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Treatise of Geochemistry chapter 

Hydrothermal vents serve as a primary interface between the cold deep ocean and the warm oceanic crust. While early research showed that seawater-​rock interactions add to or remove elements from seawater during the generation of hydrothermal fluids, consideration of these fluid fluxes alone does not relay the total impact that hydrothermal systems have on seawater geochemistry. In addition, hydrothermal plumes, areas where hydrothermal fluids mix with ocean waters, are host to a range of particle precipitation and scavenging reactions that further modify gross hydrothermal fluid fluxes to define the total “net” hydrothermal impact on oceanic inventories. Here, we review the major discoveries made by the international GEOTRACES program regarding the geochemical transformations occurring within hydrothermal plumes. We classify each element into one of five categories based on its behavior in hydrothermal plumes, a spectrum spanning the geochemical mass balance between net hydrothermal source fluxes and net hydrothermal plume scavenging sinks. Overall, we celebrate the role that GEOTRACES has played in defining the extent and dynamics of hydrothermal plume geochemistry, which is a crucial lever for determining global hydrothermal impacts. 

This special issue of Oceanography celebrates the transformational findings of the international GEOTRACES program in chemical oceanography, 20 years after drafting of the GEOTRACES Science Plan in 2004 (GEOTRACES Planning Group, 2006). With the section cruise phase of the program ending soon, and a planned pivot toward smaller-​scale process studies, this is an opportune time to look back at the achievements of GEOTRACES during the last two decades and to highlight some of the advances in our understanding of the processes that determine the oceanic distributions of trace elements and isotopes (TEIs).