Quantifying seafloor hydrothermal fluxes and their role in global geochemical cycles

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The crust beneath the oceans is formed along mid-ocean ridges and slowly migrates away from these ridges to eventually be subducted back into the mantle. Seawater circulates through the oceanic crust, at high-temperatures near the ridge axis (forming “black-smokers”) and at lower temperatures over much of the seafloor. Reactions between the crust and circulating seawater lead to substantial changes in the composition of both the crust and the fluid. Return of these modified fluids to the ocean has a profound effect on the chemistry of seawater. My proposed research program builds on our recent work aimed at quantifying the chemical fluxes carried by these hydrothermal systems with the long-term goal of better understanding the controls on both past and future ocean chemistry. Understanding the controls on seawater composition is a central aim of the Earth Sciences because of the important role that ocean chemistry plays in many aspects of the Earth system. For example, over long timescales the composition of the ocean controls atmospheric CO2 levels and hence impacts Earth’s climate. Likewise the composition of the ocean controls the bioavailability of elements that marine organisms require as nutrients or for building shells. Hydrothermal circulation and rivers are the dominant source of chemicals into the ocean. While the last few decades have seen substantial advances in our understanding of the controls on the composition of rivers, and how rivers are likely to have changed over Earth’s history, our understanding of the chemical fluxes associated with oceanic hydrothermal systems is much more rudimentary.My research program addresses both the high-temperature hydrothermal circulation that occurs at the ridge axis and the low-temperature hydrothermal circulation that occurs off-axis. We have recently developed the most robust and rigorous approach to quantifying high-temperature chemical fluxes into the ocean, using mathematical techniques borrowed from the geophysics community. This novel approach has proved very successful not only in quantifying the chemical fluxes but also in identifying where the largest uncertainties lie – i.e. what areas need more research. Based on our results we will be focusing our ridge-axis studies on two general areas: (i) understanding the changes in fluid compositions within the upper portion of the crust as they cool and mix with seawater before venting at the seafloor; and (ii) the precipitation of material out of these fluids as they mix with seawater after venting at the seafloor. A large part of these studies will be focused around a hydrothermal system just off the west coast of Canada that is monitored in real-time using a fiber-optic cable run out of UVic, as well as being visited annually by a research ship. This new approach to studying seafloor hydrothermal systems will allow hitherto unavailable insights into their temporal variability and allow substantially improved quantification of the chemical fluxes.Because of the success of the modeling we have undertaken on high-temperature, on-axis, hydrothermal systems I plan to apply the same mathematical techniques to the low-temperature, off-axis, system. However, before we can do this we need to better understand both: (i) the timing of most fluid-rock reaction, and (ii) the effect of variations in the temperature at which these reactions occur on the chemical exchanges. To this end we will develop and apply new approaches to dating the minerals that form in the crust during alteration and apply standard and novel geochemical approaches to determine fluid-rock reaction temperature. These new data will be used, along with existing constraints, to quantify the global impact of off-axis hydrothermal circulation on ocean chemistry.