OCEAN ACIDIFICATION, VOLCANISM AND END-PERMIAN EXTINCTION: PERSPECTIVES BASED ON INTERPRETING CARBON ISOTOPE EXCURSIONS

The end-Permian extinction event coincides with the initiation of voluminous eruption of the Siberian Traps and of a negative shift in the carbon isotope ratio of marine carbonates and terrestrial organic matter most likely reflecting a substantial increase in the carbon dioxide concentration of the atmosphere and surface ocean, perhaps driving ocean acidification and hypercapnia. To assess the extent to which this perturbation may have affected ocean chemistry, namely the saturation state of the surface ocean with respect to calcite and aragonite and surface-water carbon dioxide partial pressure, we utilized the Earth system model of intermediate complexity, Genie, and forced it to conform to the initial negative shift in the carbon isotope record as preserved at Meishan, South China.

As expected, the extent of perturbation to the saturation state and pCO₂ depends on the magnitude and rate of carbon addition, which in turn is dependent on the presumed isotopic composition of the source CO₂ and the duration and magnitude of the carbon isotope shift. An additional uncertainty to which the model is sensitive is the buffering capacity of the Late Permian ocean (before the perturbation), likely reduced from that of the modern ocean because of the presumed lack of deep-sea sedimentary CaCO₃ (i.e., a very shallow CaCO₃ compensation depth). In only the most extreme cases of a purely magmatic source of CO₂ does the surface ocean become widely undersaturated with respect to CaCO₃. Sources such as volatilization of organic-C rich sedimentary rocks, thermogenic methane production, or release of biogenic methane from clathrates drive smaller reductions in saturation state and increases in pCO₂. The simulation that best matches the independent estimates of the degree of warming and quantity of CO₂ released leads to moderate reductions in saturation state but substantial increases in pCO₂of 20 ka duration, potentially sufficient to cause hypercapnia.