Ocean Basement Calcium Carbonate Veins as Recorders of Past Ocean Chemistry and the Global Carbon Cycle

数百万年来，固体地球、海洋和大气之间的碳(C)长期循环控制着大气中的二氧化碳（CO2）水平，从而控制着气候。对这一周期内C-通量的量值进行量化，就可以对造成过去全球变化的地球系统过程进行研究。在海洋中脊的海底火山活动期间，新海洋地壳的形成是板块构造周期的一个关键组成部分。板块构造周期每2亿年就会重铺地球表面的三分之二，因为地壳向俯冲带扩散，在俯冲带它被运送到下面的地幔中。海水在海洋地壳的裂缝中循环，在那里它被加热并与岩石发生反应。这些流体中的矿物质沉积在地壳中，改变了流体和岩石的化学性质。因此，流体通过地壳的“热液循环”导致海洋和地壳之间的热和化学交换。在洋中脊火山活动期间，二氧化碳气体从岩浆中释放到海洋和大气中。在“热液循环”过程中，碳酸钙（CaCO3）从地壳中的流体中沉淀下来，将二氧化碳储存在岩石中。因此，洋中脊洋壳的形成使碳在地球系统中的长期循环成为可能。此外，热液碳酸盐的化学性质反映了它们形成的流体，通过化学和同位素分析，这些碳酸盐矿物可以用来确定过去和海洋条件下海水的组成。在这项研究中，我们将使用海洋地壳和其他海底火山构造内的碳酸盐脉的分析来开发过去200万年的海洋化学的详细记录；-开发新的海水记录，记录过去气候的重要示踪剂，如Li同位素-研究CaCO3降水的物理和化学控制-量化CO2进出海洋地壳的通量和交换率。我们将利用我们的结果来模拟过去海洋地壳产生和俯冲速率、海底面积和热液交换持续时间的变化如何影响长期的全球c -循环，从而影响这些过程在控制过去气候中的作用。这项研究将解决一个关键的科学问题，即提高我们对影响气候的全球长期c循环的理解。此外，对海洋地壳中碳酸盐降水的物理和化学控制的了解将有助于人工重建和加速硅酸盐风化过程的尝试，这是一种可能的方法，可以减少大气中的二氧化碳和CaCO3，从而防止未来的气候变化。这项研究还将促进我们对海洋与下伏地壳岩石之间相互作用的认识，这些相互作用影响海洋、大气、地壳和地幔的组成，以及海洋地壳的物理性质（如孔隙度、渗透率、强度和地震性质）。了解流体-岩石相互作用如何影响地壳从隆起到俯冲带的过程，将使进入俯冲带的物质的成分和性质得以确定。这将有助于研究发生大地震的俯冲带及其相关的自然灾害（如海啸）。

The long-term cycling of carbon (C) between the solid Earth, oceans and atmosphere over millions of years controls atmospheric carbon dioxide (CO2) levels and hence climate. Quantification of the magnitude of the C- fluxes within this cycle allows the Earth system processes responsible for past global change to be investigated. The formation of new ocean crust during submarine volcanism along mid-ocean ridges is a key component of the plate tectonic cycle, which repaves two thirds of the Earth's surface every 200 million years as crust spreads towards subduction zones where it is transported down into the underlying mantle. Seawater circulates through cracks in the ocean crust, where it is heated and reacts with the rocks. Minerals are deposited in the crust from these fluids, changing the chemistry of the fluid and the rock. Consequently the 'hydrothermal circulation' of fluid through the crust results in thermal and chemical exchange between the oceans and the crust. During mid-ocean ridge volcanism CO2 gas is released from the magma to the oceans and atmosphere. During 'hydrothermal circulation' calcium carbonate (CaCO3) precipitates from the fluid in the crust, storing CO2 in the rock. The formation of ocean crust at mid-ocean ridges therefore enables long-term C-cycling through the Earth system. In addition, the chemistry of the hydrothermal carbonates reflects the fluids from which they form, and through chemical and isotopic analyses, these carbonate minerals can be used to determine the composition of seawater in the past and oceanic conditions. For this study we will use analyses of carbonate veins within the ocean crust and other submarine volcanic constructions to - Develop detailed records of past ocean chemistry for the past 200 Myrs; - Develop new seawater records of important tracers of past climate such as Li isotopes - Investigate the physical and chemical controls on the CaCO3 precipitation - Quantify the fluxes and rates of exchange of CO2 to and from the ocean crust. We will use our results to model how changes in past ocean crustal production and subduction rates, seafloor area, and the duration of hydrothermal exchange have affected the long-term global C-cycle and hence the role of these processes in controlling past climate. This research will address a key scientific issue of improving our understanding of the global long-term C-cycle that influences climate. In addition the knowledge of the physical and chemical controls on carbonate precipitation in the ocean crust will benefit attempts to artificially recreate and accelerate silicate weathering process as a possible approach to drawdown atmospheric CO2 as CaCO3, so as to prevent future climate change. This research will also advance our knowledge of the interactions between the oceans and the underlying crustal rocks, which affect the composition of the oceans, atmosphere, crust and mantle, and the physical properties of the ocean crust (e.g. porosity, permeability, strength, and seismic properties). The knowledge of how fluid-rock interaction affects the crust as it is transported from the ridges to subduction zones, will allow the composition and properties of the material entering subduction zones to be determined. This will aid studies of subduction zones where major earthquakes occur, and their associated natural hazards (e.g. tsunamis).