Award: OCE-1830033

The Santa Barbara Basin offers a setting to observe and investigate marine processes that occur when oxygen declines in concentration to the point of absence. When oxygen declines to near zero concentration, other elements sulfur, nitrogen and carbon cycle differently. This research project focused on how the loss of oxygen influences bacterial mats at the ocean floor, which in turn affect the cycling of sulfur, nitrogen and carbon using the Santa Barbara Basin as a case study. The Santa Barbara Basin experiences seasonal to intermittent formation of bacterial mats at the seafloor. In the absence of oxygen these mats accumulate nitrate from seawater which acts as a metabolic oxidant which they couple to the oxidation of sulfide. When oxygen is lost from overlying water, these bacteria thrive and nitrogen biogeochemistry shifts to production of ammonium, rather than the removal of bioavailable nitrogen as nitrogen gas. We find this dynamic further affects sediments through the depletion of iron oxides and conversion to dissolved ferrous iron, which can be lost to the overlying waters. We further find the sediment undergoes substantial shifts in its geochemistry between conditions of excess sulfide versus excess iron. These studies suggest that the cycles of oxygen loss and rejuvenation contribute to rapid release of dissolved iron, potentially influencing nutrient cycling and productivity. Furthermore, the loss of iron could have long-term biogeochemical consequences in this setting, potentially driving a shift toward a sulfide-dominated system. This research project further explored the temporal and spatial variability of oxygen and methane in the Santa Barbara Basin, as well as the seabed coverage of microbial mats under contrasting water column oxygen conditions. This was accomplished through a combination of tools that included an autonomous underwater vehicle, a remotely operated vehicle, the submersible Alvin, and ship-based water column profiling. The contrasting conditions were well captured with widespread mat accumulation during low oxygen conditions and little to no mat occurrence at the high oxygen condition. Oxygen dynamics were also identified in the form of lateral intrusion events, and potentially through internal waves. Importantly, a short term mixing event was captured over a period of weeks which allowed for investigation of the biological response. Methane was also tracked over a cycle of deoxygenation and reoxygenation, with a substantial methane accumulation during deoxygenation giving way to rapid methane consumption following the establishment of deoxygenation. These findings point to microbial dynamics that lag behind physical forcing that controls the introduction of oxygen and sustained oxygen deprivation. This research informs the interrelated processes that occur when the marine environment experiences prolonged oxygen deprivation and renewal including element cycling, nutrient dynamics, and microbial processes. By investigating these processes and the environmental context of the Santa Barbara Basin, we have contributed to a better understanding of how ocean deoxygenation is reshaping marine ecosystems. From an intellectual merit perspective, these findings are useful for understanding potential future impacts of deoxygenation on the ocean. Furthermore, from the perspective of broader impacts, this work supported research and training for postdoctoral scholars, graduate students and undergraduate students proving dozens with at-sea training experiences. Last Modified: 01/12/2025 Submitted by: DavidLValentine