Processes determining the marine alkalinity and calcium carbonate saturation state distributions

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.