. The framework for most studies of Pleistocene glacial history is a regional composite scheme constructed from a number of sites that are correlated to each other. These schemes are commonly based on event stratigraphy, climatostratigraphy and/or chronostratigraphy and may be presented as ‘glaciation curves’ along geographic transects (Fig. 8) or stratigraphic tables showing successive glacials and interglacials, stadials and interstadials. Some examples are Sachs (1953) and Svendsen et al. (2004) for Arctic Russia and Siberia, Mangerud et al. (1998) for Svalbard (Fig. 8) and Funder et al. (1998) for east Greenland. Glacial event stratigraphy (see 3.3) is not applicable everywhere in the Arctic. Large parts of northern Eurasia and Alaska were not glaciated during much of the Last Glacial cycle (Svendsen et al., 2004; Kaufman et al., 2011), which makes glacial event stratigraphy and terminology unsuitable there (Gibbard and West, 2000). Instead, other types of event sequences can be established. Sites with deposits from several climate or glaciation cycles in superposition are important for establishing regional composite schemes. Such sites provide good relative chronologies of climatic

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and environmental events, and are often subject to extensive dating efforts resulting in improved absolute chronologies. Many of these ‘multicycle sites’ are sediment successions preserved in lakes or other basins, e.g. Lake El’gygytgyn in eastern Siberia (Melles et al., 2012) and Sokli in northern Finland (Helmens et al., 2000), while some are successions exposed in coastal or fluvial cliffs, e.g. Kapp Ekholm and Leinstranda (Fig. 5) on Svalbard (Mangerud and Svendsen, 1992; Alexanderson et al., 2011a) and the Ozernaya River sites on October Revolution Island, Severnaya Zemlya (Möller et al., 2007), Fig. 2. These records have the potential to provide important links between terrestrial, marine and ice-core records. The chronology of the composite schemes is based on absolute ages where available. However, particularly where age control is poor or missing, it is common to tie events to independently dated oxygen isotope stratigraphy (e.g. Martinson et al., 1987) or ice-core based event stratigraphy, e.g. the INTIMATE event stratigraphy for the North Atlantic (Blockley et al., 2012). The adoption of the terminology of such stratigraphies, or e as is common e of the NW European chronostratigraphy (Mangerud et al., 1974; Mangerud, 1991) in other areas, may be misleading. The stratigraphic terminology may be used in ways it was not intended (cf. Björck et al., 1998), and it might give the impression of a higher-resolution record than is actually available. Recent developments in dating technology and glaciodynamic theory also question the validity of some correlations, both on a general and specific level (cf. 3.3 and 3.4). From the Arctic, new absolute ages and other data have led to that some stratigraphic schemes recently have been challenged and/or revised, e.g. Hättestrand (2008) for northern Fennoscandia, and Astakhov and Nazarov (2010) and Astakhov (2013) for northern Russia (see 3.4 above). Also, as indicated in 3.3, nearby sites may experience different glaciodynamic regimes resulting in apparently contrasting signatures from the same ice-sheet advance (cf. Landvik et al., 2005, in this volume). 4.3. Arctic Ocean deep-sea stratigraphy Establishing high-resolution age models for Quaternary Arctic Ocean sediment successions is a challenging task because, as discussed in 3.7, oxygen isotope stratigraphy cannot be applied. Therefore, a combination of other dating methods has to be applied, most of which do not give absolute ages. Instead, characteristic patterns of individual parameters are derived, leaving room for interpretation (see below and 3.5, 3.6). Lithostratigraphy. Hundreds of short sediment cores were collected in the Amerasian Basin of the central Arctic Ocean between 1952 and 1974 from Fletcher’s Ice Island, also known as T-3 (Weber and Roots, 1990). The lithology of these cores could be correlated over long distances leading Clark et al. (1980) to develop a ‘standard’ Arctic Ocean lithostratigraphy. The established age model of these cores relied on the identification of the first downcore palaeomagnetic polarity reversal, assumed to represent the Brunhes/Matuyama boundary (Steuerwald et al., 1968; Clark, 1970; Clark et al., 1980). Based on this, average sedimentation rates calculated for central Arctic Ocean cores are on the order of mm/ka (Backman et al., 2004), a view that lasted for decades. Later studies have also shown that Arctic sediment cores can be correlated over long distances in the deep central basin using characteristic variations in sediment physical, geochemical and palaeomagnetic properties as well as in microfossil content (e.g. Backman et al., 2004; Spielhagen et al., 2004; Sellén et al., 2010; Stein et al., 2010). This implies that it is possible to construct age models based on astronomical tuning of Mn layers, physical properties, magnetic inclination, bulk density, colour reflectance, marine microfossils and other indicators to a global Quaternary

Fig. 14. Correlation of magnetic inclination records from central Arctic sediment cores.

timescale (Jakobsson et al., 2000; Spielhagen et al., 2004; O’Regan et al., 2008b; Cronin et al., 2013; Löwemark et al., 2013). Palaeomagnetic records. Although there is no clear correlation between the uniform palaeomagnetic reversal patterns in the Arctic ocean (Fig. 14) and the global geomagnetic polarity timescale (GMPT), the strong correlation between many Arctic cores confirms generally continuous sedimentation in the Arctic Ocean over the time period within reach by w10 m long cores. In cases where the polarity pattern is well expressed and no thick layers of rapidly deposited sediments (e.g. turbidites, IRD units) are intercalated, magnetic inclination data can serve as a reliable backbone for an age model and for core-to-core correlation. However, due to different measurement and processing techniques, and possibly post-depositional remagnetization (Xuan et al., 2012), exact boundaries between reversals do not always align between cores when other lithostratigraphic proxies are compared (O’Regan et al., 2008a). Therefore correlations based on magnetic reversal patterns need to be supported by other high-resolution proxy measurements. Variations in magnetic, geochemical and physical properties, measured by modern logging techniques with relatively little investments of time, allow the establishment of reference records for certain regions. An excellent example is the stacked magnetic susceptibility record developed by Jessen et al. (2010) for the last 30 ka on the western Svalbard continental margin (Fig. 15), to which other cores from the area can easily be correlated. Records from the circumpolar regions of the Lomonosov Ridge also show an excellent degree of downcore coherence in grain size patterns, physical property variations and some XRF parameters (i.e. Mn). These correlations are possible between widely spaced cores (100’s km) recovered from a large range of water depths (1000e >3500 mbsl) (O’Regan et al., 2008b; O’Regan, 2011; Löwemark et al., 2012). However, significant regional differences arise because of varying mineralogical composition and sediment transport processes. For example, Sellén et al. (2010) compiled reference stratigraphies for the Southern Mendeleev Ridge, Alpha Ridge, Lomonosov Ridge, Morris Jesup Rise, and the Yermak Plateau (Fig. 2), using high-resolution magnetic susceptibility and bulk density records. While there are no clear correlations in the

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Fig. 15. Stacked magnetic susceptibility record for the western Svalbard continental margin (630e1880 m water depth), obtained from 11 sediment cores (modified from Jessen et al., 2010).

physical properties between these regions, the regional compilations do illustrate that the first downcore palaeomagnetic polarity reversal can be used as a stratigraphic marker across the central Arctic (cf. above). The old paradigm of mm/ka-scale sedimentation rates across the central Arctic Ocean was largely based on the identification of this reversal boundary as the Brunhes/Matuyama. However, other studies had suggested one order of magnitude faster (cm/ka) sedimentation rates (e.g. Sejrup et al., 1984; Jakobsson et al., 2000). An important implication of the age model of Jakobsson et al. (2000) for sediment core 96/12-1pc (Fig. 2), was that the first downcore palaeomagnetic polarity reversal must represent a brief magnetic excursion, instead of the Brunhes/Matuyama boundary. The palaeomagnetic inclination change was thus reinterpreted by Jakobsson et al. (2000) as Biwa II, at the time prescribed an age of w275 ka (Langereis et al., 1997), and has more recently been suggested to mark the ‘Pringle Falls event’ in MIS 7.5 at w238 ka (Stein et al., 2010; Channell et al., 2012). Biostratigraphy. Arguments for slower (mm/ka) or faster (cm/ka) rate scenarios are summarised by Backman et al. (2004). However, the debate about how to accomplish reasonably realistic age

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models in short cores from central Arctic Ocean sediments continued until the Arctic Coring Expedition (ACEX) carried out drilling on the Lomonosov Ridge in 2004 (Backman et al., 2006). The faster rate scenario gained strength when specimens of Evittosphaerula sp. 2 (Manum et al., 1989), representing a bioevent which at the time had a late Miocene age estimate of 5.9 Ma (Table T27 in Backman et al., 2006), were encountered at 75 m below sea floor (mbsf). This finding indicated an average sedimentation rate of approximately 1.3 cm/ka since the late Miocene on the crest of the Lomonosov Ridge. Subsequently, 10Be data (Frank et al., 2008; cf. 3.5) and five biohorizons supported the age-depth progression for the ACEX section (Fig. 16). Matthiessen et al. (2009) demonstrate a consistent late Miocene to earliest Pliocene occurrence for the ‘acme’ interval Decahedrella martinheadii from the Norwegian-Greenland Sea, the northern North Atlantic, the Labrador Sea/Baffin Bay, the Fram Strait and from the ACEX material. The average sedimentation rate is 1.4 cm/ka from the deepest 10 Be data point (117.26 mbsf, 8.10 Ma) in Fig. 16 to the sedimente water interface. The minor offsets between the two data sets are caused by either calibration and/or preservation problems. Taking these problems into account, the biostratigraphic data are considered to strongly support the age-depth progression indicated by the 10 Be data. As previously mentioned, sediment cores from many parts of the Arctic Ocean are characterised by sporadic occurrences of planktonic foraminifera and calcareous nannofossils in the uppermost few meters of the sediment stratigraphy (e.g. Herman, 1974; Worsley and Herman, 1980; Sejrup et al., 1984; Macko and Aksu, 1986; Gard and Backman, 1990). Planktonic foraminiferal assemblages show dominance of N. pachyderma (sinistral) after about 1.8 Ma in the Norwegian-Greenland Sea and in the ACEX material (Spiegler and Jansen, 1989; Cronin et al., 2008), cf. Fig. 16, thus providing only limited biostratigraphic resolution during Quaternary times. Calcareous nannofossil biostratigraphy makes it possible to recognise MIS 1 and MIS 5 in the central Arctic Ocean (Fig. 17) and occasionally MIS 3 and MIS 7 (Gard, 1986; Jakobsson et al., 2000; Backman et al., 2009) through the occurrence of the coccolith Emiliania huxleyi, which has a first appearance date at w270 ka in MIS 8 (Thierstein et al., 1977). Biostratigraphy based on calcareous plankton in the Arctic Ocean, however, is severely hampered by preservational problems beyond MIS 5. A reliable time marker is the occurrence of the benthic foraminifer Bulimina aculeata in sediments of late Early Weichselian age; the layers with B. aculeata show ages ranging mostly between 93 and 73 ka in cores with independent high-resolution stratigraphic models (Fig. 18). Other benthic foraminiferal species, such as Oridorsalis tener, Bolivina arctica and Epistimenella exigua, are useful for correlation of deep-sea Arctic Ocean Quaternary sediments in at least some regions, notably the western Arctic Mendeleev and Northwind Ridges and regions of the Lomonosov Ridge (e.g. Herman, 1973; Ishman et al., 1996; Polyak et al., 2004; Cronin et al., 2008, 2013). Future work is required to confirm the synchronous nature of foraminiferal zones characterised by unique stratigraphic occurrences or peaks in abundance of these species.

4.4. Linking terrestrial and marine records As mentioned in the Introduction, correlation of terrestrial and marine records is essential for understanding the integrated Earth system development through the Pleistocene, including identifying causes and effects and recognising feedbacks between different systems. Since marine and terrestrial stratigraphic models are independent, good correlations also provide additional confidence in the validity of age assignments. Given the dissimilar nature of

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Fig. 16. Depth versus age plot for the central Arctic drill record (ACEX) based on 10Be data (þsymbols) (Frank et al., 2008) and five biohorizons (Backman et al., 2006; Matthiessen et al., 2009). In this figure, Evittosphaerula sp. 2 of Manum et al. (1989) is referred to as Decahedrella martinheadii, following Matthiessen et al. (2009). Matthiessen et al. (2009) used the ACEX 10Be data for re-calibrating the D. martinheadii ‘acme’ interval to 5.1 Ma (top) and 7.8 Ma (base) in the central Arctic Ocean; while this figure shows the age estimates (5.9 Ma, 7.1 Ma) used by Backman et al. (2006).

Fig. 17. Late Quaternary age model and compiled chronostratigraphic data from four cores on the central Lomonosov Ridge (ACEX, PS-2185-6, 96/12-1 PC and 96/24-1SEL; see Fig. 2 for locations). All records are migrated onto the ACEX depth scale, and show a clear agreement between patterns in coarse fraction content for the last two glacial cycles. A mixture of absolute and relative dating techniques have been applied to these cores, and provide a relatively robust chronostratigraphic framework for late Quaternary sediments from this sector of the Arctic Ocean. (Figure adapted from O’Regan (2011) where references to individual datasets can be found).

terrestrial and marine records, linking marine and terrestrial records has, however, often proved difficult. For correlation of events, chronostratigraphy is most often used, but has its limits in the different materials and methods used, and in resolution. Terrestrial glacial event stratigraphies have been correlated with marine cores through IRD layers and sedimentation rates (e.g. Mangerud et al., 1998), but as mentioned above (3.3) and as discussed by Landvik et al. (in this volume), ice-sheet dynamics may make such correlations more complicated than previously assumed. In the absence of correlative dated sequences, other proxies and features have been used. For example, Murton et al. (2010) used a regional erosional surface recognised in onshore and offshore records of the Canadian Beaufort Sea as evidence in support of the hypothesis that a large deglacial meltwater outburst through the Mackenzie River triggered the Younger Dryas. Palaeomagnetic properties and magnetostratigraphy can also be used to tie marine and terrestrial events, both on shorter and longer timescales (e.g. Clark et al., 1984; Barendregt and Duk-Rodkin, 2011; Ólafsdóttir et al., 2013). One potential way to link marine and terrestrial records is by provenance analysis of marine deposits, linking specific sediment intervals to specific terrestrial source areas. A number of different methods have been used to try and tie for example a glacial source area to a corresponding marine unit. Darby and co-workers (Darby and Bischof, 1996; Darby, 2003, 2008) compiled an extensive data base for the specific chemical composition of Fe-oxide grains from various circumarctic areas, allowing for correlations between marine sediment records that received synchronous input from specific terrestrial source areas. Specific inorganic-geochemical and mineralogical bulk sediment proxies, such as clay minerals, detrital carbonates, iron oxides and coal particles, can be useful to trace material input from geochemically and mineralogically distinct source areas, e.g. the basaltic Putoran Massive in Western Siberia (Vogt et al., 2001; Vogt and Knies, 2008; Martinez et al., 2009; März et al., 2010). Also radiogenic isotopes in the terrigenous sediment fraction deposited in the Arctic Ocean (Nd, Pb, Sr) are increasingly used to trace IRD distribution or melt water discharge events (Winter et al., 1997; Haley et al., 2008; Jang et al., 2013; HillaireMarcel et al., 2013). However, these provenance-based correlations are often regionally restricted, as parts of the Arctic Ocean are influenced by different current patterns (i.e. the western Arctic

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accessible, including shallower areas, which possibly can improve future correlation between land and sea (see 5.3). In addition, our scientific tools have developed, and increased understanding raises new research questions which make it valuable to go back to known sites. As an example, studies on Svalbard within the APEXproject SciencePub (http://www.ngu.no/sciencepub) focussed on sites where earlier studies provided a well-documented lithostratigraphic framework. By re-examining these sites, building on advances in sedimentological and glaciodynamical understanding as well as using new dating technologies, new information on the dynamics and timing of past glaciations has been gained (Alexanderson et al., 2011a,b). Based on experience (e.g. the setting of other good sites) and geographical information (topography/bathymetry, extent of past ice sheets etc.) it is possible to predict the location of potential sites. New and upcoming high-resolution bathymetry data and digital elevation models based on e.g. AUV SAS (autonomous underwater vehicle with synthetic aperture sonar; e.g. Hansen et al., 2011) and LiDAR-technology (Light Detection and Ranging; Wehr and Lohr, 1999) provide more high-resolution landscape information. The improved resolution will allow the identification of smaller landforms (and potential sites) as well as more detailed morphostratigraphic (and thereby chronological) relationships (Dowling et al., 2013).

5.2. Improving chronology

Fig. 18. Age determinations for layers holding benthic foraminifers Bulimina aculeata (gray) in sediment cores from the Morris Jesup Rise (MJR; Spielhagen et al., 2004, unpublished data), Lomonosov Ridge (LR; Spielhagen et al., 2004, unpublished data; Jakobsson et al., 2001), Alpha Ridge (AR; Spielhagen et al., 2004, unpublished data), and Mendeleyev Ridge (MR; Polyak et al., 2004; Sellén et al., 2010). MIS ¼ Marine isotope (sub)stages. Question mark indicates age uncertainty for the onset of B. aculeata occurrence in PS51/038.

Beaufort Gyre versus the eastern Arctic Transpolar Drift) that do not distribute terrigenous material equally across the whole Arctic basin. A challenge for the future is therefore to improve Arctic-wide marine as well as onshore-offshore correlations. 5. Present challenges and future possibilities 5.1. Accessing the geological records Although many Arctic areas have widespread sediment successions potentially recording the Pleistocene climatic and environmental change, accessibility of the sedimentary records depend on sections made available by coastal or river erosion, or logistically demanding drilling efforts. Most readily accessible sites have been studied over the last three decades. However, the reduced sea-ice cover in the Arctic Ocean has made larger areas more easily

Due to low precision or scatter of absolute ages, the resolution of age determinations may in some cases, particularly for events older than the Last Glacial Maximum, be lower than the likely duration of the event itself. This hampers accurate correlation to any highresolution records, such as oxygen isotope stratigraphies, and thereby limits the possibility to distinguish between causes and effects, and identify interactions between land, sea and climate. In the Arctic Ocean, the recognition of higher than previously assumed average Quaternary sedimentation rates (cm/ka) has been important for the establishment of reliable age models for deep-sea sediment successions, but while the sedimentation pattern over the last 200 ka seems relatively well understood, age assignments to older deposits are still debatable (Jakobsson et al., 2000; O’Regan et al., 2008b). Better absolute age control with improved precision and accuracy is required to improve the situation. Many dating methods have problems with the availability and quality of material to be dated, e.g. the scarcity of organic material for radiocarbon dating. We cannot do anything about the geological conditions, but developments in dating technology have made a larger range of material dateable; e.g. smaller amounts of organic material and lower detection limits for 14C-dating (Walker, 2005) and single-grain analysis, pulsed measurements and new analytical protocols in luminescence dating (Duller, 2008; Ankjærgaard et al., 2010; Thiel et al., 2011). Better accuracy and precision is also gained by improved calibration of radiocarbon and cosmogenic exposure ages (Reimer et al., 2009; Young et al., 2013a). To develop and test new dating techniques, long records and integrated stratigraphic studies form a valuable platform by providing independent lithostratigraphical and chronological control. From a marine perspective, many of the problems associated with dating middle to late Pleistocene marine sediments, routinely recovered in the upper 10e15 m of sediments from icebreaker coring expeditions, can be overcome with the acquisition of longer records. As results from ACEX have highlighted for the central Arctic, radiogenic, biostratigraphic and cyclostratigraphic dating techniques are all possible when long (10’se100’s m), continuous marine sedimentary sequences are recovered by deep-sea drilling.

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Nonetheless, however low analytical uncertainties become, the geological uncertainty must still be considered. In some cases it may be large due to ambiguous association of the dated material with the event in question or to uncertainties in the temporal variation in factors influencing dating (e.g. dose rate for luminescence and ESR; cf. Skinner, 2011). This emphasises the need to better understand the fundamental physics, chemistry or biology of the material used for dating, as e.g. discussed for Mn stratigraphy above (3.6), and to use multi-proxy approaches to appreciate how the sample’s environment has changed with time, e.g. US-ESR dating (2.4). The value of many samples (ages) per sedimentary unit, landform, event or site to identify outliers and recognise patterns has been highlighted by many, e.g. Briner et al. (2005). With many samples, Bayesian age modelling, which takes stratigraphic relationships into account, can lead to more precise ages, even when applied to a combination of ages from different dating methods (Millard, 2006). 5.3. Integrating the records e concluding remarks Striving to understand whole systems and the complex interactions between the lithosphere, hydrosphere, cryosphere, atmosphere and biosphere involve the need to integrate many different records to get as a complete picture as possible. Records of different resolution and from different environmental settings need to be compared; e.g. a largely continuous IRD-record in a marine core on the shelf with a fragmented till stratigraphy from a coastal cliff section. To do this we need a good chronology. To improve our chronologies we need to improve absolute dating methods (better precision and accuracy, wider range of ages and materials). This needs to be parallelled with a better understanding of factors influencing the final age for different methods or proxies, be it physical, chemical, geological or biological processes affecting the dated material. Further development of proxies that can be found in many types of records will be very valuable since they can provide direct correlation. For all aspects, it is important to always acknowledge the geological uncertainty, which may be much larger than the analytical uncertainty. Stronger links between marine and terrestrial stratigraphies can be established by targeting future deep-sea drilling efforts closer to continental margins, where higher sedimentation rates will allow improved temporal resolution in the developed chronologies and associated palaeoenvironmental and glaciological interpretations. Lake records would also be important as potential links. Stronger links between the marine and terrestrial research communities have been gained through e.g. programmes such as APEX, and its predecessors PONAM (Polar North Atlantic Margins) and QUEEN (Quaternary Environments of the Eurasian North). The collaboration between marine and terrestrial geologists within such programmes has significantly lowered the science-cultural barrier and given many scientists a network outside their immediate research field, important not least for young researchers. The next research community we would like to involve and have better communication with is the modellers. Modelling can be used e.g. to test fieldbased data, and to predict or identify information that is needed to improve our understanding of environmental change, and could significantly move our work forward. Acknowledgements Alexanderson has written the bulk of the text and edited contributions from the co-authors, all of whom have provided input to this paper by writing separate sections or parts of sections and commenting on the manuscript in general. We are indebted to

many colleagues for discussions and comments on various parts of this review, including Lilja Rún Bjarnadóttir, Mona Henriksen, Anne Hormes, Lena Håkansson, Andrew Murray, Göran Skog and Stefan Wastegård. Two anonymous reviewers gave constructive criticism that improved the manuscript. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.quascirev.2013.09.023. References Abbott, P.M., Davies, S.M., 2012. Volcanism and the Greenland ice-cores: the tephra record. Earth-Sci. Rev. 115 (3), 173e191. Abbühl, L.M., Akçar, N., Strasky, S., Graf, A.A., Ivy-Ochs, S., Schlüchter, C., 2009. A zero-exposure time test on an erratic boulder: evaluating the problem of preexposure in surface exposure dating. Eiszeitalt. Ggw. (Quat. Sci. J.) 58 (1), 1e11. Aitken, M.J., 1985. Thermoluminescence Dating. Academic, London. Aitken, M.J., 1998. An Introduction to Optical Dating. Oxford University Press, Oxford. Aldahan, A.A., Ning, S., Possnert, G., Backman, J., Boström, K., 1997. 10Be records from sediments of the Arctic Ocean covering the past 350 ka. Mar. Geol. 144 (1e3), 147e162. Alexanderson, H., 2007. Residual OSL signals from modern Greenlandic river sediments. Geochronometria 26, 1e9. Alexanderson, H., Håkansson L. Coastal glaciers advanced onto Jameson Land, east Greenland during the late glacial-early Holocene Milne Land stade. Polar Res. (in press). Alexanderson, H., Murray, A.S., 2012a. Luminescence signals from modern sediments in a glaciated bay, NW Svalbard. Quat. Geochronol. 10, 250e256. Alexanderson, H., Murray, A.S., 2012b. Problems and potential of OSL dating Weichselian and Holocene sediments in Sweden. Quat. Sci. Rev. 44, 37e50. Alexanderson, H., Johnsen, T., Murray, A.S., 2010. Re-dating the Pilgrimstad interstadial with OSL: a warmer climate and a smaller ice sheet during the Swedish Middle Weichselian (MIS 3)? Boreas 39 (2), 367e376. Alexanderson, H., Landvik, J.Y., Ryen, H.T., 2011a. Chronology and styles of glaciation in an inter-fjord setting, northwestern Svalbard. Boreas 40 (1), 175e197. Alexanderson, H., Landvik, J.Y., Molodkov, A., Murray, A.S., 2011b. A multi-method approach to dating middle and late Quaternary high relative sea-level events on NW Svalbard e a case study. Quat. Geochronol. 6, 326e340. Alexanderson, H., Ingólfsson, Ó., Murray, A.S., Dudek, J., 2013. An interglacial polar bear and an early Weichselian glaciation at Poolepynten, western Svalbard. Boreas 42, 532e543. Andersson, T., Forman, S.L., Ingólfsson, Ó., Manley, W.F., 1999. Late Quaternary environmental history of central Prins Karls Forland, western Svalbard. Boreas 28, 292e307. Andersson, T., Forman, S.L., Ingólfsson, Ó., Manley, W.F., 2000. Stratigraphic and morphologic constraints on the Weichselian glacial history of northern Prins Karls Forland, western Svalbard. Geogr. Ann. 82A (4), 455e470. Andreev, A.A., Grosse, G., Schirrmeister, L., Kuzmina, S.A., Novenko, E.Y., Bobrov, A.A., Tarasov, P.E., Ilyashuk, B.P., Kuznetsova, T.V., Krbetschek, M., Meyer, H., Kunitsky, V.V., 2004. Late Saalian and Eemian palaeoenvironmental history of the Bol’shoy Lyakhovsky Island (Laptev Sea region, Arctic Siberia). Boreas 33 (4), 319e348. Ankjærgaard, C., Jain, M., Thomsen, K.J., Murray, A.S., 2010. Optimising the separation of quartz and feldspar optically stimulated luminescence using pulsed excitation. Radiat. Meas. 45 (7), 778e785. Arkhipov, S.A., 1989. A chronostratigraphic scale of the glacial Pleistocene of the West Siberian North. In: Skabichevskaya, N.A. (Ed.), Pleistotsen Sibiri. Stratigrafia i mezhregionalnye korrelatsii. Nauka, Novosibirsk, pp. 19e30 (in Russian). Arnold, L.J., Roberts, R.G., 2011. Paper I e optically stimulated luminescence (OSL) dating of perennially frozen deposits in north-central Siberia: OSL characteristics of quartz grains and methodological considerations regarding their suitability for dating. Boreas 40 (3), 389e416. Arslanov, K.A., Tertychny, N.I., Kuznetsov, V.Y., Chernov, S.B., Lokshin, N.V., Gerasimova, S.A., Maksimov, F.E., Dodonov, A.E., 2002. 230Th/U and 14C dating of mollusc shells from the coasts of the Caspian, Barents, White and Black seas. Geochronometria 21, 49e56. Astakhov, V.I., 2013. Pleistocene glaciations of northern Russia e a modern view. Boreas 42 (1), 1e24. Astakhov, V.I., Mangerud, J., 2005. The age of the Karginsky interglacial strata on the lower Yenisei. Dokl. Earth Sci. 403 (5), 673e676. Astakhov, V., Nazarov, D., 2010. Correlation of upper Pleistocene sediments in northern west Siberia. Quat. Sci. Rev. 29 (25e26), 3615e3629. Backman, J., Jakobsson, M., Løvlie, R., Polyak, L., Febo, L.A., 2004. Is the central Arctic Ocean a sediment starved basin? Quat. Sci. Rev. 23 (11e13), 1435e1454. Backman, J., Moran, K., McInroy, D.B., Mayer, L.A., the Expedition 302 Scientists, 2006. In: Proceedings of the Integrated Ocean Drilling Program, vol. 302. Integrated Ocean Drilling Program Management International, Inc.

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