GEOCARBSULF: A COMBINED MODEL FOR PHANEROZOIC ATMOSPHERIC O2 AND CO2

A model for the combined long-term cycles of carbon and sulfur has been constructed which combines all the factors modifying weathering and degassing of the GEOCARB III model for CO2 (Berner and Kothavala, 2001) with rapid recycling and oxygen dependent carbon and sulfur isotope fractionation of an isotope mass balance model for O2 (Berner, 2001). New isotopic data for both carbon and sulfur are used and new feedbacks are created by combining the models. Sensitivity analysis was done by determining (1) the effect on O2 of varying the dependence of organic matter and pyrite weathering on erosion rate and the degree of exposure of coastal lands by sea level change; (2) the effect on O2 of using different data for carbon isotope fractionation during photosynthesis; (3) the effect on sulfur isotope fractionation of varying the size of O2 feedback during bacterial sulfate reduction: (4) the effect on CO2 of adding rapid recycling to the weathering of organic matter and carbonates. (Rapid recycling treats carbon and sulfur weathering in terms of young rapidly weathering rocks and older more slowly weathering rocks) Results show little change in the trend of atmospheric CO2 over the Phanerozoic from the results of GEOCARB III. There are generally very high values for CO2 in the early Paleozoic, low values in the late Paleozoic during the Permo-Carboniferous glaciation, a very sharp peak at the Permo-Triassic boundary, and a more-or-less continuous decline of CO2 from the Jurassic to the present. These results are in general agreement with independent estimates of CO2 levels. . Atmospheric oxygen shows a major broad late Paleozoic peak with a maximum value of about 30% O2 in the Permian, a secondary less-broad peak centered near the Silurian/ Devonian boundary, variation between 15% and 20 % O2 during the Cambrian and Ordovician, a very sharp drop from 30% to 15% O2 at the Permo-Triassic boundary, and a more-or less continuous rise in O2 from the late Triassic to the present. These results correlate well with evolutionary events for both plants and animals (P. D.Ward, in preparation).