Isotopic chemostratigraphy of the microbialite-bearing Permian-Triassic boundary section in the Zagros Mountains, Iran

Carbon isotopic chemostratigraphy and a Sr-isotope datum provide age-constraints to the uppermost Permian-lowermost Triassic section in central Zagros Mountains, Iran. The 77-m-thick carbonate-dominated section contains two microbialite beds that consist of abiotic carbonate crystals and possible calcimicrobes. The textures resemble those reported from the lowermost Triassic elsewhere, which were likely developed under unusual environmental and ecological conditions after the end-Permian extinction. Our results show that the lower section up to the lower microbialite bed has high δ¹³C values of + 3 to + 4‰, which are typical values for the latest Permian. From 3 m above the base of the lower microbialite, the values abruptly decreases to - 0.3‰. In the upper section, the δ¹³C curve records another negative excursion and gradual increase to + 2.0‰. The curve coincides with the global trend from other Permian-Triassic boundary sections, and places the chemostratigraphic boundary within the lower microbialite. The δ¹³C negative excursion postdates geochemical and/or ecological changes that triggered the microbialite development. If the trigger was increased alkalinity by a vast carbon flux, the source carbon should not have significantly light value of δ¹³C.