PARALLEL TRENDS IN ORGANIC AND INORGANIC CARBON ISOTOPES ACROSS THE PERMIAN TRIASSIC BOUNDARY

Stable carbon isotope ratios in both inorganic and organic reservoirs have been widely applied to model environmental and sedimentological changes on a global scale. Most studies dealing with major extinction events have used the record of inorganic carbon. In this paper we report the relation between shifts in carbon-13 content of organic matter and coexisting carbonate fractions at a major extinction event, the Permian/Triassic boundary. We found that both deltaC-13(carb) and deltaC-13(org) of the surface ocean varied dramatically across the boundary, but the fractionation DELTAC-13 between organic matter and carbonate remained constant. This result appreciably restricts the interpretation of changes in the carbon cycle during this critical interval. The new data are best explained by a combination of two mechanisms for variation in deltaC-13(carb): (1) burial and erosion of organic carbon, with a long time constant: and (2) sequestration of organic carbon into shallow and deep oceanic reservoirs, with a shorter time constant. For application to our case, the first mechanism is limited by possible buildup of marine PCO2, which would increase the isotopic fractionation factor. The second mechanism is limited in application to short-term transient variations in deltaC-13. Modelling of the carbon cycle and its variations of deltaC-13 must take both mechanisms into account.