Organic geochemical studies in the McMurdo Dry Valleys, Antarctica

I. Matsumoto Genki

doi:10.20612/rog.23.24.0_53

I review the organic geochemical studies of lake waters and sediments, soils, Beacon Supergroup of Gondowanaland sediments and cryptoendolithic microbial communities of the McMurdo Dry Valleys in southern Victoria Land, Antarctica. Total organic carbon contents in the anoxic lake waters of meromictic lakes are extremely high due to the accumulation of refractory organic matter from microbial degradation together with dissolved salts. Vertical distribution of physicochemical properties reveals the zonation of microbial communities in the lakes. The composition of organic components in the lakes and ponds varies largely, reflecting differences in microbial communities and their activities. Significant amount of long-chain (>C₁₉)n-alkanes and n-alkanoic acids are detected in some lake and pond sediments, implying that microorganisms are important sources of their long-chain components in natural environments on the earth. Unusually, the predominant sterol in the McMurdo Dry Valleys is frequently 24-ethylcholest-5-en-3β-ol (C₂₉ cholesterol) which is synthesized by green algae, cyanobacteria and/or diatoms. As expected a series of phenolcarboxylic acids related to the lignin of vascular plants are absent but p-hydroxybenzoic acid is predominant phenolcarboxylic acids in the lakes and soils. <br>The occurrence of matured isomers of steranes and triterpanes, the paucity of n-alkenoic acids and visual kerogen in soil samples from the McMurdo Dry Valleys suggest that organic components in the soil samples are derived from erosion of Beacon Supergroup sedimentary rocks and past biological debris containing vascular plant waxes as well as wind transported cyanobacterial mats rather than living organisms. <br>Long-chain n-alkanes and n-alkanoic acids are found in Beacon Supergroup samples from the McMurdo Dry Valleys. Steranes, triterpanes and visual kerogen results of Beacon Supergroup samples from Allan Hills imply that organic materials in the sedimentary environments are contributed mainly by vascular plants with some influence of microorganisms, whereas those of Carapace Nunatak sample are largely due to fern spores. Variable thermal maturities of steranes probably reflect thermal effects of basaltic dikes on the Beacon Supergroup in these areas in Jurassic time. Thermal stresses of the formation of the Beacon Supergroup prior to basaltic intrusion have been estimated to be quite low. <br>Long-chain anteiso-alkanes and anteiso-alkanoic acids are often major components in cryptoendolithic microbial communities originating probably from microorganisms, such as bacteria and lichens in moderate pH conditions (pH 3-5). Normal alkenoic/alkanoic acid ratios are probably useful marker for the fossilization of cryptoendolithic microbial communities. Thermally matured triterpanes and steranes from fossilized associations on Mount Fleming strongly suggest the presence of Gondowanaland sediments formed during Devonian and Jurassic (400-180 million years ago).

Organic geochemical studies in the McMurdo Dry Valleys, Antarctica

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