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

海洋地壳是由海底火山形成的洋中脊形成的，随着时间的推移，海洋地壳逐渐扩散，形成了覆盖地球表面三分之二的海洋盆地。海水在冷却的地壳裂缝中循环，带走热量，并与岩石发生化学反应。随着时间的推移，这种“热液循环”影响着海洋和地球的组成。随着地壳年龄的增长，沉积物在上面堆积，海水沿着这些沉积物和下面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（Mg）是海水中含量第二丰富的阳离子，由河流提供给海洋，然后在海洋地壳热液蚀变和沉积物中富镁碳酸盐（即白云岩）的沉淀过程中被去除。这些相同的过程也直接影响全球碳收支，使Mg在重建过去的气候变化时成为一个有用的代理，这是了解我们星球未来演变的关键。在地质“岩石记录”中，白云岩是海洋镁的主要储存物，尽管白云岩的形成范围在过去的1.5亿年里显著减少。然而，在火山地壳上的沉积物中发现了白云岩，在那里热液流体-岩石交换正在发生。破译该白云岩组的时间、温度和镁的来源，对于确定其形成方式和成因，以及是否为重要的镁储层具有重要意义。镁同位素提供了地球不同储集层中镁元素来源的地球化学指纹图谱。然而，在沉积物-基底界面附近形成的白云岩的镁同位素组成尚未确定。科学的海洋钻探使我们能够对海洋沉积物和深埋在它们下面的海洋地壳进行取样。国际海洋发现计划（IODP）的390和393探险队将在南大西洋的6个地点进行取样，恢复700万到6100万年前在大西洋中脊形成的海洋地壳，并提供一个独特的机会来研究沉积物和海洋地壳之间接触处的白云岩形成。我将使用回收的岩心来重新评估过去6000万年海洋的镁预算，并确定白云岩的形成是否仍然对现代海水化学具有重要的控制作用。

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.