Award: OCE-1829844

NSFGEO-NERC: Collaborative Research: Using Time-series Field Observations to Constrain an Ocean Iron Model Iron is an essential micronutrient for marine phytoplankton, and the supply of dissolved iron (DFe) sets a limit on primary production over as much as one third of the surface ocean. Availability of DFe thus exerts important controls on the ocean-atmosphere balance of carbon dioxide, hence global climate, and the ocean ecosystem. As such, there is a need to include and accurately represent DFe in numerical models of the Earth system, in order to predict how future environmental changes will impact ocean biology and biogeochemistry. Contemporary numerical models struggle to reproduce the observed spatial and temporal variations in DFe in the ocean, due to an imperfect mechanistic understanding of the processes that mediate the transformations between the various chemical forms of iron in the oceanic water column. This project aimed to fill this knowledge gap by measuring the major chemical forms of dissolved and particulate iron in the water column at the open-ocean location of the Bermuda Atlantic Ocean Time-series Study (BATS) over one full annual cycle. This study site was chosen because previous observations have revealed a large and consistent seasonal cycle in the concentration of DFe in the upper water column, which is driven by physical and biological processes that are well characterized by the monthly data collected by the ongoing BATS research program. By combining observations of the major inputs and forms of dissolved and particulate iron in the water column during the winter, spring, summer and fall of 2019, together with numerical simulations using a state-of-the-art ocean biogeochemical model, we developed a new conceptual framework for the processes that control the concentration of DFe in the ocean. Specifically, these processes require two largely independent pools of DFe, one being DFe that is chemically complexed or stabilized by dissolved organic compounds called ligands, and the other being colloidal iron oxyhydroxides that can aggregate to form authigenic particulate iron phases and eventually sink, thereby removing DFe from the water column. Whereas previous research had emphasized organic ligands as playing a dominant role in controlling oceanic DFe concentrations, our results suggest that colloidal iron oxyhydroxides play a similarly major role in regulating DFe in the ocean. When these two different DFe pools were represented in the numerical model, it was able to accurately simulate the distribution and seasonal variations of DFe in the BATS region, where previously it had failed (Figure 1). Moreover, the new model formulation significantly improves model DFe simulations at the ocean-basin scale, suggesting that our new conceptual paradigm has general applicability for iron in the ocean. Broader impacts of this project have included the training and mentoring of undergraduate and graduate students, and early-career researchers, as well as the mentoring of middle-school girls in an ocean science focused summer program. In addition, the improved understanding of the ocean iron cycle has advanced our ability to predict how ocean biology and chemistry will respond to future environmental changes, such as a warming climate and changes in the supply of iron-bearing dust to the surface ocean. Figure 1. Observed and modeled DFe concentrations versus depth for the BATS region in March, May, August and November 2019. Red crosses are DFe data, black lines are solutions for the original model formulation with varying total ligands derived from DOC. While blue lines are solutions for the new model formulation with prognostic strong ligands (solid) or DOC derived total ligands (dashed line). Last Modified: 12/21/2023 Submitted by: RodneyJohnson