Lake Untersee is one of the largest perennially ice-covered lakes in Dronning Maud Land. We investigated the energy and water mass balance of Lake Untersee to understand its state of equilibrium. The thickness of the ice cover is strongly correlated with sublimation rates; variations in sublimation rates across the ice cover are largely determined by wind-driven turbulent heat fluxes and the number of snow-covered days. Lake extent and water level have remained stable for the past 20 years, indicating that the water mass balance is in equilibrium. The lake is damned by the Anuchin Glacier and mass balance calculation suggest that subaqueous melting of terminus ice contributes 40–45% of the annual water budget; since there is no evidence of streams flowing into the lake, the lake must be connected to a groundwater system that contributes 55–60% in order to maintain the lake budget in balance. The groundwater likely flows at a rate of ~8.8 × 10−2 m3 s−1, a reasonable estimate given the range of subglacial water flux in the region. The fate of its well-sealed ice cover is likely tied to changes in wind regime, whereas changes in water budget are more closely linked to the response of surrounding glaciers to climate change.

Perennially ice-covered lakes that are tightly sealed from the atmosphere represent a unique group of polar lakes. In these lakes, the δD-δ 18 O evolution of the water column and steady-state conditions are controlled by rates of recharge and freezing at the bottom of the ice cover. We developed a recursive model (FREEZCH9) that takes into account the changing salinity in the water column as a result of freezing and mixes the recharge water to the residual water in well-sealed perennially ice-covered lakes. Our model is tested against datasets from Lake Vostok and is used to assess the δDδ 18 O mass balance of Lake Untersee and evaluate if the lake is in isotopic steady-state. Our FREEZCH9 simulations fit well with the predicted δD-δ 18 O values of Lake Vostok's upper water column and the overlying accreted ice. Simulations with FREEZCH9 also suggests that Lake Untersee is in isotopic steady-state and that its two input sources (i.e., subaqueous terminus melting of the Anuchin Glacier and subglacial meltwater) have similar δD-δ 18 O composition. Our modeling demonstrates that Lake Untersee most likely did not receive additional input from surface streams during the last 300-500 years. FREEZCH9 may be also used to determine if any groundwater systems of the McMurdo Dry Valleys are fully or partially recharged by subglacial lakes.

perennially ice-covered lakes that host benthic microbial ecosystems are present in many regions of Antarctica. Lake Untersee is an ultra-oligotrophic lake that is substantially different from any other lakes on the continent as it does not develop a seasonal moat and therefore shares similarities to sub-glacial lakes where they are sealed to the atmosphere. Here, we determine the source of major solutes and carbon to Lake Untersee, evaluate the carbon cycling and assess the metabolic functioning of microbial mats using an isotope geochemistry approach. The findings suggest that the glacial meltwater recharging the closed-basin and well-sealed Lake Untersee largely determines the major solute chemistry of the oxic water column with plagioclase and alumino-silicate weathering contributing < 5% of the Ca 2+-na + solutes to the lake. the tic concentration in the lake is very low and is sourced from melting of glacial ice and direct release of occluded co 2 gases into the water column. The comparison of δ 13 c tic of the oxic lake waters with the δ 13 c in the top microbial mat layer show no fractionation due to non-discriminating photosynthetic fixation of HCO 3 in the high pH and carbon-starved water. the 14 C results indicate that phototrophs are also fixing respired CO 2 from heterotrophic metabolism of the underlying microbial mats layers. The findings provide insights into the development of collaboration in carbon partitioning within the microbial mats to support their growth in a carbon-starved ecosystem. Numerous perennially ice-covered lakes have been inventoried in Antarctica, including in the McMurdo Dry Valleys (MDV), Bunger Hills, Vestfold Hills, Schirmacher Oasis, and Soya Coast 1,2. These lakes have varied chemistries as a result of source water and Holocene history of the lakes, but many are oligotrophic and support benthic cyanobacterial mats and heterotrophic bacterial communities 3-5. Primary productivity is often limited by light attenuation through the ice-cover and nutrient availability (e.g., C and P), however, summer moating and streams provide seasonal recharge of nutrients to the lakes 4,6. Analysis of carbon isotopes (e.g., δ 13 C, 14 C) can provide insights about the source of carbon and transformations of organic matter as photosynthesis and remineralization control the isotopic composition of most organic matter. For example, carbon isotope geochemistry of dissolved inorganic carbon (δ 13 C DIC) and organic carbon (δ 13 C DOC) have been used to trace carbon sources and cycling in the MDV lake ecosystem 3,7-9. Lake Untersee is a 169-m deep ultra-oligotrophic lake that is substantially different from other lakes in Antarctica 10,11. It is recharged by subaqueous melting of glacial ice and subglacial meltwater, and the lake remains ice-covered with no open water along the margin (summer moating) that would provide access to nutrients, CO 2 and enhanced sunlight 12-14. In the absence of large metazoans, photosynthetic microbial mats cover the floor of the lake from just below the ice cover ...

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−1 dry 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−1 dry soils). The δ13Corg of 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 NO3 with 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 δ13CCaCO3 values 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.

We present the basic theory of stable isotopes (δ( 18 O) and δ(D)) of freezing water solutions in the environment set within a water isotope-augmented version of FREZCHEM(V15). We validate this model with a couple of examples. The isotopecapable FREZCHEM is simplified to run much faster using set-piece initial chemistries to calculate the freezing temperature of the remaining water. The fast version is embedded in a semi-empirical model for residual liquid water in sub-zero soils. A uniform specific soil column is driven with a defined seasonal temperature wave. The co-isotopes of the residual water and ice are calculated as a function of time and depth. The model is applied to clayey soils of the sort sampled in detail in a 10 (10 to 20 m) core suite from the Mackenzie Delta, Canada. The stable isotopes are compared to the model ones and the match is quite good except in the uppermost 2.5 m, where an upturn in the model δ( 18 O) curve is smaller. If the upper 2.5 m of icy soil is populated with "modern" water, and below by late glacial meltwater the size of the upturn in δ( 18 O) starting at 2.5 m is reproduced by the model.