Model based Paleozoic atmospheric oxygen estimates: a revisit to GEOCARBSULF

Geological redox proxies increasingly point towards low atmospheric oxygen concentrations during the early Paleozoic Era, with a subsequent protracted rise towards present-day levels. However, these proxies currently only provide qualitative estimates of atmospheric O₂ levels. Global biogeochemical models, in contrast, are commonly employed to generate quantitative estimates for atmospheric O₂ levels through Earth’s history. Estimates for Paleozoic pO₂ generated by GEOCARBSULF, one of the most widely implemented carbon and sulfur cycle models, have historically suggested high atmospheric O₂ levels throughout the Paleozoic, in direct contradiction to competing models. In this study, we evaluate whether GEOCARBSULF can predict relatively low Paleozoic O₂ levels. We first update GEOCARBSULF by adopting the recent compilation of the δ¹³C value of marine buried carbonate and replacing the old formulation of the sulfur isotope fractionation factor with empirical sulfur isotope records. Following this we construct various O₂ evolution scenarios (with low O₂ levels in the early Paleozoic) and examine whether GEOCARBSULF can reproduce these scenarios by varying the weathering/degassing fluxes of carbon and sulfur, or carbonate δ¹³C. We show that GEOCARBSULF can, in fact, maintain low-O₂ (even 1–5% atm) levels through the early Paleozoic by only varying the carbonate δ¹³C within 2 standard deviation (SD) bounds permitted by the geological record. In addition, it can generate a middle–late Paleozoic rise in O₂ concentration, coincident with the diversification of land plants. However, we also argue that tracking atmospheric O₂ levels with GEOCARBSULF is highly dependent on carbonate carbon isotope evolution, and more accurate predictions will come from an improved C isotope record.