Diagenetic origin of carbon and oxygen isotope compositions of Permian-Triassic boundary strata

Bulk carbonate δ^(13)C and δ^(18)O compositions of profiles across Permian–Triassic (P–T) boundary sections in China, Italy, Austria, and Iran show wide varieties of trends. The δ^(13)C depletions occur in all sections and range from 2 to 8‰ PDB in magnitude. These excursions take place over intervals ranging from less than 0.1 to more than 40 m. The δ^(18)O values may increase or decrease toward the P–T boundary, but decrease sharply by 2–9‰ PDB at or above the boundary. Cross-plots of δ^(13)C and δ^(18)O values from all sections show positive covariance. Wide differences in magnitudes, trends, and position of the excursions relative to the boundary, as well as the covariance patterns suggest that P–T boundary δ^(18)O and δ^(13)C values are partially or entirely diagenetic in origin, formed in association with exposure surfaces. This interpretation implies that P–T boundary sections studied till date were subaerially exposed before, during, and after the mass extinction, resulting in the removal of strata containing key information about the extinction mechanism. This inference is consistent with the paleontological studies that have shown the presence of gaps at the boundary, and further supported by the sharp lithologic changes observed at virtually all P–T boundary sections. Subaerial exposures are documented by detailed sedimentologic and isotopic studies from central Tethyan sections in Abadeh and Shah Reza in Iran. Proposed P–T boundary extinction models are based on isotopic values that are diagenetic in origin and stratigraphic sections that are incomplete, leading to extinction mechanisms with little physical supporting evidence