Steranes and triterpanes in the Beacon Supergroup samples from southern Victoria Land in Antarctica

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Scientific paper

Steranes and triterpanes in Beacon Supergroup samples (sedimentary rock and silicified wood) from Allan Hills and Carapace Nunatak of southern Victoria Land in Antarctica were studied to elucidate sources of organic materials, sedimentary paleoenvironment and thermal history after deposition. Relative abundances of C 27 , C 28 and C 29 steranes and visual kerogen results of Beacon Supergroup samples from Allan Hills imply that organic materials in the sedimentary paleoenvironments are contributed mainly by vascular plants with some influence of microorganisms, while those of the Carapace Nunatak sample may be largely due to fern spores. The pristane / phytane and pristane / heptadecane ratios of the samples were generally close to unity and between 0.50 and 0.99, respectively, suggesting that the sedimentary paleoenvironment was shallow lacustrine with alternating oxic and anoxic conditions. The (22 S /22 R )-17 (H),21 (H)-C 31 -C 33 triterpane ratios are approximately at thermal equilibrium values ( ca. 1.5) in most samples, while the (20 S /20 R )-5 (H), 14 (H), 17 (H)-C 29 sterane ratios and the (20R + 20S)-5 (H), 14 (H), 17 ( H )/5 ( H ), 14 (H), 17 (H)-C 29 sterane ratios vary from 0.0 to 1.1 and from 0.0 to 1.4, respectively. Most of the (20 S /20 R )-5 (H), 14 (H), 17 (H)-C 29 sterane ratios did not reach thermal equilibrium values. The correlation coefficient between the (20 S /20 R )-5 (H), 14 (H), 17 (H)-C 29 sterane ratios and (20R + 20S)-5 (H), 14 (H), 17 ( H )/5 ( H ), 14 (H), 17 (H)-C 29 , sterane ratios is very high (0.96). These variable maturities probably reflect thermal effects of basaltic dikes on the Beacon Supergroup at Allan Hills and Carapace Nunatak during Jurassic time. Thermal stresses on the Beacon Supergroup prior to basaltic intrusion have been estimated to be quite low, so the paleotemperatures of this formation have been quite low.

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