Hydrothermal Systems: Processes Controlling Vent Fluid Chemistry and Implications for Global Biogeochemical Cycles

Hydrothermal circulation through oceanic lithosphere is a fundamental control on the chemistry of the oceans, yet the compositions of vent fluids vary widely, and their net effect on global elemental cycles remains enigmatic. As seawater percolates down into young oceanic lithosphere, it undergoes a series of water–rock reactions across a broad thermal range, from low-temperature alteration in the recharge zone to high-temperature exchange in the reaction zone, with resulting vent fluids diverging significantly in composition from the initial seawater. Drawing evidence from direct sampling of hydrothermal fluids and vent deposits, laboratory experiments, and modeling, we highlight that variable vent fluid chemistry arises from differences in host-rock lithologies, the nature of the hydrothermal heat source, and pressure-temperature conditions within the hydrothermal system. Building on this foundation, we then examine how these fluids influence global elemental cycles across timescales: their effect on the biological carbon pump, primarily through Fe and Mn fertilization (short timescales), as well as their influence on secular variation in seawater composition, as recorded by carbonates, evaporites, and fluid inclusion records (geological timescales). Regardless of timescales, the evidence compiled here reinforces hydrothermal circulation as a first-order control on ocean chemistry. However, key questions remain open: the relative importance of axial (high-temperature) and ridge flank (low-temperature) processes in global biogeochemical cycles, and the role of additional controls (such as authigenic clay formation and submarine groundwater discharge) in shaping seawater chemistry through time.