Biogeochemistry of trace metals in deep sea hydrothermal vents

Hydrothermal vents in the sea floor release large volumes of hot, metal-rich fluids into the deep ocean. Until recently, it was assumed that the majority of the metal released was incorporated into sulfide or oxide minerals, and that the net flux of hydrothermally-derived metals to the open ocean was negligible. However, mounting evidence suggests that organic compounds bind to and stabilize metals in hydrothermal fluids, increasing trace metal flux to the global ocean.

In situ measurements reveal that hydrothermally-derived chromium, copper and iron bind to organic molecules upon mixing with seawater. Geochemical model simulations based on data from two hydrothermal vent sites suggest that complexation significantly increases metal flux from hydrothermal systems. According to these simulations, hydrothermal fluids could account for 9 and 14% of the deep-ocean dissolved iron and copper budgets respectively. A similar role of organic complexation can be implied for the hydrothermal fluxes of other metals such as manganese and zinc. The source and identity of these organic ligands awaits further investigation. (Sylvia Sander)