Feedbacks between oceanic hydrothermal systems and the Earth system

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The crust beneath the oceans forms along mid-ocean ridges, migrating slowly away from these ridges to eventually be subducted into the mantle. Seawater circulates through the oceanic crust at high-temperatures (>350°C) near the ridge axis forming "black-smokers" and potentially economic sulphide deposits with unique chemosynthetic ecosystems. Seawater also circulates through the crust at low temperatures (<50°C) over much of the seafloor in "off-axis" hydrothermal systems. Reactions between the crust and circulating seawater in both on- and off-axis hydrothermal systems lead to substantial changes in the composition of the crust and the fluid. Return of the modified fluids to the ocean has a profound effect on the chemistry of seawater, and alteration of the crust is significant for many solid earth geochemical cycles (e.g. adding volatiles to the mantle wedge and inducing mantle heterogeneity). My proposed research program builds on our recent work on chemical fluxes carried by on- and off-axis hydrothermal systems, aiming to determine the links and feedbacks between hydrothermal fluxes and the environment (e.g. bottom seawater temperature and composition). To better understand the environmental controls on element fluxes between the crust and ocean associated with on-axis high-temperature hydrothermal systems we will use two approaches. First, we will use seagoing studies to investigate how high-temperature hydrothermal fluids interact with the overlying water column in so-called hydrothermal plumes. These are formed by extreme dilution (~10,000:1) of black smoker fluid and form ~200 m above the seafloor along mid-ocean ridges. Second, we will use the rock record of high-temperature fluid-rock reaction in the crust to test models that suggest that at times in the past, when the major ion chemistry (e.g. Ca, Mg, sulphate) of seawater was different, the composition of black smoker fluids was also different. To better understand the feedbacks between low-temperature, off-axis, hydrothermal systems and environmental conditions we will use a three-pronged approach. First, we will undertake laboratory experiments to determine the dissolution rate of basalts in seawater at different temperatures and using different starting materials. Second, we will undertake field studies to determine if laboratory dissolution rates differ from natural dissolutions rates due to the very different timescales involved. Finally, we will study ancient ocean crust to determine whether there is a geological record of changing crustal alteration with changing environmental conditions. Finally, all investigations of environmental controls on hydrothermal fluxes require that we understand the past temperature and composition of the deep ocean. We will use both rigorous data compilation approaches, and analysis of the composition of upper oceanic crust that was altered at different times during the Phanerozoic, to improve constraints on paleo-seawater compositions.