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

点击翻译按钮获取中文摘要

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.