Collaborative Research: Isotopic constraints on and modelling of the Subtropical and Equatorial Pacific Ocean iron cycle

About This Grant

Iron (Fe) is an essential element for life, and its availability limits biological productivity in large areas of the ocean. As a result, the ocean iron cycle is closely tied to the ocean carbon cycle. Progress in understanding the ocean iron cycle has been slower than for some other elements because it is present at very low concentrations and care must be taken to avoid contaminating samples. The international GEOTRACES program has, over the last twenty years, greatly accelerated progress in marine iron geochemistry. Capabilities have also been developed for measuring the stable isotopic composition of iron in seawater. In this project, a team of researchers from the University of South Florida and the University of Hawaii will measure iron isotopes in samples from three GEOTRACES expeditions in the Pacific Ocean. This will produce an unprecedented data set for iron isotopes. Combining these data with ocean biogeochemical modeling, the team will address important questions about the sources of iron to the ocean and about biological cycling of iron. The project will support several early-career scientists, including postdoctoral researchers and undergraduate students at each institution. Project data will be made publicly available through the Biological and Chemical Oceanography Data Management Office and in GEOTRACES data products. Iron plays an essential role in driving oceanic biogeochemical cycles and is thought to limit phytoplankton growth over large regions of the surface oceans, particularly the Southern Ocean, where primary productivity mediates deep ocean-atmosphere carbon dioxide exchange, and the eastern Equatorial Pacific, where iron feeds a highly productive ecosystem. Deep iron sources are key to the marine iron cycle, especially in the deep Pacific Ocean where hydrothermal venting from the East Pacific Rise (EPR) has been suggested to supply dissolved iron to the Southern Ocean via upwelling. However, key questions remain about sources, longevity/speciation, and transport of this dissolved iron. Samples have been collected during two GEOTRACES zonal sections, from South America to Australia along 10°S (GP21 in 2021) and Ecuador to Papua New Guinea along the Equator (GP11 in 2023). Samples are also available from one GEOTRACES-compliant cruise (GPc03) in the western subtropical Pacific. Using a combination of state-of-the-art iron isotope measurements and biogeochemical modelling, the team will address the following questions: 1) Using iron isotopes as a tracer, what is the relative supply of dust, sediment, and hydrothermal-derived iron to different regions of the shallow ultra-oligotrophic South Pacific Gyre and Equatorial upwelling zone, and how does iron supply vary in space and time? 2) How do microorganisms fractionate iron isotopes as they take up iron? How does the extent of this fractionation relate to the (temporally and spatially variable) nutrient limitation regimes and the degree of upper ocean iron recycling in the Equatorial-South Pacific? 3) What is the flux, isotope signature, and location of dissolved iron supplied to Pacific Deep Waters (PDW) by hydrothermal venting along the EPR and the Tonga/Kermadec arc? What are the stabilization mechanisms, isotopic changes, and longevity of this dissolved iron with transport? How do dissolved iron and iron isotopes evolve in PDW as it travels southward along the coast of South America and into the Southern Ocean? The results are expected to transform understanding of this essential micronutrient. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.