Excess dissolved Ca in the deep ocean: a hydrothermal hypothesis

Variations in seawater alkalinity and dissolved calcium provide information essential to establishing the integrated deep ocean calcium carbonate dissolution flux. High-precision Ca measurements reported here confirm earlier suggestions that the deep ocean contains higher levels of dissolved Ca than expected from calcium carbonate dissolution only. I propose that the mid-depth Ca excess is a manifestation of the circulation of seawater through hydrothermal systems at mid-ocean ridges. Qualitative agreement with δ3He distributions, and evidence for deep water Mg depletions, support this hypothesis. The magnitude of the mid-depth Ca excess implies a total hydrothermal flux larger than expected from the corresponding excess in 3He, and points to the relative importance of low-temperature diffuse fluxes versus high-temperature discrete fluxes for elements such as Ca. The implied total hydrothermal flux for Ca is consistent with its oceanic mass balance requirements, and resolves several long-standing problems associated with our understanding of the chemical evolution of the ocean, and in particular its low alkaline content.