Ice Wedges Research Articles

Topography Controls Variability in Circumpolar Permafrost Thaw Pond Expansion

AbstractOne of the most conspicuous signals of climate change in high‐latitude tundra is the expansion of ice wedge thermokarst pools. These small but abundant water features form rapidly in depressions caused by the melting of ice wedges (i.e., meter‐scale bodies of ice embedded within the top of the permafrost). Pool expansion impacts subsequent thaw rates through a series of complex positive and negative feedbacks which play out over timescales of decades and may accelerate carbon release from the underlying sediments. Although many local observations of ice wedge thermokarst pool expansion have been documented, analyses at continental to pan‐Arctic scales have been rare, hindering efforts to project how strongly this process may impact the global carbon cycle. Here we present one of the most geographically extensive and temporally dense records yet compiled of recent pool expansion, in which changes to pool area from 2008 to 2020 were quantified through satellite‐image analysis at 27 survey areas (measuring 10–35 km2 each, or 400 km2 in total) dispersed throughout the circumpolar tundra. The results revealed instances of rapid expansion at 44% (15%) of survey areas. Considered alone, the extent of departures from historical mean air temperatures did not account for between site variation in rates of change to pool area. Pool growth was most clearly associated with upland (i.e., hilly) terrain and elevated silt content at soil depths greater than one meter. These findings suggest that, at short time scales, pedologic and geomorphologic conditions may exert greater control on pool dynamics in the warming Arctic than spatial variability in the rate of air temperature increases.

Synchronous Isotopic Curves in Ice Wedges of the Batagay Yedoma: Precision Matching and Similarity Scoring

ABSTRACTThe purpose of the study is to compare the results of the stable isotope analysis (δ18O, δ2H, and deuterium excess values) of parallel and synchronous ice wedges exposed in one outcrop in the upper part of the Batagay Yedoma (67.58° N, 134.77° E) and to estimate their precision, that is, a measure of similarity of independent isotope results of two ice wedges. Detailed sampling of two syngenetic ice wedges (IW‐17 and IW‐20) for isotope analysis was carried out in the exposure of the thermo‐erosional ravine in the southeastern part of the Batagay megaslump. In total, results of 105 measurements for IW‐17 and 59 measurements for IW‐20 were obtained. According to 14C AMS ages of organic microinclusions in the ice‐wedge ice, the studied ice wedges began to form no later than 42 cal ka BP. The termination of their growth was dated to about 11.7 cal ka BP. For both ice wedges, generally low isotope values were obtained. For the IW‐17, the mean values are −34.38‰ for δ18O, −264.82‰ for δ2H, and 10.2‰ for dexc. For IW‐20, the mean isotope values are −34.09‰ for δ18O, −262.91‰ for δ2H, and 9.84‰ for dexc. Statistical tests showed significant correlations between the isotope data in the upper parts of ice wedges that may be explained by similar conditions during their growth at this stage. Our results allow us to refine the solutions to the two main issues studied of the Late Pleistocene Yedoma: the age of the ice wedges, the accuracy of the isotope profiles, and the paleotemperature interpretation of the isotope records.

ISOSCAPES AND PALEOISOTHERMS OF THE HOLOCENE MEAN JANUARY AIR TEMPERATURE IN THE NORTHWESTERN SIBERIA (BASED ON STABLE OXYGEN ISOTOPE COMPOSITION OF ICE WEDGES)

Holocene ice wedges at 16 sites in the Northwestern Siberia were studied, the age of ice wedges was determined, and stable isotope data ( 18O and 2Н values) for all studied ice wedges have been summarized. The δ2H-δ18O ratio and the dexc values for ice wedge ice indicate good preservation of the isotope signal of winter precipitation in ice, which allows to use the obtained δ18O values for paleotemperature reconstructions. The isoscapes (lines of equal 18O values) created for the Holocene ice wedges and modern ice veinlets are generally submeridional; the 18O values decrease from west to east of the study area. Taking into account a new Holocene tripartite division scheme it was shown that the mean January air temperature (TmJ) in the Northwestern Siberia ranged approximately from -21 to -30°С during Greenlandian and the first half of the Northgrippian stage of the Holocene (11,4 to 6 cal ka BP) and approximately from -24 to -27°С from the end of the Northgrippian - to the Meghalayan stage of the Holocene (5,2 to 0,9 cal ka BP). The Holocene isotherms are close to the modern sub-meridional position and show an eastward decrease in TmJ values. Winter climatic conditions in the Northwestern Siberia were generally stable during the Holocene, meanwhile TmJ was on average 1-2°C lower than modern ones.

Identifying ice wedges in ground-penetrating radar data from the Cold Regions Research and Engineering Laboratory permafrost tunnel, Fox, Alaska

The warming of Alaska’s permafrost has been leading to thawing within its discontinuous permafrost. This can cause ice wedges to melt, resulting in thermokarst formations such as sinkholes and landslides, which are causing damage to Alaskan infrastructure. To avoid developing further infrastructure in areas containing ice wedges, it is necessary to improve methods of detecting subsurface ground ice. Ground-penetrating radar (GPR) is a nonintrusive remote sensing method of locating and characterizing permafrost and other subsurface features that are not evident on a cut face or surface. When applied to the detection of ice wedges at the Cold Regions Research and Engineering Laboratory permafrost tunnel in Fox, Alaska, a few identifying features have been observed which corresponded with prior research. We first use GPR reflections of areas along the tunnel walls holding known ice wedges to verify the identifying features of an ice wedge within GPR data. We use these reflections as test data to identify possible ice wedges in locations where they were not observed on the tunnel walls.

Optically‐Stimulated‐Luminescence Ages and Paleo‐Environmental Implications of Relict Frost Wedges in North–Central Bohemia, Czech Republic

ABSTRACTRelict frost wedges are widespread and valuable indicators of past environmental conditions that have been extensively dated across central and western European lowlands over the past few decades, but their timing in the Czech Republic is poorly known. Here, we present optically‐stimulated‐luminescence (OSL) ages for seven relict frost wedges situated at four study sites in north–central Bohemia, Czech Republic (49.9992–50.4956°N, 13.3736–16.0011°E, 230–350 m above sea level). The OSL ages indicate that the frost wedges developed during two phases in marine isotope stage 2: an older phase that peaked at 23.6 ± 2.9 ka and a younger phase that peaked at 16.1 ± 1.3 ka. Both phases probably experienced cold, dry and windy conditions that were mostly associated with <0°C mean annual air and ground temperatures and the presence of at least discontinuous permafrost, which is in line with similar central and western European records and other paleo‐environmental archives. The new OSL ages constitute the first extended dataset on the timing of frost wedges in this undersampled area between the Fennoscandian and Alpine ice sheets, which is essential for reconstructing past permafrost extents and climate conditions, as well as for validating models of past permafrost dynamics in central Europe.

Rapid Ice‐Wedge Collapse and Permafrost Carbon Loss Triggered by Increased Snow Depth and Surface Runoff

AbstractThicker snow cover in permafrost areas causes deeper active layers and thaw subsidence, which alter local hydrology and may amplify the loss of soil carbon. However, the potential for changes in snow cover and surface runoff to mobilize permafrost carbon remains poorly quantified. In this study, we show that a snow fence experiment on High‐Arctic Svalbard inadvertently led to surface subsidence through warming, and extensive downstream erosion due to increased surface runoff. Within a decade of artificially raised snow depths, several ice wedges collapsed, forming a 50 m long and 1.5 m deep thermo‐erosion gully in the landscape. We estimate that 1.1–3.3 tons C may have eroded, and that the gully is a hotspot for processing of mobilized aquatic carbon. Our results show that interactions among snow, runoff and permafrost thaw form an important driver of soil carbon loss, highlighting the need for improved model representation.

Среднеянварская палеотемпература воздуха в период формирования Сеяхинской едомы 30—13 тыс. калиброванных лет назад

Study of the yedoma strata with thick syngenetic ice wedges on the eastern coast of the Yamal Peninsula at the mouth of the Seyakha River, including radiocarbon dating of the enclosing sediments and ice wedge ice and determination of isotope composition of ice, showed that between 30 and 13 thousand calibrated years ago, the mean January air temperature was on average 10°C lower than the current one and varied from –35° to –33°C. The accumulation of yedoma strata on the northern plains of Western Siberia is evidence of the glacial cover absence in the area during the last glacial maximum.

Contemporary formation of ice-wedge pseudomorphs during the expansion of a thermokarst lake in Old Crow Flats, Yukon, Canada

Ice-wedge pseudomorphs (IWPs) are thermokarst structures that form by the secondary infilling of cavities left by the slow melting of ice wedges. Contemporary IWP formation in periglacial environments informs our understanding of past processes and dynamics implied by their presence in the stratigraphic record. However, contemporary IWPs are seldom studied as they are difficult to locate and observe due to the general lack of surface expression and because they can deform with time or be confused with sediment wedges. This study presents detailed information about the morphology and soil composition of IWPs that formed beneath a thermokarst lake in Old Crow Flats, northern Yukon (Canada), and provides the first conceptual model of lacustrine IWP development. Seven IWPs were unearthed along a transect perpendicular to a former eroding lakeshore in a recently (2019), partially drained lake basin. The IWPs were between 76 cm and 122 cm high and had varying morphologies, generally forming triangles pointing down or T shapes. They had a much higher organic matter content and were composed of slightly coarser sediment than their host sediment, characteristics that were shared by basin floor surface sediment found in the near-shore zone (<15 m) of the former lake. Within five meters of the former shoreline, some troughs showed pronounced subsidence due to the melting of remnant wedge ice following lake drainage. Based on ice-wedge dimensions, bank height, shore erosion rates, and field measurements of sublake active-layer thickness prior to lake drainage, it is likely that ice wedges were entirely melted by the development of the sublake active layer, prior to talik development. Together, the results indicate that the IWPs formed within 5 to 10 m of the eroding shore bank, 9 to 18 years after lake submergence, and were filled with the organic-rich lacustrine sediment that accumulated in the near-shore zone due to sediment sorting by wave action following the erosion of peaty polygonal tundra. This study highlights that IWPs can form during the natural evolution of thermokarst lakes in regions with stable permafrost and are not necessarily indicators of conditions leading to generalized permafrost degradation.

Post-fire stabilization of thaw-affected permafrost terrain in northern Alaska

In 2007, the Anaktuvuk River fire burned more than 1000 km2 of arctic tundra in northern Alaska, ~ 50% of which occurred in an area with ice-rich syngenetic permafrost (Yedoma). By 2014, widespread degradation of ice wedges was apparent in the Yedoma region. In a 50 km2 area, thaw subsidence was detected across 15% of the land area in repeat airborne LiDAR data acquired in 2009 and 2014. Updating observations with a 2021 airborne LiDAR dataset show that additional thaw subsidence was detected in < 1% of the study area, indicating stabilization of the thaw-affected permafrost terrain. Ground temperature measurements between 2010 and 2015 indicated that the number of near-surface soil thawing-degree-days at the burn site were 3 × greater than at an unburned control site, but by 2022 the number was reduced to 1.3 × greater. Mean annual ground temperature of the near-surface permafrost increased by 0.33 °C/yr in the burn site up to 7-years post-fire, but then cooled by 0.15 °C/yr in the subsequent eight years, while temperatures at the control site remained relatively stable. Permafrost cores collected from ice-wedge troughs (n = 41) and polygon centers (n = 8) revealed the presence of a thaw unconformity, that in most cases was overlain by a recovered permafrost layer that averaged 14.2 cm and 18.3 cm, respectively. Taken together, our observations highlight that the initial degradation of ice-rich permafrost following the Anaktuvuk River tundra fire has been followed by a period of thaw cessation, permafrost aggradation, and terrain stabilization.

RADIOCARBON AGES OF PLANT REMAINS IN MASSIVE GROUND ICE AND UNDERLYING SEDIMENTS OF THE BARROW PERMAFROST TUNNEL, ALASKA

ABSTRACT Massive ground ice found in the Barrow Permafrost Tunnel at 3–7 m depths from the surface has been interpreted as an ice wedge and used to reconstruct early Holocene environmental changes. To better understand the development of this ground ice, we conducted radiocarbon dating for 34 samples of plant remains from the massive ground ice and underlying sediment layer. A significantly large gap in the measured radiocarbon ages (more than 24 ka) between massive ice and the underlying sediment layer throughout the tunnel profile suggested at least two possibilities. One is that the lower and older sediment layer had thrust upwards at the boundary between intruding ice wedge and adjacent sediment, and the growing ice had pushed the sediment sideways. Another is that erosional events had removed surface materials at about 12–36 ka BP (14–41 cal ka BP) before the overlaying sediment layer with massive ground ice developed. The overall distribution of radiocarbon ages from the massive ice supported the ice-wedge hypothesis as a formation mechanism, although our results showed several age inversions and large fluctuations. Dating of densely spaced samples revealed two ground-ice regions with similar ages around 11–11.5 and 10–10.5 ka BP divided by a relatively narrow region of transitional ages along the tunnel long-axis. This distribution may be explained by a possible misalignment between the sampling direction and the ice-wedge growth line or by intermittent ice growth with repeated cracking at more random locations than the classic ice-wedge growth model suggested.

A unified model for frost wedging in an open fissure under unidirectional freezing

The propagation of joints and fissures, included by the frost heaving pressure arising from the water-ice phase transition is considered as frost wedging. This process is widely observed and plays a crucial role in weathering of jointed hard rocks in cold regions. Despite some fundamental results achieved in former experimental studies, a unified theoretical model is still absent, hindering a thorough understanding of this process. To further explore the mechanism of frost wedging during the freezing process, this paper firstly carries out a frost heave test on a rock block bearing a single open fissure. The results indicate that the evolution of frost heaving pressure in the fissure can be divided into four stages: sealing of the open fissure by ice wedge, ice wedge slipping, crack propagation, as well as stabilization of the ice-filled fissure. Based on the laboratory test results, a unified model for the frost heaving pressure in a single open fissure is established, which comprehensively takes into account the mechanical interaction between ice, water and rock. The model is validated by the test result, and it can reasonably describe the formation of the sealing conditions and the process of crack propagation. Fracture of the ice-rock interface near the free surface increases the likelihood of ice wedge slip, while frost wedging mainly damages geological bodies during crack propagation.

EARLY HOLOCENE OXYGEN ISOTOPE CHRONOLOGIES (11,267–6420 CAL BP) FROM ICE WEDGE AT CHARA, TRANSBAIKALIA

ABSTRACTAccelerator mass spectrometry radiocarbon (AMS 14C) dating was used for determining the age of wedge ice. It has been found that between 11,270 and 6420 cal BP, or the Greenlandian and Northgrippian stages of the Holocene, ice wedges grew syngenetically in sandy deposits with gravel in the Chara River valley. The variations of δ18O values in the ice wedges are about 8‰, from –25.5‰ to –18.8‰. Based on the stable isotope composition of ice wedges, paleotemperature reconstructions revealed that the mean January temperature was as low as –38°C during the coldest periods of the early half of the Holocene and as high as –28°C during the warmer periods.

Investigation of ice wedge bearing capacity based on an anisotropic beam analogy

Ships and offshore structures operating in ice-covered waters need to be able to break ice. The hulls of ships and offshore structures are often designed to be inclined so that an out-of-plane force can be incurred to initiate flexural failure. During this process, an in-plane force co-exists as the result of the hull inclination, which can affect the bending process and alter the magnitude of interaction force. Additionally, the loading process can be rapid and incur dynamic effect within the ice sheet, resulting in higher peak loads compared to the scenario of quasi-static loading. The effect of in-plane force and loading speed, however, are often neglected in the modelling of the icebreaking process in order to enable simplified solutions of analytical form, which leads to deviations in the estimation of ice loads. This paper aims to investigate the effects of in-plane force and loading speed on the ice bearing capacity during the icebreaking process. This is conducted via the Finite Element Analysis (FEA) with the Cohesive Element Method (CEM). An anisotropic beam analogy concept is proposed to model a 3D ice wedge with a 2D beam which has varying properties along the axial direction. The analogy enables fast and stable simulation of ice crack propagation through its thickness direction, while maintaining properties of the ice wedge in 3D. The analogy is verified with Nevel's closed-form solution for a narrow wedge resting on an elastic foundation. After that, the beam is loaded with a rigid wall with different inclination angles and loading speeds. The results reveal that the in-plane force and loading speed have significant influence on the icebreaking process, resulting in different bearing capacity and overall energy consumption. Based on the numerical results, a regression study is conducted to derive a correction coefficient accounting for the joint effect of in-plane force and loading speed on the bearing capacity, which can be incorporated into ice-structure interaction simulation models.

Europa's Double Ridges Produced by Ice Wedging

AbstractDouble ridges are sprawling features observed globally across the icy surface of Europa. They consist of two topographic highs flanking a trough. The topographic relief of the ridges is approximately 100 m, and the ridges extend up to hundreds of kilometers in length. The interior structure and dynamics of Europa's ice shell are currently poorly constrained. Therefore, accurate models for the formation of these prominent surface features can be useful for determining how the ice shell operates. We hypothesize that double ridges form as a result of incremental ice wedging. We use both analytical and numerical finite element models to quantify the deformation that occurs as an ice wedge grows incrementally within the ice shell. We show that incremental growth of the ice wedge results in surface deformation that matches the size and shape of typical Europan double ridges, including their topographic relief and surrounding troughs. We find that as the depth of the ice wedge increases, double ridges become broader and shorter. We explore the possibility of local and non‐local sources for the liquid water that freezes to produce the wedge and ultimately argue in favor of local sources of liquid water within the ice shell.

Ground Penetrating Radar Survey of Arctic Polygonal Wedge Structures

Ground Penetrating Radar Survey of Arctic Polygonal Wedge Structures

Pollen spectra in Holocene ice wedges in the floodplain of the Lyakkatosyo River (Eastern Yamal, Russia)

Pollen spectra in Holocene ice wedges in the floodplain of the Lyakkatosyo River (Eastern Yamal, Russia)

Electrical Resistivity Tomography Investigation of Permafrost Conditions in a Thermokarst Site in Fairbanks, Alaska

&lt;abstract&gt; &lt;p&gt;The degradation of permafrost poses severe environmental threats to communities in cold regions. As near-surface permafrost warms, extensive topographic variability is prevalent in the Arctic and Sub-Arctic communities. Geologic hazards such as thermokarst are formed due to varying rates of permafrost degradation, resulting in ground subsidence. This gradual subsidence or abrupt collapse of the earth causes a danger to existing infrastructure and the economic activities of communities in cold regions. Understanding the causes of thermokarst development and its dynamics requires imaging its underground morpho-structures and characterizing the surface and subsurface controls. In this study, we conducted a two-dimensional (2D) electrical resistivity tomography (ERT) survey to characterize the permafrost conditions in a thermokarst prone site located in Fairbanks, Alaska. To increase the reliability in the interpretability of the ERT data, borehole data and the depth-of-investigation (DOI) methods were applied. By using the 2D and three-dimensional (3D) ERT methods, we gained valuable information on the spatial variability of transient processes, such as the movement of freezing and thawing fronts. Resistivity imaging across the site exhibited distinct variations in permafrost conditions, with both low and high resistive anomalies observed along the transects. These anomalies, representing taliks and ice wedges, were characterized by resistivity values ranging from 50 Ωm and above 700 Ωm, respectively. The results from this study showed the effectiveness of ERT to characterize permafrost conditions and thermokarst subsurface morpho-structures. The insights gained from this research contribute to a better understanding of the causes and dynamics of thermokarst, which can be instrumental for engineers in developing feasible remedial measures.&lt;/p&gt; &lt;/abstract&gt;

Morphological and Spectral Characterization of Lunar Regolith Breakdown due to Water Ice

Remote sensing observations of the Moon suggest that the lunar polar regolith environment is affected by several natural processes that may cause the regolith in these regions to become more porous and fine particulate. One of these processes may be the mechanical breakdown of regolith particles through the interaction of water ice and regolith by frost wedging. We present morphological and spectral analyses of high-fidelity lunar regolith simulants LHS-1 (lunar highlands simulant-1) and LMS-1 (lunar mare simulant-1) that have been exposed to varying concentrations of water ice (1, 10, and 30 wt%) over extended periods of time (1, 3, and 6 months) to evaluate the extent at which lunar regolith may be weathered by ice-regolith interactions in the Moon’s polar regions. To characterize changes in regolith particle morphology, we explored grain size and shape parameters with the CILAS ExpertShape suite and characterized the abundance and evolution of clinging fines with scanning electron microscopy and energy dispersive X-ray spectroscopy. Reflectance spectra were taken from 1.0–22.5 μm (444.4–10,000 cm−1) to characterize any differences in spectral features that may occur as a result of regolith breakdown. Both the morphological and spectral investigations display trends that show simulant particle degradation as a function of composition, increasing water concentration, and freezing time. Our study demonstrates that the lunar regolith is susceptible to mechanical breakdown in the presence of water ice and that water ice is likely a contributor to the weathering environment within permanently shadowed regions on the lunar surface.

Humus component of pedosediments at the end of the Sartan cryochron in the Nadym River basin (West Siberia)

The aim of the study. To assess the potential of humus substances from the pedosediments in the polygonal ice wedges pseudomorphs as indicators of the paleoecological conditions of their formation during the period of their functioning within the surface soil, using the case of the Razduv object of the end of the Sartan cryochron (corresponding to MIS-2) in the basin of Nadym River (West Siberia). Important comment: In this paper the terms “humus”, “humus component of soils” and the “system of humus substances” are used as synonyms. Location and time of the study. The research was conducted in the Nadym district of the Yamalo-Nenets Autonomous Okrug (YaNAO), at the Razduv site 15 km to the north-east of the Nadym city, during the late summer 2018. Methods: The study is based on a new methodological approach to the diagnostics of the bioclimatic conditions at the end of Sartanian cryochron in one of the northern territories of West Siberia, based on the humus substances of pedosediments which serve as indicators of paleoenvironment. Main results. It was established that the applied indicator features of the formation conditions of the studied pedosediments, based on indicators of the composition and spectral properties of their humus component, correlated well with each other, in their totality reflecting the cold humid conditions of soil formation, similar to those for modern soils of different tundra subzones of the West Siberia. Conclusions. The study of the specific features of composition, structure and properties of humus substances from the pedosediments in the pseudomorphs after polygonal ice wedges, dated back to the end of the Sartanian cryochron at the key section of Razduv and revealing the indicators of their formation conditions, demonstrated that the characteristics of the humus substances system and in particular one of its components, i.e. humic acids, obtained by independent methods, have high diagnostic significance for reconstructing paleoclimatic and paleoecological conditions of their formation. Characteristics of modern soils and paleosols from various parts of North Siberia, from northern Trans-Urals to the coastal lowlands in the north of Yakutia, were used as a recent reference dataset for diagnostics and reconstruction of the bioclimatic conditions of paleosol development on the land surface during MIS 2. It was confirmed that the studied samples, attributed to redeposited soil materials, i.e. pedosediments, due to their morphology in fact had undergone the phase of pedogenesis that is evidenced by the presence of humus substances, including humic acids with their typical outline of FTIR-spectra with a characteristic set of the absorption bands, individual ratio of the main structural elements and similar variation of the quantitative parameters of the fluorescence emission. The closest similarity of the composition and proportion of the main components of the humus substances system was found between the samples of the Sartanian pedosediments and the modern soils formed in the tundra conditions of West Siberia. The similarity of the main humus substances ratio, as well as the main structure-forming elements and values of all fluorescence characteristics of humic acids, especially for the samples with higher humus content and those affected by permafrost, was revealed. These results confirm that at the end of the Sartanian cryochron the pedogenesis in the vicinities of Razduv section was similar to that is now going on in the region located to the north of the study site.

Январская палеотемпература воздуха в период МИС-3-2 в северо-восточной Якутии, реконструированная по высокоразрешающей записи изотопного состава сингенетических повторно-жильных льдов Батагайской едомы

The work is aimed at determining the age of ice wedges, as well as at characterizing winter temperature fluctuations during the formation of the Yedoma based on the isotopic composition of ice wedges in the unique outcrop of the Batagay mega-slump. The researchers carried out a detailed sampling from the Upper Ice Wedge Complex of the Batagay Yedoma, determined the isotopic composition in three ice wedges at different altitudes. The results of radiocarbon dating of organic matter from ice wedge confirm the time of syngenetic formation of the upper-tier ice wedges in the range from 27.2 to 30.8 thousand cal. years. Ice wedges of the lower tier, according to the results of radiocarbon dating of ice accumulated 44.7—47.9 thousand years ago simultaneously at different levels: 274—268 m and 224—227 respectively. The study of yedoma deposits and ice-wedge ice made it possible to obtain data on climatic fluctuations in a region with the most severe continental climate. Direct radiocarbon dating of syngenetic ice wedges provided dating of ice growth in the Batagay yedoma, data on the content of stable isotopes in the ice demonstrated stable and very harsh conditions of the winter season. The air paleotemperature in Batagay in January 47—42 and 30—25 thousand cal. years ago was consistently low about –51°C.