Award: OCE-1636332

There are three regions of the oceans where oxygen concentrations drop to negligible values at depths as shallow as 100m. They are the Eastern Tropical North Pacific Ocean (ETNP) (off southern Mexico), the Eastern Tropical South Pacific (ETSP) off Peru, and the Arabian Sea. We don’t know what defines their boundaries, or controls the internal cycling of oxygen, nitrogen and other elements. In this project, we focused on the cycles of multiple elements within the ETNP – the least well studied of these systems. We included elements (sulfur, iodine, iron, cadmium, manganese, nickel, aluminum, titanium and the lanthanide group elements) with chemistries that are profoundly altered by the presence or absence of oxygen, but in different ways. Their comparative behavior shows how this regime works as a system. We focused on two aspects – the role of processes on sinking particles and the role of inputs from the continental shelf. We began the study focusing on sinking particles and processes originating "on site". However, after five cruises and a great deal of data, we realized that our findings were most readily explained by inputs from the continental shelf. In the ETNP, the most active anaerobic regime is the boundary region between the shelf sediments and overlying water. Our data suggest that eddies along the continental shelf-slope break are moving water – enriched with dissolved and particulate elements – hundreds of kilometers offshore over a narrow range of densities and depths. Large particles containing insoluble elements like titanium and aluminum were found far offshore, where they exhibited sharp maxima just below the depth where oxygen runs out. Since large particles sink rapidly, such features could only occur if the physical processes transporting water offshore were also fast. Oceanographers often refer to the circulation within the ETNP and similar regimes as "sluggish", but that really means the water doesn’t reach the surface and become replenished with oxygen very often. There is evidently quite a bit of movement within the oxygen-depleted depths. The linkage between offshore transport and the sediment-water interface is clearly demonstrated by examining two elements that have a benthic source – manganese and iodine. We studied the relationship between iodide and manganese on two transects off the Mexican coast. The two elements show identical sub-surface enrichments over a very narrow range of depths, extending far offshore. We discovered that the water enriched in manganese and iodide has a characteristic temperature (130 C) and is salty relative to the waters above and below. It is a persistent water mass feature and physical oceanographers call it "13C water". This salty water forms a dense wedge that inhibits penetration by the oxygen-rich waters above it. Further south, off Costa Rica, the 13C water is less pronounced, and oxygen penetrates much deeper. But 13C water re-emerges in the ETSP where it is important in the upper boundary of that system. A critical characteristic is that the 13C water intersects with the majority of the shelf sediments, essentially providing a conduit for the transport of materials offshore. Interestingly, these 13C water plumes are also the most biologically active part of the anaerobic water column, and form its upper boundary. More work is needed to determine if 13C water and its linkage to the shelf influences these boundaries. That is important, because there is an ongoing debate about what controls these boundaries and whether they are likely to expand or contract in a warming ocean. A linkage between the shelf and the interior of the ETNP has societal implications, because shelf sediments are supplied by major rivers that may be diverted or otherwise diminished in the future. The ETNP is important as a source of dissolved iron into the interior of the ocean. Measurements of iron isotopes indicated enrichment in the light isotope, suggesting that the iron came from oxygen-free conditions on the shelf. However, iron and manganese become decoupled beyond the shelf, even in the absence of oxygen. Soluble iron is oxidized by nitrate in the absence of oxygen, whereas manganese is not. Consequently, iron plumes do not extend as far offshore. We made the first measurements of the rates of this process during the course of this project. Some of these measurements were made in devices suspended within the ocean (as opposed to the ship’s laboratory) which provided confidence that perturbations to the microbial community and oxygen contamination were minimized. We found that the oxidized iron sinks but is remobilized at depth, forming broad deep plumes that may reach the surface far away from their point of formation. Our project provides insight into how this previously understudied system works and how linkages with the continent may affect it in the future. Five Ph.D students from USC will use results in their dissertations. An archive of preserved samples are available for future work. Last Modified: 12/30/2020 Submitted by: James W Moffett