Ice wedges in the Yana Highlands of interior Yakutia – the most continental
region of the Northern Hemisphere – were investigated to elucidate changes
in winter climate and continentality that have taken place since the Middle Pleistocene. The
Batagay megaslump exposes ice wedges and composite wedges that were sampled
from three cryostratigraphic units: the lower ice complex of likely
pre-Marine Isotope Stage (MIS) 6 age, the upper ice complex (Yedoma) and the
upper sand unit (both MIS 3 to 2). A terrace of the nearby Adycha River
provides a Late Holocene (MIS 1) ice wedge that serves as a modern reference
for interpretation. The stable-isotope composition of ice wedges in the MIS 3 upper ice complex at Batagay is more depleted (mean
Interior Yakutia is currently the most continental region of the Northern
Hemisphere. In Verkhoyansk, the cold pole of the Northern Hemisphere,
mean monthly air temperature ranges exceed 60
While biological proxies such as pollen or plant macrofossil remains can be used to reconstruct summer climate, winter conditions in interior Yakutia may be inferred only from ice wedges, which can be found in Middle and Late Pleistocene as well as Holocene permafrost deposits. Ice wedges provide winter temperature information due to their specific seasonality, with frost cracking in winter and crack infilling by snowmelt in spring (Opel et al., 2018). They integrate the stable-isotope composition of cold-season precipitation (Meyer et al., 2015), which in high northern latitudes is mainly a function of temperature (Dansgaard, 1964). However, a knowledge gap exists about the winter paleoclimate of interior Yakutia, because ice-wedge studies in the Yana Highlands – between the Verkhoyansky and Chersky ranges – began only recently (Vasil'chuk et al., 2017). In contrast, other sites in east Siberia (west Beringia) have been extensively studied over recent decades, including the coastal lowlands to the north (e.g. Meyer et al., 2002a, b; Opel et al., 2017b), the Kolyma region to the east (e.g. Vasil'chuk et al., 2001; Vasil'chuk, 2013; Vasil'chuk and Vasil'chuk, 2014) as well as central Yakutia to the south (e.g. Popp et al., 2006) (Fig. 1). Ice-wedge studies have been carried out also in east Beringia, e.g. in Alaska, Yukon, and the Northwest Territories (Kotler and Burn, 2000; Meyer et al., 2010b; Fritz et al., 2012; Lachniet et al., 2012; Porter et al., 2016).
Here, we report cryostratigraphic field observations of ice wedges and composite wedges made during a reconnaissance expedition from 27 July to 5 August 2017 together with stable-isotope data from the Batagay megaslump and Adycha River floodplain (Fig. 1). New radiocarbon ages from organic remains in ice wedges and host sediments help constrain the late Quaternary chronology. The observations and ages elucidate the history of wedge development, past winter temperatures, and continentality starting from the Middle Pleistocene.
Neither study site has been glaciated during at least the last 50 000 years,
and the Batagay megaslump provides access to permafrost formations starting from the
Middle Pleistocene (Ashastina et al., 2017; Murton et al., 2017). The
present mean annual air temperature at Batagay is
The Batagay megaslump (67.58
Overview photograph of the Batagay megaslump showing the study sites of Ashastina et al. (2017) and Murton et al. (2017) and this study. Photograph taken by Alexander Gabyshev in 2015.
The slump exposes Pleistocene and Holocene permafrost formations ranging in age from MIS 6 (or older) to MIS 1 (Ashastina et al., 2017, 2018; Murton et al., 2017). Above slate bedrock and a basal diamicton, four major cryostratigraphic units contain ice wedges and/or composite wedges (Fig. 3, Table 1).
Cryostratigraphic units of the Batagay megaslump as presented by Ashastina et al. (2017) and Murton et al. (2017) and the reconciled stratigraphic framework used in this study and proposed for future studies.
The lowest unit is a pebbly dark sand, 3–7 m thick, with ice wedges at least 2–3 m high and 1 m wide, truncated by a thaw unconformity. This lower ice complex has neither been dated nor sampled previously for wedge ice.
The lower sand unit above reaches about 20 m in thickness and is
composed of yellowish pore-ice cemented (fine) sand with grey horizontal
bands. It contains tall, narrow syngenetic ice wedges up to 0.5 m wide. The
middle part of the unit was dated to
The overlying upper ice complex is 20–25 m thick and dominated by
huge syngenetic ice wedges up to few metres wide and at least several metres
high within silty and sandy deposits. Finite radiocarbon ages from
The upper sand is up to about 20 m thick and consists of pore-ice cemented
brown to grey sand with narrow (
A near-surface layer 1–1.5 m thick of brown sand and modern soil covers the
sequence. This layer was dated to
According to Murton et al. (2017), exposed floodplains of proximal rivers such as the Batagay and Yana, within 2 and 10 km, respectively, of the slump are the assumed major source of the sediments exposed in the headwall, which implies upslope directed transport by wind. Periglacial and nival processes on nearby hillslopes may also have contributed to sediment supply (Ashastina et al., 2017).
For a Late Holocene reference, we studied an ice wedge at the actively
eroding Holocene to recent bank of a small cut-off channel of the
Adycha River (67.66
Cryostratigraphic observations of ice wedges, composite wedges, unit contacts, and sediments at the Batagay megaslump and a terrace of the Adycha River provided a framework for sample selection. A total of six ice wedges and composite wedges from the lower ice complex, the upper ice complex, and the upper sand were described in detail and sampled at two sections of the slump (Figs. 2, 3). Section 1 was studied in a gully in the icy badlands of the slump (Fig. 4), and section 2 was studied in a headwall slope segment (Fig. 5). Composite wedges of the lower sand unit were inaccessible due to dangerous outcrop conditions, and no wedges were observed in the near-surface layer. At the Adycha River, we sampled a single ice wedge.
Schematic cryostratigraphic sketch of section 1, Batagay megaslump.
Schematic cryostratigraphic sketch of section 2, Batagay megaslump.
Ice samples about 4 cm wide, mostly in horizontal profiles, were obtained by chainsaw. Details about the studied ice and composite wedges are given in Table S1 in the Supplement. The ice samples were melted on site in freshly opened standard Whirl-Pak sample bags, and the meltwater filled up 30 mL PE bottles that were then tightly closed and stored cool until stable-isotope analysis.
In addition, water from small streams draining the slump was sampled in
several parts of the slump floor and the main outflow stream (
The stable oxygen (
We picked organic material from our ice-wedge and host sediment samples for
radiocarbon dating at the MICADAS
Samples were packed in tin capsules and combusted individually using an
Elementar vario ISOTOPE EA (elemental analyser). If samples were smaller
than approximately 200
The present study confirms cryostratigraphic observations about ice wedges and composite wedges from earlier studies (Ashastina et al., 2017; Murton et al., 2017) and provides new observations about ice-wedge relationships across contacts between the cryostratigraphic units, as outlined below.
In the lower ice complex, ice wedges are truncated by a thaw unconformity, though the toes of some narrow syngenetic composite wedges in the overlying lower sand unit extend down across the contact (Fig. S1 in the Supplement). A single example of clear ice sharply overlying the shoulder of an ice wedge in the lower ice complex (B17-IW1, Fig. 4) contained brown plant remains and round, few-millimetre diameter organic bodies identified as hare droppings. This ice lacked the foliation characteristic of wedge ice and is interpreted as pool ice. The basal contact of the lower ice complex was not observed.
In the lower and upper sand units, narrow syngenetic wedges vary in apparent width from a few centimetres to about 0.5 m. Width commonly varies irregularly with height along individual wedges, sometimes gradually, sometimes abruptly. Wedge height varies from a few metres to at least 12 m. Wedges tend to be oriented approximately at right angle to colour bands in the sand, such that wedges in the upper sand unit – whose bands dip downslope, parallel to the ground surface – are characteristically subvertical, with their tops inclined downslope (Fig. S2). The wedge infills grade between end members of ice-wedge ice and icy sand wedges, with slightly sandy ice-wedge ice (i.e. composite wedges) as the most common type.
Near the base of the upper ice complex, syngenetic ice wedges, a few metres wide, tend to narrow downwards in the lower several metres of the unit and terminate with irregular or flattish or U- to V-shaped toes over a vertical distance of about 1–3 m (Fig. S2). At the northeast part of the slump headwall, however, the base of the upper ice complex is a sharp contact that cuts down into and truncates bands in the underlying lower sand unit over a vertical distance of up to at least several metres (Fig. 3). As a result, the upper ice complex thickens substantially downslope.
The top of the upper ice complex displays a variety of wedge relationships with the overlying upper sand unit (Fig. S3). Some wide syngenetic wedges terminate along planar to gently undulating contacts that are horizontal to gently dipping and have apparent widths of about 1–3 m; some taper upward into narrow wedges characteristic of the upper sand unit; some taper irregularly upward, marked by shoulders up to about 0.5 m wide; and some end upwards with a narrow offshoot. These changes occur over a vertical distance of about 1–3 m.
A small truncated ice wedge (B17-IW1; 0.5 m wide, 1.1 m high) from the lower
ice complex was sampled in section 1 (Fig. 4) near the slump bottom, about
50 m below the ground surface and about 5 m below the altitude level in the
lower sand unit dated by luminescence to
Co-isotopic
Stable-isotope (
n/a – not applicable
The isotopic composition of the intrasedimental ice (
Radiocarbon dating of twigs, Cyperaceae stems, and roots from a sediment
sample 0.5 m above the truncated ice wedge yielded nonfinite ages of
Radiocarbon ages of organic remains in ice wedges (sample ID includes IW) and host sediments of the Batagay megaslump and at the Adycha River. The samples were radiocarbon dated as gas targets (lab ID ends with 1.1) and graphite targets (lab ID ends with 2.1).
The isotopic composition of intrasedimental ice (
Two large syngenetic ice wedges from the overlying ice-rich upper ice complex in Sect. 2 (Fig. 5) in the southern part of the slump show more
depleted isotope values as compared to the lower ice complex (Table 2,
Fig. 6). Ice wedge B17-IW6 (about 0.5 m wide, sampled about 26 m below
surface, b.s.) shows a mean isotopic composition of
Unidentified plant (bract fragments and roots) and insect (complete pieces
and fragments of elytron) remains from ice wedge B17-IW6 as well as
Cyperaceae remains and roots from host sediment at the lowest studied level
in section 2, about 26 m b.s., both revealed nonfinite ages (
Two narrow sand–ice wedges (composite wedges) and one ice wedge were sampled
from the rather ice-poor upper sand unit in section 2 (Fig. 5) at the
southern part of the slump. Composite wedge B17-IW2 (0.25 m wide) was
sampled 2.6 and 2.8 m b.s., and composite wedge B17-IW3 (0.2 m wide) was
sampled 2.0 m b.s. The stable-isotope composition of the composite wedges
from the upper sand unit differs from those of the lower ice complex and the
upper ice complex (Table 2, Fig. 6). The mean values are
Ice wedge B17-IW4 (about 0.4 m wide and containing a few thin sand veins)
was sampled in the upper sand 2.0 m b.s. and exhibits a more enriched
stable-isotope composition (mean values
The intrasedimental ice (
Two radiocarbon ages (analysed as gas) of small samples of
The samples from the outflow stream draining the slump show average
stable-isotope values of
The samples of summer precipitation at the Batagay megaslump show much more
enriched stable-isotope compositions than the wedges, as expected (Table 2).
Mean values are
Ice wedge A17-IW3 was about 0.3 m wide and sampled about 1.4 m above river
level and about 2 m below the ground surface. Its mean stable-isotope values
are
Rootlets and larch needles from the host sediment were dated to
To date, the exceptionally long record of permafrost history under highly continental climate conditions as exposed in the Batagay megaslump is still not entirely established in terms of its geochronology. Previous studies by Ashastina et al. (2017) and Murton et al. (2017) provide the general differentiation into four main units with ice and/or composite wedges overlain by Late Holocene cover deposits, which are the lower ice complex (pre-MIS 6), the lower sand unit (MIS 6), the upper ice complex (MIS 3–2), and the upper sand unit (MIS 3–2). Thaw unconformities as observed in sampled sections of this study (Figs. 4, 5), but also in previous studies (Ashastina et al., 2017; Murton et al., 2017), question the chronological continuity of the archive. Such (thermo-)erosional features are often observed in late Quaternary permafrost chronologies (e.g. Wetterich et al., 2009) and are most likely caused by intense permafrost degradation during warm stages such as the last interglacial with widespread thermokarst (Reyes et al., 2010; e.g. Kienast et al., 2011) or exceptionally warm interstadials but might also relate to palaeotopography and respective accumulation and erosion areas.
The Batagay megaslump, on account of its huge dimensions and rapid erosion,
holds potential to identify additional records and units in future studies.
With the present study and additional radiocarbon dating, further confidence
of the existing stratigraphy is reached, although the wedge-ice-derived age
information requires careful interpretation. Due to the formation mechanism
and downward directed growth of wedge ice, organic remains from inside the
ice are commonly younger than those of the host deposits at the same
altitude (i.e. sampling depth). The risk of dating redeposited organic
material, that entered into or later froze onto the surface of ice recently
exposed in a slump, is common, in particular in erosional features such as
slump-floor gullies. It can be seen in the truncated lower ice
complex (Fig. 4) where an age of about 16
A distinct unit of wood and plant remains up to 3 m thick and traceable
along the exposure is situated near the top of the lower sand unit and below
the upper ice complex. Radiocarbon dating of this layer revealed an infinite
radiocarbon age of
The expected difference between ice-hosted and sediment-hosted ages of the
same sample depth is clearly seen in the upper ice complex, where
organic remains from the ice wedge are dated to about 25
The new radiocarbon ages from the upper sand unit largely confirm
the assumed accumulation period of this unit from about 36 to 26
Interestingly, no indications for an upper sand unit have been found at the
top of the modern headwall on the upslope part of the slump. Additionally,
there are no significant Holocene deposits exposed at all in the upper part
of the slump. However, carcasses of
The lower ice complex is not dated so far. Given the dating results
of the overlying lower sand unit, it might correspond to the Yukagir Ice
Complex from Bol'shoy Lyakhovsky Island, which has been dated by
radioisotope disequilibria (
The shape and composition of the wedges in the Batagay megaslump correspond to the assumed genesis of the respective units and may also provide paleoclimate information (Table 4). Both the lower and upper ice complex units are characterized by ice wedges up to several metres wide, whereas both the lower and upper sand units contain tall narrow wedges. The former indicates more stable surface conditions with lower rates of sand aggradation and sufficient meltwater supply during the formation of both ice complexes that allowed more horizontal growth of ice wedges. The tall narrow wedges of both sand units, in contrast, point to rapid upslope-directed aeolian deposition of sand and therefore a predominant vertical ice-wedge growth in persistent polygonal patterns. The downslope inclination of narrow syngenetic wedges in the upper sand unit indicates upward growth subvertically at a right angle to the aggrading depositional (hillslope) surface. The tall and narrow wedges in the sand units are similar in size and shape to chimney-like sand wedges in thick aeolian sand sheets in the Tuktoyaktuk Coastlands of western Arctic Canada, attributed to rapid aggradation of aeolian sand and rapid wedge growth (Murton and Bateman, 2007). The composite wedges in the lower and upper sand units imply dry conditions with only little meltwater supply. However, as most ice wedges exhibit a significant sediment content, a steady supply of windblown sediment and a rather thin snow cover during ice-wedge formation is likely.
Summary of past climate in the Yana Highlands. Palaeo-ecological interpretations are from Ashastina et al. (2018) and based on fieldwork in 2014.
At the top of the upper ice complex, ice wedges tend to narrow and partly transform into narrow composite wedges of the upper sand unit (Fig. S3). This indicates a rather gradual change of the deposition regime towards higher sedimentation rates and possibly lower meltwater supply due to drier conditions. Interestingly, the polygonal pattern has not been affected by this change, as indicated by the consistent frost-cracking positions. The upward transition of wedges from the upper ice complex to upper sand unit, however, was interrupted episodically by thaw, producing a number of thaw unconformities at different depths. In contrast, the erosional event truncating the lower ice complex seems to have changed the polygonal pattern in which the lower sand unit has been deposited.
A major erosional surface attributed to gullying by water flowing down a palaeo-hillslope is inferred from the large concave-up lower contact of the upper ice complex in the northeast part of the headwall (Fig. 3). Numerous gullies on the present hillslope near the megaslump are indicated on satellite images between 1968 and 2010 (Kunitsky et al., 2013), which suggests that gullies are characteristic landforms of such terrain under present environmental conditions. Water is supplied to such gullies by snowmelt in spring and rainfall and melt of ground ice in summer. Erosion of the underlying lower sand unit provided substantial accommodation space for development of an unusually thick upper ice complex with wide ice wedges above it. Stratigraphically, this erosional surface is at a similar level as the upper woody debris layer that is thought to be of last interglacial age (Ashastina et al., 2017, 2018), which in turn suggests that the erosion also probably took place during warm (i.e. interglacial) conditions.
Unfortunately, no comprehensive modern precipitation stable-isotope data are
available for the Yana Highlands. The nearest sites with available data are
Zhigansk, about 500 km to the west (Kurita, 2011);
Tiksi, about 500 km to the northwest (Kloss, 2008); and Yakutsk, about
650 km to the south (Kloss, 2008; Papina et al., 2017). These
sites show similar isotopic characteristics for the cold season (October to
March). Mean
The Yana Highlands' ice-wedge stable-isotope data form two major clusters
(Fig. 6, Table 2). The regression of the first cluster (cluster 1) plots
below but mainly parallel to the GMWL (
Generally, the high
The regional palaeoclimatic implications for the Yana Highlands drawn from
our ice-wedge
No palaeoclimatic information based on ice-wedge isotopes is available yet
from Batagay for the MIS 4 (Zyryan) and MIS 2 (Sartan) stadials. On Bol'shoy
Lyakhovsky Island of the New Siberian archipelago, ice wedges from both
periods show
When interpreting ice-wedge stable isotopes in terms of absolute
palaeotemperatures, one has to keep in mind that the isotopic composition of
the oceanic moisture source has changed over glacial–interglacial
timescales, with enrichment in
In general, the inferred lower winter temperatures meet the expectations of an increased continentality during the Late Pleistocene cold stages as well as partly drier conditions indicated by the formation of composite wedges. Palaeo-ecological analysis (plant macrofossil remains, pollen, and beetles) of Batagay megaslump deposits (Ashastina et al., 2018) indicates generally much drier conditions compared to recent times (Table 4), in particular during the cold stages. Tree and beetle indicators imply warm summers for most units, whereas macrofossil remains indicate cold to very cold winters for the upper ice complex. These patterns are generally in line with our ice-wedge interpretation and support the hypothesized higher continentality.
To assess the climate and continentality across much of west Beringia during
the MIS 3 interstadial, we compare our Yana Highlands ice-wedge
stable-isotope data to a dataset of 17 other ice-wedge sites (Figs. 1,
7). Most of the wedges are from the Yedoma Ice Complex
(Schirrmeister et al., 2011b, 2013), which is
widely distributed in east and central Siberia. We selected all available
ice wedges per study site that provide both
Comparison of stable-isotope data from Siberian ice wedges attributed to the MIS 3 interstadial (about 50 to 30 kyr), comprising data from the western Laptev Sea (Magens, 2005), the Lena River Delta (Schirrmeister et al., 2003, 2011a; Wetterich et al., 2008; Thomas Opel, unpublished data), the central Laptev Sea (Meyer et al., 2002a; Hanno Meyer and Thomas Opel, unpublished data; Schirrmeister et al., 2017), the New Siberian Islands and the Laptev Strait (Lutz Schirrmeister and Hanno Meyer, unpublished data; Wetterich et al., 2014; Opel et al., 2017b), the Kolyma Lowland (Strauss, 2010), the Yana Highlands (this study; Vasil'chuk et al., 2017), and central Yakutia (Lutz Schirrmeister, unpublished data; Popp et al., 2006). The new data from Batagay are marked in red. Further information is given in Table S3.
The peculiarity of the Yana Highlands is also apparent in the
Map of mean
Map of mean
Little is known about
The mean
The
Our stable-isotope data clearly show that winter temperatures in the Yana Highlands during MIS 3, represented by the upper ice complex (Yedoma) at Batagay, and the Holocene were distinctly lower than at other ice-wedge study sites in coastal and central Yakutia. This indicates the persistence of enhanced continentality of the Yana Highlands region during at least part of the Late Pleistocene in west Beringia and during the Holocene. The stable-isotope data from narrow composite wedges of the upper sand unit (MIS 3–2) and an old ice wedge from the lower ice complex (pre-MIS 6) are less indicative and require additional studies.
High-resolution systematic sampling and dating now need to be carried out
for all cryostratigraphic units of the Batagay permafrost sequence to
validate our findings of increased continentality during MIS 3 and the
Holocene, to improve the temporal resolution of the Batagay ice-wedge
record, and to elucidate the palaeoclimatic history from other time slices.
Of particular interest are the lower and uppermost parts of the upper ice complex, likely representing MIS 4 and 2 stadials, as well as more detailed
studies of the yet-undated lower ice complex. To establish reliable
ice-wedge chronologies for the upper ice complex and upper sand unit,
radiocarbon dating should include macro remains, dissolved organic carbon,
and
The new ice-wedge data presented in this paper as well as
the ice-wedge data used for spatial comparisons are available at
The supplement related to this article is available online at:
TO initiated and designed the present study and wrote the paper with contributions by the other co-authors. TO, JBM, and KA sampled and described ground ice and host sediments, supported by PPD and VB. HM carried out stable-isotope analyses and supported the interpretation. HG and GM conducted the radiocarbon dating. FG provided GIS analysis and maps. SW and LS collected ice-wedge data for comparison and supported data analysis and interpretation. All co-authors contributed to the final discussion of the results and interpretations.
The authors declare that they have no conflict of interest.
We would like to thank Erel Strutchkov for support in fieldwork, as well as Luidmila Pestryakova and Waldemar Schneider for the organization of export of samples. Mikaela Weiner supported stable-isotope analysis. Thomas Opel and Sebastian Wetterich acknowledge funding from the German Research Foundation (DFG grants OP217/3-1, OP217/4-1, and WE4390/7-1, respectively). Frank Günther was supported by ERC no. 338335 and by DAAD with funds from BMBF and the EU's Marie Curie Actions Programme, REA grant agreement no. 605728 (P.R.I.M.E.). We acknowledge three anonymous referees and the editor, Denis-Didier Rousseau, for constructive comments, which helped us to improve the manuscript.
This research has been supported by the Deutsche Forschungsgemeinschaft (grant nos. OP217/3-1, OP217/4-1 and WE4390/7-1) and the European Research Council (PETA-CARB, grant no. 338335).The article processing charges for this open-access publication were covered by a Research Centre of the Helmholtz Association.
This paper was edited by Denis-Didier Rousseau and reviewed by three anonymous referees.