New paleogeographic and degassing parameters for long-term carbon cycle models

Long-term carbon cycle models are critical for understanding the levels and underlying controls of atmospheric CO2 over geological time-scales. We have refined the implementation of two important boundary conditions in carbon cycle models, namely consumption by silicate weathering and carbon degassing. Through the construction of continental flooding maps for the past 520 million years (Myrs), we have estimated exposed land area relative to the present-day (f(A)), and the fraction of exposed land area undergoing silicate weathering (f(AW-fA)). The latter is based on the amount of exposed land within the tropics (+/- 10 degrees) plus the northern/southern wet belts (+/- 40-50 degrees) relative to today, which are the prime regions for silicate weathering. We also evaluated climate gradients and potential weatherability by examining the distribution of climate-sensitive indicators. This is particularly important during and after Pangea formation, when we reduce f(AW-)f(A) during times when arid equatorial regions were present. We also estimated carbon degassing for the past 410 Myrs using the subduction flux from full-plate models as a proxy. We further used the subduction flux to scale and normalize the arc-related zircon age distribution (arc-activity), allowing us to estimate carbon degassing in much deeper time. The effect of these refined modelling parameters for weathering and degassing was then tested in the GEOCARBSULFvolc model, and the results are compared to other carbon cycle models and CO2 proxies. The use of arc-activity as a proxy for carbon degassing brings Mesozoic model estimates closer to CO2 proxy values but our models are highly sensitive to the definition of f(AW-)f(A). Considering only variations in the land availability to weathering that occur in tropical latitudes (corrected for arid regions) and the use of our new degassing estimates leads to notably higher CO2 levels in the Mesozoic, and a better fit with the CO2 proxies. (C) 2021 The Author(s). Published by Elsevier B.V. on behalf of International Association for Gondwana Research.

Automated summary

Long-term carbon cycle models play a crucial role in understanding the evolution of atmospheric CO2 levels over geological time scales. This study refines boundary conditions related to silicate weathering and carbon degassing, and evaluates the impact of these factors on the carbon cycle model. By investigating continental flooding maps and climate-sensitive indicators, the study provides insights into the changes in land area exposure and potential weathering effects over the past 520 million years. The use of arc-activity as a proxy for carbon degassing helps to bring Mesozoic model estimates closer to CO2 proxy values.