Detection of Low-Temperature Hydrothermal Fluxes by Seawater Mg and Ca Anomalies

Geochemical fluxes into and out of the ocean control its chemical composition. Measurements of the magnesium (Mg) content of seawater, an assumed "conservative" element in the ocean, reveal mid-depth Mg depletions in the vicinity of the East Pacific Rise. The magnitude of the anomalies suggests that fluxes associated with the low-temperature circulation of seawater through axial mid-ocean ridge systems are much larger than the high-temperature axial component. A higher total axial hydrothermal flux provides a mechanism that simultaneously satisfies the mass balance requirement of several major seawater constituents.