Award: OCE-1829761

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 ocean 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. Despite this known importance, the mechanisms of dust dissolution and the identity of the organic molecules solubilizing Fe remain poorly understood. This study developed new chemical analyses combined with laboratory and field experiments to better understand how natural organic molecules present in seawater enhance the release of Fe from desert dust, what organic molecules are produced by microbes in response to dust, what is the isotopic fractionation of Fe associated with dust dissolution, and ultimately what is the role of organic molecule-mediation dissolution in determining the effect of dust on marine nutrient and carbon cycles. The findings of this project provided a better understanding of the factors that control organic matter cycling and supply dissolved iron in the surface ocean. Ultimately, these constraints have the potential to improve our ability to explain and predict changes in metal distributions, ocean carbon cycling, and ocean productivity by more accurately representing organic carbon and atmospheric supply of iron. The molecular forms of iron and organic matter were surveyed in the North Atlantic Ocean and the Oregon coast to identify the source and fate of these molecules. Results from three expeditions indicated that distinct organic ligands which stabilize iron are supplied by biological production in the surface ocean and from marine sediments. In the open ocean, these analyses provided the first molecular level information on fractions of dissolved organic matter that have different lifetimes within the water column. While these fractions had been previously inferred from seasonal dynamics in concentration measurements, their chemical differences and the underlying reason behind their different lifetimes had remained unknown. The findings of this study indicate that the fractions with lower water solubility are more rapidly removed from the water column. These more rapidly cycling molecules may have a higher propensity to form marine snow gels that entrain metals and facilitate their removal, either by sinking to depth or by enhancing consumption by bacteria. Understanding these differences in the chemical makeup of organic matter and metals in the ocean helps us explain why they accumulate and how they might change over time, which is important for understanding marine ecosystems and the planet's carbon cycle. Laboratory experiments informed by this oceanographic field work were also conducted to investigate the rates and mechanistic controls over dust dissolution by environmentally relevant organic molecules in the ocean. These studies revealed that the rate of iron exchange between different chemical forms is slow relative to the rate of biological cycling, challenging previous hypotheses that equilibrium exchange processes govern iron cycling. Experiments conducted in collaboration with the Conway lab at the University of South Florida determined rates of dust dissolution by organic molecules in seawater and demonstrated that this dissolution process did not result in a large fractionation of iron isotopes. These findings indicated that other processes govern iron isotope distributions throughout the surface ocean. This work was incorporated into a published historical perspective review paper that synthesized current knowledge on the role of organic molecules in governing the supply of micronutrient metals to the global ocean. Beyond these scientific advancements, this project contributed to science education and public scientific literacy. This project trained three graduate students, a postdoctoral scholar, and three undergraduates in advanced chemical analysis techniques broadly relevant to chemical and environmental scientific disciplines and careers. These students gained hands-on experience with field/lab work, data analysis, and synthesis through research projects, contributing to new knowledge and publications. High school students participated in this work through the OSU Summer Experience in Science and Engineering for Youth program. They were exposed to university research, diverse science career paths, and the excitement of scientific discovery. This exposure aims to increase access to higher education and motivate students to pursue science and engineering fields. Findings from this project were also incorporated into lecture and discussion material as part of an undergraduate chemical oceanography course to illustrate crosscutting principles of how ocean elemental cycles and biological productivity intersect. To inform public discourse of ocean iron fertilization, a webcast public presentation discussed the science and controversies surrounding this potential carbon dioxide removal strategy. This outreach effort is timely as the United States and other countries explore broader solutions to climate change. Last Modified: 12/28/2023 Submitted by: ReneBoiteau