Collaborative Research: Coupling of physical and chemical processes in the shelf to basin transport of iron and iodine off Washington and Oregon

生产性近海水域和相对非生产性近海水域之间的边界不是离海岸距离的简单函数，而是由生物、化学和海洋物理之间的复杂相互作用决定的。这种相互作用是控制海洋对粮食供应和全球碳循环贡献的基本属性。在夏季的几个月里，海水温度较高，氧气含量较低，海底附近的生物分解会消耗可用氧气，并将沉积物中的铁释放到上面的海水中。在海洋更深的部分，峡谷和其他崎岖的表面特征会影响洋流，并产生湍流，迅速将铁从近海的大陆架上移走。这种铁反过来可以提高几英里之外的生物生产力。该项目将研究(1)大陆架海底分解过程如何释放海洋生物所需的铁等元素，以及(2)洋流如何与大陆架和坡度相互作用，将大陆架上覆水域移至近岸，以提高生产力。根据加州和俄勒冈海岸20多年来的模型模拟，科学家们将收集和分析铁和碘的样本，以证明该模型的发现。这些模型显示，铁通过羽流从大陆架运往公海。碘之所以被纳入这项研究，是因为它可以追踪从低氧沉积物到大陆架的输入。此外，研究结果将用于改进当前模型中使用的假设。该项目将支持四名研究生和几名本科生的全部或部分研究。科学家们通过Cool项目(实验室外的化学海洋学)和冰上基金会的学生参与了高中生的指导。来自南加州大学和加州大学洛杉矶分校的科学家将研究华盛顿和俄勒冈海岸的铁(Fe)和碘(I)地球化学，那里有高有机质氧化速率、低溶解氧浓度和高沉积铁通量。这个项目假设从陆架到盆地的穿梭飞机控制着从富铁陆架到北太平洋内陆的铁的供应。对2007-2017年间运行的区域海洋模拟系统(ROMS)和生物地球化学元素循环模式(BEC)的模拟表明，这条海岸线是洋盆铁的主要来源，表现出由极地潜流产生的次表层涡流引起的深部和浅部铁羽。这个模型的结果将被用来预测铁的分布如何季节性地演变，并指导2021年到该地区的两次航次期间的采样。根据ROMS-BEC模型，铁的库存在夏末最高，此时底水氧气最低，而库存在冬季和早春最低。模型产生的深部羽流与纳米比亚、秘鲁和阿拉斯加湾报告的类似。这项工作将对深度高达4500米的地方进行采样，并寻找这样的羽流。深部羽流在全球铁的预算中很重要，因为它们可能在铁有限的高纬度水域露出水面。碘也将被测量，因为像铁一样，它在沉积物水界面积累，并在还原条件下释放。来自陆架沉积物的碘羽流将被用作横向铁平流的半保守示踪剂。巡航还将评估来自河流(特别是哥伦比亚河)的直接输入与减少沉积物的相对重要性，因为这些输入是季节性分离的。该奖项反映了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。

The boundary between productive coastal waters and relatively non-productive offshore waters is not a simple function of distance from the coast, but is determined by a complex interplay between biology, chemistry, and ocean physics. This interplay is a fundamental property that controls the oceanic contribution to the food supply and the global carbon cycle. During summer months when waters are warmer with low oxygen, biological decomposition near the seafloor consumes available oxygen and releases iron from sediments into overlying seawater. In deeper parts of the ocean, canyons and other rugged surface features influence currents and create turbulence that rapidly moves iron from the shelf offshore. This iron in turn can enhance biological productivity miles away. This project will investigate (1) how decomposition processes on the continental shelf seafloor release elements like iron required for ocean life and (2) how ocean currents interact with the continental shelf and slope to move waters overlying the shelf offshore to enhance productivity. Based on model simulations from the California and Oregon coasts done over 20 years that show transport of iron via plumes from the shelf to the open ocean, scientists will collect and analyze samples for iron and iodine to prove the findings of the model. Iodine has been included in the study because it can track inputs from low oxygen sediments to the shelf. In addition, results from the study will be used to improve assumptions used in the current model. The project will support the research in whole or in part of four graduate students and several undergraduate students. The scientists are involved with High School student mentoring, through Project COOL (Chemical Oceanography Outside the Lab) and the Students on Ice Foundation. Scientists from the University of Southern California and the University of California, Los Angeles will study iron (Fe) and iodine (I) geochemistry along the Washington and Oregon Coasts, where high organic matter oxidation rates, low dissolved oxygen concentrations, and high sedimentary Fe fluxes on the continental shelf occur. This project hypothesizes that a shelf-to-basin shuttle controls the supply of Fe from the Fe-rich shelf into the interior of the North Pacific Ocean. Simulations of a Regional Oceanic Modeling System (ROMS) coupled with the Biogeochemical Elemental Cycling model (BEC) run between 2007-2017 showed that this coastline is a major source of Fe to the ocean basin, exhibiting both deep and shallow Fe plumes that arise from subsurface eddies generated by poleward undercurrents. Results from this model will be used to predict how the distribution of Fe evolves seasonally and guide sampling during two cruises in 2021 to the region. According to the ROMS-BEC model, the Fe inventory is highest in late summer, when bottom water oxygen is lowest, and the inventory is lowest in winter and early spring. Model-generated deep plumes resemble those reported off Namibia, Peru and the Gulf of Alaska. This work will sample depths up to 4500m and look for such plumes. Deep plumes are important in global Fe budgets and because they may outcrop in high latitude waters that are Fe limited. Iodine will also be measured because, like Fe, it accumulates at the sediment water interface and is released under reducing conditions. Plumes of iodine arising from shelf sediments will be used as a semi-conservative tracer for lateral Fe advection. The cruises will also assess the relative importance of direct inputs from rivers (especially the Columbia River) versus reducing sediments, since these inputs are decoupled seasonallyThis 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.