Award: OCE-1658042

The purpose of this research was to build computer models to help us understand the cycling of iron (Fe) in the global ocean. Fe is an important nutrient required by phytoplankton (algae) that grow in ocean surface waters and extract carbon dioxide from the atmosphere. However, the processes that supply iron to the surface ocean remain poorly understood. Iron inputs to the ocean from atmospheric dust, seafloor sediments, and hydrothermal vents, are difficult to quantify, as is the timescale of iron removal from seawater due to its tendancy to stick to sinking organic detritus. Our computer models were designed to teach us about the ocean Fe cycle by determining the balance of processes required to explain a growing database of Fe concentration measurements, as well as measurements of other "complementary" trace elements. These elements often share some sources and sinks with Fe, but otherwise exhibit simpler cycling behavior than Fe, which has complicated chemistry and biological functions. Our computer models were designed to be fast and efficient enought to run very quickly, allowing thousands of simulations to determine the processes that best explain the data. By modeling the oceanic aluminum (Al) dustribution, we learned important lessons about the supply of trace elements from dust: almost half of the global source is focused in the Atlantic Ocean downwind of the Sahara Desert, and the trace element supply to remote regions of the ocean is buffered by the enhanced solubility of dust that has undergone chemical processing in the atmosphere. The total supply of Fe to the ocean from dust was found to be at the low end of previous estimates, and falls short of the biological Fe demand in most of the ocean (Fig. 1). By modeling isotopes of the element thorium, we learned that physical stirring ("resuspension") of seafloor sediments acts as a major source of trace elements to the water column, especially in regions where sandy sediments accumulate rapidly at the seafloor, due to sediment inputs from land. We estimates that iron inputs from this mechanism are larger than previously recognized, and combined with other sedimentary processes make the seafloor the largest single source of Fe to the ocean. By modeling the trace metal zinc (Zn), we learned about the role of sinking organic detritus in redistribution trace elements through the water column, by collecting molecules near the surface and releasing them at depth after the particles sink - a set of processes referred to as "reversible scavenging". Additional work led by our collaborators examined the pathways of Fe transport from hydrothermal vents towards the ocean surface, and concluded that the majority of this hydrothermal Fe is trapped in the deep ocean due to the reversible scavenging process, rather than emerging at the ocean surface. Taken together, all of our results paint a picture of the ocean Fe cycle in which sediments are the dominant source to the ocean as a whole, but atmospheric dust plays an outsized role in supplying Fe to surface ocean ecosystems, because Fe supplied from sediments and vents is efficiently scavenged before reaching the surface. We estimate the oceanic lifetime of Fe to be 15-45 years (Fig. 2), towards the low end of previous estimates. Broader impacts of this project include: (1) The development of a simple modeling framework that has been made widely available to the scientific community to help interpret datasets and test hypotheses; (2) Training of a graduate student, who was supported through most of her Ph.D. by this award; (3) Development of teaching materials for oceanography classes at the University of Rochester; (4) Development of an outreach workshop focused on ocean chemistry and climate change, offered twice to high school students from the low-income Rochester City Schools District. Last Modified: 11/16/2023 Submitted by: ThomasSWeber