Retrogressive thaw slumps

"Several types of massive ground ice, including ice wedges and intrasedimental ice, observable within the cliff wall of a retrogressive thaw slump located on the southern coast of Herschel Island. This exposure has a headwall that measures in excess of 22 m high and is over 1.3 km long. This cliff exposure has been increasing in size on a yearly basis since it was first measured in 1950 from air photographs". Credit Dave Fox

As ice-rich permafrost and glacial terrain thaws, the melting ground ice causes the land surface to collapse through a series of processes resulting in the formation of an irregular land surface, called thermokarst, composed of hummocks and hollows.[6]

Retrogressive thaw slumps are the "most active geomorphic features" of thermokarst permafrost terrain".[7] Permafrost dates back hundreds of thousands of years; thermokarst and its features—such as retrogressive thaw slumps—which are slope failures, have been initiated by terrain disturbance associated with clear-cutting forests, the construction of seismic lines and roads.[8]

The retrogressive thaw slump forms on massive ice or ice-rich permafrost, which is often covered in a layer of tundra vegetation under which a layer of peat may lie.[9] The RTS surface is convex and is located on the shoulder of the hillslope.[10]

The most thawing occurs on south- and west-facing slopes.[11] Ballantyne describes how, as scarp ice thaws it causes rapidly-evolving retrogressive slope failure or slumping. This landslide "exposes a fresh face" of ice-rich permafrost.[11] As thawing takes place, the ice-rich, steep, erosional headscarp retreats as it collapses. An active layer of basal sediment accumulates[9] flowing down a low-gradient slump floor. This flows downslope as it "collapses to the base of the exposure".[12] The floor or base of the retrogressive thaw slump is covered in then sediment—mudflows and braided hills.[9] As the headscarp progressively retreats, the slump floor extends.[9]

As the slopes thaw, the ice-rich permafrost is exposed and turns into a mud slurry.[12] Thermokarst processes may cause lakes to enlarge, peatlands to collapse and landslides or thaw slumps to develop."[12][lower-alpha 1] "Retrogressive thaw slumps are among the most active geomorphological features in permafrost terrain."[13]

A 2009 study classified slumps as active, stable, and ancient. An active slump is one that has a clearly-defined headwall and bare areas; and a stable slump is one that has clearly-defined boundaries and is completely covered in vegetation. The headwall relief of an ancient slump is a subdued scar on the terrain that is covered in tundra vegetation.[14]

RTS morphology comprises a vertical headwall, an inclined headscarp, a floor filled with flow deposits, and a lobe that conveys thawed sediments downslope.[1] RTS morphology comprises the headwall, headscarp; a floor, and a lobe.[15][1] The vertical headwall is steep and ice-rich; the downsloped headscarp is a "low-angled scar zone" composed of thawed slurry; the lobe is a tongue of debris in active slumps which is composed of saturated materials that flowed downslope.[15]

Retrogressive thaw slumps are slope failures due to abrupt thawing of ice-rich permafrost.[16] They have also been called ground-ice slumps,[17] thermocirques,[18] tundra mudflows,[19] retrogressive flow slides,[20] and bi-modal flows.[21][13] These terms are no longer recommended by the National Snow and Ice Data Center (NSIDC). [21]

Yedoma are deposits of highly organic-rich[22] and ice-rich permafrost with ice content representing from 50 to 90% of its volume.[23] Much of the yedoma deposits have been frozen since 10,000 years ago, in the Pleistocene age.[22] As of 2011, the Yedoma domain covered 1,000,000 square kilometres (390,000 sq mi; 100,000,000 ha; 250,000,000 acres) of the northern permafrost zone,[24] mostly in Siberia, including northern Yakutia, and also in Alaska, and in the north of Canada, including the Yukon Territories.[25][22][lower-alpha 2]

A retrogressive thaw slump is a slow landslide caused by thawing yedoma.[26][27] Because yedoma deposits are ice-rich, they are "especially prone to rapid-thaw processes" and "highly vulnerable to disturbances such as thermokarst and thermo-erosion processes".[28][lower-alpha 3]

According to the definition of the Multi-Language Glossary of Permafrost and Related Ground-Ice Terms compiled by the International Permafrost Association (IPA)'s Terminology Working Group, "retrogressive thaw slumps consist of a steep headwall that retreats in a retrogressive fashion due to thawing, and a debris flow formed by the mixture of thawed sediment and meltwater that slides down the face of the headwall and flows away."[29]

Retrogressive thaw slumps are forms of the permafrost or glaciated regions and may be found in the Northern Hemisphere and the Tibetan Plateau, from the Himalayas to northern Greenland, in northern Canada and Alaska.[16][30][31] RTSs "are commonly found on the banks of northern rivers and lakes and along the arctic coast, especially where undercutting is active."[13]

Alaska

Arctic Coastal Plain, Teshekpuk Lake, Alaska by Brandt Meixell, USGS

Russia

Batagaika Crater

Two-thirds of Russia's territory consists of permafrost terrain—which represents that largest share in the world.[26] It is here in central Yakutia in the East Siberian taiga in Verkhoyansky District of the Sakha Republic that the largest retrogressive thaw slump, the Batagaika Crater is located.[26] Because of its massive size—it is one-kilometre-long and 100 metres (330 ft) deep and growing annually[38]—the Batagaika Crater, has been called a "megaslump"—a large retrogressive thaw slump.[39][38] The crater is a feature of a 'thermokarst depression'; in contrast to other thermokarst depressions in permafrost terrain—including those found in the north of Canada—the Batagaika Crater is much deeper—from "two-to-three times deeper".[27]

In 2016, University of Sussex's scientist and professor, Julian Murton, led an expedition to undertake a pilot study of the Batagaika Crater—"one of 'most important' sites in the world for the study of permafrost".[27] The local residents living near the crater refer to it as a "gateway to Hell."[26] The Yakutian people, believe that the crater is the door way to under world, one of three worlds, which include the upper and middle worlds.[27]

The bottom layer of permafrost sediment has been estimated to be "at least six hundred and fifty thousand years old", based on luminescence dating of drill bores extracted by Murton and his team.[26] Murton said that this means that the permafrost "survived the previous interglacial period, which began some hundred and thirty thousand years ago...The oldest permafrost in Eurasia has been kicking around for over half a million years...Seeing as it survived intense global-warming events in the past, it must be pretty resilient."[26]

While permafrost is resilient, it is not invulnerable. Batagaika Crater began to form in the 1960s, following clear-cutting of a large forested area. It has been growing 100 feet (30 m) a year since then.[26] In 2008, there was major flooding in the area which accelerated to growth of the depression.[27]

In 2016 and 2017, a group of researchers studied retrogressive thaw slumps in Northeast Siberia: one at Kurungnakh Island in the Lena River Delta and the second in Duvanny Yar near the Kolyma River. In 2018, an international team of scientists, found a nematode in a Pleistocene squirrel burrow in the Duvanny Yar outcrop that was estimated to be about 32,000 years old. The scientists thawed the nematodes; it revived and began moving and eating, making it one of the oldest living multicellular animals on Earth.[40][41]

As the climate warms, "terrain-altering thermokarst" retrogressive thaw slumps represent the "most rapid and dramatic changes" in permafrost regions.[43]

Permafrost in the Northern Hemisphere, which covered approximately 23,000,000 square kilometres (8,900,000 sq mi; 2.3×10⁹ ha; 5.7×10⁹ acres), representing 24% of the terrestrial area, as of 1997[44] is vulnerable to climate warming.[28] Of the estimated 950 billion tonnes of carbon in permafrost, the yedoma domain stores just a little less than 50% of this carbon.[22]

RTS "represent a particularly dramatic landscape response that is expected to intensify in magnitude and frequency with future climate change."[4] They "contribute large volumes of materials downslope to lakes, drainage networks and coastal zones."[4]

Retrogressive thaw slumps development is one result of global warming as Arctic temperatures rise rapidly rise and permafrost thaws.[45] For this reason, permafrost environments are extremely vulnerable to climate change in the Arctic.[46][lower-alpha 4][lower-alpha 5] [lower-alpha 6]

RSTs thaw "hectares of permafrost annually."[3][15] RTSs "typically retreat and expand at high rates".[16] RTSs damage "infrastructure, and releas[e] carbon preserved in frozen ground."[16]

"Advances in remote sensing techniques, and their application in a broad suite of change detection studies, indicate recent increases in the rates and magnitude of thermokarst including retrogressive thaw slumping, lake expansion and the transformation of frozen peatlands to collapsed wetlands."[2]

In Canada, computerized alert systems are being installed to monitor humidity, temperature and other factors at RTSs that threaten infrastructure, such as the Alaska Highway. One of the first of these alert systems includes several in northern Quebec and one at the Takhini Slump. There are plans in place to set up systems along the Dempster Highway, in the Yukon Territories and the Northwest Territories.[49] The Takhini Slump, located 30 kilometres (30,000 m) north of Whitehorse is "thawing at faster rates than ever before", according to Fabrice Calmels and his research team who came to the area in 2019, when the Takhini Slump was 95 metres (312 ft) from the Alaska Highway; in 2021 it was 40 metres (130 ft) away.[49]

A 2022 Earth System Science Data report described the implementation of a number of sensors and integrated observation dataset used to monitor the RSSs "hydrological and thermal deformation" to mitigate infrastructure damage to the Qinghai-Tibet Engineering Corridor engineering projects.[37]

"[I]ncreased knowledge on the dynamics of thaw slump activity is required to better characterize their impact on the environment."[3] According to a 1990 study, woodchip insulation on ice-rich slopes were used to slow down slumping along the "pipeline route from Norman Wells, NWT, to Zama, Alberta."[15][15]