RE-OS SYSTEMATICS OF ARCTIC BLACK SHALES REVEAL EXTRAORDINARY CONDITIONS APPROACHING THE PERMO-TRIASSIC BOUNDARY

The end-Permian mass extinction left a marked bio/chemo stratigraphic record that has focused a diversity of geoscientists on a single issue. Detailed Re-Os studies spanning Upper Permian black shale horizons from the Mid-Norwegian shelf and East Greenland provide the first precise depositional ages and record of the Os isotopic composition for latest Permian seawater (Georgiev et al. 2010). Here we discuss the geochemical evidence for an extraordinary sequence of paleoenvironmental changes leading up to the foremost mass extinction in all of Earth history.

Three intervals of organic-rich shale taken from a drill core penetrating the Mid-Norwegian shelf yield precise and indiscernable Re-Os isochrons at 252 Ma. Three associated and well-constrained initial ¹⁸⁷Os/¹⁸⁸Os ratios of ~0.6 are lower than the 0.68-0.83 range for Middle Triassic seawater (Xu et al. 2010). A dramatic and systematic increase in ¹⁸⁷Re/¹⁸⁸Os ratios (from ca. 100 to >6000) approaching the Permian-Triassic boundary is attributed to rapidly rising Re in combination with stable Os concentrations in these shales. For comparison, ¹⁸⁷Re/¹⁸⁸Os ratios typical of black shales are ca. 1 to 600, and rarely exceed 1300.

Drill core from the Ravnefjeld Formation in East Greenland provides shale horizons that yield an identical 252 Ma age and the same exceptional ¹⁸⁷Re/¹⁸⁸Os ratios. These data affirm a proposed stratigraphic correlation between the mid-Norwegian shelf and facing units in East Greenland, now separated by the North Atlantic Ocean.

Published Re-Os data for Late Permian shales from Poland (Pasava et al. 2010) and China (Yang et al. 2004), with far less precise Re-Os ages are here noted to possess the same exceptionally high ¹⁸⁷Re/¹⁸⁸Os. We suggest a global Late Permian seawater ¹⁸⁷Re/¹⁸⁸Os notably higher than the present-day value of ~4250 (Peucker-Ehrenbrink & Ravizza 2004). Experimental work by Yamashita et al. (2007) suggests that highly reducing conditions produce high ¹⁸⁷Re/¹⁸⁸Os in shales. We suggest that the exceptionally high ¹⁸⁷Re/¹⁸⁸Os ratios for the Permian-Triassic shales require additional factors, such as elevated temperature and/or acidity of the Late Permian bottom waters.

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