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去除元素的过程。长期以来，在全球海洋硅（Si）预算中存在一个“缺失汇”，即沉积物的去除似乎无法平衡河流的输入。几十年前，人们假设海洋沉积物中的“逆风化”可能是这种缺失的汇。在这一过程中，发生在陆地上的风化过程，即硅从矿物中去除并溶解在水中的过程，将被逆转，这些矿物将通过形成粘土在海洋沉积物中重新形成。这一过程的证据是很难获得的，直到最近的研究使用先进的测量技术表明，全球规模的海洋逆风化可以解释所有失踪汇项在全球硅预算。如果得到证实，这意味着逆风化将是迄今为止发现的最大的海洋硅个体汇，其中大部分埋藏发生在海洋的一个相对较小的区域，即陆地-海洋界面。此外，海洋逆风化速率的持续上调对其他元素（如铁、铝）的固存和其他沿海过程（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逆风化发生速度的最重要因素，并开发新方法来评估野外环境中的这一过程。 除了科学追求，该项目将支持早期职业研究人员，博士后研究员，研究生和本科实习生。它还将通过参加科学博览会和为希望接受海洋科学教育的学生提供年度奖学金，支持高中外联活动。该项目将开展一项社区推广活动，每年在亚特兰大科学节期间使用，这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，研究人员建议了解海洋次生粘土形成的驱动因素，并使用一种新的硅和锂双稳定同位素方法来促进现场反应程度的确定。总体目标是：1）更好地约束地球化学因素、动力学和由矿物产生的二氧化硅（bSiO 2）形成次生粘土的机制;这将通过使用纯矿物相、硅藻bSiO 2和人工海水进行受控实验室实验来完成;（2）通过野外沉积物、硅藻bSiO_2和海水的围隔生态系统培养实验，检验分离的地球化学因子的有效性; 3）实验确定在海洋沉积物条件下的次生粘土形成过程中，实验室衍生的Li和Si同位素分馏是否有效。 这项工作解决了国家研究理事会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“海洋地球化学和物理过程如何促成今天的气候及其变化，以及这个系统在下一个世纪将如何变化？“这些结果对于理解调节海洋元素封存的因素具有根本的重要性（例如Si、C、Fe、Al、Mg、K）和通过海洋CO2演化（逆风化反应的副产品）驱动全球气候的那些，该奖项反映了NSF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进行评估，被认为值得支持审查标准。

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.