Volcanic CO2 outgassing scenarios

To control the mean atmospheric concentration of CO2 in the model, we used small changes in the prescribed volcanic CO2 outgassing. The present-day volcanic outgassing used in the model is 5.3 Tmol C/year, which is the value that balances the weathering rate (26). The value of volcanic CO2 outgassing for ~3 Ma ago has been derived from fitting modeling results to the benthic δ18O stack. For that, we used an ensemble of transient model simulations initialized at 3.2 Ma ago and run for 500 ka driven by orbital variations, with continents fully covered by regolith and prescribed constant volcanic CO2 outgassing ranging from 5.3 to 6.5 Tmol C/year. The different values of volcanic CO2 outgassing lead to different modeled CO2 concentrations. A best fit of modeled benthic δ18O to the stack of (3) during interglacials is used to constrain the value of CO2 outgassing to 6.2 Tmol C/year for the time interval between 3 and 2.7 Ma ago. Some recent studies suggest that Antarctica could have contributed up to 20 m of sea-level equivalent during the late Pliocene/early Pleistocene (47, 48). That would translate in a ~0.2‰ reduction in δ18O, which, in turn, would lead to lower volcanic CO2 outgassing values ~6.0 Tmol C/year to be more appropriate for the early Pleistocene. As a consequence, modeled atmospheric CO2 concentration would also be lower by ~30 ppm. We then constructed four different scenarios for the temporal evolution of volcanic CO2 outgassing between 3 Ma ago and the present day. All scenarios consist of a long-term decrease from 6.2 Tmol C/year at 3 Ma ago to 5.3 Tmol C/year at present, but the timing and rate of the decrease vary between scenarios (Fig. 1C). A decrease from 6.2 to 5.3 Tmol C/year corresponds to a decrease of less than 20%. Because even the value for the present-day volcanic outgassing is known with an uncertainty of more than 100% (~3 to 10 Tmol C/year) (28), changes in volcanic outgassing represent a possible candidate to explain the long-term CO2 decrease. However, the aim of our study is not to explain the causes of the CO2 decrease, and other processes such as changes in weathering (7) or organic carbon burial in deep-sea sediments (29) would also be potential candidates to explain the long-term CO2 decline.

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