Reconstruction of the Global Carbon Cycle Fluxes from the Devonian to the Present

Stabilizing mechanisms of Earths climate are key for the persistence of life over geological timescales. To prevent catastrophic excursions of temperature, the Earth thermostat theory suggests that carbon dioxide (CO2) fluxes into the atmosphere, mainly by solid Earth degassing, should be largely compensated by continental and seafloor weathering as well as the burial of organic carbon (C). CO2-driven temperature changes, increasing or decreasing CO2 consumption via silicate weathering, is thereby considered as the main negative feedback. Here, we use reconstructions of paleogeography and the distribution of the major climatic zones combined with proxy data of atmospheric CO2 and oxygen to calculate the fluxes of the global C cycle from 400 Ma ago to the present. For the flux calculation, the parametrizations from state-of-the-art biogeochemical models are used [1]. While providing a data-driven and spatially explicit estimation of the major C fluxes, we assess whether the Earth thermostat theory of balancing C in- and outputs into the atmosphere has been fulfilled. We show that the modelled atmospheric C mass balance fluctuates around zero, in general support of the Earth thermostat theory. Nevertheless, imbalances of up to 1.6e+13 mol C a1 are modelled with largest discrepancies during the Devonian, the Triassic-Jurassic boundary, the mid-Cretaceous and the early Cenozoic. We further test the sensitivity of the C fluxes to uncertainties in the proxy data and to different process parametrizations. The modelled mass balance is less sensitive to variation in the CO2 proxy data, climate sensitivity and global runoff compared to variation in the solid Earth degassing rate, the organic C burial rate and the plant-mediated silicate weathering enhancement. Despite its relevance, the terrestrial and marine organic C production is represented very simplistically in current continuous biogeochemical models. A better representation of these processes, including evolutionary aspects like thermal adaption, will help to understand the evolution of atmospheric CO2 throughout the Phanerozoic. [1] Mills, B.J.W., Donnadieu, Y., Goddéris, Y., 2021. Spatial continuous integration of Phanerozoic global biogeochemistry and climate. Gondwana Research, in press. https://doi.org/10.1016/j.gr.2021.02.011.