A fresh look at the pre-industrial air-sea carbon flux using the alkalinity budget

Abstract. The disparities in estimates of the ocean carbon sink, whether derived from observations or models, raise questions about our ability to accurately assess its magnitude and trend over recent decades. A potential factor contributing to this inconsistency is the pre-industrial air-sea carbon flux, which is thought to arise globally from an imbalance between riverine discharge and sediment burial of carbon. The characterization of this flux is essential for isolating the anthropogenic component of the total air-sea carbon flux estimated from observations; however, it remains highly uncertain, limiting confidence in the impactful applications of the Global Carbon Budget (GCB). In this study, we propose a fresh look at the pre-industrial air-sea carbon flux using the alkalinity budget. We demonstrate the relevance of a novel theoretical framework that directly enables the calculation of the riverine/burial-driven pre-industrial carbon outgassing using both carbon and alkalinity budgets. We also introduce a practical framework to evaluate the spatial distribution of this flux through a series of ocean biogeochemical simulations. Our reassessment, grounded in existing carbon and alkalinity budgets, yields an estimated riverine/burial-driven pre-industrial carbon outgassing of 0.49 [0.34; 0.70] PgC yr^-1, which is lower than the most recent central estimate of 0.65 ± 0.30 PgC yr^-1. This adjustment partially reduces the disparities between observation-based and model-derived estimates of the anthropogenic ocean carbon sink. Using a composite simulation derived from a linear combination of our sensitivity experiments, we reassess the spatial distribution of this flux, attributing 29 % to the southern region (south of 20° S), 40 % to the inter-tropical region (20° S–20° N), and 31 % to the northern region (north of 20° N). Notably, these findings represent an intermediate distribution compared to those used in the GCB over time, with recent values at 14 %, 64 %, and 22 %, and historical values at 49 %, 25 %, and 26 %. Addressing the current inconsistencies between the combined carbon and alkalinity budgets is thus an urgent priority for building confidence in the global riverine/burial-driven pre-industrial carbon outgassing, and intermodel comparisons are required to constrain its regional distribution.