MAGMATISM-INDUCED EXPANSION OF SULFIDIC WATERS IN THE PANTHALASSA PERMIAN-TRIASSIC OCEAN
Geochemical signals in the section indicate establishment of strongly euxinic bottom water conditions shortly after a regional (and perhaps global) transgression reinitiated sedimentation in the Late Permian. Euxinic signals are most prominent in the uppermost Permian sediments, and gradually wane up section into the lowermost Triassic strata. Based on the low hydrogen index of the organic matter, we suggest that the amount of organic carbon in the sediments reflects mostly changes in the terrestrial vegetation and not primary marine productivity.
Unusual sulfur-iron-organic carbon patterns characterize the Upper Permian – lowermost Triassic part of the section. These may form in a strongly euxinic ocean with prolific bacterial sulfate reduction leading to near-complete decomposition of predominantly labile marine organic matter. Alternatively, widespread oxidation of methane by sulfate reducers could explain observed variations in sediments near the PTB.
Comparison of new Re-Os isotope data from Opal Creek with published Permian-Triassic data reveals the potential for a global high Re and high Re/Os in Upper Permian shales. At Opal Creek, drastic and permanent declines in Re concentrations, and Re/organic matter, Re/Al, and Re/Os ratios, are associated with deposition of a pyrite layer and overlying siltstones shortly above the base of the section and before the mass-extinction event. This interval also marks dramatic and permanent declines of phosphorous content, reduced sulfur and total nitrogen isotope ratios, and a prominent positive carbon isotope ratio excursion in the minimal organic carbon present in the rocks. Initial Os isotope ratios decline from ~0.55 to ~0.35 in this interval, indicating a possible magmatic trigger for the intensification and expansion of Late Permian euxinia. Stronger magmatic pulses recorded by lower initial Os ratios followed the main extinction.
Acknowledgements – Supported under CHRONOS project. We thank T. Algeo for providing samples from his Opal Creek collection.