Biomarker and isotopic trends in a Permian-Triassic sedimentary section at Kap Stosch, Greenland
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NOTICE: this is the author’s version of a work that was accepted for publication in Organic Geochemistry. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Organic Geochemistry . 43, (Feb), 2012. http://dx.doi.org/10.1016/j.orggeochem.2011.10.010
We report a geochemical study of a composite sedimentary section that captures the Permian-Triassic (PT) transition at Kap Stosch, East Greenland. The samples were from the original paleontological collection of early PT researchers. The rocks, which include samples from four proximal outcrop localities, were deposited during the Late Permian and Early Triassic at the margin of the Boreal Sea with a depositional hiatus and erosional event of unknown duration. Bulk geochemical measurements for most of the samples show good correlation between S2 and TOC% which, combined with low Tmax values, indicate that the organic matter (OM) that formed contemporaneously with sediment deposition is of relatively low maturity. Significant changes through the PT transition include a pronounced switch in the δ13C of TOC from high values near -24% to lower values averaging 32%, that is matched by a significant increase in the hydrogen index (HI) of the kerogen. The Permian samples containing 13C enriched OM also have low Rock-Eval HI values and anomalous pyrograms, indicating that the kerogen is heterogeneous in terms of source and maturity, as confirmed by microscopic analysis of the kerogen concentrates.Samples from the Permian section contain an abundance of black angular fragments of woody tissue in addition to gymnosperm pollen and spinose acritarchs of the Vittatina-Association (Balme, B., 1979. Palynology of Permian-Triassic boundary beds at Kap Stosch. Meddeleleser om Gronland 200, 1-36). In contrast, black woody tissue is rare in samples from the Early Triassic section with well preserved gymnosperm and lycopod pollen and spores of the Protohaploxypinus and Taeniaesporites associations. Biomarkers indicate moderate maturity for Permian samples, with the C27 sterane 20S/(20S + 20R), C31 homohopane 22S/(22S + 22R) ratio and Ts/(Ts + Tm) values all being higher than those for Triassic sediments. The marked switch in maturity indicators across the PT transition suggests an unconformity consistent with palynological observations. The pristane/phytane values are low and the homohopane index values high, indicating that anoxic conditions prevailed throughout deposition of the sediments.Additionally, markers of photic zone euxinia (i.e. isorenieratane, crocetane and 2,3,6-aryl isoprenoids) were present in all samples and all show maximum abundance closest to the PT transition. The C33 n-alkyl cyclohexane, a potential event marker for the onset of the biotic crisis in the Late Permian, was found in samples at, and immediately following, the paleontological PT transition. Despite the distinct change in lithology across the PT transition, the redox and Chlorobi-derived biomarkers indicate that photic zone euxinic conditions prevailed throughout the deposition of the Kap Stosch sedimentary sequence.
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