GEOCHEMISTRYAND PETROGRAPHY OF THERMALLY METAMORPHOSED ANTARCTIC COAL: IMPLICATIONS FOR 13C -DEPLETED METHANE RELEASE
Large δ13C excursions present at the Permian-Triassic boundary are thought to indicate a considerable release of isotopically light carbon into the atmosphere (Retallack and Jahren, 2008). The largest of these excursions (-22.2 ‰) was measured in organic matter from Antarctica (Retallack and Jahren,...
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| Format: | Others |
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OpenSIUC
2012
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| Online Access: | https://opensiuc.lib.siu.edu/theses/821 https://opensiuc.lib.siu.edu/cgi/viewcontent.cgi?article=1829&context=theses |
| Summary: | Large δ13C excursions present at the Permian-Triassic boundary are thought to indicate a considerable release of isotopically light carbon into the atmosphere (Retallack and Jahren, 2008). The largest of these excursions (-22.2 ‰) was measured in organic matter from Antarctica (Retallack and Jahren, 2008). Antarctic coals are known to be heavily intruded by Jurassic dikes and sills, and the δ13C values of the organic matter may have been influenced by later thermal alteration. In order to evaluate the influence of rank and maceral content on isotopic composition, a total of 335 samples described as Permian-age "coal" were obtained primarily from the United States Polar Rock Repository. Most of the organic matter from Permian coal seams in Antarctica has been extensively altered after burial by localized high heat flow and, in some cases, contact metamorphism associated with dikes and sills; this thermal alteration has likely changed the δ13C values of the organic matter. The rank of the samples prior to intrusion is estimated to be medium to high volatile bituminous. The majority of the samples analyzed (96%) have been altered to above low volatile bituminous rank based on vitrinite reflectance, most (83%) are semi- to meta-anthracites, and a few have been altered to anisotropic cokes. The samples do not follow the typical burial maturation geochemical track, as they are higher in volatile matter (%, daf) and O (%, dmmf), and lower in H (%, dmmf) than coals of the same rank that have undergone normal burial maturation. Carbon stable isotopic data indicate a weak correlation with coal rank as well as with the amount of pyrolitic carbon. Although the isotopically lightest samples measured in this study are located within what is interpreted to be the Permian-Triassic boundary, the effects of thermal alteration of organic matter on δ13C values must be considered in any interpretation of Permian-Triassic atmospheric conditions. |
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