The Late Pleistocene palaeoenvironmental evolution in Northern Eurasia through the prism of the mollusc shell-based ESR dating evidence

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The Late Pleistocene palaeoenvironmental evolution in Northern Eurasia through the prism of the mollusc shell-based ESR dating evidence Anatoly Molodkov Research Laboratory for Quaternary Geochronology, Department of Geology, Tallinn University of Technology, Tallinn, 19086, Estonia

A R T I C LE I N FO

A B S T R A C T

Keywords: ESR and IR-OSL dating Mollusc shells Chronology Late Pleistocene Northern Eurasia Eurasian Arctic

Long-term investigations devoted mostly to the Eurasian Arctic sedimentary deposits have provided dated climatic sequence within the Late Pleistocene period. Most of the dates (ca. 315) were obtained by electron spin resonance (ESR) method on marine mollusc remains taken directly from transgressive marine sedimentary deposits. Here, it is shown that the climatic signature revealed in the studied sedimentary sequences can be compared by proxy- and chronologic correlation with the climatic signals recorded in different parts of the continent from deposits of marine, freshwater, and terrestrial origin. The resulting climate-chronostratigraphic record allow the last interglacial to be placed approximately between 145–140 ka and 70 ka with concentration of overwhelming majority of the warm climate-related dates (ca. 82%) in the second half of Marine Isotope Stage (MIS) 5 in the time range between 110 and 70 ka. This evidence coincides with integrative multidisciplinary results derived from the south-eastern coast of the Gulf of Finland. They show unambiguously that the second half of the MIS 5 sequence, at least between ca. 94 ka and 71 ka, reflects the fully interglacial character of the studied deposits. The climate-chronostratigraphic record within MIS 3 displays a sequence of three successive palaeoenvironmental events (59.0–52.0 ka, 47.5–40.0 ka and 32.4–24.8 ka), which are interpreted as the consequence of large-scale climate amelioration during which marine sedimentation occurred on palaeo-shelves what is now dry land. The consistency of the parallel comparative dating results obtained in this study by different methods, both the numerical and relative, exemplify the potential of palaeodosimetric dating methods mainly used in this study — mollusc shell-based electron spin resonance (ESR) and feldspar-based infrared optically stimulated luminescence (IR-OSL) — to chronologically organize the sequence of the Late Pleistocene palaeoenvironmental events, and to improve understanding of the palaeoenvironmental evolution during this period.

1. Introduction Accurate age determination and correlation of sediments is one of the most actual trends in the Quaternary geology. At the same time, the chronology of deposits and corresponding palaeoenvironmental events, reliable correlation of various strata within the both the local, regional and the wider territories remain a serious problem. The existing dating methods do not always allow a sufficiently accurate and reliable temporal comparison of the different stratigraphic schemes, especially beyond the practical limit of the 14C dating (ca. 50 ka; Muscheler et al., 2014). The development and implementation of dating methods that are able to work in a much wider time range would be highly desirable for solving still existing problems, even for the most well-studied period of the Quaternary –– the Late Pleistocene. There remain many questions regarding chronology, landscape and climatic features of the warm and cold phases of different magnitudes and duration reconstructed for this

period. A careful consideration of this problem leads to the conclusion that there is a deep conviction that at least climatic and paleogeographic changes are appropriately reflected in temporal variation of oxygen isotope ratios (δ18O) derived from sea-floor sediment cores (Shackleton, 1969). According to the key concept of climate change in the Late Pleistocene formed under the influence of deep-sea oxygen isotope curves (Mangerud et al., 1979), which are believed to directly reflect global ice-volume fluctuations, the last continental interglacial period was regarded as the deep-sea equivalent of the Marine Isotope Substage (MIS) 5e, which lasted from about 130 ka to 117 ka (Mangerud, 1989). This subdivision is regarded as the most favourable period of the entire MIS 5 with the climate warmer than it is today (Rovere et al., 2016). Accordingly, it is believed that the last glacial (Weichselian) stage started at around 117 ka (MIS 5d). It included three glaciations: the early Weichselian (100–80 ka), the Middle Weichselian (60–50 ka) and the Late Weichselian (25–15 ka) (Mangerud, 1989)

E-mail address: [email protected]. https://doi.org/10.1016/j.quaint.2019.05.031 Received 30 May 2018; Received in revised form 23 May 2019; Accepted 26 May 2019 1040-6182/ © 2019 Published by Elsevier Ltd.

Please cite this article as: Anatoly Molodkov, Quaternary International, https://doi.org/10.1016/j.quaint.2019.05.031

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large-scale climatic changes and, therefore, can be regarded as a highly sensitive recorder of these changes in the past. There can be found different indications of climate changes in the Quaternary, which are potentially derivable from diverse proxies in terrestrial, limnic, glaciological as well as marine sedimentary records of this region. Of particular importance can be multi-method approach to the timing of at least the most prominent periods in the history of the Late Pleistocene palaeoenvironmental evolution. For these purposes, it is mandatory to apply reliable numerical dating methods and set up an accurate and precise chronological framework for climatically indicative sediments. Reliable ages are fundamental to place changes in climates, landscapes, flora and fauna in their correct temporal sequence, to understand the tempo and mode of geological and biological processes. A recent publication (Molodkov, 2012) has demonstrated the accuracy potential of the currently available dating techniques by comparison and crosschecking between different dating methods, both absolute and relative. A remarkable coincidence of the electron spin resonance (ESR), infrared optically stimulated luminescence (IR-OSL), optically-stimulated afterglow (OSA, Jaek et al., 2003), U–Th and 14C dating results obtained on mollusc shells, K-feldspar and quartz grains and taken from the same enclosing sediments confirms a good potential to improve our understanding of the Late Pleistocene palaeoenvironmental evolution in Northern Eurasia, with a special focus on the climatically highly sensitive Eurasian Arctic palaeo-shelf area. Eurasian Arctic continental margin palaeoenvironmental records consisting of climate indicative deposits and precise chronology could provide a valuable data to improve the reconstruction of past sea level and climate oscillations. Among the numerous datable material, marine mollusc shell skeletal remains are the most promising objects for the study of palaeoenvironmental evolution, especially in the Eurasian Arctic, with its abundance of different age marine transgressive deposits. The use of those genetically homogeneous and wide spread sea-level and palaeoenvironmental proxy indicators for age determination of enclosing sediments and relevant marine and terrestrial palaeoenvironmental events as well as for temporal correlations between terrestrial and marine records seems highly promising because it permits to construct chronostratigraphic and chronological framework of the Quaternary at least at a transcontinental scale. On the other hand, comparison of the reliable results obtained on shell-bearing deposits with major features of marine and terrestrial palaeoclimatic records can furnish the clue in solving of some topical problems of geochronology, which may be highly complicated, but be of major importance. As it was mentioned above, molluscs are among the most diverse and abundant animal groups, inhabiting many aquatic and terrestrial environments. Due to their diversity, abundance, good preservation in the enclosing deposits, and presence along wide latitudinal and altitudinal gradients, mollusc shells can itself serve as good environmental indicators and proxies to provide important information on past climate events and sea level changes even in the absence of instrumental data to measure these changes directly. In the present study, the Late Pleistocene palaeoclimatic record is mostly based on the mollusc shell-based ESR chronostratigraphy of climate-controlled marine deposits and related multidisciplinary factual data. Application of the ESR method for mollusc shell dating purposes has the following main advantages:

during which the sea level fell to around 60 m below present (Lambeck and Chappell, 2001; Lambeck et al., 2002a, b). In this time, the large-scale ice sheet that formed over Scandinavia (Hirvas, 1991), spread eastwards along the northern present-day landsea margin of the Eurasian Arctic. The huge territories in Northern Eurasia drained because shorelines of the Arctic seas moved off the coast northward by up to hundred kms. But now it became known that even in the central area of the Fennoscandian glaciations (N Finland) MIS 5 is characterized by three long forested intervals, broadly corresponding likely to MIS 5e, 5c and 5a (Helmens, 2014). Together with other data from different depositional environments it argues in favour of entire MIS 5 as the last interglacial. These data also show that the division between the MIS 5e and the so called early Weichselian glaciation period (MIS 5d–a), within which, according to many authors, the Scandinavian and Barents-Kara ice sheets attained their maximum size and the ice front of the latter reached the eastern part of the Taimyr Peninsula (Svendsen et al., 2004), may does not correspond to the most probable palaeoenvironmental situation during MIS 5. Marine deposits of Northern Eurasia are palaeoenvironmentally the most important for Quaternary studies, as they are located in a climatesensitive area, are characterized by a wide distribution, continuous sedimentation, and by usually well-preserved subfossil material. Among the latter, mollusc shells are often found in uplifted coastal marine deposits. Dating of these subfossils from transgressive deposits can provide an independent chronology of sea level/climate changes. In other words, the raised mollusc-rich marine deposits can be considered to be a proxy indicator of the climate change that should be reflected both in marine and terrestrial records. In this respect, the numerical dating of mollusc shells could be of considerable value. In this context, it seems expedient to consider all the results obtained in the Research Laboratory for Quaternary Geochronology (RLQG) on the relatively well-studied time interval of the Late Pleistocene, mainly focusing on those obtained when accompanied by other independent dating methods both numerical, geological and biological ones within the Marine Isotope Stages 5 and 3. Especially since that despite the long-term research, thoughts about the palaeoenvironmental history of this period, in particular MIS 5, remain rather contradictory or are diametrically opposed (e.g., Helmens, 2014: entire MIS 5 as the last interglacial; Otvos, 2014, and many others: short MIS 5e interglacial and a long glacial MIS 5d – MIS 2 interval). The same uncertainty is observed concerning MIS 3. Estimates based on deep-sea chronologies and U–Th dating of coral reefs indicate that during this time interval sea level oscillated between ∼90 and ∼50 m below present sea level (Shackleton, 1969, 1987; Lambeck et al., 2002a, b; Yokoyama and Esat, 2011). On the other side, several studies indicate the occurrence of MIS 3 ice-free conditions during early part of this period even in Fennoscandia (Helmens and Engels, 2010). Exactly in the area, which was traditionally regarded as a centre of Fennoscandian glaciation throughout at least the entire Pleistocene, including period between 70 ka and 15–10 ka ago (Hirvas, 1991). However, reconstructions of climate in northeast Finland have yielded unambiguous results, indicating present-day summer temperatures in northeast Finland (Alexanderson et al., 2008; Helmens and Engels, 2010; Wohlfarth and Näslund, 2010), and indicate that lowland Scandinavia and even most of the circum-Baltic region were almost free of glacier ice during MIS 3 (Houmark-Nielsen, 2010). Sea levels were also found to be considerably higher in the seas situated in the extraglacial areas of Northern Eurasia (Belluomini et al., 2002; Iannace et al., 2003; Sasaki et al., 2004; Zander et al., 2006; Gracia et al., 2008; Zhao et al., 2008; Frigola et al., 2012; Rodriguez-Lazaro et al., 2017) than those indicated by traditional sea level curve. One of the most promising ways to solve the existing problems is to shift the palaeoenvironmental and chronological investigations to Eurasian Arctic palaeo-shelf area, which was always very responsive to

• the dating possibilities of the shells samples of different species, • • 2

genesis (marine, freshwater, terrestrial) and composition (calcite, aragonite); unlimited number of repeated measurements of one and the same sample; a relatively low amounts of shell material needed for analysis; sometimes 300 mg are enough for a comprehensive analysis of Pleistocene shells, although for Holocene shells, it is desirable to have 1–2 g;

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Fig. 1. Typical ESR spectra of the aragonitic marine (solid line in A) and calcitic freshwater (B) shell samples. The analytical signal at g = 2.0012 (dotted in A) is extracted from the conventional shell spectrum (solid) by over-modulation (OM) detection method (Molodkov, 1988, 1993). The OM-signal intensity is normalised in this part of the figure with respect to the peak intensities of the conventional ESR spectra of the shell recorded at much smaller modulation amplitude of 0.25 mT. Radiation-induced signal in ESR-spectrum of calcite freshwater mollusc shells is superimposed by strong Mn2+ signals, which mask weak radiation-induced lines (B). Analytical line at g = 2.0012 is extracted by phase sensitive detection (PSD) technique (C).

• wide

(both structural forms of calcite and aragonite) mentioned in the present paper, were made at the RLQG, using an advanced version of the method (Molodkov, 1986, 1988; 1989, 1993; 2001; Molodkov et al., 1998). After decades of intensive research in the field of chronology of the Quaternary period (see, e.g., Molodkov and Bolikhovskaya, 2010, and references therein), the author of this article tends to believe that one of the most promising materials for ESR dating of many geological formations are exactly the subfossil mollusc shells. These mollusc remains can provide Quaternary scientists with valuable palaeoenvironmental data from both marine, terrestrial, and lacustrine sources. The ESR dating method of the mollusc shells consists in a direct measurement of the number of radiation-induced paramagnetic centres. The lattice of the biogenic shell carbonate has no these centres at the time of its formation, but the ionizing radiation from the shell itself and the surrounding environment (enclosing matrix and cosmic ray) causes the gradual generation and subsequent accumulation of paramagnetic centres during its burial in the sediments. The number of these longlived (about 108–109 years for temperature conditions of the Arctic regions; Molodkov, 1988, 1989, 2001) centres is directly related to the total radiation dose that the shell has received, and, therefore, to the age of the shell and of the enclosing sediments. An overview of the ESR dating procedure used in this study is presented in Molodkov et al. (1998). A brief outline of the procedure is given below. The shells for the ESR measurement were washed in water; the remnant clay minerals were removed by ultrasonic bath; the shells were then measured for thickness and etched by 10% acetic acid to remove α-irradiated surface, washed repeatedly in water and dried at room temperature. The dried shell samples were gently ground by pestle in an agate mortar, sieved in order to separate the fraction of 75–400 μm, washed five-six times with water to remove adhered < 75 μm particles, then allowed to dry at room temperature again. After preparation, each sample was divided into 11 aliquots. Of those 10 were irradiated by calibrated doses ranging from 100 to 1000 Gy with radiation steps of 100 Gy, then annealed for 3 h at 80 °C to

applicable time range, covering a greater part of the Quaternary –– from 200 to 300 years (Bitinas et al., 2018) to 1–2 million years (unpublished data, obtained by the author from Caspian Sea sediments).

This paper presents an overview of the use of ESR dating of mollusc shells in palaeoenvironmental studies as proxies in the Late Pleistocene palaeoenvironmental and palaeoclimatic reconstructions, mostly in the northern part of Eurasia. 2. Materials and methods In this paper, a study of environmental changes in the northern part of Eurasia was carried out mainly using mollusc shell-based ESR dating method accompanied by other climate proxies, particularly by palaeontological. In such circumstances, being numerically dated, the mollusc shells can be served as representative natural archives. In the interval of the Late Pleistocene more than 315 age determinations were carried out mainly for marine transgressive deposits of Eurasian Arctic palaeo-shelf. However, only those age determinations will be considered in the present paper in detail that are accompanied with the data of parallel dating methods, absolute or relative. 2.1. Mollusc shell-based ESR dating method This method has been applied in the present study to date shells of different mollusc species and origin taken directly from the sedimentary deposits of interest. Since Ikeya and Ohmura (1981) first recognized the mollusc shell material as a possible dating object by electron spin resonance, the ESR method has been gradually improved and has become the major tool for mollusc-based chronostratigraphy for shell-bearing deposits of various genesis beyond and within 14C dating range (see, e.g., Ikeya, 1985, 1993; Grün, 1989). ESR-datings of all marine, freshwater and terrestrial mollusc shells 3

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the use of the dose-rate conversion factors of Guérin et al. (2011) showed that the ages of the samples remained unchanged to one decimal place. The external alpha contribution is made negligible by etching away the alpha-irradiated surface (ca. 20 μm) of the shell. For detecting and identifying naturally occurring radioactive elements in the surrounding matrix, a laboratory multichannel gamma-ray spectrometer with a 150 mm diameter × 100 mm thick low-background sodium iodide crystal was used. Representative sediment samples of about 1.0–1.5 kg in weight were used for assessment of the gamma and beta contribution to the external dose rate. The contribution of cosmic rays to the total dose rate was calculated according to Prescott and Hutton (1994). The intensity of cosmic rays decreases with depth. In order to take into account the increase in the thickness of the deposits during burial, we used half of the present depth for calculation of cosmic ray dose as an approximation of the mean burial depth over the dated period. The water content used for dose rate calculation was estimated from the in situ water content. It was determined by weight loss of a sample upon drying in a drying oven at 110 °C until a constant weight is reached. Internal dose rate was normally calculated basing on the ICP-MS determination of U-concentration in the shells assuming early (i.e. soon after burial) uptake of uranium into the shell. The percentage of the beta dose was estimated taking into account the average thickness of the shell. An α-ray efficiency (k–value) of 0.15 was used (Kai and Ikeya, 1989). The ESR ages of the samples were calculated using software developed by author. At present, the ESR dating method applied in this work usually provides overall analytical precision of up to about 10% (at one sigma confidence level), when taking into account the standard errors assumed for every parameter used in the age calculation.

allow fading of any possible short-lived ESR signals induced by laboratory irradiation, and analysed with an ESR spectrometer (X-band) at room temperature. It was determined that overwhelming majority of the shells studied were composed of aragonite, and displays typical ESR spectra (Fig. 1A, grey line). Adequate quantification of the paramagnetic radiation-induced centre concentration was obtained from the peak-to-peak amplitude of the g = 2.0012 analytical signal (Fig. 1A, dotted line). This signal is derived by over-modulation of the ESR spectra of the shell sample by use of large (up to 1 mT) modulation amplitude instead of the magnetic field settings traditionally used on recording of the first derivative of the ESR absorption signal to limit the distortion of the lineshapes. The over-modulation (OM) detection technique (Molodkov, 1988, 1993) allows to extract and enhance the relatively wide analytical line at g = 2.0012, and to suppress the narrower, interfering radiation-induced signals at g1–g4 and the broader signal at g5 (Fig. 1A), eliminating, or at least maximally minimizing, superposition effects from these lines. The method applied assures dosimetric read-out relevant to the intensity of the true absorption signal (see, e.g., Fig. 1 in Molodkov, 1988), which is a more precise and adequate parameter compared to the overlapping peak intensities of the high-resolution ESR derivative spectra traditionally employed. In addition, this method considerably enhance sensitivity that makes it possible to extend the range of applicability of the method due to the possibility of dating geologically much younger (the Late Holocene) samples. The line at g = 2.0012 was recorded with a sweep width of 100 mT, a scan speed of 3.7 mT/min, and time constant of 0.01 s. The microwave power used for these measurements was 2 mW, with 100 kHz magnetic field modulation at 1 mT. Accurate reconstruction of palaeodose, P, was performed by extrapolating the regression curves to zero ESR intensities. The multiplealiquot additive-dose (MAAD) protocol was applied to construct dosedependent curves. Additive-dose growth curves were constructed using single saturation exponential fitting of the 11 experimental dose points. Curve fitting and statistical analyses were conducted using Origin 8 (Origin Lab) software. The data points used for regression are the means of up to ten measurements of the g = 2.0012 ESR signal (Molodkov, 1988, 1993) for each aliquot after successively shaking test tube with an aliquot before each measurement. Besides marine molluscs, terrestrial and freshwater ones are also good indicators of environmental condition changes. The relative abundances of either group of species provide important information about palaeoenvironmental conditions during their life. However, shells of various species of this group of molluscs are frequently composed of calcite. Their ESR spectra are strongly affected by the superposition of a very intense Mn2+ signals which mask relatively weak radiation-induced lines (Fig. 1B). In this case concentration measurements of the 2.0012 centres were performed using the phase sensitive detection (PSD) technique (Molodkov, 1988, 1993), that allows analytical signal to be extracted from disturbing non-radiation induced components of the ESR spectra. This technique consists of ОМ, HMP (high-microwave-power) methods and precise ( ± 0.5 dgr) detector phase shift, providing the greatest suppression of manganese signals in the ESR spectra of calcitic shells and separation of the analytical line at g = 2.0012 (Molodkov, 1988). ESR spectra of these shell samples were normally recorded with a sweep width of 200 mT, a scan speed of 1.9 mT/min in the region of g = 2.00, and a time constant of 0.01 s. The microwave power used for dosimetric reading was up to 100 mW with 100 kHz magnetic field modulation at 1 mT. The external beta and gamma contributions to the total dose rate, D, were estimated from the contents of the natural radioactive elements (238U+235U, 232Th and 40K) in the sediments assuming secular equilibrium corrected for the measured in situ water content using the dosecorrection factors of Adamiec and Aitken (1998) and taking into account for beta-dose attenuation in thin layers (Grün, 1986). A check on

2.2. IR-OSL dating method This method allows determining the numerical age of the enclosing sediments using potassium feldspar, one of the most abundant minerals in the Earth's crust. The “luminescence clock” used by IR-OSL dating is the trapped charges (electrons in the present case) in the crystal structure of the mineral, the grains of which behave in the enclosing deposits as palaeodosimeters. The basic premise is that the latent luminescence in this mineral (or, in other words, population of trapped charges) was initially reduced almost to zero by daylight exposure during predepositional transport of the mineral grains (luminescence clock zeroing). The IR-OSL age of deposits is the time which has elapsed since the rock-forming mineral grains were last exposed to light. The luminescence age can be calculated knowing the total absorbed energy and the energy absorption rate. Further discussion of these and related principles is provided, e.g., by Huntley et al. (1985) and Aitken (1985, 1998). The IR-OSL measurements of the mineral grains extracted from the sample to be dated were made in the laboratory at room temperature with a specialized measurement system, the main element of which is the IR-OSL reader. The sediments were prepared for the luminescence analysis according to standard laboratory procedures (Aitken, 1985). Briefly, K-feldspar grains in the size range 100–150 μm were extracted from the sediment under subdued filtered light in the laboratory following a procedure that included wet sieving, heavy liquid flotation (collecting 2.54–2.58 g cm−3 fraction) and treatment by 20–40% HCl acid to remove carbonates. The alpha-affected surface layer of the Kfeldspar grains was removed by etching in 10% HF for 15 min. The IR-OSL was measured with a computer-controlled reader using 860 nm stimulation by short 3 s laser pulses. The IR-stimulated luminescence from K-feldspar was measured in the 380–430 nm wavelength range using a combination of 3 mm SZS-22 (blue-green), 3 mm PS-11 (purple) and 2 mm FS-1 (violet) colour glass filters manufactured by the LZOS, JSC (Lytkarino Optical Glass Factory). 4

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the largest dataset for the Late Pleistocene warm climate-related shellbearing deposits of various geographic origins. In Section 4 of this paper, it will be focused mainly on six areas that are located far apart from each other and which are assured by crosscomparison data to the greatest extent: West Siberian Arctic, the European North-East, the Kola Peninsula, the south-eastern coastal area of the White Sea, and the eastern shore of the Baltic Sea.

Palaeodose determinations were made by extrapolating the doseresponse curves to zero IR-OSL intensities using the MAAD protocol (up to 66 aliquots, 15 mg per aliquot, 11 dose points). Additive-dose growth curves were constructed using natural and up to 10 laboratory dose points fitted by a single saturation exponential function. Each dose point consists of measurements of up to six separate aliquots. Aliquots of each sample were gamma irradiated to a maximum dose of 1000 Gy with radiation steps of 100 Gy. Preheating of the K-feldspar samples or any other thermal treatment before the measurements was not applied. Instead, samples were stored for about 1 month at room temperature to allow the decay of postradiational phosphorescence (for details, see Molodkov and Bitinas, 2006). The external beta and gamma contributions to the total dose rate, D, were estimated in the laboratory from the contents of the natural radioactive elements (238U+235U, 232Th and 40K) in the sediments assuming secular equilibrium corrected for the measured in situ water content using the dose-correction factors of Adamiec and Aitken (1998). The internal beta dose from the decay of 40K and 87Rb within Kfeldspar grains (up to ca 700 μGy/a) was obtained from the concentration estimates recommended by Huntley and Baril (1997) and Huntley and Hancock (2001), respectively, and using the beta attenuation factors reported by Mejdahl (1979) and Brennan (2003). The upper limit of the potassium feldspar-based IR-OSL dating method can widely vary and may extend up to 700 ka (author's data published in Annual Geochronological Bulletin, 2017), depending on burial conditions and the physical properties of the mineral. Geologically very young samples were recently successfully analysed and dated in correct stratigraphic order at about 0.6 and 0.3 ka (Molodkov, unpublished data from the north-east of the Gulf of Finland).

4. Results and discussions The formation of the Pleistocene deposits in the Eurasian Arctic area was strongly affected by the numerous sea level fluctuations. Transgressions and regressions of the World Ocean caused essential reorganisation of landscapes in the coastal zone, and rather rapid alternations of marine and continental conditions of sedimentation. In the beginning of the Late Pleistocene, a large transgressive epoch coinciding with climate amelioration occurred in this region. During the transgression, known (see, e.g., Knipowitsch, 1900; Lavrova, 1924) in the north of Eurasia as the Boreal (lat. Borealis – “northern”), its waters dashed farther onto the mainland along the deep ancient valleys, forming ingressive sounds — estuaries, and overflooding low areas between rivers. This transgression led to the formation of large epicontinental basins, such as the White Sea, Pechora, West Siberian and the Taimyr ones (see Molodkov and Bolikhovskaya, 2009, Fig. 2). In the cold epoch, which was accompanied by regression of the sea, palaeogeographical conditions sharply changed and the shore line moved to the offshore. The events of such exclusively dynamical Late Pleistocene history can be reconstructed on the basis of study of deposits by the complex of methods. Among them the crucial place is taken by methods of numerical chronology, which in contrast to conventional geological methods permit unequivocally determine the timing of palaeoenvironmental events and to correlate sedimentary sequences within the limits of extensive territories. Continental, estuarine and marine beds of the last interglacial have been identified in the Eurasian Arctic in great numbers of sites in diverse environments. Important and, until recently, most difficult to solve problem of this vast area was exactly a question about ages of marker marine horizons, and correlation of coeval deposits, striped by numerous boreholes and outcrops. Well-preserved mollusc shells are often found in these marine deposits. Within each depositional sequence species composition of mollusc assemblages changes from cold-water to warm-water ones, followed again by cold-water assemblages (see, e.g., Dyke et al., 1996). Dating of these subfossils can provide an independent chronology of sea level/climate. In turn, these subfossils can also serve as good environmental indicators and proxies to provide important information on past climate events and sea level changes reflected in marine transgressive records. Although there is abundance of other numerical age determinations on these and associated deposits, for instance, by luminescence methods (Svendsen et al., 2004; Astakhov and Nazarov, 2010; Astakhov and Mangerud, 2014). However, this approach alone does not, unfortunately, bear any useful palaeoenvironmental information and in this respect the method is practically dumb from the point of view of palaeoenvironmental reconstructions. The best possibility to sharply improve the situation is to use methods, which are able to involve palaeoenvironmental proxies for more reliable palaeoclimatic reconstructions. Such a combination may have a greater potential to achieve maximal synergetic effect. In this respect, the mollusc shell-based ESR dating method can be regarded as higly advantageous because combines numerical chronology and climate sensitive mollusc fauna. Being ESR dated, it is good potential to produce an independent mollusc-based chronology for multiple periods, characterized by world-wide warming, ice cap meltings, global sea level rise causing marine transgressions during which large basins occupied vast areas of the Northern Eurasia coast. Besides,

2.3. OSA dating OSA (Optically Stimulated Afterglow) refers to measurements in which single short (ms) stimulation is followed by longer readout of the subsequent luminescence (afterglow) signal over ms to s time scales. For quartz, optically stimulated afterglow was observed by Jaek et al. (1999). The OSA stimulation spectrum of quartz reveals the exponential rise in OSA response intensity in the whole studied spectral range from 1100 to 250 nm. OSA measurements were made for shorter-wavelength bands between 400 and 250 nm (Jaek et al. (1999)). The intensity of the OSA signal was defined as the number of photons measured per second after a 10 ms pause when stimulation was ended (Jaek and Vasilchenko, 2002). The major advantage of OSA over conventional optically stimulated luminescence is that the intensity of the afterglow is measured after the end of the pulse of the stimulated light, which allows overcoming the problem of spectral separation of the luminescence output from stimulating light. As well as in the case of IR-OSL measurements, for OSA measurements is also necessary to store the sample after irradiation for about 1 month at room temperature. The measuring apparatus is described in Jaek et al. (1999). However, in our studies OSA method applied to quartz grains using the multiple-aliquot additivedose (MAAD) protocol is considerably restricted due to a large dispersion of data points on the dose-response curves leading to an increase of the age uncertainty. 3. Study area The ESR-based proxy record of the climate and sea level changes over the Late Pleistocene period has been derived from a large number of shell subfossils. Most of them come from the palaeo-shelf deposits of the Eurasian continental margin, from the New Siberian Islands in the East (∼150°E) and to the eastern shore of the Baltic Sea in the West (∼20°E). Some dating results on freshwater mollusc shell samples from interglacial lacustrine deposits and terrestrial mollusc (land snail) subfossils have also been obtained. Altogether these dated shells form 5

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Fig. 2. Map of the study areas (I –– West Siberian Arctic region; II –– the European North-East; III –– Kola Peninsula and the south-eastern coastal area of the White Sea; IV –– the eastern shore of the Baltic Sea) showing location of the investigated sections, in which mollusc shells of MIS 5 in age were found and analysed by ESR (black circles). The grey circles indicate sites in which mollusc shells of MIS 5 in age were also found but not considered in the present paper. Location of the Voka section (diamond) where interglacial sediments of the same age were found and analysed by IR-OSL is also shown. Limits of the MIS 4 glaciation (dashed curve) according to Zarrina (1991), and of the MIS 5b glaciation (dotted curve) according to Zubakov (1992). For related details see subsubections 4.1.1., 4.1.2., and the last paragraph of subsubsection 4.1.4.

first Late Pleistocene interglacial climatic optimum in the West Siberian Arctic region. On the other hand, according to many authors, the huge Late Pleistocene Barents-Kara ice sheet extended along the Russian Arctic from the White Sea shores in the west (Larsen et al., 2006) to the Taimyr Peninsula in the east (Svendsen et al., 2004) attaining its maximum size as early as 90–80 ka ago when it is believed the ice front reached far onto the continent. To resolve these controversies, new comprehensive studies of the Late Pleistocene transgressive sediments have been carried out in the lower reaches of the Yenisei River, which through the Yenisei Gulf, that is a large and long estuary, flows into the Kara Sea. Field studies were performed in collaboration with the Gramberg All-Russia Scientific Research Institute for Geology and Mineral Resources of the Ocean (VNIIOkeangeologia, St. Petersburg), Institute of Earth's Cryosphere (Tyumen), and Department of Geography of the Moscow State University. Spore-and-pollen and benthic foraminifera assemblages were studied at the Central Laboratory for Mining and Geology (Syktyvkar). Diatoms were studied at VNIIOkeangeologia and Moscow University. Data on the mollusc fauna were obtained by A. Krylov from the PolarGeo JSC, and A. Voronkov from the Norwegian Polar Institute. Mollusc shells and organic remains were U–Th dated at the Laboratory of Geochronology of the St. Petersburg University, and shells by ESR at the Research Laboratory for Quaternary Geochronology (RLQG), Department of geology, Tallinn University of Technology. Enclosing sediments were dated in the RLQG using K-feldspar-based IROSL and quartz-based OSA. In accordance with the stratigraphical, lithological, and paleontological features these marine sediments correlate with the Late Pleistocene interglacial and the Kazantsevo horizon in the Unified Regional Stratigraphic Scheme of the Quaternary Deposit of the West Siberian Plain (2000). The most noteworthy in the context of the present paper are the age determinations achieved for samples taken from the stratotype section 1303, sections 0827 and 0923 (see Fig. 3 for location). It is important to note that part of the age determinations was obtained in RLQG by parallel dating of the samples by two, and in some cases, by three independent methods — ESR, IR-OSL and OSA. The results obtained for the studied sediments are presented in Table 1. From the Table it is seen that the overwhelming majority of the dates obtained by luminescence and ESR methods on 25 samples falls within the range of about 104 to 70 ka, i.e. within the second half of MIS 5. Shell samples and the enclosing sediments taken from the

fossil marine mollusc faunas from the Arctic region are of special interest because may often contain indicator species, which can be a useful source of valuable information about environmental changes at high latitudes during the Quaternary period. Many mollusc species are able to respond quickly to these changes and, therefore, can be used as proxies for the marine climate in the Arctic. 4.1. Marine Isotope Stage 5 4.1.1. West Siberian Arctic region The largest number of our mollusc shell-based ESR datings accompanied by the dating results provided with the use of IR-OSL analysis of potassium feldspar grains, OSA analysis of quartz grains, and 230Th/U analyses of marine mollusc shells was obtained from the stratotype and some key sections of Late Pleistocene Kazanian marine sediments in the lower Yenisei River region (Fig. 2). Despite the fact that Kazantsevo marine deposits in northern West Siberia, which are characterized by thermophilic faunas (Troitsky, 1966) and taiga spore-and-pollen spectra (Zagorskaya et al., 1965; Volkova et al., 2002), are of great stratigraphic importance for the entire Siberian Arctic, their depositional history remain debated (e.g., Astakhov, 2006, 2013; Astakhov and Nazarov, 2010) in spite of numerous detailed investigations previously carried out for the long time on numerous sections throughout the Western Siberia (e.g., Sachs, 1953; Troitsky, 1966; Slobodin, 1970). The ages of the sediments were mostly determined by 14C dating (Kind, 1974), but until recently chronology of the region remains poorly defined due to the lack of accurate numerical ages beyond the practical limit of the method. This is a serious gap in the knowledge of the development of this palaeoenvironmentally important geographical region taking into account that Kazanian marine sediments, widely distributed in the high Arctic, are characterized by unusually warm-water foraminifera (Gudina, 1969; Levchuk, 1986) and presence of boreallusitanian to boreal mollusc species, including Cardium edule, Arctica (Cyprina) islandica, Astarte borealis, Mya truncata, Hiatella arctica, Macoma calcarea, Lacuna vincta, Buccinum undatum, Euspira pallida, etc. (Lavrova, 1946, 1961; Lavrova and Troitsky, 1960). Among them the Arctica (Cyprina) islandica inhabits now much more westward in the more favourable conditions of the shelf seas surrounding the Kola and Scandinavian peninsulas (Zatsepin and Filatova, 1961). This species is established as a bright representative (Troitsky, 1966) of the boreal group of molluscs that allowed V. Sachs (1953) and S. Troitsky (1966) to correlate the formation of the enclosing strata with the time of the 6

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Fig. 3. Schematic geological cross-section of the study outcrops 1303, 0827, 0923 and 1305 from West Siberia Arctic region and Yu-VII from European North-East, showing location of the sampling points and ages of the collected samples, in ka; (1) sand; (2) aleurite; (3) sandy clay; (4) soil; (5) loam; (6) sandy loam; (7) mollusc shells; (8) unconformity surface. Outcrop 1303, located at Kazanka River, is the stratotype of the Kazantsevo interglacial in Siberia; Outcrop 0411, located on the Cape Shaitansky, is the MIS 3 site to be discussed in subsection 5.1. (adapted from Gusev et al., 2011, 2016 and Zarkhidze et al., 2010 with additions).

IR-OSL at 92.6 ± 7.1 and 71.1 ± 5.5 ka, respectively. Sediments of section 0827 at the depths of 11 m and 9 m are dated by IR-OSL at 76.6 ± 6.0 ka and 73.2 ± 5.8 ka, respectively. Shells of four different species of bivalve molluscs, including Arctica (Cyprina)

Kazantsevo stratotype section 1303 (Fig. 3) at a depth of 6 m were dated in parallel by ESR and IR OSL at 84.7 ± 7.0 ka and 85.6 ± 6.7 ka, respectively, and at a depth of 4 m at 78.4 ± 6.4 and 77.6 ± 6.0 ka, respectively. Sediments from the depths of 7 m and 2 m are dated by

Table 1 ESR and luminescence dating results and radioactivity data for mollusc shells and enclosing sediment samples from the sections located on the eastern bank of the Yenisei River and in the middle reach of the More-Yu River. U, Th and К are the uranium, thorium and potassium content in the sediment as determined from laboratory gamma spectrometry (modified from Gusev et al., 2016 and Zarkhidze et al., 2010 with additions). No.

Sample no.

Material/section/depth (m)

Method

Age (ka)

U (ppm)

Th (ppm)

K (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

RLQG RLQG RLQG RLQG RLQG RLQG RLQG RLQG RLQG RLQG RLQG RLQG RLQG RLQG RLQG RLQG RLQG RLQG RLQG RLQG RLQG RLQG RLQG RLQG RLQG RLQG RLQG RLQG RLQG RLQG RLQG

sand/0827/4.0 sand/0827/4.0 sand/0827/7.0 sand/0827/7.0 sand/0827/9.0 sand/0827/11.0 sand/0923/3.2 sand/0923/12.5 sand/1303/2.0 sand/1303/4.0 sand/1303/6.0 sand/1303/7.0 sand/1305/1.5 Arctica islandica/0827/7.0 Astarte borealis/0827/7.0 Macoma calcarea/0827/7.0 Mya truncata/0827/7.0 Macoma calcarea/0923/3.2 Hiatella arctica/0923/3.2 Clinocardium ciliatum/0923/3.2 Neptunea despecta/1303/4.0 Astarte borealis/1303/4.0 Astarte borealis/1303/6.0 Neptunea despecta/1305/1.5 Astarte borealis/1305/1.5 Sand/Yu-VII/6.5 Sand/Yu-VII/11.5 Astarte borealis/Yu-VII/9.2 Astarte borealis/Yu-VII/11.5 Astarte borealis/Yu-VII/6.5 Astarte borealis/Yu-VII/13.5

IR-OSL OSA IR-OSL OSA IR-OSL IR-OSL IR-OSL IR-OSL IR-OSL IR-OSL IR-OSL IR-OSL IR-OSL ESR ESR ESR ESR ESR ESR ESR ESR ESR ESR ESR ESR IR-OSL IR-OSL ESR ESR ESR ESR

71.1 ± 5.5 70.1 ± 14.3 71.9 ± 5.6 71.3 ± 17.3 73.2 ± 5.8 76.6 ± 6.0 87.5 ± 6.8 103.5 ± 8.0 71.1 ± 5.5 77.6 ± 6.0 85.6 ± 6.7 92.6 ± 7.1 93.5 ± 7.3 76.2 ± 6.0 79.3 ± 6.7 74.0 ± 6.3 70.1 ± 5.9 84.2 ± 7.1 93.0 ± 7.8 87.5 ± 7.2 79.2 ± 6.8 78.4 ± 6.4 84.7 ± 7.0 95.3 ± 8.2 94.2 ± 7.8 109.8 ± 6.9 88.2 ± 5.4 85.0 ± 9.1 90.3 ± 10.9 107.6 ± 12.4 91.5 ± 10.5

0.44 0.44 1.13 1.13 0.63 1.69 0.60 0.55 0.66 0.51 0.60 0.61 0.70 1.13 1.13 1.13 1.13 0.60 0.60 0.60 0.51 0.51 0.60 0.70 0.70 0.29 0.53 1.18 0.80 0.38 1.02

2.43 2.43 5.48 5.48 1.76 5.59 3.24 3.13 3.29 1.99 3.56 2.68 3.33 5.48 5.48 5.48 5.48 3.24 3.24 3.24 1.99 1.99 3.56 3.33 3.33 1.32 2.51 3.59 3.48 1.55 4.35

1.58 1.58 1.87 1.87 1.91 1.73 1.93 1.82 1.78 1.52 2.26 1.55 2.06 1.87 1.87 1.87 1.87 1.93 1.93 1.93 1.52 1.52 2.26 2.06 2.06 1.44 1.44 1.69 1.32 1.44 1.75

2042-081 2042-012 2043-081 2043-012 2044-081 2045-081 2074-052 2075-052 2253-094 2254-094 2270-094 2306-094 2255-094 449-061-A 449-061-B 449-061-C 449-061-D 455-052-А 455-052-B 455-052-C 489-094-A 489-094-B 490-094 491-104-A 491-104-B 1478-103 1608-124 316-042 317-042 318-042 320-042

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deposits (Zarkhidze et al., 2010). The existence of sea terraces of the same height also on the Taimyr Peninsula prove that the sea basin was not isolated, but occupied significant areas of Eurasian North, which was proved also by previous researchers of the Boreal transgression (e.g., Lavrova and Troitsky, 1960). The Yu-VII section was one of the studied were the numerical ages of some of the deposits were determined both in parallel by ESR and IR-OSL methods and separately by ESR. Palynological spectra of the Yu-VII section conforms the Late Pleistocene flora of the forest-tundra zone (Zarkhidze et al., 2010). Among the mollusc shells found in the deposits Astarte borealis, A. montagui and Macoma calcarea species dominate. Broken shells of Arctica islandica, Clinocardium ciliatum, Mytilus sp. and juvenile forms of Cerastoderma edule were also encountered (Zarkhidze et al., 2010). Shell and sediment samples taken from the upper unit of the sandy sediments at a depth of 6.5 m (Fig. 3, Table 1, SM1 and SM2) were dated in parallel by ESR to 107.6 ± 12.4 ka (RLQG 318-042), and by IR-OSL to 109.8 ± 6.9 ka (RLQG 1478-103). The ESR ages of the underlying aleurite-sandy thickness lying beneath unconformity surface increases consistently with depth in the section from 85.0 ± 9.1 ka (9.2 m, RLQG 316-042) to 91.5 ± 10.5 ka (14 m, RLQG 320-042). At a depth of 12.8 m sediments were also dated in parallel by ESR to 90.3 ± 10.9 ka (RLQG 317-042), and by IR-OSL to 88.2 ± 5.4 ka (RLQG 1608-124). Despite the age inversion, which is most likely explained by neotectonic movements, which are very active in the area, it is clear that marine sedimentation was the leading geological process in this area in the first half of the Late Pleistocene (Bolshiyanov, 2006). Based on a variety of analyses, including lithologic, macro- and microfaunistic, diatom, pollen and chemical, it can be concluded that at least between 110 and 85 ka the study area was not affected by significant glacier influence.

islandica, taken at a depth of 7 m were dated by ESR from 79.3 ± 6.7 ka to 70.1 ± 5.9 ka (74.9 ± 2.3 ka in average). Enclosing sediments at a depth of 7 m were dated in parallel by two luminescence methods –– IR-OSL and OSA –– using two main rock-forming minerals –– feldspar (71.9 ± 5.6 ka) and quartz (71.3 ± 17.3 ka), respectively. Two shells (Astarte borealis) from a depth of 4 m were dated by U–Th at 48.9 ± 4.3 ka and 41.5 ± 4.0 ka and two Arctica islandica at 90.9 ± 9.0 ka and 53.0 ± 3.1 ka (Gusev et al., 2011), i.e. 58.6 ± 12.4 ka in average on four shells. Feldspar and quartz from the same depth were analysed in parallel by IR-OSL and OSA, and yielded 71.1 ± 5.5 ka and 70.1 ± 14.3, respectively. Shell samples of three different mollusc species taken from section 0923 at a depth of 3 m were dated by ESR, and enclosing sediments by IR-OSL, also in parallel. The results of the parallel dating are as follows: IR-OSL — 87.5 ± 6.8 ka, ESR for shells of three different mollusc species — 93.0 ± 7.8 ka, 87.5 ± 7.2 ka, and 84.2 ± 7.1 ka, or 88.2 ± 2.9 ka in average for these three shells. A shell sample taken from a depth of 1.2 m were U–Th dated at 79.7 ± 5.0 ka. Sediment sample taken from the bottom of the section was IR-OSL dated at 103.5 ± 8.0 ka and is the older one in the numerical ages array obtained in the present study from the marine transgressive sediments along the eastern bank of the Yenisei River. The composition of mollusc fauna collected from the above sections consists of relatively shallow, warm to moderately warm-water, boreal and subarctic species. Boreal representatives are Arctica (Cyprina) islandica, Mytilus edulis, Zirphea crispata (for more details see Gusev et al., 2016). According to the ages obtained by luminescence and ESR dating, the duration of sedimentation of the mollusc-bearing deposits lasted about 22 ka (see Table 1) that corresponds roughly to MIS 5a-b. The greatest number of the largest specimens of Arctica (Cyprina) islandica shells were met in section 0827. The sediments containing these shells are referred in the Western Siberia Arctic to the final (MIS 5a) stage of the warm-water Kazantsevo transgression (Gusev and Molodkov, 2012). Studies of spore-and-pollen assemblages in marine sediments may often face certain problems. Nevertheless, the analysed samples was found to contain quite a bit of in situ material corresponding mainly to taiga or forest tundra that differs from the spore-and-pollen spectra in over- and under-lying strata. According to the spore-and-pollen content, forests consisted mainly of spruce and birch, with minor pine and alder (for more details see Gusev et al., 2016). Highest percentages of Picea obovata and Pinus sylvestris in zone dated by luminescence and ESR methods at about 90–80 ka may indicate warming trend in the study area during this period. The spore-and-pollen spectra dominated by spruce correspond to taiga vegetation common to Late Pleistocene Kazantsevo time, when birch-spruce, pine-spruce, and the spruce forests grew in the area. Taken as a whole, our results obtained using ESR for age determination of the mollusc shell-bearing marine deposits, clearly demonstrate that transgressive sediments with abundant boreal malacofauna including representative species Arctica islandica accumulated in the study area during the second half of MIS 5. These results and those obtained from other palaeoenvironmental proxies indicate the climate much warmer than today, and eliminate the possibility of existence of a glacial environment in the lower reaches of the Yenisey River and, moreover, overriding of the region by the Barents-Kara Ice Sheet during this period.

4.1.3. ESR dating of marine deposits in the eastern periphery of the Fennoscandian Shield (White Sea coastal area) The formation of the Pleistocene deposits in the White Sea area (see Figs. 2 and 4A for locations) was strongly affected by main environmental factors, such glaciations and sea level fluctuations. In the complicated history of the area, the deposits of the first Late Pleistocene Boreal transgression serve as an excellent key horizon for palaeogeographical and stratigraphic conclusions and correlations. 4.1.3.1. Kola Peninsula. One of the areas of particular interest in this region is the southern coastal area of the Kola Peninsula. Traces of the first Late Pleistocene transgression — the Boreal –– is distinctly recorded here in a number of type sections. The most remarkable assemblages of the Late Pleistocene flora and fauna are represented by the Varzuga outcrop located on the south coast of the Kola Peninsula at about 66.4°N, 36.6°E (Fig. 4A) on the right bank of the Varzuga River. The deposits of interest form a lens of fossil-bearing sandy clay traced in a type section T-16 of the outcrop (Fig. 4B) over some 25 m. Boreal or mainly boreal species account for 39% of the total fauna found at the bottom of the lens, 39% are arcto-boreal and 22% are arctic and mainly arctic (Gudina and Yevzerov, 1973). The arctic forms Astarte borealis and A. crenata var. crebricostata dominate the assemblage. The composition of the fauna suggests a littoral or upper sublittoral origin for these deposits, under hydrobiologic conditions somewhat more favourable than those of the White Sea at the present time (Gudina and Yevzerov, 1973). At the base of the lens, V. Gudina (Gudina and Yevzerov, 1973) found a large number of foraminifera representing 44 species. This assemblage is dominated by a cibicidid-trifarinid-cassidulinid association and comprises many lusitanian, boreal-lusitanian and boreal forms, e.g. Globulina inaequalis, Guttulina lactea, Sigmomorphina undulosa, Hyalinea balthica, Elphidium excavatum, Trifarina angulosa, Lagena semilineata, L. sulcata, Elphidium boreale, etc.

4.1.2. The European North-East Geological investigations were carried out here in the middle reach of the More-Yu River. They were aimed on determination of age and origin of the deposits composing 80–100 m terrace (Zarkhidze et al., 2010). Some of the deposits revealed nonconformity with dislocated sandy aleurite or sandy deposits. A chemical compound of deposits unequivocally indicates formation of the sediments in a sea water of various salinity. Index fossils reveal the Kazantsevo (MIS 5) age of the 8

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Fig. 4. (A) Location of the profiles studied in the White and Barents Sea coastal areas: 1 – Varzuga; 2 – locality 3; 3 – Lodma; 4 – Zaton and Z-23; 5 – borehole 34; 6 – locality 30; (B) Sketch of section T16 located on the right Varzuga River bank: I – Ponoy Bed; II – Strelna Bed; III – lens of sandy clay; IV — till of the last glaciation; V – postglacial deposits; (C) Chronology and lithology of the sedimentary sequence of the Zaton section studied in the eastern part of the White Sea coast. Pollen zones are after Devyatova (1982). 1 – soil; 2 – sand; 3 – till; 4 – sandy clay; 5 – shingle and pebbles; 6 – sampling point for ESR dating; 7 – sandy silt; 8 – silty sand; 9 – clay; 10 – sampling interval and ESR age, ka (adapted from Molodkov and Raukas, 1988).

admixture of alder in the lower part of the sandy clay. At present, birchpine and pine forests dominate in the lower reaches of the Varzuga River. Therefore, in the period when the deposition of sandy clay started, the climate has been similar or even warmer than that at present. In order to provide crucial chronological data needed to reconstruct the temporal position of this marine interglacial event and to elucidate the true stratigraphical position of the corresponding deposits, which are comprehensively and interdisciplinary investigated, a sediment

In the sandy clay component of the deposits (Fig. 4B) G. Blagoveshchensky (Lavrova, 1960) found numerous pollen grains derived from woody plants. Spores (33–64%) usually dominate over pollen of woody (31–50%) and herbaceous (1–17%) plants. In the total pollen of woody taxa (except the lower part of the spectrum) birch dominates, making up most (48–65%) of the flora. Besides, pine (13–37%), alder (1–10%), dwarf birch (3–13%), willow (1–9%) and spruce (1–6%) were also found among that group. The pollen spectrum considered reflects the phase of birch forest-tundra and sparse birch-pine forest with an 9

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example, to marine successions should be handled with a certain caution while interpreting the regional vegetation history. For example, the M2–M8 pollen zone has been assumed to cover the entire last interglacial period which lasted from about 127 to 70 ka (Gerasimov and Velichko, 1982). However, after introducing a new chronostratigraphic framework of NW Europe the last interglacial became correlated with substage 5e only. Moreover, already in 1992 V. Zubakov (1992) demonstrated that during substage 5b (ca. 88 ka; compare with the data of Svendsen et al., 2004) the Fennoscandian ice sheet could have reached far out of the sea floor of the White Sea, possibly to a position about 300 km inland from the White Sea coast (Fig. 2; for more details, see Molodkov and Raukas, 1998). Unfortunately, lack of accurate datings has been the main handicap in answering the existing questions related to the Late Pleistocene palaeoenvironmental history of this region. Thus, it seems quite understandable that new chronologically supported evidences may show whether or not the Fennoscandian ice sheet ever overrun the eastern part of the White Sea coast during MIS 5 and may help to solve, to some extent, the problem concerning the distribution and duration of the last interglacial seas in the vast areas of NE Europe and further east. The Zaton outcrop is located in the Arkhangelsk District, NW Russia, on the left bank of the Mezen River about 1.5 km upstream from the Zaton settlement (Fig. 4A). The marine formations in this outcrop are well preserved and abundant in malacofauna, which is richer and more thermophilous than the present-day fauna in the same area, which now is subarctic. It contained a lot of thermophilous boreal (Arctica islandica, Littorina littorea, Capulus hungaricum, Mytilus edulis, Cardium fasciatiim, C. echinatum, Macoma baltica, Astarte sulcata, A. borealis, Lucina borealis) and boreal-lusitanian and lusitanian (Nassa reticulata, Bittium reticulatum, Cardium edule, C. fasciatum, C. paucicostatum, Anomia straita) species (Devyatova, 1982; Lavrova, 1961). In the outcrop the fauna for dating was taken from silty-sandy deposits at the depths of 6.4 m to 3.8 mm (Fig. 4C). Shells of six different mollusc species were analysed: Arctica islandica (n = 3), Astarte crenata, Macoma calcarea (n = 3), Neptunea despecta, Pecten islandicus. Analytical data and ESR dating results are listed in Table 3. The shells analysed gave concordant dates within the time span from about 111 ka to 90 ka in average. IR-OSL was not applied to determine the numerical age of the deposits from this section. The results obtained show that the whole complex of marine sediments accumulated continuously during the time of Boreal transgression. The visible part of underlying marine clays, which accumulated most likely under deep water conditions, seems to be older than 111 ka. Pollen analysis of the marine sediments (Devyatova and Loseva, 1964; Devyatova, 1982) at the Zaton site indicate climatic amelioration during the interglacial upwards in the profile. Last interglacial is commonly characterized by seven pollen

Table 2 ESR dating results and radioactivity data for mollusc shells and enclosing sediment samples from the T-16 section located on the right bank of the Varzuga River (modified from Molodkov and Yevzerov, 2004). Sample no.

RLQG RLQG RLQG RLQG

309-042 310-042 311-042 312-042

U

Th

K

Age

(ppm)

(ppm)

(%)

(ka)

1.50 1.50 1.50 1.50

8.73 8.73 8.73 8.73

2.22 2.22 2.22 2.22 Average

112.5 ± 8.0 103.0 ± 11.0 94.0 ± 6.5 110.0 ± 10.3 104.9 ± 4.6

sample with a variety of marine shell species was collected from these fossil-rich marine deposits. Mollusc shell samples were taken at a depth of 16 m, about 30 cm from the bottom of the lens, for dating by ESR. Analytical data and ESR dating results are listed in Table 2 and SM1. The ESR analysis was performed on broken and whole well-preserved aragonite shells belonging to four different marine mollusc species. The average of four ESR age determinations on different shells is 104.9 ± 4.6 ka. Thus, the data available and the results obtained clearly indicate that the time of the formation of the sandy clay, when the climate has been similar or even warmer than that at present, can be confidently correlated with the interglacial marine transgression during the second half of MIS 5. The dating results from the Varzuga outcrop are corroborated also by the results obtained from the eastern part of the White Sea coast (Fig. 4A) at the several sites, including Zaton and locality 3, as well as on the Kolguev Island (loc. 30) and Kanin Peninsula (borehole 34) located in the south-eastern part of the Barents Sea where three Arctica (Cyprina) islandica shell specimens were only ESR-dated at 71.5 ± 6.0 ka (loc. 3), 111.0 ± 9.0 ka and 100.0 ± 10.0 ka (loc. 30) and 87.8 ± 8.5 ka (borehole 34) (Molodkov and Raukas, 1998). 4.1.3.2. South-eastern coastal area of the White Sea. In this area the most famous section — the Zaton — was studied in particular detail. The results of the study together with the materials from the Western Siberia allow to get additional valuable data concerning the distribution and age of the Boreal Sea. Until recently, the stratigraphic position and correlation of the deposits has been mainly based on the palynological data. Seven assemblage zones (M2–M8) of a wide geographical and temporal extent (Grichuk, 1961, 1989) have been established for the last interglacial in the central area of the East European Plain. However, the palynologically studied sections alone do not provide sufficient evidence for solving the chronostratigraphical problems. In many cases geographic variations and non-climatic factors inherent, for

Table 3 ESR dating results and radioactivity data for mollusc shells and enclosing sediment samples from the Zaton section located in the Arkhangelsk District, NW Russia (modified from Molodkov and Raukas, 1988). Sample no.

Mollusc

Age (ka)

U (ppm)

Th (ppm)

K (%)

92.0 ± 6.0 92.0 ± 7.0 82.0 ± 6.0 95.0 ± 15.0 92.0 ± 9.0 90.0 ± 8.0 90.0 ± 8.0 90.4 ± 1.9 120.0 ± 8.0 109.0 ± 7.0 105.0 ± 7.0 111.3 ± 5.0

0.75 0.55 0.55 0.55 0.55 0.55 0.55

2.34 2.05 2.05 2.05 2.05 2.05 2.05

0.93 1.09 1.09 1.09 1.09 1.09 1.09

0.41 0.41 0.41

1.62 1.62 1.62

1.10 1.10 1.10

species RLQG RLQG RLQG RLQG RLQG RLQG RLQG

24-124 25-124 26-124 27-124 28-124 29-124 30-124

Macoma calcarea Macoma calcarea Arctica islandica Pecten islandicus Macoma calcarea Arctica islandica Neptunea despecta Average

RLQG 31-124 RLQG 32–124A RLQG 32–124B

Astarte crenata Arctica islandica Arctica islandica Average

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assemblage zones (c–i after Milthers, 1928; M2–M8 after Grichuk, 1961). The following pollen sequence from bottom to top is distinguished in the study area:

2015), suggesting different duration of this period and uncertainty of the chronostratigraphical position of the Late Pleistocene interglacial deposits. Interglacial sediments in the Netiesos section with a thickness of about 6 m are rich in organic matter. The distribution of the sediments indicates formation in the bay of a large lake. The sediments are overlying by fine sands, which are attributed to the following cool period, probably MIS 4 in age. Multidisciplinary investigations of the Late Pleistocene sedimentary sequence of the Netiesos section including geochemical, thermoluminescence, palaeontological (plant macro- and micro-remains, diatoms, fishbones), palaeomagnetic (Baltrūnas et al., 2013) and malacological (Sanko and Gaigalas, 2004, 2007) were performed on numerous sediment samples taken from lake-and-bog deposits of the section. This interdisciplinary approach enabled the establishment of the most pronounced changes in the Late Pleistocene environmental conditions. In order to provide age constraints on the palaeoenvironmental events multimethod chronology was applied for this sequence providing it with a coherent numerical time framework. The potassium feldspar-based IR-OSL dating method was applied to 8 samples of sandy deposits from the outcrop in the depth range between 12.2 m and 3.9 m (Baltrūnas et al., 2015). Location of the first two of them between 12.0 and 10.5 m is shown in Fig. 5B. The ESR method was applied to date freshwater mollusc shells taken directly from the gyttja and peat situated in the lower part of the section. The shells were represented here by tiny, thin-walled freshwater Pelecypod and Gastropod species: Valvata piscinalis, V. cristata, Bithynia tentaculata, Radix limosa, Radix auricularia, Lymnaea stagnalis, Acroloxus lacustris, Gyraulus albus, G. laevis and Sphaerium corneum species. In contrast to marine mollusc shells, all the freshwater shells studied were composed of calcite, and displayed typical ESR spectra with a characteristic hyperfine sextet associated with the Mn2+ ion in shell carbonate (Fig. 1B). The phase sensitivity detection (PSD) technique (Molodkov, 1988, 1993) was used to enhance the analytical line at g = 2.0012, and to suppress the manganese signals as well as the interfering radiation-induced signals in the region of g = 2.00. The analytical data and the results of ESR dating are reported in Table 4. In most cases, ESR ages were determined on up to three shells of different mollusc species taken from the same sampling point. Two groups of shell samples were collected from gyttja, and the third was taken from the overlying peat layer. The ages of two groups of the shell samples from the gyttja are practically the same, 112.5 ± 10.8 ka and 112.1 ± 25.9 ka, which is not surprising because the difference in the

c(M2) - birch and pine with an admixture of spruce; d(M3) - pine and birch with an admixture of oak, elm and hazel; e(M4a) - pine with an admixture of oak and elm, appearance of hazel; fα(M4b) - oak and elm, the lower maximum of hazel; fβ(M5) - linden with abundant oak, elm and hornbeam, the upper maximum of hazel; g(M6) - hornbeam with an admixture of linden, oak, elm, hazel and spruce; h(M7) - the upper maximum of spruce with an admixture of broadleaved trees; i(M8) - pine with an admixture of spruce and birch. Pollen assemblage of M8 zone was formed by vegetation which characterizes transition from forest to periglacial type. Zone M9 marks the ultimate onset of severe periglacial conditions in the beginning of MIS 4. According to Devyatova’s (1982) paleobotanical analysis, sediments at the depths of ca 5–4 m in the Zaton section dated at 90.4 ± 1.9 ka (n = 7) were deposited during pollen zones M5–M6. Lusitanian-boreal species such as Cardium edule and С. fasciatum, were also found in this part of the Zaton section (Devyatova and Loseva, 1964). As a whole, it indicates that the formation of the mollusc-bearing strata of these ages occurred during the last interglacial climatic optimum when climate was warmer than it is at the present day in this area.

4.1.4. ESR evidences from freshwater environments on the eastern shore of the Baltic Sea One of the most famous sections of the last interglacial in this area –– the Netiesos –– is located in southern Lithuania on the right bank of the Nemunas River (∼54.65° N; ∼24.05° E; for location see Fig. 5A). Earlier sedimentological works on this section (e.g. Gaigalas et al., 2005) indicated that lake littoral and deep sedimentation environment existed during the greater part of the last interglacial. The bog formation started in the second part of the interglacial under the dry but warm climatic conditions. However, the upper boundary of this interglacial period has not yet been definitively defined. It still varies in Lithuania from 70 ka (Gaigalas, 1994) to 117 ka (Baltrūnas et al.,

Fig. 5. (A) Location of the profiles studied on the eastern shore of the Baltic Sea; (B) Chronology and lithology of the sedimentary sequence of the Netiesos section in southern Lithuania. Pollen zones are after Kondratienė (1996). 1 – sand; 2 – peat; 3 – gyttja with peat; 4 – gyttja; 5 – sandy silt; 6 – mollusc shells (adapted from Baltrūnas et al., 2013).

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and Bolikhovskaya, 2009, 2010; Molodkov, 2012). In conclusion of this section, it should also be noted one more important fact, namely, all the evidence above coincide very well with integrative multidisciplinary results derived from the south-eastern coast of the Gulf of Finland, at the Voka site. They show unambiguously that the fully interglacial climate conditions (pollen zones M6 – M8, after Grichuk, 1961) existed in this area during the second half of the MIS 5, at least between ca. 94 ka and 71 ka (Bolikhovskaya and Molodkov, 2014; Molodkov and Bolikhovskaya, 2015). No evidence was found for glacial activity during MIS 5b – MIS 4 at this site.

Table 4 ESR/IR-OSL results and radioactivity data for the samples from the Netiesos site (modified from Baltrūnas et al., 2013). Sample no.

Dating method

Age (ka)

U (ppm)

Th (ppm)

K (%)

RLQG RLQG RLQG RLQG RLQG RLQG

IR-OSL IR-OSL IR-OSL ESR ESR ESR

66.2 ± 5.1 89.8 ± 6.8 93.1 ± 8.4 101.5 ± 11.5 112.5 ± 10.8 112.1 ± 25.9

1.24 0.73 0.44 0.64 1.32 1.32

3.21 2.28 1.58 0.93 3.62 3.15

1.71 1.03 1.09 0.30 0.97 1.02

2054-111 2053-111 2052-111 2215-095 2200-095 2224-095

5. ESR evidence from a series of warm climate-related terrestrial and marine events during the MIS 3 period

sampling depth is relatively small, 0.8 m. Shell samples from the peat layer are dated to 101.5 ± 11.5 ka. This result coincides closely with the ages of 108.8 ± 8.7 and 105.7 ± 10.0 ka of the peat that were obtained by the U–Th method using leachate (L/L) analysis (Gaigalas et al., 2005). Two samples from the sands overlying the peat at the depths of 12.1 m and 10.5 m were dated by IR-OSL at 93.1 ± 8.4 and 89.8 ± 6.8 ka, respectively. It is noteworthy here that, according to five more IR-OSL data, layer of the fine-grained sands between the depths of 10.1 and 4.2 m were deposited in the cold period from approximately 66 to 64 ka (for more details, see Baltrūnas et al., 2013). This layer was interpreted as deposited by in stagnant water basin, and characterized by the spread of cryophilous and hydrophilous vegetation (plants that can survive in a cold and moist climate) (Kondratienė, 1965). This period meets roughly the middle part of MIS 4, which is associated with the first post-MIS 5 significant deterioration of the climate, during which glaciers is thought to have reached as far south as approximately 53–52°N in the study region (Zarrina, 1991). However, no signs of glacigenic deposition were observed in the Netiesos section (∼54°N) during this severe climatic event. As, by the way, they were not observed also in the Voka section (Molodkov and Bolikhovskaya, 2011, 2015), situated on the southeastern coast of the Gulf of Finland (59.4°N), i.e. much more north that the Netiesos (for locations see Fig. 2). In addition, data indicate that deposits corresponding to the initial and final phases of this cold stage (MIS 4), as well as to the whole MIS 3 (∼59–24 ka), are missing in the Netiesos section. The pollen assemblage of the last interglacial in Lithuania have been divided into five (M1–M5) pollen zones (Kondratienė, 1996). During the pollen zone M3 broad-leaved species dominated. This period of optimal climatic conditions, during which temperatures were higher than now, is divided into three phases: the oak phase (М3a), the lime phase (M3b) and the hornbeam phase (M3c). The palaeobotanical examination of the sediments in the Netiesos section allowed to identify pollen composition (interval 15.5–12.9 m, Fig. 5B) that is clearly related to three subzones of the climatic optimum of the interglacial (Kondratienė, 1996). At this time, the territory was covered by deciduous forests and numerous thermophilic water and terrestrial plant species were present. In addition to the Netiesos (see Fig. 5A), similar results were obtained on freshwater mollusc shells collected from interglacial peaty sandy loam in the Gailiūnai site (118.0 ± 15.0 ka), from gyttja in the Valakampiai site (113.0 ± 3.0 ka) and from lake-and-bog deposits in the Jonionys site (109.5 ± 8.5 ka) (Gaigalas and Molodkov, 2002). Besides, 18 samples from inter-till sands from various sites in Lithuania were IR-OSL-dated between 114.3 ± 7.4 ka and 76.5 ± 4.9 ka (Molodkov et al., 2010; Damušytė et al., 2011). Thus, the results obtained from the Netiesos and other interglacial section from this area convincingly testify that temporal position of the optimal climatic zones M3a–M4 of the last interglacial in the SouthEastern Baltic region coincides perfectly with the ESR/IR-OSL palaeoclimatic records obtained for the second half of MIS 5 on directly ESRand IR-OSL-dated warm climate-related deposits along the climatesensitive Arctic and Subarctic regions of Northern Eurasia (Molodkov

The Marine Isotope Stage 3 (∼59–24 ka, Martinson et al., 1987), which corresponds to the second half of the last glacial period of the West European stratigraphic scheme, is traditionally characterized by a cold climatic epoch during which sea-level reached a stand of about 90 to 60 m below its present one. Such a low sea level implies a significant expansion of the ice sheets in polar and temperate latitudes due to accumulation of the isotopically lighter water extracted from the ocean. The main nucleation centres of massive cover glaciation, which expands into previously unglaciated areas of continental north-western Europe and further to the east, are northern Fennoscandia and the Barents–Kara Sea area. Judging from some recent publications (e.g., Larsen et al., 2006), the Kara Sea Ice Sheet covered significant areas of the adjacent land areas between 55 ka and 45 ka. If we assume the absence of the Kara glacier, then coastlines had to move well offshore northward from the coastal line, in some areas up to several hundred kilometres due to sea-level drop of 90–60 m during MIS 3 (Shackleton, 1969, 1987; Lambeck et al., 2002a, b; Yokoyama and Esat, 2011). In both cases, findings of any boreal marine fauna, and, even more so, warm-water one, dated as MIS 3 in age would be highly unlikely in the present-day Siberian Arctic coastal zone. However, already in the early work of V. Sachs and K. Antonov (1945), the so-called Karginsky (MIS 3) interglacial sedimentary sequences have been distinguished in the Western Siberia's Arctic area. Numerous 14C dates obtained in this area on vegetable detritus, wood and peat samples taken from alluvial deposits, data from marine faunabearing deposits, and findings of such plant remains as Carex sp., С. sect. Vignea, Alnus truticosa, Betula sect. Nanae, Comarum palustre and many others climate proxy indicators suggest that climate during several optimum warm periods of the Karginsky time was as warm as and possibly warmer than today (Kind, 1974). Three main waves of warming were recorded within MIS 3 lasted from 55 to 50 to 27 cal. 14C ka BP. The warmest one (ca 55–48 ka cal. 14C ka BP, Kind, 1974) was probably the time of the maximum marine transgression and the climate warmer than today. Sea flooded wide areas of the biggest West Siberian peninsulas — Yamal, Gydan, and Taimyr Fig. 2). The MIS 3 stratigraphic level here is characterized by typical malacofauna association dominated by arctic and arctic-boreal species such as Astarte borealis, Cardium ciliatum, Macoma calcarea, Mya truncata, and others. The foraminiferal association included not only boreal and arcticboreal but also boreal-lusitanian and lusitanian species (Levchuk, 1986), which nowadays occur at the western shore of the Kola Peninsula in the zone influenced by North Atlantic warm current (Arkhipov et al., 2005). Some researchers also believe that the further east — in the Laptev Sea shelf area — the sea level in this time was also close to the modern level, or even slightly higher than today (Alekseev, 1997). The time between about 47 and 38 cal. 14C ka BP is referred to as the Malokheta warming, which also represents a period with the climate warmer than today. Between about 34 and 27 cal. 14C ka BP climate became cooler, but still is referred to as the third –– LipovkaNovoselovo –– warm phase. This timeframe reconstruction of the 12

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Fig. 6. Map of the study areas (I –– West Siberian Arctic region; II –– Kola Peninsula and the south-eastern coastal area of the White Sea; III –– the eastern shore of the Baltic Sea) showing location of mollusc shells (closed circles) and marine sediments (diamonds) dated at MIS 3 by mollusc shell-based ESR, U––Th (open circle) and feldspar-based IR-OSL, respectively. The location of the two sites where foraminiferas were dated by AMS14C is represented by two squares, placed in the northern part of the Taimyr Peninsula. The figures indicate the ages, in ka.

warm phases within the mid-MIS 3: (1) optimum of the Malokheta warming (Section 258) and (2) onset of the Lipovka-Novoselovo warming (Section TKh-32). These dates are also broadly consistent with the second and third ESR clusters corresponding to two of three main phases of the MIS 3 transgression in this region. Further to the east, in north-eastern Siberia, sediments of the freshwater basin, composed of sands with the presence of vegetable detritus, were found on the Sardakh Island located in the Lena River Delta (72,6° N, 127,3° E). Organic material (vegetable detritus) from the sands has been 14C dated as more than 41.7 ka BP (LU-4890). IR-OSL age of the enclosing sand was determined as 45.6 ± 3.5 ka, RLQG 1755-027 (Bolshiyanov et al., 2013).

Karginsky (MIS 3) period as an interglacial (or megainterstadial) one is based on a great number of conventional 14C dates obtained mostly from sediments along the Lena River, Upper Yenisey River area (Kind, 1974), and, in the recent years, further to the west, in the European North-East (Andreicheva and Marchenko-Vagapova, 2017; Zaretskaya et al., 2019). Important evidences of the natural environment changes in terms of time and space for MIS 3 were obtained from ESR analysis of mollusc shells taken mostly along the continental margin of the Eurasian North.

5.1. Western Arctic Siberia In Fig. 6 are shown the locations of the sampling sites in coastal zone of Eurasian north from which collected shell samples were dated to MIS 3. Dating results (Table 5) revealed that ESR ages of the shells within MIS 3 are grouped into a sequence of three age intervals of 58.7–52.0 ka, 47.0–40.0 ka and 32.4–24.8 ka, which are correlated fairly well with those determined from calibrated radiocarbon dates (Fig. 7). Besides ESR, two IR-OSL dates were obtained from the shallowwater sand from the key section 0411 on the Cape Shaitansky (71.2° N, 82.3° E) located in the Yenisey River mouth area: 57.2 ± 3.9 ka and 45.8 ± 3.2 ka (Gusev et al., 2010). The dated sand is foraminifera-free but contains wood remains, weeds, sponges, diatoms and megaspores. Spore-and-pollen spectra evidence for warm environment with predominantly taiga vegetation. Trees make up the most part of the spectra. A specimen of Hiatella arctica shell collected from the site 0824 located ca 65 km south of the 0411 section is U–Th dated to 55.7 ± 3.5 ka (LUU-544) (Gusev et al., 2011). As important evidence for the existence of favourable palaeoenvironmental conditions and sea level stands close to the present level during MIS 3 can also serve two AMS14C age determinations on marine sediments obtained from two sites located in the northern part of the Taimyr Peninsula (Fig. 6). These sediments were directly AMS14С-dated using foraminifers as material for dating. Individual foraminifera was extracted directly from two marine sections (Gusskov et al., 2008). Foraminiferal assemblages are unambiguously interpreted as boreal (site 258, 76.5° N, 103.8° E) and arcto-boreal (site TKh-32, 75.3° N, 100.1° E) ones (Gusskov et al., 2008). The dates obtained from the sites (39.0 ± 11.0 ka BP or 41.3 ± 0.8 cal ka BP, АА-59333, site 258, and 31.3 ± 0.4 ka BP or 35.3 ± 0.5 cal ka BP, АА-59332, site TKh-32, respectively) undoubtedly confirm MIS 3 ages corresponding to two

5.2. South-eastern coast of the White Sea From Fig. 6 it can be seen that in the south-eastern coastal area of the White Sea, already mentioned in the MIS 5 chapter, there are four dates falling within MIS 3. Three of them were obtained on three different mollusc species — aragonitic Hiatella arctica, dated at 41.1 ± 1.0 ka and 43.0 ± 4.0 ka, calcitic Modiolus modiolus, dated at 47.5 ± 4.3 ka, and Mytilus edulis, whose shell contains both calcite and aragonite layers, dated at 53.4 ± 5.0 ka using aragonite one. On average, it takes 43.3 ka. These shells were taken from the Lodma outcrop (64.8° N; 41.8° E; see Fig. 4A) at a depth of 15 m in the lowermost part of the 3.5-m-thick marine sand layer, which in turn is overlain by 12-m-thick poorly sorted (diamict) deposits (Molodkov and Raukas, 1998). Another Hiatella arctica shell sample collected in the vicinity of the Zaton settlement from the Z-23 section (65.7° N; 44.3° E) situated in the lower course of the Mezen River was ESR dated at 54.0 ± 4.7 ka. These datings from the White Sea south-eastern coastal area support the opinion of a number of researchers that the deposits of the second large Late Pleistocene transgression, which took place during MIS 3, occur in this region. Finishing consideration of the palaeoenvironmental situation in the most northern part of Eurasia during MIS 3, it should be noted that assumed two cold periods between of three warm waves of interglacial rank during MIS 3 had a more complicated climatic structure than it could be expected. For instance, the detailed palynological analysis and IR-OSL datings of the samples taken from the already above-mentioned Voka section have provided convincing evidence of the occurrence of two severe and two considerably milder climate intervals even in the 13

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Table 5 Results of ESR dating of marine mollusc shells (Nos. 1–26) and IR-OSL dating of sediments (Nos. 27–34), which form three clusters (see Fig. 7) within the MIS 3. Nos.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

RLQG ID

326–103 342–103 341–103 405–039 325–103 286–079 044–047 189-095 A 287–129 189-095 B 358–094 349–073 190–095 021–124 416–119 432–090 402–039 308–060 128–109 015–124 284–079 418–090 285–079 348–073 344–073 375–039 2072–042 2073–042 1755–027 1796–048 1797–048 1998–090 1999–090 2000–090

Field No.

CL5b Sz02-23a Sz02-8a V-1-2 CL5a 98039 S2/L7 LDM-1 1186/8 LDM-1 AO 5/2 1387/1 LDM-1 111/b K-18 HP/E-10-2 Z-23-9 9026/6 613/5 29/a 98034 HP/E-08-1 98038 9/3 D-3/L3 SP07-2026 0917-1, 2.0 m depth 0917-1, 2.5 m depth 1610/1 0411, 5.0 m depth 0411, 11.0 m depth 46766/1, 5.4 m depth 46766/2, 5.8 m depth 46766/3, 6.1 m depth

Location

Dated material

Changeable Lake, SZ Ozernaya River Delta, SZ Ozernaya River Delta, SZ Varzuga River, KP Changeable Lake, SZ Oscar Peninsula, T Novorybnoye, KB Lodma River, WSE Ravicha Peninsula, T Lodma River, WSE Ozernaya River, SZ New Siberia Islands Lodma River, WSE Izvestnyakovaya River, SZ Kamenka River, KP Hatay, SET Mezen River, WSE Mikhailova Peninsula, T Ostantsovaya River, T Ozernaya River, SZ Oscar Peninsula, T Hatay, SET Oscar Peninsula, T Gusinaya River, T Kamenka River, KP Leinstranda shore, BP Sibiryakova Island, SKS Sibiryakova Island, SKS Lena River Delta Cape Shaitansky, YB Cape Shaitansky, YB Šventoji, W Lithuania Šventoji, W Lithuania Šventoji, W Lithuania

Hiatella arctica Hiatella arctica Hiatella arctica Astarte borealis Hiatella arctica Bathyarca glacialis (?) Yoldia lenticula Hiatella arctica Portlandia arctica Hiatella arctica Astarte borealis Astarte borealis (?) Modiolus modiolus Hiatella arctica Astarte borealis Donax (S.) trunculus Hiatella arctica Astarte borealis Hiatella arctica Hiatella arctica Hiatella arctica Spisula (S.) subtruncata Hiatella arctica Hiatella arctica Astarte borealis Hiatella arctica sand sand sand sand sand sand sand sand

Age, ka

24.8 25.9 26.0 26.0 28.4 32.4 40.0 41.1 42.0 43.0 44.1 47.0 47.5 52.0 52.0 53.5 54.0 54.7 55.0 56.0 56.0 56.0 56.4 57.0 58.7 59.0 41.0 45.8 45.6 45.8 57.2 43.7 48.4 48.8

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

2.1 2.5 2.3 2.6 2.7 2.7 6.0 3.8 4.0 4.0 3.6 3.3 4.3 4.0 4.3 4.4 4.7 4.5 18.0 4.0 5.2 4.5 5.2 4.2 4.4 4.7 3.2 3.5 3.5 3.2 3.9 4.0 4.5 6.2

U

Th

K

(ppm)

(ppm)

(%)

1.28 0.38 1.41 2.20 1.22 0.99 2.53 0.90 1.39 0.90 1.96 1.99 0.90 0.70 1.71 0.58 0.63 0.79 0.80 0.60 0.96 0.67 0.99 0.56 1.57 2.12 0.49 0.74 0.45 0.52 0.19 1.22 1.99 1.79

3.60 1.33 4.67 8.07 3.31 4.20 10.50 3.17 3.46 3.17 7.79 13.44 3.17 3.40 9.48 0.00 0.56 2.85 1.34 1.70 4.12 0.07 4.20 2.93 8.67 12.59 2.80 3.50 2.85 3.27 1.95 5.70 4.85 5.86

1.62 0.61 1.97 2.53 1.59 1.05 2.00 1.20 1.76 1.20 2.53 3.00 1.20 0.80 2.46 0.20 0.47 1.00 0.76 0.55 1.03 0.17 1.05 0.96 2.32 3.15 1.58 1.70 1.77 1.71 1.83 1.21 1.18 1.42

SZ – Severnaya Zemlya, KP – Kola Peninsula, OP – Oskar Peninsula, T –Taimyr Peninsula, KB – Khatanga Bay, WSE – south-eastern coastal area of the White Sea, SET – south-eastern Turkey, BP – Brøggerhalvøya Peninsula, Svalbard, SKS – south of the Kara Sea, YB – Yenisei Bay.

principles or/and techniques give the same age, this is powerful evidence that the methods applied work properly and that the ages obtained by these methods are most likely accurate. Comparison of parallel dating results by two main dating methods used in the present study –– ERS and IR-OSL –– shows good consistency of the ages obtained (Fig. 8). All these coincidences support each other giving the dating results a high degree of reliability. This may be further evidence Fig. 7. Three important periods of climate warming (early Karginsky, Malokheta, Lipovka-Novoselovo) and high sea-level stands in the West Siberian Arctic during MIS 3 according to 14C and mollusc shell-based ESR dates (circles). Marine Isotope Stages are after Martinson et al. (1987).

relatively narrow time span between 39 and 33 ka. Besides, two minor short-term warmings, noted on a continuous time scale constructed on the base of the age-depth model at 34.2 ka and between 33.7 and 37.8 ka, were recognized within the last cold phase lasting from 35.3 ka to 32.6 ka (Bolikhovskaya and Molodkov, 2007).

6. The reliability of the dating results Given the importance of accurate and precise age determinations to identify and temporally constrain the most debatable Late Pleistocene palaeoenvironmental events, carrying out of a comparative study was strongly required in order to validate the dating methods used in the present work, and to confirm whether the results derived are reliable and accurate. One of the best ways to check the dating results is the use of different dating methods on the same sediment sample and materials included in it. If two or more dating methods based on different

Fig. 8. Comparison of parallel dating results obtained by two main independent dating methods used in the present study –– ERS and IR-OSL. The figures indicate the ages, in ka. 14

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that the methods applied work properly and that the ages obtained by these methods is, and will most likely be accurate. As an additional test, ESR dating on shells of different mollusc species was used in this work. Table SM1 show some these comparison results in detail. Ages of these shells, as well as everyone else in this work, were calculated assuming early uptake of uranium into the shell (i.e. soon after burial). The validity of this approach is one of the important questions since the radiation generated by this internal uranium is the major contributor to the internal dose rate. Therefore, the age depends on the uranium uptake model. While different U-uptake histories are possible, three main models for the U-uptake into shells and other biominerals are frequently considered: Early U-uptake (EU), where all the U that is measured today was accumulated during mollusc's life and soon after death; Linear U-uptake (LU) assumes that the U has been accumulated continuously in linear manner over burial time; Recent U-uptake (RU), where most U-uptake occurs just before the present. As was shown by several studies (see, e.g., Broecker, 1963; Blanchard et al., 1967; Kaufman et al., 1971; Choukri et al., 1995), U incorporation into mollusc shells occurs mostly during mollusc's life and the very beginning of fossilization. U content in living marine molluscs is normally less than 0.5 ppm (Kaufman et al., 1971). According to our own data on ca 460 shells, U content averages about 0.8 ± 0.7 ppm in 95%, and 0.5 ± 0.4 ppm in 82% of them. It implies that shell samples, in which U is taken up in living state and then very soon after burial, remains, in most cases, almost constant at least during the Late Pleistocene. In this case, internal component of radiation dose forms normally only a smaller part of the total dose. For the shells presented in this study this comprises ca 9% in average. Actual uncertainty for each individual shell has been taken into account when calculating the total age uncertainty. Therefore, the assumption that shell absorbs all its maximal U content measured today rapidly after death (EU model) is generally valid. On the other hand, if the linear uptake (LU) model (Ikeya, 1982) is assumed, it means that U is taken up from the lowest level (i.e. from 0 ppm), and its content increase at a constant rate, culminating in the present-day measured concentration. However, this approach cannot be considered correct because it contradicts the existing data on the initial concentration of uranium in shells, which is not equal to zero, and in fact generally leads approximately to a halving of the of the U content in comparison to the real one when calculating the internal dose rate, and hence leads to overestimation of the age. Consequently, ages that are calculated on the base of an EU uptake model will seemingly result in lower age estimates in comparison with ages calculated based on the assumption of a LU uptake, on the one hand, however, in fact, in much accurate age estimations on the other. Of special interest in this regard are the latest results obtained on the shell samples of four different species (RLQG 449-061A-D) collected from the same sampling point (Fig. 3, Table 6). The results demonstrate that even within the same environment different shells may behave differently: the shell RLQG 449-061A reveal a relatively large amounts of uranium (3.85 ppm), whereas, the other three shells accumulated uranium in much lower (0.26–0.53 ppm, i.e. by about one order of magnitude) concentrations. At the same time, the ages determined for these four shells are turned out to be almost the same. This may indicate that U content in these shells soon after burial was close or equal to the measured one. In doing so, shell RLQG 449061A has accumulated uranium at its early stage of formation in concentrations far exceeding those expected, i.e. less than 0.5–0.8 ppm. This fact of the early incorporation of uranium in such a high concentration is indirectly confirmed also by a significantly higher palaeodose (185 Gy) generated under the influence of the early incorporation of a significant amount of uranium. For comparison, accumulated radiation doses in other three shells are much lower (103–140 Gy) in approximate accordance with Uin ranging from 0.26 to 0.53 ppm. It can also serve as an indirect indication of the different

Table 6 The interspecies mollusc shell-based ESR dating results and some radioactivity data for the dated shells. P is the palaeodose; Uin is the uranium content in the shells. #

ID

P

Uin

ESR age

RLQG

(Gy)

(ppm)

(ka)

1 2 3 4

449-061A 449-061B 449-061C 449-061D

184.7 103.0 139.0 129.3

3.85 0.26 0.37 0.53

76.2 79.3 74.0 70.1

5 6 7

455-052A 455-052B 455-052C

163.5 141.0 185.0

0.74 0.55 1.47

84.2 ± 7.1 93.0 ± 7.8 87.5 ± 7.2

8 9

489-094A 489-094B

134.0 82.7

1.70 0.17

79.2 ± 6.8 78.4 ± 6.4

10 11

491-104A 491-104B

111.8 165.5

0.11 0.87

95.3 ± 8.2 94.2 ± 7.8

± ± ± ±

6.0 6.7 6.3 5.9

ability of the shells to accumulate uranium in the initial period of time even being in the same environmental conditions. A similar picture is observed for other three groups of different shell species, each taken from the same sampling point: RLQG 455-052A-C, 489-094A, B, 491104A, B. A comparison of the dating results show a good conformity of the ages within each group of the shells collected from a particular sampling point(s), with the age data within each group being within the error limits of the method. In the general case, the most reliable indication to understand the uranium behaviour in the shells during their burial history can be considered not only a high or low U content in them, but also results of parallel dating of sediments by different methods, or for different shells species taken from the same sampling point. The correct chronostratigraphic sequence and coincidence of the results for a series of shell or parallel dating, obtained sequentially from different depths, can serve as an indication of the openness/closeness of the dated shells with respect to uranium. In case of violation of the chronostratigraphic sequence of the ages or if there is a significant difference in the results of parallel dating, shell(s) with outlying age(s) may be suspected of having open system behaviour and, therefore, should be dated using the ESR open system (ESR-OS) method (Molodkov, 1988). A few cases, including those considering relatively rare recent U-uptake (RU), have been described, for example, in Alexanderson et al., 2011); Doğan et al., 2012; Molodkov (2012), and in Table 7. Recent U-uptake can be recognized, in particular, by anomaly high U content in shells. We encountered such a case when trying to date a Hiatella arctica shell (RLQG 383-039) taken on NW Svalbard, which was characterized by a very high (19.1 ppm) uranium content. As it was shown in an early work (Molodkov, 1988), the present-day 230 Th activity in the shell may serve as a good indicator of time-averaged U content in the shell. Thermal ionisation mass spectrometry (TIMS) measurements of the shell have shown that the 230Th activity in the shell is low (6.84 * 10−3 Bq). The time-averaged U content in this shell is calculated to be 0.4 ppm, which gives an ESR-OS age of 405 ± 31 ka (Table 7). The TIMS and ESR-OS results obtained indicate a very recent (Late Holocene) uptake of uranium in this shell (for further details see Alexanderson et al., 2011). A Spisula subtruncata shell sample (RLQG 418-090) was collected from marine terraces in the Eastern Mediterranean and dated by ESROS to 56.0 ± 4.5 ka. The results show that this Spisula subtruncata shell behaved nearly as a closed system (see Table 7). The calculated timeaveraged U content in this shell is similar to measured values. Another shell (Donax trunculus, RLQG 432-090) is also ESR-OS dated at 53.5 ± 4.4 ka despite that this shell sample demonstrates much more complicated behaviour, as it seen from Table 7 (for further details see 15

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Table 7 ESR closed (CS) and open (OS) system results for MIS 11 and MIS 3 shells and radioactivity data for enclosing sediment samples. P is the palaeodose; U–Th is the Useries dating result for the shells; Uin–m is the measured uranium content in the shells; Uin–av is the time-averaged uranium content in the shells; U, Th and К are the uranium, thorium and potassium content in the enclosing sediments (adapted from Alexanderson et al., 2011) (RLQG 348-039) and Doğan et al., 2012). Lab. No.

RLQG 348-039 RLQG 418-090 RLQG 432-090

Mollusc

P

ESR-CS

ESR-OS

U–Th

Uin-m

Uin-av

U

Th

K

species

(Gy)

(ka)

(ka)

(ka)

(ppm)

(ppm)

(ppm)

(ppm)

(%)

Hiatella arctica Spisula (S.) subtruncata Donax (S.) trunculus

379.5 71.4 32.3

72.1 ± 5.6 58.6 ± 4.7 43.5 ± 3.5

405.0 ± 31.0 56.0 ± 4.5 53.5 ± 4.4

2.3 55.9 23.9

19.10 3.68 1.53

0.41 3.87 0.78

1.90 0.67 0.58

1.05 0.07 0.00

0.53 0.17 0.20

close to the present one or somewhat higher, judging from the fact that a significant part of marine shells dated to MIS 3 were found on the shores of the Arctic seas, in estuaries and river valleys. It indicates the penetration of sea water into depressions of the relief of coastal land during sea level rise. And this phenomenon bore, likely, a wide geographical extent. Based on good consistency of the results obtained in the present comparative study by both the palaeodosimetric and radiometric dating methods, it can be concluded that there is a good potential to improve our understanding of the Late Pleistocene palaeoenvironmental evolution, with a special focus on the mollusc shellbased electron spin resonance dating method implemented in climatically highly sensitive areas. Now that we are certain of the ages, we have a solid basis for further extended multidisciplinary research.

Doğan et al., 2012). 7. Conclusions On the basis of multi-method approach and using various independent sources of climate and chronostratigraphic information, the main Late Pleistocene palaeoclimatic events were identified in the vast areas of Northern Eurasia from about 150° E to about 20° E. The ESR analysis of mollusc shells collected from layers attributable to warm climate episodes and high sea level stands enabled to identify several palaeoclimatic levels which may be correlated with climatic signals recognizable by various climate proxies including palaeontological (plant macro- and micro-remains), and malacological ones. Climate signals from these proxies show a good agreement with high sea level stands dated directly by ESR and other dating methods applied or referred to in the present study, both numerical and relative (ESR-OS, IROSL, OSA, U––Th, 14C, AMS14C, palaeomagnetic, lithostratigraphic, biostratigraphic; see Molodkov, 2012, for more details). Combined, they make it possible to trace confidently most relevant (or at least most prominent) palaeoclimatic signals within the entire time interval under consideration. The use of the mollusc shell-based ESR method has provided both the independent data on the main climatic changes during the Middle and Late Pleistocene, and their absolute chronology that made it possible the spatial and temporal large-scale intercomparison with climatic signals derived from different geological archives (see, e.g., Molodkov and Bolikhovskaya, 2002, 2006, 2009, 2010). One of the valuable features of the mollusc shell-based ESR dating method is the possibility of self-verification of the results by carrying out additional age determination on several shells of different mollusc species taken from the same sedimentary sample. On the basis of the data obtained, the last interglacial is placed approximately between 145―140 ka and 70 ka. Temporal distribution of the reported ESR ages within MIS 5 demonstrates that the overwhelming majority of the dates (ca. 87%, according to the data considered in the present paper) are concentrated in the second half of MIS 5 in the time range between 110 ka and 70 ka. This fact may lead to the conclusion that the palaeoenvironmental status of MIS 5e is not still fully clear and needs to be carefully explored in the future studies supported by multi-method approaches. Besides, some of the numerous ESR results on marine mollusc shells and IR-OSL results on marine sediments, mainly from Eurasian Arctic area, revealed that the ages obtained are clearly grouped into a sequence of three age clusters within MIS 3: 58.7–52.0 ka, 47.0–40.0 ka and 32.4–24.8 ka. They are correlated fairly well with those derived from calibrated radiocarbon dates obtained in the Siberian Arctic and further to the west. Numerous 14C dates obtained in this area on vegetable detritus, wood and peat samples taken from alluvial deposits, data from marine fauna-bearing deposits, and findings of indicative plant remains and many others climate proxy indicators suggest that climate during three optimum warm periods during MIS 3 was likely as warm as and possibly warmer than today. Proceeding from the factual material represented in the present paper, it can be assumed that sea level, at least trice during MIS 3, was

Acknowledgements During this work I have collaborated with many colleagues, and I wish to extend my warmest thanks to all of them, in particular, Prof. Dr. N. Bolikhovskaya, Prof. Dr. A. Bitinas, Dr. habil. V. Baltrūnas, Prof. Dr. Yu. Bolshiyanov, Dr. A. Damušytė, Prof. Dr. U. Doğan, Dr. V. Doronichev, Dr. L. Golovanova, Prof. Dr. A. Gaigalas, Dr. E. Gusev, Dr. O. Korsakova, Dr. V. Yevzerov, Prof. Dr. P. Möller, Prof. Dr. A. Raukas, Dr. L. Semenova. I'm also grateful to colleagues Tatyana Balakhnichova and Marina Osipova for their contribution to the laboratory work reported here. Special thanks are due to Dr Mathieu Duval and three anonymous reviewers of the manuscript, whose thoughtful comments and suggested revisions were enormously helpful in completing this work. For many years this work was supported by several grants from the Estonian Science Foundation. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.quaint.2019.05.031. References Adamiec, G., Aitken, M., 1998. Dose-rate conversion factors: update. Ancient TL 16 (2), 37–50. Aitken, M.J., 1985. Thermoluminescence Dating. Academic Press, London. Aitken, M.J., 1998. An Introduction to Optical Dating. Oxford University Press. Alekseev, M.N., 1997. Paleogeography and geochronology in the Russian eastern arctic during the second half of the quaternary. Quat. Int. 41/42, 11–15. Alexanderson, H., Eskola, К.О., Helmens, К.F., 2008. Optical dating of a Late Quaternary sediment sequence from Sokli, northern Finland. Geochronometria 32, 51‒59. Alexanderson, H., Landvik, J.Y., Molodkov, A., Murray, A.S., 2011. A multi-method approach to dating middle and late Quaternary high relative sea-level events on NW Svalbard – a case study. Quat. Geochronol 6, 326–340. Andreicheva, L.N., Marchenko-Vagapova, T.I., 2017. New data on the natural environment of the middle and late neopleistocene interglacial periods in the East of the European subarctic region of Russia. Stratigr. Geol. Correl. 25 (6), 679–695. Annual Geochronological Bulletin, 2017. VSEGEI, Saint Petersburg. Arkhipov, S.A., Volkova, V.S., Zolnikov, I.D., Zykina, V.S., Krukover, A.A., Kulkova, L.A., 2005. Chapter 4. West Siberia. In: In: Velichko, A.A., Nechaev, V.P. (Eds.), Cenozoic Climatic and Environmental Changes in Russia, vol 382. Geological Society of America Special Paper, pp. 67–88. Astakhov, V.I., 2006. Chronostratigraphic subdivisions of the Siberian upper Pleistocene. Russ. Geol. Geophys. 47 (11), 1186–1199. Astakhov, V.I., 2013. Pleistocene glaciations of northern Russia—a modern view. Boreas

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