Evidence for in-situ methane production in ice based on anomalous isotope analyses
Abstract
Studying microbial ecology at low temperatures is important for understanding the limits of life processes as well the search for extraterrestrial life. Glacial ice sheets are special habitats where microbes have been preserved for geologically significant periods of time. Glaciers provide three distinct environments for microbial ecosystems. Subglacial lakes beneath the East Antarctic ice sheet provide one of the most intriguing environments that have yet to be explored. The upper portion of a glacier is formed from eolian derived (wind blown) materials (snow, impurities and microbes). Bulk impurity levels tend to be less than a few ppm, cell densities generally below 100 cells/ml and surface temperatures are generally below -15oC. Subglacial environments (lowest 20m), on the other hand, tend to have (by comparison with the overlying glacier ) extremely high impurity concentrations, cell densities on the order of 106 cells/ml, and temperatures close to the pressure melting point (~ 0oC). Microbial communities in the subglacial environments are comprised of eolian derived organisms that have traveled vertically through the ice sheet as well as organisms that inhabited the soil/rock environment before the glacier formed. Cell density measurements in glacier ice are fairly straightforward given proper cleaning techniques. Whether or not the cells in a glacier are able to grow (or at least maintain their metabolic functionality) while immured in the glacier has yet to be determined. This question remains unanswered largely because the metabolic rates of microbial communities in ice have not been measured in the lab. One way to infer in-situ microbial activity in ice is to analyze the elemental and isotopic composition of gaseous metabolic byproducts that are retained in the ice matrix. We present two case studies in which the measured methane (CH4) concentration and isotope values in ice result from in-situ production. Methane measurements spanning the last 25kyr from the Sajama ice core from central Bolivia (18oS, 69oW, 6542masl), for example, were 1X-5X higher than contemporaneous values recorded in polar ice cores [Campen et al., 2003]. \delta13CH4 values from five discrete depths were compared to corresponding measurements made on the Taylor Dome ice core and suggest the additional (in-situ) CH_{4} in the Sajama samples has an average isotopic composition of -63.2±2.8‰ . For reference, atmospheric δ ^{13}CH_{4} values range from -42 to -45/pm over this period. The Sajama isotope values are characteristic of methanogenic CH_{4} emitted from most terrestrial ecosystems. The second case study revolves around ice that was recovered from a perennially ice covered lake in the McMurdo Dry Valleys, Antarctica. Previous work on ice from Lake Bonney demonstrated a rich microbial consortium located ~2m below the surface [Priscu et al., 1998]. Methane isotope analyses were made on ice from this depth interval to identify the presence of microbially produced CH_{4}. δ ^{13}CH_{4} and δ DCH4 results suggest the CH4 arises from acetogenic CH4 production as opposed to CO2 reduction. Campen, R.K., T. Sowers, and R.B. Alley, Evidence of Microbial Consortia Metabolizing Within a Low-Latitude Mountain Glacier, Geology, 31 (No. 3), 231-234, 2003. Priscu, J.C., et al., Perennial Antarctic Lake Ice: An oasis for life in a polar desert, Science, 280, 2095-2098, 1998.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2004
- Bibcode:
- 2004AGUFM.B23C..02S
- Keywords:
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- 4815 Ecosystems;
- structure and dynamics;
- 1827 Glaciology (1863)