Quantifying seafloor hydrothermal fluxes and their role in global geochemical cycles

海洋下面的地壳是沿着大洋中脊形成的，然后慢慢地从这些海脊迁移出去，最终被俯冲回地幔。海水在洋壳中循环，在海脊轴线附近的高温下(形成“黑烟”)，在大部分海底上空温度较低。地壳和循环海水之间的反应导致地壳和流体的组成都发生了实质性的变化。这些经过改良的流体返回海洋，对海水的化学成分产生了深远的影响。我提议的研究计划建立在我们最近旨在量化这些热液系统携带的化学通量的工作的基础上，长期目标是更好地了解对过去和未来海洋化学的控制。了解海水组成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发挥着重要作用。例如，在很长的时间尺度上，海洋的组成控制着大气中的二氧化碳水平，从而影响地球气候。同样，海洋的组成控制着海洋生物作为营养物质或建造贝壳所需元素的生物可获得性。热液循环和河流是进入海洋的化学物质的主要来源。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已经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。我的研究计划既研究发生在脊轴的高温热液循环，也研究发生在轴线外的低温热液循环。我们最近开发了一种最可靠和最严格的方法，利用从地球物理学界借用的数学技术，量化进入海洋的高温化学通量。事实证明，这种新的方法不仅在量化化学通量方面非常成功，而且在确定最大的不确定因素--即哪些领域需要更多研究--方面也非常成功。根据我们的结果，我们将把我们的脊轴研究集中在两个一般领域：(1)了解地壳上部在海底喷发之前冷却并与海水混合时流体成分的变化；(2)这些流体在海底喷发后与海水混合时物质的沉淀。这些研究的很大一部分将集中在加拿大西海岸附近的一个热液系统，该系统使用UVIC外的光缆进行实时监测，并由一艘科考船每年访问。这种研究海底热液系统的新方法将使人们能够深入了解它们的时间变异性，并大大改进对化学通量的量化。由于我们对高温轴上热液系统进行的建模取得了成功，我计划将同样的数学技术应用于低温离轴系统。然而，在我们能够做到这一点之前，我们需要更好地了解：(I)大多数流体-岩石反应的时间，以及(Ii)这些反应发生的温度变化对化学交换的影响。为此，我们将开发和应用新的方法来确定蚀变过程中在地壳中形成的矿物的年龄，并应用标准的和新的地球化学方法来确定流体-岩石反应温度。这些新数据将与现有的限制一起用于量化离轴热液环流对海洋化学的全球影响。

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.