Characterization of Particles Recovered From a 4 Km Ice Core Above Lake Vostok, Antarctica

Nine samples of glacial ice from distinct glacial maxima and interglacial periods have been recovered from depths ranging from 179 to 3351 meters above Lake Vostok, Antarctica, representing a record of { ~}420,000 years, and three samples of ice from depths of 3540-3590 meters interpreted as having accreted from Lake Vostok, have been analyzed to determine the mineralogic and biologic constituents of these ice cores. Known quantities of ice were melted and particulate matter was collected on 0.2 micron filters for imaging and analysis using cryo-SEMBSE and EDS techniques. The mineralogy of the glacial ice generally follows continental crustal abundances, with modal quartz (19-65%)> alkali feldspar (16-42%)>plagioclase (5-25%). Combined micas (bio-musc-chlor) range from 1-43%, with the higher values recorded in interglacial intervals. Volcanic glass constitutes up to 34% of some samples. Most of the analyzed grains are sub-angular to sub-rounded and occur in the size range of 1-5 microns; 5-10 micron particles of angular volcanic glass and ternary feldspars occur in both glacial and interglacial intervals. In addition, one grain with a Fe-Ni composition is interpreted as a micrometeorite, and fibrous grains of a Mg-Fe silicate are interpreted as enstatite "whiskers". Organic matter occurs irregularly throughout the ice column and includes fragments of diatom frustules, mats of organic debris, and rare rods and coccoids that are most likely microbial. In contrast, sediments in the accreted ice typically have grain sizes <1 micron, with modal biotite (73%)> quartz (13%)> Kspar (9%)>plagioclase (2%). Rod-shaped bacterial cells are readily observed on the filter substrate and attached to mats of organic debris (Priscu et al., 1999). There is a clear distinction in particle composition and morphology and the type of organic material found in the glacial ice compared with accreted ice. Sediments in the glacial ice contain minerals representative of typical continental crust, with little evidence of sorting by grain size or composition. Airborne modifications of the modal mineralogy of the glacial ice may occur through episodic infusions of volcanic ash or crystallites, enrichment of micas through aeolian sorting, and rare capture of meteoric particles. Enrichment of biotite in the accreted ice is interpreted as the result of differential settling in the lake waters prior to accretion. The relatively large abundances of bacteria in the accreted ice implies that Lake Vostok may support microbial life, whereas rare microbes and fragments of diatom frustules in the glacial ice may have been transported and deposited by aeolian processes.