CARBON, SULFUR AND OXYGEN ACROSS THE PERMIAN-TRIASSIC BOUNDARY

The results of a theoretical isotope mass balance model are presented for the time dependence of burial and weathering-plus-degassing fluxes within the combined long term carbon and sulfur cycles. Averaged data for oceanic d13C and d34S were inputted each million year from 270 to 240 Ma (mid-Permian to mid-Triassic) in order to study general trends across the Permian-Triassic boundary. Results show a drop in the rate of global organic matter burial during the Late Permian and a predominance of low values during the Early-to-Middle Triassic. This overall decrease with time is ascribed mainly to epochs of conversion of high biomass forests to low biomass herbaceous vegetation resulting in a decrease in the production of terrestrially derived organic debris. Additional contributions to lessened terrestrial carbon burial were increased aridity and a drop in sea level during the Late Permian which led to smaller areas of low-lying coastal wetlands suitable for coal and peat deposition.

Mirroring the drop in organic matter deposition was an increase in the burial of sedimentary pyrite, and a dramatic increase in the calculated global mean ratio of pyrite- S to organic-C. High S/C values resulted from an increase of deposition in marine euxinic basins combined with a decrease in the burial of low-pyrite associated terrestrial organic matter. The prediction of increased oceanic anoxia during the Late Permian and Early Triassic agrees with independent studies of the composition of sedimentary rocks

Weathering plus burial fluxes for organic carbon and pyrite sulfur were used to calculate changes in atmospheric oxygen. The striking result is a continuous drop in O2 concentration from about 30% to 13% over a twenty million year period. This drop was brought about mainly by a decrease in the burial of terrestrially-derived organic matter. but with a possible contribution from the weathering of older organic matter on land. It must have exerted a considerable influence on animal evolution because of the role of O2 in respiration. Some examples are the extinction of many vertebrates, loss of giant insects and amphibians and the restriction of animals to low elevations. It is concluded that the extinction of plants may have contributed to the extinction of animals.