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Organic Carbon in a Late Permian to Earliest Triassic Lacustrine Environment: Implications for Organic Matter Type, the Carbon Cycle, and Global Change across the Permian-Triassic Boundary

Thomas, Stephanie 1; Tabor, Neil 1
1 Earth Sciences, Southern Methodist University, Dallas, TX.

More than 1 km of Upper Permian and Lower Triassic strata outcrop in the Turpan Basin, NW China. These strata were deposited within a fluvio-lacustrine environment where paleosols and other climate-sensitive sedimentologic features indicate an especially noticeable trend towards arid conditions across the Permian-Triassic Boundary. Organic matter samples were collected from the Wutonggou, Guiodikeng, & Jiucaiyuan fms and were classified as either (1) identifiable vascular plant matter, where biological structures can be recognized macroscopically or (2) non-identifiable organic matter, where no biological structures are recognized by the naked eye.

Interpreted depositional environment (fluvial, flood plain, lacustrine) was used to elucidate the origin of organic matter. Furthermore, macroscopically unidentifiable organic matter was prepared for bulk geochemical Rock-Eval analysis to determine the kerogen type, i.e. a terrestrial vascular matter or lacustrine algal matter signature.

Stable carbon isotope analysis of organic matter provides additional clues to lake evolution and the global carbon cycle. Variations among organic matter δ13C values are strongly related to photosynthetic process. Although algae and Permian-Triassic land plants likely used C3 photosynthesis, algal photosynthesis is modified by the use of HCO3-, which provides a very different isotopic signature than land plants, which generally take CO2 directly from the atmosphere.</P><P>THUS, δ13C values are expected to co-vary with type, i.e. whether the organic matter is derived from algal, fungal, or vascular plant matter. Specifically, lacustrine algal matter δ13C values should be relatively positive (-20 ‰), whereas terrestrial vascular C3 plant matter should be relatively more negative (-27‰). Preliminary results demonstrate a large shift in δ13C values near the perceived Permian-Triassic Boundary, from -24‰ to -31‰. This shift likely does not indicate a transition between organic matter types, but is plausibly caused by incorporation of atmospherically-derived light carbon that is mechanistically linked to the end-Permian life crisis.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009