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主题是了解向海水输送元素和从海水中移除元素的过程。长期以来，全球海洋硅(Si)预算中一直存在着一个“缺失的汇”，因为对沉积物的清除似乎没有平衡来自河流的输入。几十年前，人们推测海洋沉积物中的“逆风化”可能就是这个失踪的下沉。在这个过程中，陆地上发生的风化过程将被逆转，这些矿物将通过形成粘土在海洋沉积物中重新组成。这一过程的证据很难获得，直到最近，使用先进测量技术的研究才表明，全球海洋逆风化的规模可以解释全球硅预算中缺失的所有汇项。如果得到证实，这意味着反向风化将是迄今为止已发现的最大的海相硅沉淀池，大部分埋葬发生在海洋的一个相对较小的区域--海陆界面。此外，继续向上修正海洋反向风化速率对其他元素(例如铁、铝)的封存和其他沿海过程(例如海洋酸化，因为二氧化碳是反向风化过程的副产品)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，并开发新的方法在现场环境中评估这一过程。除了科学追求，这个项目还将支持一名早期职业研究员、一名博士后研究员、一名研究生和本科生实习生。它还将通过参加科学博览会和为希望学习海洋科学教育的学生提供年度奖学金来支持高中推广活动。该项目将开发一项社区推广活动，每年在亚特兰大科学节期间使用，亚特兰大科学节是佐治亚州最大的科学博览会，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能力。在这个项目中，研究人员建议了解海相次生粘土形成的驱动因素，并促进使用一种新的硅和锂稳定同位素方法来确定野外反应程度。总体目标是：1)更好地约束硅藻产生的二氧化硅(BSiO)形成次生粘土所涉及的地球化学因素、动力学和机制；这将通过使用纯矿物相、硅藻bSiO_2和人工海水进行受控的实验室实验来实现；2)通过利用野外沉积物材料、硅藻bSiO_2和海水进行中观培养实验来测试单独的地球化学因素的有效性；以及3)通过实验确定在海洋沉积条件下的次生粘土形成过程中实验室获得的Li和Si同位素分馏是否有效。这项工作涉及美国国家研究委员会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，具体而言是“海洋生物地球化学和物理过程如何对今天的气候及其变异性做出贡献，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控海洋元素封存的因素(例如，硅、碳、铁、铝、镁、钾)以及那些通过海洋沉积物中反向风化反应的副产品海洋二氧化碳释放来驱动全球气候的因素具有重要意义。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.