Award: OCE-1757045

Iron (Fe) is a critical micronutrient that regulates ocean primary productivity and ultimately the global carbon cycle. Various sources of iron to the ocean have been identified, including dust, hydrothermal vents, and release from sediments. The factors controlling the magnitude of dissolved iron release from sediments over a defined area in a specific time period -termed the benthic iron flux- are not fully understood. In particular, the contribution of sandy sediments, which dominate on the continental shelves, is poorly constrained. These permeable sediments allow for the advective transport of water into, through and out of these deposits. They are also inhabited by numerous bottom-dwelling organisms, for example, worms, clams, and shrimp. These organisms mix the sediment and pump water into and out of the sediment in a process called "bioirrigation", thus potentially enhancing the release of iron to the overlying water. The isotopic composition of iron in ocean water can help identify its origin as the different iron sources have distinct isotope signatures. The objective of this study was to elucidate the interconnected roles of sediment permeability, bioirrigation, and additional factors (such as organic carbon availability and oxygen concentrations in overlying waters), in controlling the magnitude of the dissolved iron flux and its isotope composition. We carried out experiments with living organisms and irrigation mimics and used time-lapse oxygen imaging and benthic chamber incubations to study oxygen dynamics and iron fluxes from the sediments in different seasons. We developed novel dissolved Fe traps that accumulate the released iron and found that this method greatly increases the accuracy of flux estimates for iron and Fe-associated elements such as phosphorus. A key finding of our project is that sandy sediments, in contrast to previous assumptions, can be characterized by high benthic iron fluxes and must be taken into account when considering sources to the global ocean iron inventory. Bioirrigation dramatically enhanced fluxes indicating that the activities of benthic macrofauna play a key role in controlling the magnitude of benthic iron fluxes from permeable sediments. Our results also demonstrate that pumping patterns of benthic macrofauna, the depth where oxygen is introduced into the sediment, the area of sediment that experiences intermittent oxygenation and the magnitude of iron fluxes vary seasonally. The occurrence of low-oxygen conditions in overlying waters can further enhance iron fluxes from sandy sediments. However, it is the interplay between the activity of the benthic macrofauna and the environmental conditions that can drive benthic iron fluxes to values that are an order of magnitude higher than what has been previously reported from continental margin sediments. The iron isotope composition of the benthic iron flux varies seasonally, driven by differences in the sedimentary biogeochemical sulfur and iron cycles. Bioirrigation further modifies the isotope signal by altering the amount of dissolved iron that is oxidized at the sediment-water interface. Our project also reveals that accurate measurements of benthic iron fluxes require careful method assessment as significant losses of dissolved iron within chamber systems that are typically used need to be accounted for. Approaches such as the novel Fe traps we developed are well-suited for examining iron fluxes from sands. The project significantly advances our understanding of sedimentary iron cycling within and fluxes from permeable sediments. The isotopic signatures of these fluxes will help to further constrain contributions of different iron sources to the ocean iron inventory. Last Modified: 06/30/2023 Submitted by: Laura Wehrmann