Carbon Budget and Carbonate Saturation in Global Paleo-ocean

The relationships between the global carbon cycle and paleo-climates on short and long time scales have been based on studies of accumulation rate of different components of the sedimentary carbon reservoir, as well as reconstructions of the geographic distributions of carbon burial. Variations in the rate and proportion of carbonate burial through Phanerozoic time have been attributed to the influence of tectonics on eustasy, atmospheric CO2 concentration, MOR hydrothermal flux, and weathering and riverine flux. Variability in biologic production (Corg and Ccarb) through geologic time also had a significant impact on surface ocean chemistry and the sedimentary carbon record. In this study, a geochemical model with Phanerozoic atmospheric pCO2 and surface ocean temperature reconstructions from the literature was used to estimate the history of variations in surface-ocean chemistry (degree of saturation with respect to calcite and aragonite). The results show that, using present-day values of ocean salinity and alkalinity, the Early Paleozoic and Middle Mesozoic oceans were undersaturated with respect to CaCO3. For the present-day values of supersaturation (IAP/Ksp) of 3.0 to 4.5, paleo-alkalinity of ocean water would have been up to 2.5 times greater than at present, although the pH values of surface ocean water would have been somewhat lower than the present value. This alkalinity factor is consistent with a higher calcium concentration (up to 2.5x) due to increased MOR circulation and also higher salinity (up to 1.5x) attributed by other authors to segments of the geologic past. Our model results indicate that pCO2 was a contributing factor to shifts between calcite and aragonite saturation of seawater, however, additional changes in alkalinity were also needed to maintain supersaturation. In addition to MOR circulation, continental weathering of crystalline and older carbonate rocks was likely an important mechanism for maintaining supersaturation of surface-ocean water, particularly during times of increased carbonate storage.