DEPLETION OF REDOX-SENSITIVE ELEMENTS IN THE MEISHAN SECTION, CHINA: PROXY EVIDENCE FOR LARGE-SCALE OCEANIC ANOXIA ACROSS THE PERMO-TRIASSIC BOUNDARY?

We have undertaken a detailed sampling and analysis across the Permo-Triassic boundary at the Meishan B section, adjacent to the GSSP, China, to assess environmental change at the P-Tr boundary (Bed 27) and the main extinction horizon (Bed 25). A total of 38 samples was taken from a 2.3-metre interval of limestone, shale and dolomitic marl, from Bed 24a to Bed 32, and analysed by XRF for major and trace elements. A subset of 26 samples, predominantly dark-coloured mudrocks, was analysed for Re and Os abundances and ¹⁸⁷Os/¹⁸⁸Os.

Initial ¹⁸⁷Os/¹⁸⁸Os_(t=250Ma) ranges from 0.3 to 2, with no consistent pattern up-section. The data thus provide no evidence at Meishan for major perturbation of seawater ¹⁸⁷Os/¹⁸⁸Os in this region of Palaeotethys, at least at the scale of the sampling. This is unlike early Jurassic or end-Cretaceous boundary sections which record substantial and possibly global shifts in seawater ¹⁸⁷Os/¹⁸⁸Os.

The section between Beds 24 and Bed 27 shows major changes in elemental abundances that partly accompany the documented excursion in δ¹³C. Redox- and biologically-sensitive elements such as Cu, Ni, P, Zn and V show strong fluctuations in abundance throughout Bed 24, even when abundances are normalised with Al₂O₃ to minimise the effects of carbonate dilution. In Beds 26 and the lower part of Bed 27, however, the Al₂O₃-normalised concentrations of these elements fall by an order of magnitude, and remain consistently low in the overlying Triassic marls. Re and Os show similar dramatic decreases. Whilst these decreases are partly an artefact of Al-normalisation, the changes are accompanied by large increases in ratios involving lithogenous elements such as rare-earth elements, Nb, Zr, Th and Ti.

The elemental data thus record a dramatic change in conditions at Meishan, beginning at Bed 26. One cause may be an increase in the flux of terrigenous material, but this is not reflected by the ¹⁸⁷Os/¹⁸⁸O. Alternatively, there was a sudden reduction of redox-sensitive elements in the seawater column, removed by oceanic anoxia in distal, but contiguous, water masses. If the Meishan seas had open exchange with at least Paleotethys, then the anoxic event could have been global, as suggested by other studies.