Collaborative Research: Understanding substrate limitation and Lithium and Silicon isotope fractionation during secondary clay formation in marine systems

化学海洋学领域的一个长期研究课题是了解向海水输送元素和从海水中取出元素的过程。长期以来，全球海洋硅（Si）收支中一直存在一个“缺失的汇”，即向沉积物的转移似乎没有平衡来自河流的输入。几十年前，人们假设海洋沉积物中的“反向风化”可能是这个缺失的汇。在这个过程中，发生在陆地上的风化过程，即硅从矿物质中除去并溶解在水中，将被逆转，这些矿物质将通过形成粘土在海洋沉积物中重新组合。这一过程的证据很难获得，直到最近才有研究使用先进的测量技术表明，海洋逆风化的全球量级可以解释全球硅收支中所有缺失的汇项。如果得到证实，这意味着逆风化作用将代表迄今为止发现的最大的海洋硅汇，其中大部分埋藏在海洋的一个相对较小的区域，即陆海界面。此外，海洋逆风化速率的持续向上修正对其他元素（如铁、铝）的封存和其他沿海过程（如海洋酸化，因为二氧化碳是逆风化过程的副产品）也有影响。该项目旨在了解影响反向风化发生速度的最重要因素，并开发新的方法来评估现场环境中的这一过程。除了科学研究之外，该项目还将支持一名早期职业研究员、一名博士后研究员、一名研究生和一名本科生实习生。它还将通过参加科学博览会和为希望从事海洋科学教育的学生提供年度奖学金的方式，支持高中的外展活动。这个项目将开发一个社区外展活动，每年在亚特兰大科学节期间使用，亚特兰大科学节是格鲁吉亚最大的科学博览会，向公众展示科学和技术。最后，它将在美国建立硅同位素测量的能力。在这个项目中，研究人员建议使用一种新的硅锂双稳定同位素方法来了解海洋次生粘土形成的驱动因素，并促进野外反应程度的确定。总体目标是：1)更好地约束硅藻生成二氧化硅（bSiO2）次生粘土形成的地球化学因素、动力学和机制；这将通过使用纯矿物相、硅藻bSiO2和人工海水进行受控实验室实验来完成；2)利用野外沉积物、硅藻bSiO2和海水进行中环境培养实验，验证孤立地球化学因子的有效性；3)实验确定实验室推导的Li和Si同位素分馏在海洋沉积条件下次生粘土形成过程中是否有效。这项工作解决了国家研究委员会2015-2025年海洋科学十年调查中确定的八个海洋科学优先事项之一，特别是“海洋生物地球化学和物理过程如何影响当今的气候及其变化，以及这个系统在下个世纪将如何变化？”这些结果对于理解调节海洋元素封存（如Si、C、Fe、Al、Mg、K）的因素以及通过海洋沉积物中逆风化反应的副产物海洋CO2演化驱动全球气候的因素具有重要意义。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

A long-standing topic of investigation in the field of chemical oceanography is understanding the processes that deliver elements to, and remove them from, seawater. There has long been a "missing sink" in the global marine silicon (Si) budget in that removal to sediments did not appear to balance the inputs from rivers. Several decades ago, it was postulated that "reverse weathering" in marine sediments could be this missing sink. In this process, the weathering process that takes place on land, whereby silicon is removed from minerals and dissolved in water, would be reversed and these minerals would be reconstituted in marine sediments through the formation of clays. Evidence for this process was very difficult to obtain, and only recently have studies using advanced measurement techniques shown that the global magnitude of marine reverse weathering could account for all the missing sink term in the global Si budget. If validated, this means reverse weathering would represent the largest individual sink for marine Si identified to date, with most of this burial occurring in a relatively small area of the ocean, the land-sea interface. Moreover, the continued upward revision of the marine reverse weathering rate has implications for the sequestration of other elements (e.g. iron, aluminum) and for other coastal processes (e.g. ocean acidification, as carbon dioxide is a byproduct of the reverse weathering process). This project aims to understand the most important factors affecting how fast reverse weathering occurs, and developing new approaches to evaluate this process in the field environment. Beyond the scientific pursuits, this project will support an early career researcher, a postdoctoral investigator, a graduate student, and undergraduate interns. It will also support high school outreach through science fair participation and annual scholarships for students wishing to pursue Marine Science education. This project will develop a community outreach activity to be used annually during the Atlanta Science Festival, Georgia's biggest science fair that showcases science and technology to the public. Finally, it will build capacity for silicon isotope measurements in the U.S.In this project, the investigators propose to understand the driving factors of marine secondary clay formation and facilitate the determination of reaction degree in the field using a novel dual silicon and lithium stable isotope approach. The overarching goals are: 1) to better constrain the geochemical factors, kinetics, and mechanisms involved in secondary clay formation from diatom-produced silica (bSiO2); this will be done by conducting controlled laboratory experiments using pure mineral phases, diatom bSiO2, and artificial seawater; 2) to test the validity of the isolated geochemical factors by conducting mesocosm incubation experiments using field sediment materials, diatom bSiO2, and seawater; and 3) to experimentally determine whether laboratory-derived Li and Si isotope fractionations are valid during secondary clay formation under marine sediment conditions. This work addresses one of the eight Ocean Sciences Priorities identified in The National Research Council's 2015-2025 Decadal Survey of Ocean Sciences, specifically "How have ocean biogeochemical and physical processes contributed to today's climate and its variability, and how will this system change over the next century?" These results have fundamental importance to understanding the factors regulating marine elemental sequestration (e.g. Si, C, Fe, Al, Mg, K) and those driving global climate through oceanic CO2 evolution, a byproduct of the reverse weathering reaction, in marine sediments.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.