Quantifying seafloor hydrothermal fluxes and their role in global geochemical cycles

海洋下面的地壳是沿着洋中脊形成的，并慢慢地从这些脊移开，最终俯冲回地幔中。海水在海洋地壳中循环，在山脊轴附近的高温下（形成“黑烟”），在大部分海底的较低温度下。地壳和循环海水之间的反应导致地壳和流体的成分发生了实质性的变化。这些经过改造的液体返回海洋对海水的化学性质产生了深远的影响。我提出的研究计划建立在我们最近的工作基础上，旨在量化这些热液系统所携带的化学通量，其长期目标是更好地了解过去和未来海洋化学的控制因素。了解对海水成分的控制是地球科学的中心目标，因为海洋化学在地球系统的许多方面起着重要作用。例如，在很长的时间尺度上，海洋的成分控制着大气中的二氧化碳水平，从而影响地球的气候。同样，海洋的组成也控制着海洋生物作为营养物或造壳所需元素的生物可利用性。热液循环和河流是化学物质进入海洋的主要来源。虽然在过去的几十年里，我们对河流组成的控制以及河流在地球历史上可能发生的变化的理解取得了实质性的进展，但我们对与海洋热液系统相关的化学通量的理解要初级得多。

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.