UK-IODP Moratorium Award for Lewis Grant - Shipboard Scientist Expedition 390: The role of ridge flank dolomitization in the ocean Mg budget

洋壳是在海底火山大洋中脊形成的，随着时间的推移逐渐扩散，形成覆盖地球表面三分之二的海洋盆地。海水通过冷却地壳的裂缝循环，带走热量，并与岩石发生化学作用。这种“热液循环”随着时间的推移影响着海洋和地球的组成。随着地壳的老化，沉积物在其上方积累，海水沿着这些沉积物与下层地壳之间的接触流动，使这里成为海水-岩石化学交换的重要场所。镁是海水中第二丰富的阳离子，由河流提供给海洋，然后在洋壳的热液改造和沉积物中富镁碳酸盐(即白云岩)的沉淀过程中被去除。这些过程也直接影响了全球碳收支，使镁成为重建过去气候变化的有用指标，这是理解地球未来演化的关键。在地质记录中，白云岩是海洋镁的主要储存来源，尽管在过去的1.5亿年里，白云石的形成程度显著下降。然而，在紧邻火山壳之上的沉积物中发现了白云石，那里正在进行热液流体-岩石交换。破译这个白云岩建造的时间、温度和镁的来源，以确定它是如何形成的和为什么形成的，以及它是否是一个重要的镁储集层，这一点很重要。镁同位素提供了地球上不同储集层中镁的来源的地球化学指纹。然而，形成在沉积物-基底界面附近的白云石的镁同位素组成尚未确定。科学的大洋钻探使我们能够对海洋沉积物和深埋在海底的洋壳进行采样。国际海洋发现计划(IODP)第390次和第393次探险将横跨南大西洋，钻探六个地点的横断面，恢复700万至6100万年前在大西洋中部海脊形成的洋壳，并为研究沉积物与洋壳接触处的白云石形成提供了一个独特的机会。我将使用回收的岩心重新评估过去6000万年来海洋的镁收支，并确定白云石的形成是否仍然对现代海水化学产生重大控制作用。

Ocean crust is created at submarine volcanic mid-ocean ridges and spreads away over time to form the ocean basins that cover two thirds of Earth's surface. Seawater circulates through cracks in the cooling crust, removing heat and chemically interacting with the rocks. This 'hydrothermal circulation' influences both the composition of the oceans and Earth through time. As the crust ages, sediment accumulates above it, and seawater flows along the contact between these sediments and the underlying crust, making this an important site for seawater-rock chemical exchange. Magnesium (Mg) is the second most abundant cation in seawater and is supplied to the oceans by rivers, before being removed during hydrothermal alteration of ocean crust and precipitation of Mg-rich carbonates (i.e., dolomite) in the sediments. These same processes also directly influence the global carbon budget, making Mg a useful proxy when reconstructing past climate change, which is key to understanding the future evolution of our planet.In the geological 'rock record', dolomite was a major store for oceanic Mg, although the extent of dolomite formation has decreased significantly over the last 150 million years. However, dolomite has been found within the sediments immediately overlying the volcanic crust, where hydrothermal fluid-rock exchange is occurring. It is important to decipher the timing, temperature, and the source of Mg of this dolomite formation to determine how and why it forms, and whether it is an important Mg reservoir. Mg isotopes provide a geochemical fingerprint of the source of the Mg in different reservoirs in the Earth. However, the Mg isotopic composition of dolomites formed close to the sediment-basement interface has not yet been determined.Scientific ocean drilling allows us to sample marine sediments and the oceanic crust buried deep below them. International Ocean Discovery Program (IODP) Expeditions 390 and 393 will drill a transect of six sites across the South Atlantic, recovering ocean crust formed at the Mid-Atlantic ridge between 7 and 61 million years ago and providing a unique opportunity to investigate dolomite formation along the contact between sediments and the ocean crust. I will use the recovered cores to reassess the Mg budget of the oceans over the last 60 million years and determine whether dolomite formation still exerts a significant control on modern seawater chemistry.