Award: OCE-1829643

Iron (Fe) is a crucial nutrient for microbial growth in the oceans, impacting the carbon cycle and the climate system, but Fe does not dissolve readily in seawater and so its availability limits phytoplankton growth over much of the surface oceans. One of the most significant ways by which Fe reaches the surface oceans is through deposition of wind-blown dust; however, for this Fe to be available for biological growth, it must dissolve and be kept in solution bound to organic molecules or as colloids. Despite this known importance, the mechanisms of dust dissolution and the identity of the organic molecules keeping Fe in solution remain poorly understood. This award focused on using analysis of field samples and global biogeochemical ocean modelling to better understand how Fe is released from atmospheric dust into surface ocean waters, the impact of this dust on surface waters, and how the aerosol source and dissolution mechanisms affect the resultant iron isotopic signature of the dissolved Fe pool. Scientists from the University of South Florida, Oregon State University, and Old Dominion University collected natural North Atlantic dust at the Tudor Hill Tower on Bermuda over a yearly cycle and characterized the weekly bulk and water- or acid- soluble Fe content and isotopic composition of the dust. This time series provides insight on the relative contribution of atmospheric Fe to the region from both natural Saharan dust and human-derived combustion emissions (highly seasonally-variable), as well as the applicability of different leaching methods for studying atmospheric Fe. The time series of dust deposition was paired with analysis of seasonally-collected water column depth profiles (0 to 1000 m) for dissolved Fe isotope composition, allowing comparison of the primary atmospheric iron isotope signal with that following the deposition of the same dust into seawater. Finally, experiments with marine siderophores were carried out to investigate the direct role of specific natural organic ligands in mediating dust dissolution and the Fe isotopic signature of the surface waters. While atmospheric depoition of elements (including Fe) to the North Atlantic Ocean is characterized by the dominance of natural Saharan dust, especially in summer months, the larger North Pacific Ocean receives a large flux of both natural and anthropogenic atmospheric material from Asia and North America, but with very strong seasonal gradients. In this project, in collaboration with scientists at the University of Tokyo and Japanese GEOTRACES, analysis of isotopic composition of atmospheric Fe and surface seawater across the North Pacific Ocean highlighted the importance of biological uptake in controlling the surface dissolved Fe pool. Further, using collaborative global biogeochemical modelling with scientists at the University of Liverpool, it can be shown that the impact of atmospheric Fe on the surface Fe and Fe isotope cycle is highly dependent on ocean regime. This contrast betwen ocean basins provided a useful opportunity to further inform how oceanic processes in surface waters act to determine both the size and isotopic composition of the dissolved Fe pool. The outcomes of this work will provide a framework for models and interpretation of large-scale field studies such as GEOTRACES, as well as enhancing the research community's ability to interpret aerosol and oceanic iron isotope data. The project supported two early-career investigators, as well as mentorship, professional development, and training of two postdoctoral fellows, an undergraduate technician, and three graduate students. Findings were communicated to a range of local, national, and international communities. Last Modified: 01/04/2024 Submitted by: TimothyMConway