Award: OCE-1829981

The Santa Barbara Basin, off the coast of Southern California, is a unique marine environment increasingly impacted by ocean deoxygenation. This process, characterized by declining oxygen levels in ocean waters, is having significant impacts on marine ecosystems and biogeochemical cycles, including nutrient availability, microbial activity, and the health of marine life. Our research focuses on how deoxygenation in the basin's deep, oxygen-deficient waters influences sulfur-oxidizing microbial mats, element cycling, and nutrient dynamics. Deoxygenation and the Development of Bacterial Mats A distinctive feature of the Santa Barbara Basin is the seasonal formation of bacterial mats at the sediment-water interface. These mats use nitrate to oxidize sulfide in the absence of oxygen. When the bottom waters become anoxic, these bacteria thrive, shifting the sediment from a predominantly denitrifying to an ammonium-producing environment. This shift in microbial activity is associated with changes in sediment chemistry, including the depletion of iron oxides and the release of dissolved iron from the sediment. Our studies suggest that the cycles of deoxygenation and reoxygenation in the basin's bottom waters contribute to very high fluxes of dissolved iron, further influencing nutrient cycling and the overall health of the ecosystem. Shifting Geochemical Conditions in the Sediment As oxygen levels in the Santa Barbara Basin further decline, we observe significant changes in the geochemistry of the sediment. Below the basin's sill, which is located about 475 meters deep, the sediment undergoes dramatic shifts in its redox state, moving from an iron-rich to a sulfide-rich environment. This state is marked by high rates of sulfate reduction in the sediment, which in turn leads to the production of sulfide. Despite these high rates of sulfate reduction, the accumulation of iron sulfides in the sediment is lower than expected, suggesting that reactive iron is being lost to the water column during anoxic conditions and is not being replenished during re-oxygenation. The loss of iron could have long-term consequences for the basin's biogeochemistry, potentially driving a shift towards more sulfidic condition, highlighting the need for further study of these changes and their potential consequences for marine life. The Role of Oxygen in Marine Biogeochemistry In addition to studying the impacts of deoxygenation on sediment and microbial processes, we have also focused on better understanding the spatial and temporal variability of oxygen in the Santa Barbara Channel. Using advanced technologies such as autonomous underwater vehicles (AUVs) and remotely operated vehicles (ROVs), we collected high-resolution data on oxygen levels in the channel and reconstructed oxygen concentrations in the bottom boundary layer. This new data provides the first detailed map of dissolved oxygen levels across the Santa Barbara Channel, including the bottom layer of the basin. By reconstructing oxygen levels in this bottom layer, we were able to update our understanding of iron release from the seafloor by incorporating new measurements of iron fluxes. Our findings suggest that the release of iron from the seafloor, driven by changes in bottom-water oxygen, contributes significantly to the biogeochemistry of the region --- estimated at approximately 24,000 tons of iron per year. This research is crucial for understanding how changes in oxygen levels can affect the cycling of key nutrients like iron and nitrogen, which, in turn, influence the productivity and health of marine ecosystems. The Role of Iron in Marine Productivity Iron is a crucial micronutrient for phytoplankton, the tiny marine algae that form the foundation of the ocean's food web. Phytoplankton rely on iron for photosynthesis, and their growth can be limited by its availability in surface waters. The Santa Barbara Basin is a hotspot for the release of iron from its sediments, especially in areas where bottom waters become oxygen-deprived, or anoxic. Our research shows that iron is released in large quantities from the seafloor, with fluxes as high as 4.9 millimoles per square meter per day. These iron releases can enhance phytoplankton productivity in surface waters by alleviating iron limitations. However, the effects of this increased iron availability may be partially offset by other nutrient limitations, such as nitrogen, which can reduce the overall impact of this positive feedback. Conclusion Our research sheds light on the complex interactions between deoxygenation, element cycling, nutrient dynamics, and microbial processes in the Santa Barbara Basin. By studying this climate-vulnerable region, we gain a better understanding of how ocean deoxygenation is reshaping marine ecosystems. These findings are crucial for predicting the broader impacts of deoxygenation on global ocean biogeochemistry and marine life, helping to inform strategies for managing and protecting marine environments in a changing climate. Last Modified: 12/13/2024 Submitted by: TinaTreude