Effects of paleoseawater composition on chemical and isotopic exchange at mid-ocean ridges

古海水成分对洋中脊化学和同位素交换的影响

• 批准号: 1737186
• 负责人: Donald DePaolo
• 金额: $ 32.72万
• 依托单位: University of California-Berkeley
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2017
• 资助国家: 美国
• 起止时间: 2017-08-01 至 2019-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1737186&HistoricalAwards=false
• 关键词: Effects; paleoseawater; composition; chemical; isotopic

The chemical composition of the oceans holds clues to important aspects of the history of climate, life, and mountain building on the Earth. Although we now know that the amount in seawater of certain elements like calcium, magnesium and sulfur, which are minor, but significant, constituents of seawater, have changed by large amounts and cyclically over many millions of years, we still do not know what causes these changes. One process that can contribute to, and also be affected by, the changes is the circulation of seawater through the fractured oceanic crust near mid-ocean ridges. This research is aimed at understanding how changes in dissolved S, Mg, and Ca in seawater affect the large geothermal systems that stretch around the globe on the ocean floor. Better understanding of could lead to breakthroughs in our understanding of why Earth's climate oscillates over long time periods and how mid-ocean ridge geothermal systems transfer chemical elements between the deep Earth and the oceans. The research involves laboratory experiments that reproduce the conditions and processes deep within hot oceanic crust and models how seawater circulates through the rocks and changes as chemical reactions occur. In terms of broader impacts, the experiments will provide thermodynamic and kinetic data that can be used in geothermal resource development and in studies of water-rock interaction that occur in other contexts, such as nuclear waste storage. New data on Sr partitioning in secondary minerals could have broad applicability to studies of hydrothermal/geothermal systems. Additional impacts include support of analytical and experimental research facilities at that benefit students at the University of California and many other scientists and students from other institutions. The educational value of the project is substantial in that it connects geological processes with oceanography and climate, and integrated knowledge of chemistry, hydrology and geology.Mid-ocean ridge hydrothermal systems are a major contributor to the ocean's geochemical cycle. The chemical exchange that occurs as seawater circulates through basalt and gabbro of varying temperatures affects the balance of ions in seawater and influences the carbon cycle through the coupling of dissolved Ca and Mg and carbonate sedimentation. Understanding the geologic history of the oceans and the relationships between seawater chemistry and climate requires that chemical exchange between seawater and oceanic crust at and near mid-ocean ridges be understood, not only at present, but back in time through Earth's history. Differences in the chemical composition of seawater in the geologic past could have had a significant effect on the seawater-basalt exchange in seafloor hydrothermal systems, especially at times when seawater Mg and SO4 were lower, and Ca and Sr were higher than at present. Consideration of these differences could cause major changes in the way we interpret records from past oceans, in particular the Sr isotopic evolution of seawater. It may also change how we view the causes and consequences of changes in seawater composition through Earth history. There is evidence that Sr isotopic exchange between seawater and ocean floor rocks changed substantially during periods when the Mg/Ca and Ca/SO4 ratios of the ocean were different than they are today. Hence modern seafloor hydrothermal systems may not be representative of most of the Phanerozoic. To better understand how seawater Mg-Ca-SO4 chemistry affects MOR hydrothermal experiments using reactors and mineral/fluid compositions that mimic seafloor hydrothermal systems will be run at varying fluid compositions. Run products will be analyzed geochemically and the results will be incorporated into the water-rock interaction modeling code TOUGHREACT, a hydrological modeling code that will be modified to accommodate conditions of seafloor hydrothermal systems.

海洋的化学成分为地球上气候、生命和造山历史的重要方面提供了线索。虽然我们现在知道海水中某些元素的含量，如钙、镁和硫，它们是海水中少量但重要的成分，在数百万年的时间里发生了巨大的周期性变化，但我们仍然不知道是什么导致了这些变化。有一个过程可以促成这种变化，同时也会受到这种变化的影响，那就是海水通过洋中脊附近断裂的海洋地壳的循环。这项研究旨在了解海水中溶解的S、Mg和Ca的变化如何影响遍布全球海底的大型地热系统。更好的理解可能会导致我们对地球气候在长时间内振荡的原因以及海洋中脊地热系统如何在地球深处和海洋之间转移化学元素的理解取得突破。这项研究包括实验室实验，再现高温海洋地壳深处的条件和过程，并模拟海水如何在岩石中循环，以及化学反应发生时的变化。就更广泛的影响而言，这些实验将提供热力学和动力学数据，可用于地热资源开发和在其他情况下（如核废料储存）发生的水岩相互作用的研究。次生矿物中锶分配的新数据对热液/地热系统的研究具有广泛的适用性。其他影响包括对分析和实验研究设施的支持，这使加州大学的学生和许多其他科学家以及来自其他机构的学生受益。该项目的教育价值是巨大的，因为它将地质过程与海洋学和气候联系起来，并整合了化学，水文和地质学的知识。洋中脊热液系统是海洋地球化学循环的主要贡献者。海水在不同温度的玄武岩和辉长岩中循环时发生的化学交换影响了海水中离子的平衡，并通过溶解的Ca和Mg与碳酸盐沉积的耦合影响了碳循环。了解海洋的地质历史以及海水化学与气候之间的关系，需要了解海洋中脊及其附近的海水与海洋地壳之间的化学交换，不仅要了解目前的情况，还要了解地球历史上的情况。地质历史时期海水化学成分的差异可能对海底热液系统中海水-玄武岩交换有重要影响，特别是在海水Mg和SO4较低，Ca和Sr较高的时期。考虑到这些差异可能会导致我们解释过去海洋记录的方式发生重大变化，特别是海水的锶同位素演化。它还可能改变我们对地球历史上海水成分变化的原因和后果的看法。有证据表明，在海洋Mg/Ca和Ca/SO4比值与今天不同的时期，海水和海底岩石之间的Sr同位素交换发生了实质性变化。因此，现代海底热液系统可能不能代表显生宙的大部分。为了更好地了解海水Mg-Ca-SO4化学反应如何影响MOR热液实验，将在不同的流体组成下使用反应器和矿物/流体组成模拟海底热液系统。运行的产品将进行地球化学分析，结果将纳入水岩相互作用建模代码TOUGHREACT，这是一个水文建模代码，将被修改以适应海底热液系统的条件。