Environmental controls on mid-ocean ridge hydrothermal fluxes

High temperature hydrothermal fluxes at mid-ocean ridges are thought to be an important component of oceanic biogeochemical cycles. However, little consideration has been given to how these fluxes vary as a consequence of changing environmental conditions over Earth history. Here we consider how changes in sea level and ocean chemistry are likely to have impacted on-axis, high-temperature, hydrothermal fluxes focusing on Phanerozoic conditions. Changes in sea level lead to changes in hydrostatic pressure near the base of hydrothermal systems where both peak fluid-rock reaction temperatures and phase separation occur. In general, higher global sea level will lead to higher peak temperatures of fluid-rock reaction. Additionally, phase separation at higher pressure tends to lead to formation of a more Cl-rich vapor, that constitutes a larger mass fraction of the system. These combined factors may serve to significantly modify hydrothermal fluxes even for sea level changes on the scale of 100 m. Changes in ocean chemistry can also affect axial hydrothermal fluxes in several ways. Seawater sulfate contents control the amount of anhydrite that forms, which has both physical (porosity filling) and chemical effects. The most important aspect of ocean chemistry in controlling the composition of high-temperature vent fluids may be ocean salinity. If evaporite formation and dissolution has changed ocean salinity substantially over the Phanerozoic, hydrothermal fluxes could have been greatly modified. Ocean chemistry also plays a large role in controlling processes operating in hydrothermal plumes and hence the net flux of elements into and out of the ocean associated with hydrothermal systems. We conclude that there is a need for substantial further work to quantify the effects of sea level and ocean chemistry on high-temperature hydrothermal fluxes, including the development of more robust models that integrate field, laboratory and theoretical observations.