Sulfur isotopic evidence of large release of hydrogen sulfide during P-T transition

The Permo-Triassic mass extinction is the most devastating event in the history of life on Earth and occurred circa 251 Ma ago. The exact cause of extinction remains uncertain. In order to understand the cause of extinction, we studied the sulfur isotopes and redox sensitive elements from the P-T section of Spiti valley of Himalaya, India. In Spiti valley, 1-10 cm of ferruginous band of sediments separates the Permian shale from Triassic limestone. Analyses of redox sensitive elements such as As, Mo, As, Ni, Sb, Th, Mn and Fe show clear evidence of anoxia or euxinia. The total sulfur content of overlying Triassic limestones varies from 0.1 to 0.25% with δ34S values between -17.7 to -6.4 %. The underlying shales contains 0.2 to 0.4 wt% of sulfur with δ34S of sulfate ranging from -16 to -19.8 %. Whereas sulfate content from the boundary are an order of magnitude higher and varies from 1% to 2.5 wt % with δ34S ranging between -13 to -15.4 %. Both above and below the extinction boundary, the δ34S of pyrites are in general lighter than their sulfate counterpart and this most likely results from bacterial reduction of sulfate. The trend is reversed at the boundary where the pyrites are much heavier than sulfate. We believe that this heavy sulfur isotope composition is due to preferential loss of light sulfur isotope during hydrogen sulfide loss to atmosphere. This occurred when chemocline which separates sulfidic deep water from sulfate rich (oxygenated) water abruptly moved upward to the surface. This migration emits copious amounts of H2S to the atmosphere which may be an associated cause for the extinction.