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Continental-scale ice sheets have covered Antarctica since an interval of global cooling near the Eocene–Oligocene boundary around 33.9 million years ago (Ma). However, the sequence of events that led to the emergence of the persistent ice sheet in modern East Antarctica remains disputed. The transition to permanent polar aridity in high elevations of East Antarctica is critical to our understanding of the threshold response of glacial systems in Antarctica to changes in surface temperature at lower elevations. Here we constrain the onset of the polar aridity—which was probably necessary for regional ice-sheet stability—by assessing meteoric beryllium-10 profiles in mid-Miocene and late Quaternary soils at three sites situated 1,200–1,800 metres above sea level in the McMurdo Dry Valleys. Interpreting these profiles as indicators of water infiltration, we find that meteoric beryllium-10 entered mid-Miocene soils as late as the late Miocene. Reconstructions based on palaeo-active-layer thickness and known thresholds of meteoric beryllium-10 mobility suggest late Miocene summer temperatures of 7–10 °C with annual precipitation >10 mm. Therefore, the high elevations have probably been under a hyper-arid polar climate since the late Miocene (~6 Ma) and not the middle Miocene (13.8–12.5 Ma) as indicated by some previous reconstructions. Together, our findings indicate that high elevations of the McMurdo Dry Valleys probably experienced warm and wet climatic intervals from ~14.0 to 6.0 Ma, which reconciles observations of coastal warmth and reduced ice in the Ross Embayment. This finding also suggests that the McMurdo Dry Valleys may be more susceptible to climate change than anticipated.

ABSTRACTClimate change is impacting lakes and ponds in Antarctica. To that end, we investigated the distribution and chemical composition of ice-covered ponds in Untersee Oasis, East Antarctica. Thirty-nine ponds were inventoried, and the thirteen sampled ponds have low total dissolved solutes (TDS) with a Ca(Na)-SO4or Na-Cl geochemical facies. Tritium and radiocarbon measurements of the total inorganic carbon (TIC) suggest that the ponds are recharged by modern snowmelt. Hierarchical cluster analysis grouped the ponds based on the amount of potential incoming solar radiation reaching their surface. Ponds receiving higher insolation develop moats or completely lose their ice cover during summer, have lower pH, and higher TDS, δ18O, total inorganic carbon, and δ13CTIC,which suggest CO2availability is not limiting benthic photosynthetic activity. The ponds that receive lower insolation retain their full ice cover and remain well-sealed to direct exchanges with the atmosphere, have high pH, and lower TDS and TIC, and likely host a CO2-starved benthic microbial ecosystem. Therefore, the state of the ponds’ ice cover evolved their hydrochemistry along different trajectories. The findings can be used to predict the trajectory of the chemical composition of the ponds and effect on biological productivity as the phenology of their ice cover shifts under a warming climate.

Recent climate warming is contributing to permafrost degradation and vegetation change; however, little is known about the legacy of Holocene landscape change on contemporary soil biogeochemical conditions. In permafrost soils of northwestern Canada, widespread permafrost degradation occurred during the early Holocene warm interval and its impacts on soil biogeochemistry are archived in the paleo‐active layer. Here we show contrasting profiles of soil soluble chemistry and organic carbon composition at sites affected by different types of permafrost degradation. At sites that experienced increased depth of thaw, the relict active layer contained a lower abundance of soluble ions than the underlying undisturbed permafrost; however, both the relict active layer and undisturbed permafrost contained mainly old recalcitrant organics suggesting that minor microbial degradation of organics had occurred. At sites that experienced past thaw slumping, the relict active layer had a higher solute content and contained both young‐degradable and old‐recalcitrant organics due to the integration of slumped surface organic mats into the colluvial soils or vegetation re‐colonizing the surface of the former slump. Our results show that permafrost degradation that occurred during the early to mid‐Holocene have preconditioned the biogeochemical conditions in near‐surface permafrost soils such that significant local variability exist following past landscape changes. Thus, determining the state of soil soluble chemistry and SOC in permafrost should be done within a paleo‐landscape change framework to better forecast CO2–CH4emissions and solutes release from thawing permafrost. Effects of Holocene landscape changes on soil biogeochemistry are archived in relict active layerDifferent types of permafrost degradation have varying effects on soil biogeochemical conditionsHolocene landscape changes preconditioned biogeochemical conditions in permafrost soils and the potential consequences of future thaw

Little is known about the abundance and source of soil organic carbon and biogeochemical cycling in permafrost soils from the ultraxerous environment of the Dry Valleys of Antarctica. Here we investigate the distribution, source and cycling of organic carbon, total nitrogen and carbonates in the icy permafrost soils of University Valley, Quartermain Mountains. Results indicate that organic carbon content is lowest in icy soils from the perennially cryotic zone (<40 μg g−1dry soils) and higher in the icy soils from the seasonally noncryotic zone, where the highest concentrations were found in the warmer‐wetter section of the valley and near a frozen pond (up to 313 μg g−1dry soils). The δ13Corgof organic carbon in the icy soils showed that it is derived from the weathering of Beacon Supergroup sandstone that hosts active endolithic communities. The C:N ratios in icy soils formed two populations: one with ratios <5 and the other with ratios near the Redfield ratios. The low C:N ratios suggest that physicochemical processes dominates these soils, as supported by the absence of microbial activity and atmospherically deposited NO3with minimal postdeposition modification. The near Redfield C:N ratios can be explained by physical processes (translocation of SOC in the soils from snow meltwater) or balanced microbial activity. The latter is supported by the δ13CCaCO3values of carbonates that suggest a contribution from microbially respired endolith‐derived organic matter, providing indirect evidence of heterotrophic activity in permafrost soils from an ultraxerous environment. Organic carbon, total nitrogen, and carbonate contents follow ground surface temperature zones and are the lowest reported in the Dry ValleysOrganic carbon in the icy soils originates from the weathering of Beacon Supergroup sandstone that hosts active endolithsδ13CCaCO3in some of the icy soils suggests a contribution of microbial respiration of endolith‐derived organic matter

In the upper McMurdo Dry Valleys, 90% of the measured ice table depths range from 0 to 80 cm; however, numerical models predict that the ice table is not in equilibrium with current climate conditions and should be deeper than measured. This study explored the effects of boundary conditions (air versus ground surface temperature and humidity), ground temperature cycles, and their diminishing amplitude with depth and advective flows (Darcy flow and wind pumping) on water vapor fluxes in soils and ice table depths using the REGO vapor diffusion model. We conducted a series of numerical experiments that illustrated different hypothetical scenarios and estimated the water vapor flux and ice table depth using the conditions in University Valley, a small high elevation valley. In situ measurements showed that while the mean annual ground surface temperature approximates that in the air, the mean annual ground surface relative humidity (>85%ice) was significantly higher than in the atmosphere (~50%ice). When ground surface temperature and humidity were used as boundary conditions, along with damping diurnal and annual temperature cycles within the sandy soil, REGO predicted that measured ice table depths in the valley were in equilibrium with contemporary conditions. Based on model results, a dry soil column can become saturated with ice within centuries. Overall, the results from the new soil data and modeling have implications regarding the factors and boundary conditions that affect the stability of ground ice in cold and hyperarid regions where liquid water is rare. Ice table depths in the University Valley consistent with present‐day conditionsGround surface temperature and humidity must be used, not those in the air

Some of the coldest and driest permafrost soils on Earth are located in the high-elevation McMurdo Dry Valleys (MDVs) of Antarctica, but little is known about the permafrost microbial communities other than that microorganisms are present in these valleys. Here, we describe the microbiology and habitable conditions of highly unique dry and ice-cemented permafrost in University Valley, one of the coldest and driest regions in the MDVs (1700 m above sea level; mean temperature −23 °C; no degree days above freezing), where the ice in permafrost originates from vapour deposition rather than liquid water. We found that culturable and total microbial biomass in University Valley was extremely low, and microbial activity under ambient conditions was undetectable. Our results contrast with reports from the lower-elevation Dry Valleys and Arctic permafrost soils where active microbial populations are found, suggesting that the combination of severe cold, aridity, oligotrophy of University Valley permafrost soils severely limit microbial activity and survival.

Some of the coldest and driest permafrost soils on Earth are located in the high-elevation McMurdo Dry Valleys (MDVs) of Antarctica, but little is known about the permafrost microbial communities other than that microorganisms are present in these valleys. Here, we describe the microbiology and habitable conditions of highly unique dry and ice-cemented permafrost in University Valley, one of the coldest and driest regions in the MDVs (1700?m above sea level; mean temperature -23?°C; no degree days above freezing), where the ice in permafrost originates from vapour deposition rather than liquid water. We found that culturable and total microbial biomass in University Valley was extremely low, and microbial activity under ambient conditions was undetectable. Our results contrast with reports from the lower-elevation Dry Valleys and Arctic permafrost soils where active microbial populations are found, suggesting that the combination of severe cold, aridity, oligotrophy of University Valley permafrost soils severely limit microbial activity and survival.

The article reports on the IceBite project which aimed to develop technology for the exploration of the subsurface environment of Mars and to enhance the understanding on the physical, chemical and biological dynamics in dry and ice-bearing permafrost like that of which is in the upper Dry Valleys of Antarctica. Procedural elements and statistical information related to understanding the behaviour of ice-bearing permafrost of vapour-diffusion origin on Earth and Mars are discussed.