A probabilistic assessment of calcium carbonate export and dissolution in the modern ocean

Abstract. The marine cycle of calcium carbonate (CaCO₃) is an important element of the carbon cycle and co-governs the distribution of carbon and alkalinity within the ocean. However, CaCO₃ export fluxes and mechanisms governing CaCO₃ dissolution are highly uncertain. We present an observationally constrained, probabilistic assessment of the global and regional CaCO₃ budgets. Parameters governing pelagic CaCO₃ export fluxes and dissolution rates are sampled using a Monte Carlo scheme to construct a 1000-member ensemble with the Bern3D ocean model. Ensemble results are constrained by comparing simulated and observation-based fields of excess dissolved calcium carbonate (TA*). The minerals calcite and aragonite are modelled explicitly and ocean–sediment fluxes are considered. For local dissolution rates, either a strong or a weak dependency on CaCO₃ saturation is assumed. In addition, there is the option to have saturation-independent dissolution above the saturation horizon. The median (and 68 % confidence interval) of the constrained model ensemble for global biogenic CaCO₃ export is 0.90 (0.72–1.05) Gt C yr⁻¹, that is within the lower half of previously published estimates (0.4–1.8 Gt C yr⁻¹). The spatial pattern of CaCO₃ export is broadly consistent with earlier assessments. Export is large in the Southern Ocean, the tropical Indo–Pacific, the northern Pacific and relatively small in the Atlantic. The constrained results are robust across a range of diapycnal mixing coefficients and, thus, ocean circulation strengths. Modelled ocean circulation and transport timescales for the different set-ups were further evaluated with CFC11 and radiocarbon observations. Parameters and mechanisms governing dissolution are hardly constrained by either the TA* data or the current compilation of CaCO₃ flux measurements such that model realisations with and without saturation-dependent dissolution achieve skill. We suggest applying saturation-independent dissolution rates in Earth system models to minimise computational costs.