To reconstruct palaeoclimate and palaeoenvironmental conditions in the
northeast Siberian Arctic, we studied late Quaternary permafrost at the
Oyogos Yar coast (Dmitry Laptev Strait). New infrared-stimulated luminescence
ages for distinctive floodplain deposits of the Kuchchugui Suite
(112.5
The wide tundra areas of the northeast Siberian Arctic lowlands are characterized by deep permafrost that results from cold continental climate conditions in west Beringia during the late Pliocene and Pleistocene when this region remained non-glaciated (Schirrmeister et al., 2013). Ice complex (IC) deposits formed in polygonal tundra environments with syngenetic ice-wedge growth during different periods of the late Quaternary in non-glaciated Beringia (Tumskoy, 2012; Schirrmeister et al., 2013). The most prominent IC of late Pleistocene age is called Yedoma IC (MIS4-3), but older IC formations are known such as the Yukagir IC of MIS7 age (Schirrmeister et al., 2002a) and the Buchchagy IC of MIS5 age (Wetterich et al., 2016). The ice-rich permafrost in this area contains huge amounts of ground ice. Syngenetic ice wedges are the major component. Vertically foliated ice wedges are formed by polygonal frost cracking due to thermal contraction of soils in winter and the subsequent filling of cracks with water in spring (e.g. Leffingwell, 1915; Lachenbruch, 1962). Snowmelt is the main source of the water that enters the frost crack, quickly refreezes there due to the negative ground temperatures, and forms a vertical ice vein. Depending on climate and site-specific environmental conditions minor sources may include varying proportions of densified snow or hoar-frost accretion (St-Jean et al., 2011; Boereboom et al., 2013). The periodic repetition of frost cracking and ice-vein formation results in ice-wedge growth in width and, if synchronous to sedimentation at the surface (syngenetic ice wedges), also in height.
Ice wedges may serve as paleoclimate archives (e.g. Mackay, 1983;
Vaikmäe, 1989; Meyer et al., 2002b; Vasil'chuk, 2013), in particular in
regions with a limited availability of climate archives. They can be studied
by means of stable isotopes (Mackay, 1983). Due to rapid freezing in the
frost crack preventing fractionation (Michel, 1982), the isotopic composition
of each single ice vein is directly linked to atmospheric precipitation, i.e.
winter snow, and, therefore, indicative of the climate conditions during the
corresponding cold season. However, isotopic fractionation in the snow cover
might impact the stable-isotope composition of wedge ice as well but is
considered to be negligible for the purpose of this study. Hence, the stable
isotope ratios of oxygen (
In contrast, intra-sedimental ice forming cryostructures (pore ice as well as segregated-ice lenses and layers) in syngenetic permafrost originate from the freezing of soil moisture in the seasonally thawed active layer. Soil moisture is fed by varying proportions of different water sources such as summer rain and winter snow as well as meltwater of the thawed active layer ice (Mackay, 1983; Vaikmäe, 1989). Additionally, soil moisture is subject to evaporation processes and numerous freeze–thaw cycles before it enters the perennially frozen state. Hence, the stable isotope composition of pore and segregated ice has undergone several fractionation processes until the final freezing during permafrost aggradation. It therefore cannot be interpreted straightforwardly as a climate proxy (Wetterich et al., 2014, 2016). Nevertheless, the isotopic composition of pore and segregated ice has been successfully interpreted in terms of general climate trends such as long-term warming or cooling (Schwamborn et al., 2006; Dereviagin et al., 2013; Porter et al., 2016).
In the Siberian Arctic Laptev Sea region, comprehensive studies of ice-wedge and partly pore- and segregated-ice stable isotopes of stratigraphic units accessible in coastal exposures have been carried out in the last years at the Mamontova Khayata section of the Bykovsky Peninsula (Meyer et al., 2002a) and on the south coast of Bol'shoy Lyakhovsky Island close to the Zimov'e River mouth (Meyer et al., 2002b). Selected stratigraphic units have been studied at Cape Mamontov Klyk (Boereboom et al., 2013), at Bol'shoy Lyakhovsky Island (Wetterich et al., 2011, 2014, 2016), at the Oyogos Yar Coast (Opel et al., 2011), and in the Lena River delta (Schirrmeister et al., 2003b, 2011a; Wetterich et al., 2008; Meyer et al., 2015). To verify the obtained palaeoclimate results on different timescales and to assess their spatial and temporal representativity, additional extensive ground-ice stable-isotope records are needed.
As for all climate archives, reliable chronologies are crucial for
ground-ice-based palaeoclimate studies. However, direct dating of ice wedges
(Vasil'chuk et al., 2000) is challenging, in particular for the pre-Holocene.
Mostly, there is only little particulate organic material for radiocarbon
dating preserved in ice wedges. Therefore, air-bubble CO
Overview map of the Laptev and East Siberian seas region (top) and detailed map the Oyogos Yar coast with extent of Yedoma Ice Complex remnants and the position of the coastal exposure presented in Fig. 2 (bottom, background image: false colour infrared RapidEye mosaic August 2010; modified after Günther et al., 2013).
To address the issue of temporal and spatial representativity of ground-ice stable-isotope records, we present in this paper new data from different late Quaternary stratigraphic and chronological units at the Oyogos Yar Coast of the Dmitry Laptev Strait. Based on new geochronological and cryolithological information we discuss the cryostratigraphy. We interpret the variability in new ice-wedge and pore- and segregated-ice stable isotope data and discuss the relevance of ground-ice stable isotopes in terms of paleoclimate and environmental history. We relate our ground-ice stable-isotope data to previously published data from Bol'shoy Lyakhovsky Island in order to generate a Dmitry Laptev Strait ground-ice isotope record and compare it to large-scale climate changes. Furthermore, we shed light on the potential of ice-wedge isotope data as tools for stratigraphic correlations between different study sites.
The Dmitry Laptev Strait connects the Laptev and East Siberian seas (Fig. 1), and its coasts have been the subject of geographical and geological research for more than 100 years (Bunge, 1887; von Toll, 1897; Romanovskii, 1958c, a, b). The north shore of the Dmitry Laptev Strait, i.e. the south coast of Bol'shoy Lyakhovsky Island, represents one of the best-studied Quaternary permafrost sites in northeast Siberia. To reconstruct the environmental dynamics of west Beringia since the mid-Pleistocene, extensive studies of the exposed frozen sediments and ground ice have been carried out (Arkhangelov et al., 1996; Kunitsky, 1996; Meyer et al., 2002b; Schirrmeister et al., 2002a, 2011b; Andreev et al., 2004, 2009, 2011; Tumskoy, 2012; Wetterich et al., 2009, 2011, 2014, and 2016 and references therein). In addition, the permafrost exposures of the Oyogos Yar mainland coast at the south shore of the Dmitry Laptev Strait have been studied, but less extensively (Ivanov, 1972; Gravis, 1978; Konishchev and Kolesnikov, 1981; Kaplina and Lozhkin, 1984; Tomirdiaro, 1984; Nagaoka et al., 1995; Wetterich et al., 2009; Kienast et al., 2011; Opel et al., 2011; Schirrmeister et al., 2011b; Rudaya et al., 2015, and references therein).
Interglacial and interstadial warm periods promote extensive permafrost thaw and subsequent surface subsidence mainly due to ground-ice melt. Such processes, termed thermokarst (e.g. Kaplina, 2009), have substantially influenced the study area during the last interglacial and since the late glacial–Holocene transition. Thawed and refrozen deposits are called taberite or taberal deposits (Kaplina, 2009) and underlie thermokarst-lake deposits (Fig. 2). Due to varying deposition regimes over time as well as spatially and temporally variable patterns of permafrost degradation (and also aggradation) during warm periods, permafrost sequences are often not continuous. This often complicates geochronological interpretations as deposits of consecutive late Quaternary periods may not be found superimposed upon each other but at laterally different positions and in different altitudes.
Synopsis of the stratigraphic units exposed on the Dmitry Laptev Strait according to the latest compilation by Tumskoy (2012) and updates from this study. Strata in bold font were not found at the Oyogos Yar coast (OY) but are described from Bol'shoy Lyakhovsky Island (BL). MIS refers to Marine Isotope Stage and SIW to syngenetic ice wedges. Radiocarbon ages are given as mean calibrated ages b2k (before 2000 CE). IRSL: infrared-stimulated luminescence; AMS: accelerator mass spectrometry; LGM: Last Glacial Maximum.
Further information is given in the following studies from both sides of the Dmitry Laptev Strait: a – this study; b – Schirrmeister et al. (2011b); c – Andreev et al. (2009); d – Wetterich et al. (2009); e – Opel et al. (2011); f – Wetterich et al. (2011); g – Wetterich et al. (2014); h – Andreev et al. (2004); i – Wetterich et al. (2016); j – Schirrmeister et al. (2002a).
Following Tumskoy (2012), perennially frozen deposits exposed at the Dmitry Laptev Strait span from MIS7 to the Holocene (Table 1). Yedoma Ice Complex deposits (MIS 3–2) are preserved in sections elevated up to 35 m a.s.l., while thermokarst basins up to 15 m a.s.l. high exhibit lacustrine and palustrine deposits of the late glacial and Holocene periods. Lacustrine deposits of the Krest Yuryakh Suite, which are commonly assigned to the last interglacial (Wetterich et al., 2009; Kienast et al., 2011; Tumskoy, 2012) as well as the Buchchagy Ice Complex (MIS 5e–b; Wetterich et al., 2016) are preserved either below Yedoma Ice Complex or below late glacial to Holocene thermokarst-lake and thermokarst-basin palustrine deposits (Fig. 2). The stratigraphic position of floodplain deposits assigned to both the Kuchchugui Suite (MIS6; Tumskoy, 2012) and the Zyryanian (MIS4) (Andreev et al., 2004, 2009) is still under debate.
The main landscape elements, i.e. Yedoma Ice Complex uplands as well as thermokarst basins (alasses) may be cut by thermo-erosional gullies and river valleys and are subject to rapid coastal erosion processes (Günther et al., 2013) that form steep coastal bluffs. Depending on the prevailing type of coastal erosion, thaw slumps may affect Yedoma Ice Complex deposits, shaping a thermo-terrace with thermokarst mounds (remaining sedimentary polygon-centre fillings after melting of ice wedges surrounding an ice-wedge polygon; baydzherakhs) in front of a steep wall with exposed ice wedges (Fig. 3).
The work presented here is based on material and observations of a 1-day
reconnaissance trip in 2002 (Schirrmeister et al., 2003a) and a follow-up
4-week expedition to the Oyogos Yar coast in 2007 (Schirrmeister et al.,
2008b) during which about 6 km of the Oyogos Yar coastline were studied (between 72.683
After overview surveys along the coastal bluffs ice wedges and sediment profiles from all exposed stratigraphic units were selected for extensive investigations and firstly described, photographed, and sketched.
Photographs of selected outcrops and ice wedges of different units. CW means composite wedge.
After cleaning the exposures from thawed material and debris, horizons were cryolithologically described (Murton and French, 1994; French and Shur, 2010; Murton, 2013) and samples were taken by axe and hammer in sub-profiles. The weight of the frozen sample compared to the weight of the sample after oven-drying was used to calculate the gravimetric ice content of the sediments, expressed as weight percentage (wt %) (Van Everdingen, 1998). Values higher than 100 wt % indicate ice oversaturation. From thawed sediment samples with supernatant water, we took samples for analysing the stable isotope composition of pore and segregated ice.
A handheld drilling machine (HILTI TE 5 A) equipped with a core bit was used
to obtain frozen sediment cores (150–290 cm
In total, we sampled 44 ice wedges from all exposed stratigraphic units: 7 in
2002 and 37 in 2007 (Fig. 2). Stable isotope samples from ice wedges were
taken by chain saw, by axe, or by ice screws (in horizontal profiles covering
the entire width of the studied ice
wedges) along the ice wedge's growth direction (i.e. the profiles
perpendicularly cut frost-cracking direction and near-vertical individual ice
veins). The sampling resolution varied from about 1 and about 30 cm between
different ice wedges (but remained constant within individual ice wedges),
depending on fieldwork logistics, sampling tools, and specific research
questions. The samples were either melted on site with meltwater stored in
tightly closed 30 mL PE bottles or transported as blocks in frozen state to
the cold laboratory of the Alfred Wegener Institute (AWI) in Potsdam for
sub-sampling. The melted samples were stored in a cool place
(
Organic remains enclosed in ice-wedge samples (unidentified plant remains
and lemming droppings) as well as plant remains from sediment samples were
radiocarbon dated using the accelerator mass spectrometry (AMS) facilities
at the Leibniz Laboratory for Radiometric Dating and Stable Isotope Research
(Kiel University, Germany) (Grootes et al., 2004), CologneAMS (University of
Cologne, Germany) (Dewald et al., 2013; Rethemeyer et al., 2013), and
Poznań Radiocarbon Laboratory (Adam Mickiewicz University, Poznań,
Poland) (Goslar et al., 2004). Conventional
The cores were processed for quartz and feldspar at target grain size
fractions of 20–40, 40–63, 63–100, and 90–160
Aliquots of 2 and 1 mm diameter reflect the trade-off between low grain
number per aliquot (reduced averaging of inter-grain variations) and
sufficient luminescence signal intensities. The IRSL signals of feldspars
were measured using a TL/OSL DA-20 reader (Bøtter-Jensen et al., 2003)
equipped with a
Signals were stimulated at 870 nm (IR diodes, 125
The equilibrium technique was used to prepare ice-wedge as well as pore- and
segregated-ice samples for stable-isotope analysis. Stable oxygen (
For this study, we considered only ice wedges with a clear stratigraphic relation to one of the studied units and at least three samples. Exceptions were made only for a small Holocene ice wedge at the top of the Yedoma Ice Complex (two samples) as well as for narrow modern ice wedges, i.e. single veins or groups of ice veins representing the youngest ice-wedge growth stage (one to two samples). Following this, from the 44 sampled ice wedges, we considered in the following only 28. Partly, ice-wedge data were only interpreted as groups (i.e. recent ice-wedge parts and modern ice veins).
In several cases, stable-isotope data, preferentially those from samples at
the edge of ice wedges showed clear signs of post-genetic fractionation
processes owing to exchange processes between wedge ice and surrounding
sediments. The respective data are characterized by distinctly elevated
Similar to ice wedges, we considered only pore- and segregated-ice stable-isotope data with a clear attribution to one of the studied units and only units with at least three data points.
Gravimetric ice content (minimum, mean, and maximum values; standard deviations) of the sediments, expressed as weight percentage (wt %) (Van Everdingen, 1998).
Within this paper, we focus on our extensive studies of cryostratigraphy and, in particular, ground-ice stable isotopes. For more detailed results and discussion of distinct sediment profiles, we refer to Wetterich et al. (2009), Schirrmeister et al. (2011b), and Wetterich et al. (2016). We also adopted most radiocarbon ages derived from sediment samples from these papers.
Eight cryostratigraphic units were distinguished during fieldwork at the Oyogos Yar coast (Table 1, Fig. 2). The studied ice wedges were assigned to five of the units based on field observations.
The oldest unit studied at the Oyogos Yar coast is represented by floodplain deposits related to the Kuchchugui Suite. The deposits consist of brownish-grey laminated silty sands and contain peat inclusions and in situ grass roots. Unit 1 varies in ice content (31–108 wt %; mean 52 wt %; Table 2). Cryostructures include structureless cryostructure (no ice inclusions visible by naked eye) as well as horizontal ice layers (10–20 mm thick, 50–100 mm apart) with wavy lenticular and irregular reticulate ice lenses (1 mm thick). A dark-brown peat soil layer covers the sequence which includes coarse (> 1 mm thick) irregular reticulate ice lenses between ice layers (10–20 mm thick and 50 mm apart).
IRSL samples Oy7-07-01 (Unit I) and Oy7-08-25 (Unit III) with all
palaeodose and dose rate parameters used for final age calculation (
Syngenetic ice wedges in Unit I are rather small and can be divided into two types. The first type occurs in the lower part of this unit and consists of 0.3 to 0.75 m wide composite wedges (sand–ice wedges) composed of alternating ice and sand veins (1 to 10 mm wide). They exhibit rounded truncated heads and are buried by the upper part of Unit I. The second type is represented by intersecting multistage ice wedges, i.e. composite wedges that transform upwards into regular syngenetic ice wedges 0.5 to 1 m wide. Their clean, transparent wedge ice contains many vertically oriented air bubbles (1-5 mm in diameter) and pronounced ice veins of 3–6 mm thickness. Additionally, epigenetic ice wedges from the overlying Unit II penetrate into the Kuchchugui deposits.
Radiocarbon ages and calibrated ages (95.4 % probability) of sediment samples from Oyogos Yar coast. NaN indicates that the calibration failed and no calibrated age is available.
IRSL analyses of feldspars (63–100
Radiocarbon dating of moss peat from the upper horizon sampled directly above composite wedge Oya IW1 revealed a non-finite age of > 44.5 cal kyr b2k for the leached residue and a mean age of 45.2 cal kyr b2k for humic acid (Table 4).
Unit II represents the Buchchagy Ice Complex. It is represented by 6 to 8 m thick brown to grey silty sands with peat inclusions and two distinct peaty horizons up to 1 m thick about 3 m apart (Wetterich et al., 2016).
The lower peaty horizon consists of brownish blue grey silty sand with numerous peat inclusions up to 300 mm in diameter. Ice layers (30 mm thick) and wavy lenticular cryostructures (1–2 mm thick, 10–15 mm apart) in between are typical. The ice content varies between 25 and 112 wt % (Table 2) in the mineral part while the peaty deposits are ice oversaturated (113–793 wt %). Brownish grey silty sand covers the lower peaty horizon and contains fewer peaty remains and some twigs. Horizontal ice layers (5–20 mm thick, 50–100 mm apart) and curved lenticular and layered cryostructures (up to 2 mm thick) are observed. The upper peaty horizon is similar to the lower one as described above.
Unit II contains syngenetic ice wedges 2 to 4 m wide and several metres deep that penetrate into Unit I. Partly they are truncated and buried below grey loam with many peat inclusions, likely representing the lower peat horizon. The wedge ice of rather dirty yellowish-grey colour contains numerous mineral inclusions and air bubbles of 1–5 mm in diameter. Ice-vein thickness is about 3-5 mm.
The only dating results available are infinite radiocarbon ages
(> 51 kyr BP and > 49 kyr BP; Table 4) for the two
distinctive peaty layers (Wetterich et al., 2016). At the southern coast of
Bol'shoy Lyakhovsky Island, opposite to the Oyogos Yar coast, the respective
peaty horizons of the Buchchagy Ice Complex show infinite radiocarbon ages
as well, whereas radioisotope (
Unit III refers to deposits of the Krest Yuryakh Suite that is commonly related to the last interglacial. Unit III comprises two types of deposits, both associated with thermokarst-lake development. The first type represents a succession of an ancient lake margin. It consists of bedded dark grey to greyish brown silts, partly rippled. Mollusc shells as well as plant detritus layers and plant inclusions (partly wood fragments) are common. The upper part shows decreasing plant detritus content and numerous mollusc shells. The cryostructure is irregular reticulate (1–2 mm thick). The second type represents lacustrine sediments filling ice-wedge casts above taberal deposits. The lacustrine deposits consist of bedded grey clayey sandy silts with brown peat lenses and alternating plant detritus layers. The cryostructures are layered (1 mm thick) and oriented parallel to the sedimentary bedding. For both kinds of deposits, the ice content varies from 15 to 66 wt % (mean 36 wt %; Table 2). The underlying taberal deposits of grey silts exhibit a structureless cryostructure.
No syngenetic ice wedges were found, but toes of younger epigenetic ice wedges, presumably related to Unit IV, were partly present.
IRSL analyses of sample Oy7-08-25 could only be based on a few aliquots (
A mean radiocarbon age of 48.3 cal kyr b2k was obtained from leached residue of wood in sample Oya-3-11 from an ice-wedge cast (Table 4).
Unit IV represents the Yedoma Ice Complex that constitutes of
grey–brown sandy silts with small (20
According to mean radiocarbon ages the Yedoma Ice Complex was deposited in the time period between about 49.4 and 36.3 cal kyr b2k (Table 4). Several age reversals have been found; hence no clear age–depth relationship could be established.
In the vast thermokarst basin, Unit V consists of taberal Yedoma Ice Complex deposits, which thawed below a thermokarst lake and refroze after lake drainage or desiccation. Unit V is represented by light-grey silts with very little plant detritus. The top of Unit V consists of a paleosol layer with twigs and peat inclusions. The cryostructure is wavy and layered (1 mm thick, 50–150 mm long, 10–20 mm apart) and the gravimetric ice content is around 40 wt % (Table 2). Whereas no syngenetic ice wedges were found, the epigenetic toes of ice wedges (related to Unit VII) were partly observed.
Radiocarbon ages of taberal Yedoma Ice Complex deposits revealed mean ages of about 46.0 and 40.1 cal kyr b2k (Table 4), which correspond to the period of Yedoma Ice Complex deposition, not the time of thermokarst-lake development.
The late glacial to Holocene sequence starts with about 2 m lacustrine deposits of Unit VI, partly filling ice-wedge casts. The cryostructure is layered and the gravimetric ice content is 30–70 wt % (Table 2). Unit VI is characterized by alternating bedding of silty fine sand and plant detritus and also contains wood fragments and mollusc shells. The lake deposits are covered by a 20 to 30 cm thick peat horizon. No syngenetic ice wedges were found, but the lower parts of ice wedges of Unit VII were.
Radiocarbon ages and calibrated ages (95.4 % probability unless otherwise indicated) of organic remains in Holocene ice-wedge samples of Unit VII in sequence C. All samples were taken at a similar depth (about 1 to 1.5 m below surface).
Radiocarbon dating revealed a late glacial age (means about 18.1 to 12.7 cal kyr b2k) of the thermokarst-lake deposits (Table 4).
The palustrine deposits of Unit VII consist of about 3 m of ice-rich greyish sandy silts, partly containing peat lenses and a pronounced brown peat horizon of 30 cm thickness and wood remains (20–30 cm in diameter). The cryostructure is layered (1 mm thick, 20–30 mm long, 50–100 mm apart). The gravimetric ice content is 50–180 wt % (mean 119 wt %; Table 2). The recent polygonal surface of the thermokarst basin is mirrored by widely distributed and actively growing syngenetic ice wedges. They are up to 3.5 m wide and up to about 8 m high with toes that reach the taberal deposits of Unit V. Their ice is mostly transparent to milky white but sometimes also dirty grey due to a higher density of sediments and organic matter. Single ice veins are 1 to 10 mm wide. The ice of the most recent ice-wedge parts as well as of the modern rejuvenation stages (up to 8 cm wide and up to 20 cm high) is more milky white due to a higher number of small air bubbles.
The palustrine deposits in the thermokarst basin accumulated over the Holocene (about 11.5 to 3.6 cal kyr b2k) with the pronounced peat horizon dated to about 9.3 cal kyr b2k (Table 4). Organic remains in ice-wedge samples indicate syngenetic ice-wedge growth over the late Holocene, i.e. since about 2.0 cal kyr b2k (Table 5).
Unit VIII constitutes the up to 1–2 m thick Holocene cover on top of the Yedoma Ice Complex. Unit VIII was only found in places representing the fillings of small initial thermokarst ponds (bylary). The deposits consist of brownish grey loam with numerous peat inclusions. The cryostructures are layered (10–20 mm thick, 20–50 mm apart) and irregular reticulate (1 mm thick). The gravimetric ice content is 54 to 118 wt % (mean 92 wt %; Table 2). The transition to the unfrozen uppermost active layer contains a 20–30 mm thick ice layer built of vertical ice needles. Unit VIII contains small, milky white syngenetic ice wedges less than 1 m wide whose toes penetrate into the Yedoma Ice Complex ice wedges (Fig. 3a). Milky white ice veins likely of Holocene origin were observed in the upper parts of two huge ice wedges of the Yedoma Ice Complex.
The Holocene cover deposits were radiocarbon dated to the early Holocene with mean ages of about 11.0 to 8.9 cal kyr b2k (Table 5).
The described cryolithological units and the respective ice wedges have been studied in three generalized stratigraphic sequences A to C (Fig. 2).
The first sequence A represents the stratigraphy of the eastern part of the study area (A in Fig. 2). It consists of Kuchchugui floodplain deposits of Unit I, Buchchagy Ice Complex (Unit II), and Krest Yuryakh thermokarst-lake deposits (Unit III), discordantly overlain by Yedoma Ice Complex (Unit IV) and Holocene cover (Unit VIII).
The second sequence, B, in the central part of the study area (B in Fig. 2) comprises Kuchchugui floodplain deposits (Unit I) covered by Buchchagy Ice Complex (Unit II) and Krest Yuryakh thermokarst-lake deposits (Unit III). The main part consists of Yedoma Ice Complex deposits (Unit IV) overlain by the Holocene cover (Unit VIII).
Sequence C in the western part of the study area (C in Fig. 2) is represented by about 10 m high thermokarst-basin outcrops and comprises Krest Yuryakh thermokarst-lake deposits (Unit III), covered by taberal Yedoma Ice Complex (Unit V) as well as late glacial to Holocene thermokarst-lake (Unit VI) and thermokarst-basin palustrine deposits (Unit VII).
Stable isotope (
The basic parameters of the studied ice wedges of different units as well as their stable-isotope data are presented in Table 6. The attribution of ice wedges to certain stratigraphic units is based on field observations and stable isotope data. Additionally, pore- and segregated-ice stable isotope data are also summarized in Table 6. Overall, both ice-wedge as well as pore- and segregated-ice stable-isotope data show a high variability over time.
The composite wedges exhibit mean
In the composite-wedge parts (Oy7-03 IW1
Pore- and segregated-ice stable-isotope values scatter between
One syngenetic ice wedge was assigned to the Buchchagy Ice Complex (Oy7-07
IW1). The mean stable-isotope values (
Pore- and segregated-ice stable-isotope values scatter between
In total, 10 ice wedges of the Yedoma Ice Complex were studied at different
altitude levels from 1.5 to about 35 m a.s.l. Close to the ground surface at
the top of the Yedoma Ice Complex as well as at the slope to the thermokarst
basin, two ice wedges (Oy7-06 IW2 and Oy7-08 IW3) showed evidence of
Holocene frost-cracking activity, i.e. Holocene ice veins. The respective
samples, i.e. those with
The overall mean stable isotope values of the Yedoma Ice Complex ice wedges
are
The pore- and segregated-ice stable-isotope values exhibit an enormous
scatter and vary between
Single
The ice-wedge stable-isotope samples from Unit VII can be assigned to four
groups: (1) older, lower sections; (2) high-resolution vertical profiles in
the younger, upper section; (3) recent, i.e. central, parts of upper
sections; and (4) modern ice veins, i.e. rejuvenation stages. The first two groups
show quite similar isotopic compositions, with mean values between
The pore- and segregated-ice stable-isotope values group between
The Holocene-influenced ice-wedge parts of the Yedoma Ice Complex show
relatively little isotope variations. Mean values vary between
The pore- and segregated-ice stable-isotope data are more enriched and
spread between
We identified 8 cryostratigraphic units at the Oyogos Yar coast, whereas 12 were found at the opposite southern coast of Bol'shoy Lyakhovsky Island (Table 1). They basically represent three main landscape types which have undergone different permafrost aggradation and degradation patterns that varied over time and in space: ice complex deposits, flood plain deposits, and thermokarst-lake and thermokarst-basin palustrine deposits. Hence, often a clear attribution of deposits and ice wedges to distinct units and their relation to each other is challenging, in particular for pre-Yedoma Ice Complex units which cannot be dated by the radiocarbon method.
At the Oyogos Yar coast we did not find deposits of the Yukagir Ice Complex
(
The radiocarbon ages of > 44.5 and 45.2 cal kyr b2k (Table 4)
point to a much younger age of Unit I in the eastern sequence A (Fig. 2)
but are close to the limit of the radiocarbon method. The stratigraphic
position of Unit I below deposits of the Buchchagy Ice Complex of Unit II
supports the age information obtained from IRSL dating, which is why we
discard the radiocarbon age.To exclude a possible attribution of Unit I in
the eastern part of the study region (sequence A) to the Zyryanian stadial
known from Bol'shoy Lyakhovsky Island (Table 1), additional age control is
greatly needed, if possible also using other approaches, e.g. uranium decay
series in ground ice (Ewing et al., 2015). The Zyryanian at Bol'shoy
Lyakhovsky Island shows similar sedimentary characteristics, pointing to
floodplain deposits (Andreev et al., 2004), but contains toes of Yedoma Ice
Complex ice wedges instead of the small truncated composite wedges found for
Unit I at Oyogos Yar. Kuchchugui ice wedges of Unit I at Oyogos Yar are
smaller and have isotopic values that are less depleted compared to ice
wedges on Bol'shoy Lyakhovsky Island (see Sect. 5.2). The
The Buchchagy Ice Complex (Unit II) is known from both sides of the Dmitry
Laptev Strait and represents ice complex formation of MIS5 age. It developed
above the Kuchchugui floodplain deposits from
The Krest Yuryakh thermokarst-lake and thermokarst-basin palustrine deposits
(Unit III) indicate warm temperatures, at least in the summer season. These
deposits filled ice-wedge casts in taberal deposits; they were encountered
within wide thermokarst basins which had developed in the degraded ice-rich
Buchchagy Ice Complex of Unit II and are therefore younger. In this paper we
present the first direct age determination for the Krest Yuryakh Suite at
the Dmitry Laptev Strait derived from deposits within an ice-wedge cast. The
IRSL for Unit III (102.4
In contrast to Bol'shoy Lyakhovsky Island, there is neither dating nor sedimentary evidence of floodplains of the Zyryanian stadial (MIS4) at Oyogos Yar (Table 1), even though the presence of such Zyryanian stadial deposits in the eastern section A cannot be ruled out and requires age control as part of future studies.
The Yedoma Ice Complex (Unit IV) started forming at Oyogos Yar from at least 50 kyr b2k, confirming earlier findings (Gravis, 1978; Kaplina and Lozhkin, 1984; Tomirdiaro, 1984; Nagaoka et al., 1995). Radiocarbon ages from Bol'shoy Lyakhovsky Island indicate even earlier ice complex formation since about 60 cal kyr b2k (Table 1) (Wetterich et al., 2014). Even though the youngest age from Unit IV at Oyogos Yar is dated to 36.3 cal kyr b2k (Table 4) a longer Yedoma Ice Complex formation can be assumed as indicated by Bol'shoy Lyakhovsky Island, where ages of 33.5 to 32.5 cal kyr 2bk have been found for the Molotkov interstadial stratum of the Yedoma Ice Complex (Andreev et al., 2009; Wetterich et al., 2014) (Table 1). It remains unclear whether the Yedoma Ice Complex formation at Oyogos Yar continued further until the Sartan stadial (or even until the end of the Pleistocene as is known from Cape Mamontov Klyk (Schirrmeister et al., 2008a) and Bykovsky Peninsula (Schirrmeister et al., 2002b)) or whether the accumulation regime has changed. On Bol'shoy Lyakhovsky Island the prevailing Yedoma Ice Complex formation moved from plain to erosional landforms such as river valleys where the Sartan stadial stratum of the Yedoma Ice Complex was formed at least between about 30 and 26.7 cal kyr b2k (Wetterich et al., 2011). At Oyogos Yar potential equivalent ice complex deposits in river valleys have not been found along the studied coastline section but may exist further east in the valley of the Kondrat'eva River.
Consequently, it is not possible to estimate whether there was substantial permafrost degradation on the top of the Oyogos Yar Yedoma Ice Complex during the postglacial warming and the duration of the potential erosional gap. The Holocene cover (Unit VIII) has been developed since 11 cal kyr b2k according to our data, which mirror Bol'shoy Lyakhovsky Island conditions (Andreev et al., 2009; Wetterich et al., 2014).
Dated thermokarst-lake deposits of Unit VI prove widespread permafrost degradation, i.e. the development of vast thermokarst basins during the last deglaciation. Thermokarst started at Oyogos Yar around 18 cal kyr b2k, about 3 kyr earlier than reported for Bol'shoy Lyakhovsky Island (Andreev et al., 2009; Wetterich et al., 2009). The lacustrine phase (Unit VI) of the studied Oyogos Yar thermokarst-basin development ended around 13 cal kyr b2k. This confirms results of earlier studies (Gravis, 1978; Kaplina and Lozhkin, 1984; Tomirdiaro, 1984; Nagaoka et al., 1995). In contrast, on Bol'shoy Lyakhovsky Island the lacustrine phase of the studied thermokarst basins ended only around 8 cal kyr b2k (Andreev et al., 2009; Wetterich et al., 2009). However, thermokarst development depends on manifold factors such as (micro-)climate, relief, substrate, ice content, and drainage. Hence, these deviations are regarded as minor given the fact that they fit into the general temporal pattern of thermokarst formation during the last deglaciation (Walter et al., 2007).
The palustrine phase (Unit VII) covers the entire Holocene, includes the distinctive peat horizon (about 9.3 kyr b2k), and continues until today. The youngest radiocarbon age of the palustrine deposits (3.6 kyr b2k) and the series of radiocarbon ages of actively growing ice wedges covering the last two millennia (Table 5) (Opel et al., 2017a) as well as the shape of the ice wedges point to a predominantly lateral ice-wedge growth in the late Holocene. This indicates rather stable surfaces in the thermokarst basin with low accumulation rates.
For all Oyogos Yar units the mean stable-isotope compositions of ice wedges
are more depleted than those of pore and segregated ice. Mean
The enormous scatter of stable isotopes of pore and segregated ice within the
Yedoma Ice Complex (Unit IV) (Fig. 5) leads to the interpretation that it
does reflect mainly secondary fractionation processes rather than climate
conditions. In particular the
In contrast, more detailed and constrained climate variability is traceable from ice-wedge stable-isotope values, in particular when a reliable age control is available. This is indicated by ice-wedge co-isotopic slopes closer to the GMWL (Table 6) and recent precipitation (Meyer et al., 2002b; Opel et al., 2011). Ice-wedge stable isotopes allow us to detect winter temperature variability on different timescales from the glacial–interglacial scale to the intra-unit scale (such as within Unit IV Yedoma Ice Complex) up to the centennial scale (such as within an ice wedge).
In the following we discuss the ice-wedge stable-isotope data from Oyogos Yar obtained within this study. In a first step we relate our data to those from Bol'shoy Lyakhovsky Island published previously (Meyer et al., 2002b; Wetterich et al., 2009, 2011, 2014, 2016) to enhance the palaeoclimatic understanding for the entire Dmitry Laptev Strait region (Fig. 6). However, one has to keep in mind that apart from a few ice wedges in Units IV (only Bol'shoy Lyakhovsky) and VII (Oyogos Yar and Bol'shoy Lyakhovsky), all ice wedges are dated only indirectly by age determinations of host sediments. Due to the downward transfer of snowmelt and corresponding stable isotope signatures into the frost cracks, which are several metres deep, ice wedges are always younger than host sediments at the exact same altitude. It is so far not possible to determine the corresponding age offsets between host sediments and studied syngenetic ice wedges for different units. Depending on deposition rates, estimated age offsets of a few hundred to a few thousand years seem to be most reasonable to us.
For our climate interpretation we use a tentative classification of mean ice-wedge
The oldest ice wedges at the Dmitry Laptev Strait are from the Yukagir Ice
Complex on Bol'shoy Lyakhovsky Island, dated to 200.9
The occurrence of composite wedges in Kuchchugui floodplain deposits (Unit I) at Oyogos Yar points to a high-accumulation regime that delivers
sufficient material (e.g. wind-blown sand) to cause the high sediment
fraction in these composite wedges. The latter is supported by the fact that
the studied composite wedges (Oy7-03 IW4 and Oya IW1) are buried and
truncated (Fig. 3f). Their convex thaw surfaces indicate a
deepening of the active layer or a local water body that leads to a melting
of the composite wedges' surface before deposition of new sediments. Their
mean
Even more severe climate conditions can be inferred from two Kuchchugui ice
wedges on Bol'shoy Lyakhovsky Island. Mean stable isotope values of about
The existence of the Buchchagy Ice Complex (Unit II) indicates cold-stage
climate conditions during its formation on both sides of the Dmitry Laptev
Strait. Very cold and stable winter climate conditions during ice-wedge
formation are confirmed by ice-wedge
The thick deposits of the Yedoma Ice Complex (Unit IV) with their huge ice
wedges formed under long-term cold-stage conditions during MIS3
(Molotkov interstadial stratum) from about 60 to about 32 cal kyr b2k. Mean
The Sartan stadial stratum of the Yedoma Ice Complex on Bol'shoy Lyakhovsky
Island formed at least between about 30 and 26.7 cal kyr b2k (Table 1) and
exhibits the lowest stable-isotope values (mean
Significantly enriched stable-isotope values were found in Holocene Oyogos Yar ice wedges in both cover deposits of the Yedoma Ice Complex (Unit VIII)
and palustrine sediments of thermokarst basins (Unit VII) (Fig. 6). Mean
The warming from the last glacial to the Holocene is also accompanied by a
slight increase (about 2 ‰ for Oyogos Yar; about
1 ‰ for Bol'shoy Lyakhovsky Island) in mean
Mean
Whereas no dating results for ice wedges from the cover deposits exist,
radiocarbon ages from ice wedges of thermokarst-basin palustrine deposits on
both sides of the Dmitry Laptev Strait indicate that the derived temperature
information can be attributed mainly to the last two millennia. It is even
possible to go into more detail. The recent parts of actively growing ice
wedges at Oyogos Yar exhibit more enriched mean
The present ice-wedge stable-isotope record of the Dmitry Laptev Strait is
based on new data from Oyogos Yar and earlier data from Bol'shoy Lyakhovsky
Island. It contains ice-wedge isotopes from seven stratigraphic units and
covers roughly 200 kyr (Table 1, Fig. 6). Other regional ice-wedge-based
reconstructions such as those from Cape Mamontov Klyk (Boereboom et al.,
2013), the Lena River delta (Wetterich et al., 2008), Bykovsky Peninsula
(Meyer et al., 2002a), or Duvanny Yar at the Kolyma River (Vasil'chuk et al.,
2001) only date back to MIS3 or MIS4 and/or contain only single
stratigraphic units (Streletskaya et al., 2015). Therefore, we additionally
compare our data to the continuous NGRIP ice-core
The cold to very cold temperatures of Yukagir Ice Complex ice wedges inferred from stable-isotope values (age about 200 kyr b2k) do not fit in with the modelled warm winter temperatures of this period. In contrast, warm summer climate conditions inferred from pollen (Andreev et al., 2004) confirm modelled summer temperatures. Considering an age offset between ice wedges and the dated peat horizon, it is likely that the ice wedges are younger and correspond to decreasing modelled winter temperatures between 200 and 180 kyr b2k (Fig. 7).
The very cold to extremely cold temperatures reflected in stable isotopes of
Kuchchugui ice wedges (Unit 1, dated to about 110 to 100 kyr b2k) do not
fit in with the modelled winter temperatures but correspond to a minimum in winter
(i.e. NDJFAM (Meyer et al., 2015)) insolation around 100 kyr b2k (Laskar et
al., 2004) (Fig. 7). Modelled cold winter periods were
either earlier (around 130 kyr b2k) or later (around 90 kyr b2k) both
accompanied by cold summer temperatures (Fig. 7) as also inferred from
Kuchchugui pollen (Andreev et al., 2011). In contrast, the NGRIP ice-core
record shows a colder period around 110 kyr b2k that might correspond to the
very low ice-wedge
The inferred very cold winter temperatures of the Buchchagy Ice Complex ice wedges also do not correspond to a corresponding cold period in modelled winter temperatures but may fit in with the cold period around 110 kyr b2k seen in the NGRIP ice core. A younger age can be ruled out as the Krest Yuryakh warm period with enhanced thermokarst processes including melting of ice wedges attributed to the Buchagy Ice Complex is centred about 102 kyr b2k (MIS5c).
All in all, our data show a rather unexpected climate variability during MIS5. Surprisingly cold winter conditions during MIS5d were succeeded by the peak interglacial warming during MIS5c leading to widespread thermokarst formation with vast thermokarst lakes.
Both the NGRIP ice-core record and the modelled temperatures (summer and
winter) show high-frequency climate fluctuations during MIS3. Even
though not temporally resolvable, the altitudinal variability in ice-wedge
isotopes at Oyogos Yar (Fig. 5) and Bol'shoy Lyakhovsky indicates that
similar climate variations likely affected the Dmitry Laptev Strait
region as well. Similar fluctuations within the same range of
The extremely cold winter temperatures reflected by ice-wedge stable
isotopes of the Sartan stadial Yedoma Ice Complex on Bol'shoy Lyakhovsky
correspond well to the Last Glacial Maximum cold period in the NGRIP ice-core record and the modelled temperatures (Fig. 7). Interestingly, such
extremely depleted
The substantial warming from the last glacial to the Holocene as captured by
modelled temperatures and the NGRIP record is also found in our Dmitry
Laptev Strait record as well as other regional ice-wedge stable-isotope
records such as Cape Mamontov Klyk (Boereboom et al., 2013), the Lena River delta (Wetterich et al., 2008), and the Bykovsky Peninsula (Meyer et al.,
2002a). The detected
The slightly increasing mean
The NGRIP record and the modelled summer temperatures show a cooling after the Northern Hemisphere early Holocene insolation maximum. In contrast, our ice-wedge data and the modelled winter temperatures verify a general Holocene winter warming trend with the highest temperatures today (Fig. 7), which is likely related to seasonal insolation and greenhouse gas forcing (Meyer et al., 2015; Opel et al., 2017a).
The present study summarizes comprehensive
stable-isotope data from ice wedges interpreted as
winter climate proxy from the Oyogos Yar mainland coast in addition to and in
comparison to pre-existing data from Bol'shoy Lyakhovsky Island in the
northeast Siberian Arctic covering the last 200 kyr. Seven distinct
generations of ice wedge are distinguished, confirming coldest winter climate
conditions during MIS5 and MIS2, warmest conditions during MIS1, and winter
climate instability during MIS3. Since dating ice wedges directly is
challenging and chronostratigraphic correlation to surrounding frozen
deposits holds difficulties, which are even more complicated by different
dating approaches beyond the radiocarbon limit (such as luminescence dating
and radioisotope disequilibria dating), further method development is needed
in ice-wedge dating and in understanding of the chronological relation
between ice wedges and host sediments (i.e. age offsets). However, in the
course of the present study, valuable geochronological data were obtained by
the IRSL dating of the deposits of the Kuchchugui stratum to MIS5d
(112.5
The data presented in this paper are available at PANGAEA
(
Thomas Opel initiated and designed the present study and wrote the paper with contributions by the other co-authors. Thomas Opel, Alexander Y. Dereviagin, Hanno Meyer, Sebastian Wetterich, and Lutz Schirrmeister sampled and described ground ice and sediments. Hanno Meyer carried out stable-isotope analyses and supported interpretation. Margret C. Fuchs conducted IRSL dating and provided interpretation. Sebastian Wetterich and Lutz Schirrmeister provided stratigraphic information and interpretation. All co-authors contributed to the final discussion of the results and interpretations obtained and have approved the final version of the paper.
The authors declare that they have no conflict of interest.
The study presented here is part of the Russian–German System Laptev Sea cooperative scientific effort. We thank our colleagues who helped during fieldwork and subsequent discussions as well as the staff of the AWI Potsdam stable isotope laboratory. We thank Andrey Ganopolski (PIK Potsdam, Germany) for providing climate-model output and Frank Günther for providing the detailed map in Fig. 1. Markus Richter (TU Dresden, Germany) and Ingrid Stein (TU Bergakademie Freiberg, Germany) supported IRSL dating. This study contributes to the project “Ice wedges as winter climate archives – towards high-quality chronologies, advanced process understanding and new paleoclimate records” (Deutsche Forschungsgemeinschaft grant no. OP217/3-1). We thank Mikhail Kanevskiy, Trevor Porter, and Go Iwahana for thorough reviews and Julian Murton for valuable comments that greatly helped to improve the paper. The article processing charges for this open-access publication were covered by a Research Centre of the Helmholtz Association. Edited by: T. Cronin Reviewed by: M. Kanevskiy, G. Iwahana, and T. J. Porter