Award: OPP-1551195

The Antarctic Peninsula, the northernmost part of the continent of Antarctica, has seen massive environmental changes in recent years, including invasions of multiple species of warmer-water animals into the peninsula?s ecosystem. In contrast, the degree of phytoplankton (plant) growth in the surface waters of the region does not appear to have changed and is now believed to be limited by the availability of the micro-nutrient iron. Given the paucity of atmospheric dust in the Southern Ocean, the current conjecture is that lacking any change in dust deposition on the ocean, the level of ecosystem productivity will remain stagnant, even though the species composition is changing. This project investigated the distribution of dissolved and solid phase iron in sediments along the western edge of the Antarctic Peninsula (i.e., the West Antarctic Shelf), as well as the biogeochemical processes occurring in these sediments that exert a major control on sediment iron distributions. In many coastal and continental margin regions, including those along the west side of the Antarctic Peninsula, sediments appear to represent a potentially important, but poorly quantified, source of iron to the overlying water column to support primary productivity. Sediment concentrations of iron are high (relative to those in seawater), and processes in the sediments may allow iron to ?leak? from the sediments to the overlying waters, which could then support productivity driven by this ?recycled? iron. The sediments on the open continental shelf on the West Antarctic Shelf receive a moderate amount of sinking organic debris which results in moderately oxidized surface sediments. Our results show that dissolved oxygen penetrates ~1 to 4 cm into the sediments and this maintains an active oxygen dependent biological community. Soluble iron is produced in these sediments below the depth of oxygen penetration, although this condition may prevent dissolved iron from escaping from these greater depths in the sediments. This occurs because when soluble iron moves upwards and reaches the oxygen-containing zone in the sediments, it tends to precipitate back into the solid phase in a variety of forms of ?rust? (i.e., amorphous, reactive iron oxides). In contrast, near-shore sediments on the West Antarctic Shelf have greater inputs of sinking organic matter, and these sediments are more oxygen-deprived (i.e., oxygen penetration depths are less than ~1 cm). This may then allow a greater fraction of sedimentary iron to be returned in soluble form to the bottom waters. The benthic flux of soluble iron released from sediments is likely the most directly ?bioavailable? form of iron for organisms to take up, however studies have shown that amorphous, reactive iron oxides are also biologically accessible to organisms by a number of different of solubilization processes. At the sites we studied we have observed that in the surface sediments (0-2 cm below the sediment surface) the concentration of total iron in the sediments is 4-5 wt% Fe and that amorphous, reactive iron oxides represent 10-20% of this total sediment iron. More importantly, the processes described above concentrate these amorphous, reactive iron oxides in the surface sediments, since below 10 cm in the sediments total iron in the sediments does not change appreciably but the relative concentrations of reactive iron oxides are roughly half what they are near the surface. Resuspension of surface sediments containing these reactive amorphous iron oxides may therefore represent another possible source of iron that is biologically accessible to iron-limited primary-producing organisms in the water column. Given the highly dynamic nature of the West Antarctic shelf region (e.g., benthic storms, bottom ice scour), this sediment resuspension flux may actually be more important than benthic fluxes of soluble dissolved iron. Overall, the project has enhanced our collective scientific knowledge by providing a more detailed view of iron dynamics in coastal and shelf sediments of the Southern Ocean. This project supported at various levels (in total) 10 faculty, students, and other workers (e.g., laboratory technicians). This included two undergraduate students and two graduate students as well as a faculty member from a predominantly undergraduate-teaching institution (University of New England). All of these individuals were active participants in the scientific work we carried out on our research cruise. The undergraduate and graduate students had never before participated in an oceanographic or earth sciences field project. So, from the point of view of our impact on these students, they gained considerable experience and first-hand knowledge of such field operations. One of the graduate students on the cruise also used samples collected on the cruise for a part of his PhD dissertation. This work (and support from this project) resulted in 4 published abstracts based on presentations at international scientific meetings, 4 published peer-reviewed articles or book chapters, 3 unpublished conference presentations (posters) and 1 Ph.D. dissertation. Two additional manuscripts are also in preparation. Last Modified: 06/12/2020 Submitted by: David J Burdige