Mathematics – Logic
Scientific paper
Jan 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998lpico.953r...6d&link_type=abstract
The First International Conference on Mars Polar Science and Exploration, Proceedings of the Conference held at Camp Allen, TX.
Mathematics
Logic
Bacteria, Brines, Cores, Depth, Freezing, Geochemistry, Ice, Ice Environments, Antarctic Regions, Ground Penetrating Radar
Scientific paper
Some of the largest lakes in the McMurdo Dry Valleys, Antarctica, have largely been ignored during past limnological studies because they were thought to be frozen solid. However, recent investigations have revealed the presence of saline water bodies beneath up to 19 m of permanent ice in two of these so-called "ice block" lakes (Lake Vida and Lake House). Lakes throughout the dry valleys that have been studied in detail more typically have ice covers ranging between 3 and 5 m. The existence of saline lakes with extremely thick ice covers is atypical, even among lakes in this region, which are themselves unique aquatic systems. These "deeply ice-covered" lakes are aquatic systems on the edge of cold-termination, and they warrant study as analogs of lakes purported to have existed on the surface of Mars in the past. Several lakes in the McMurdo Dry Valleys were presumed in the past to be frozen solid based largely on attempts at drilling the lake ice covers. Lake Vida has been the most intriguing because it is one of the two largest (in terms of surface area) lakes in the dry valleys, and yet it apparently contained no year-round liquid water at depth. Recently a ground-penetrating radar (GPR) survey was carried out on Lake Vida and another purported ice block lake, Lake House. In a large central portion of Lake Vida, the survey showed attenuation of the radar signal at approximately 19 m, suggesting saline water at this depth. Because GPR radar signals are absorbed by saline water, the depth of the water body (i.e., distance from the ice bottom to sediments) could not be determined. In Lake House, a similar water body was inferred at about 12 m depth. Ice Coring and Physical Properties: Ice cores (to 14 and 15.8 in depth) extracted in 1996 from Lake Vida contained ice bubbles with unique morphologies that were atypical when compared to other vapor inclusions in 3-5 in ice covers. Most of the vapor inclusions at depths greater than about 6 m contained hoar frost, which is indicative of prolonged exposure to a thermal gradient. At 15.8 m in the profile, wet saline ice was encountered at -11.6C (logged upon collection). The brine was later determined to be NaCl with an inferred concentration of 600 ppt, or about 17x seawater. Based on the GPR survey this brine would have been 3-4 in from the ice/liquid water interface. The GPR results show parabolic reflections in the ice starting at 16 m, which we now interpret as the start of the briny ice. A meteorological station at the west end of the lake recorded a mean annual temperature at Lake Vida of -26C. This is about 9'C colder than annual averages in Taylor and Wright Valleys during the same period. The difference occurs entirely during the winter, with the summers being very similar. The reason for the cold Victoria Valley winters appears to be a lack of foehn winds. Since local topography does not seem to be blocking these winds, we suggest that a strong winter temperature inversion in the valley forces the foehn winds to stay off the valley floor. The meteorological record thus shows that the environment at Lake Vida provides greater freezing potential than the environment of other dry valley lakes. We used these meteorological data to model the annual thermal wave in Lake Vida ice without considering the influence of the underlying water body. The modeled temperatures are compared against the actual first year's data. From this comparison it is clear that the actual temperature profile gets warmer toward the bottom, suggesting a heat source at depth. There are three probable, not mutually exclusive, candidates for this heat source: (1) localized geothermal heating, (2) ice growth at the base of the ice cover with the resultant release of latent heat, and (3) additional cooling of the water column and the release of specific heat associated with ice growth and concomitant rejection of salts, which would depress the freezing temperature of the solution in front of the advancing freezing front. Another potential but less probable explanation is that the system is not in steady state and the heat is from episodic events of water inflow. We consider (2) the strongest candidate and have calculated that it would require 17.6 cm/yr of basal freezing to generate the observed heat. Profiles of microbial biomass in the ice cores indicate that bacterial and microalgal cells (primarily filamentous cyanobacteria) are associated with sedimentary material within the ice matrices. Assays performed on ice core meltwater demonstrated that the populations of both heterotrophic and autotrophic microbes (at depths ranging from 0 to 12 m) retained metabolic potential (measured via the incorporation of radio-labeled CO2, thymidine and leucine), which was realized upon thawing of the ice samples. This suggests that the ice-bound microbial populations are capable of growth if liquid water were to become available within the permanent ice environment. Although the combination of processes that lead to the formation of active water in Lake Vida are unknown at this time, the preliminary temperature records and anecdotal observations suggest that the upper 5 m of the approximately 20-m ice cover is an "active" zone where seasonal warming, melting, and freezing occurs. Deeper in the ice, annual temperatures remain well below 0C. Thus, liquid water below the upper "active" layer is likely to be found only in association with the brine solution that was found at approximately 16 m. We have no information on the geochemistry of the brine/water column beneath or within the ice cover, so we do not know if the water provides either a reducing or oxidizing environment. Therefore, we cannot yet speculate on the types of microbial consortia that may be present. Nor do we know whether the brine contains microbial cells and/or activity. Additional information is contained in the original.
Doran Peter T.
Fritsen Chris H.
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