PETROGRAPHY OF PERMIAN-TRIASSIC BOUNDARY SECTIONS OF TURKEY, CHINA AND JAPAN: IMPLICATIONS FOR CAUSES AND ENVIRONMENTS OF THE END-PERMIAN MASS EXTINCTION

The causes of the largest extinction in Earth's history - at the end of the Permian, 251 million years ago - remain controversial. We present a global comparison of the facies succession across the Permian Triassic boundary (PTB). In Turkey microbialite framestone overlies the PTB, above a sharp truncation surface in oolite. This surface is encrusted with aragonite fans and contains no evidence of subaerial exposure. The upper half of the microbial unit is composed of wavy bedded microbial laminae with soft sediment deformation that occurred during sedimentation. The PTB in China occurs at a sharp truncation surface lacking evidence of subaerial exposure similar to that found in Turkey. However, the surface occurs beneath the base of the microbialite and no oolite is present. Here, aragonite fans are more abundant in the basal part of the microbialite and soft sediment deformation is lacking. The PTB boundary section in Japan consists of dolomitized skeletal packstone overlain by dolomitized microbialite, and coquina limestone. Unlike China and Turkey, a truncation surface is not observed in outcrop. However, petrography of the oldest microbialite sample reveals a truncation surface beneath the base of the microbialite. Aragonite fans are abundant throughout the microbialite. In some cases they nucleate on the microbial framework and radiate out into internal cavities; in other, they grow into the sediment. One sample contained small rounded grains that resembled ooids but the grains were entirely composed of radial aragonite crystal aggregates. The truncation surface, microbialites, and aragonite crystal fans formed in marine deposits globally and synchronously during and following the end-Permian mass extinction. The global formation of a truncation surface in the marine environment is consistent with a rapid delivery of CO2 and decreasing pH of seawater that caused dissolution on the seafloor. The rapid increase in CO2 is interpreted to have resulted from massive volcanic eruptions represented by the Siberian Traps. The global precipitation of calcified microbialite and aragonite fans likely form as the result of high alkalinity in the ocean resulting from CO2 buildup in the atmosphere, increased carbonate and silicate weathering on the continents and delivery of CaCO3 to the ocean by river systems.