Processes determining the marine alkalinity and calcium carbonate saturation state distributions

Abstract. We introduce a composite tracer for the marine system, Alk^*, that has a global distribution primarily determined by CaCO₃ precipitation and dissolution. Alk^* is also affected by riverine alkalinity from dissolved terrestrial carbonate minerals. We estimate that the Arctic receives approximately twice the riverine alkalinity per unit area as the Atlantic, and 8 times that of the other oceans. Riverine inputs broadly elevate Alk^* in the Arctic surface and particularly near river mouths. Strong net carbonate precipitation results in low Alk^* in subtropical gyres, especially in the Indian and Atlantic oceans. Upwelling of dissolved CaCO₃-rich deep water elevates North Pacific and Southern Ocean Alk^*. We use the Alk^* distribution to estimate the variability of the calcite saturation state resulting from CaCO₃ cycling and other processes. We show that regional differences in surface calcite saturation state are due primarily to the effect of temperature differences on CO₂ solubility and, to a lesser extent, differences in freshwater content and air–sea disequilibria. The variations in net calcium carbonate cycling revealed by Alk^* play a comparatively minor role in determining the calcium carbonate saturation state.