Batagay megaslump: A review of the permafrost deposits, Quaternary environmental history, and recent development

The Batagay megaslump, in the Yana Uplands of northern Yakutia, Russia, is the largest known retrogressive thaw slump in the world. The slump exposes a remarkable sequence of Ice Age permafrost deposits that record the interaction of colluvial, eolian and periglacial processes on a hillslope episodically forested during the last 650 ka or more in response to climate variability on glacial–interglacial timescales. Numerous bones, teeth, and occasional carcasses of Pleistocene and Holocene mammals have been recovered from the permafrost. The megaslump developed over the course of several decades in three stages: (1) gullying, (2) thaw slumping, and (3) megaslumping. After disturbance to the taiga vegetation cover in the 1940s–1960s, a hillslope gully formed by the early 1960s. The gully initiated thaw slumping along its central part during the 1980s, with the slump enlarging to megaslump (>20 ha) proportions during the 1990s. By 2019, the area of the slump had reached about 80 ha and its headwall above the slump floor was up to about 55 m high. The main geomorphic processes of slump growth are headwall ablation and thermal erosion, producing a distinctive terrain of icy badlands on the slump floor. Though much of the megaslump is rapidly growing at present, it will probably stabilize eventually as an irregular terrain characterized by sandy ridges and sand‐filled elongate depressions formed by degradation of the badlands. Comparison of the Batagay megaslump with megaslumps from northwest Canada reveals several similarities and differences in terms of their geomorphology, permafrost deposits, and Quaternary history.

Slumps whose surface area exceeds 20 ha have been distinguished as megaslumps. 3 The Batagay slump exceeds 80 ha in area and is the largest known megaslump in the world. It is growing rapidly and exposes a remarkable sequence of Ice Age permafrost deposits that extends back to the Middle Pleistocene, 4 attracting international media interest as the so-called Batagaika crater (e.g., 5,6 ). Based on the available literature, the present article reviews the regional setting and outlines the permafrost deposits and their record of late Quaternary environmental history. It focuses on the geomorphological description of the landforms and processes associated with the slump, and on the chronology of slump development.

| REGIONAL SETTING
The Batagay megaslump is in the Yana Uplands of northern Yakutia, Russia ( Figure 2). The uplands lie to the east of the Orulgan Range (part of the Verkhoyansk Range) and to the west of the Chersky Range. They reach elevations of 1,000-2,000 m above sea level (asl) and form the southern region of the Yana River basin.
Tectonically, the Batagay region is within the Verkhoyansk foldand-thrust belt, which is composed of Carboniferous to Jurassic clastic sedimentary rocks deposited on the eastern passive continental margin of the Siberian Craton. 8 The rocks in the Batagay-Verkhoyansk area are mostly Triassic and Jurassic in age. 9 Rejuvenation of Late Cretaceous thrust systems contributed to the modern mountain relief of the Verkhoyansk fold belt. Several geological faults occur near Batagay, and earthquakes of up to magnitude 4-5 are recorded, as this is part of the Verkhoyansk earthquake region.
Climatically, the Verkhoyansk region has been considered the "Cold Pole" of the Northern Hemisphere because recorded winter air temperature can drop to nearly À68 C. 10 The climate is strongly continental. At Batagay, mean winter (December-February) air temperature over the period 1988-2017 was À40.0 C, mean summer (June-August) air temperature was 13.7 C, and mean annual air temperature The Yana Uplands have remained free of glacier ice for at least the last 50,000 years (citations in Murton et al. 2017 12 ) and belonged to the unglaciated subcontinent of Beringia. Beringia was characterized by extensive silt, sand, and ice deposits frozen in permafrost beneath the land surface and covered by a mosaic of steppe and tundra plants that fed a rich animal fauna-including woolly mammoth, bison, and horse. 13 Dust storms sweeping across Beringia deposited huge amounts of silt and sand, burying the remains of many plants and some animals, which became frozen and preserved in the permafrost. 14,15 Beringia included not only lowlands 16 Glushkova,7 are indicated. Red dashed box shows the location of Figure 4 The unconsolidated deposits mapped at and within several kilometers of the megaslump are dominantly sandy. 19 The upland (above about 200 m asl) to the northeast of the Batagay River valley is underlain by sand of the Tabalakh Suite (about 95 m thick), attributed to lacustrine and alluvial deposition during the Pliocene. In the Batagay River valley itself, sand, gravel, and loam (several meters to a few tens of meters thick) have been assigned to the first, second, and third terraces above the floodplain. The sand and gravel are attributed to deposition by alluvial and solifluction processes during the Pleistocene.

| Cryostratigraphy and chronology
In outline, seven cryostratigraphic units have been identified from permafrost exposed in the slump headwall and floor (Table 1; (pIR-IRSL) dating of feldspar, accelerator mass spectrometry-based 36 Cl/Cl dating of wedge ice and radiocarbon dating of organic material ( 4 and references therein). These dating results for the ancient permafrost exposed in the Batagay thaw slump (Table 1) highlight its potential as a paleoenvironmental archive reaching back to at least the early Middle Pleistocene.

| Ground ice
The permafrost exposed in the Batagay thaw slump is very ice-rich, which promotes its high vulnerability to thaw. Kizyakov et al. 18  The wedge-ice volume (WIV) of the cryostratigraphic units shows distinct differences and largely defines the overall ice content per unit. 18 The WIV of the upper ice complex is highest, reaching 70% and resulting in up to 87% total volumetric ice content, whereas the WIV of 56% in the lower ice complex results in 79% total ice content.   This is a modern endemic of northeastern Siberia, occurring in relict, cold, very dry, exposed, and snowless steppes. 35 20 Murton et al., 4,12 Opel et al., 21 and Vasil'chuk et al. 22,23 See text, Figure 13  Beringia and found in the majority of northeast Siberian fossil assemblages. 36 Entomological remains of the Batagay permafrost sequence indicate that the site was mainly inhabited by steppe insects, while tundra species were scarce: the Arctic tundra invertebrate group is absent here, the wet tundra group comprises less than 2%, and the portion of the dry tundra group ranges between 1 and 7%. 17 By contrast, the proportion of tundra invertebrates in permafrost sequences of the coastal lowlands reaches 80%. 37 The assemblages of plant macro-and microfossils together with invertebrate remains reveal fewer tundra taxa during the Late Pleistocene at the Batagay site than records from the present coastal areas. 30-32,34,37

| Ancient DNA and biomarkers
In a pilot study, Courtin et al. 38  Kirgilyakh and Khatyngnakh ( Figure 11). The images were used by Savvinov et al. 41 to measure landform area and geometry through time ( Figure 12).
The rate of areal growth of the landform from hillslope gully through thaw slump to megaslump has been remarkably uniform ( Figure 12). Just one distinct inflexion is apparent during the early 1980s, when the growth rate increased somewhat during the transition from gully to thaw slump. Overall, the uniform growth rate of the slump is consistent with general monotonic increases in summer air temperature and summer precipitation shown in Figure 3c and d.
Disturbance to vegetation and soil resulted from deforestation and off-road vehicle activity. 41  floor is concave. 18 The channel evacuates some sandy sediment from the slump floor into the adjacent Batagay River, though the rates and volumes remain to be determined.
Based on the development of the Batagay megaslump over just several decades, as outlined above, it is clear that the Batagay megaslump is a prime example of rapid or abrupt permafrost thaw. 45 In contrast to gradual permafrost thaw, abrupt permafrost thaw such as the development of the Batagay megaslump and related greenhouse gas emissions are not accounted for in climate models. 46

| Headwall ablation
Headwall ablation occurs by solar radiation and sensible heat transfer, leading to ground-ice melt, permafrost thaw, and thus rapid slope retreat. Retreat rates depend on diurnal and seasonal weather conditions, ground-ice distribution and slump geometry. The high ice content of the permafrost deposits exposed in the slump-reaching up to 87 vol% in the upper ice complex unit-favors high rates of headwall retreat. 18 Based on the distribution of the exposed cryostratigraphic horizons the headwall can be subdivided into three types of slope:

| Thermal erosion
Thermal erosion occurs where flowing water melts ground ice by the combined effects of heat conduction and convection, and then mechanically erodes newly released sediment. 47 Thermal erosion of ice-rich permafrost can form gullies. 48,49 The main source of water is melting ground ice exposed in the headwall and along the sides of slump-floor gullies during summer. Additionally, spring snowmelt and summer rainstorms may contribute episodically to discharge. Discharge is greatest during snowmelt, hot summer days, or rainstorms.
The fine sediment eroded and suspended within turbulent ephemeral streams on the slump floor colors the water dark gray. Bedload comprises pebble-to cobble-sized clasts reworked from Quaternary sediments and eroded from the slate bedrock. The sediment released from the slump episodically dams the Batagay River, forming a temporary lake that drains abruptly.

| Future development
The Batagay megaslump is likely to grow wider and longer before eventually stabilizing. 18 Observations of the ground-ice stratigraphy and rates of headwall retreat over recent years suggest that the current expansion will continue for at least the next several years to decades but will be spatially limited by the distribution of near-surface ice-rich sediments up the hillslope. Over the next few years or more, the most active part of the slump will almost certainly unite with the separate gully to the north of the slump, which may change the slump dynamics. The icy badlands terrain in the slump floor are likely to become progressively more muted as erosion and deposition tend to promote slope decline. Local reactivation of parts of the slump floor and sides, however, are likely to follow high-magnitude erosion events, for example triggered by intense summer rainfall or rapid snowmelt. Ultimately, the terrain will not recover its initial predisturbance form, because this would require renewed growth of an ice complex to form new ice-rich permafrost, which last occurred during the Pleistocene. Instead, the terrain is likely to assume an irregular form characterized by sandy ridges and sand-filled elongate depressions formed by the degradation of the icy badlands.