Award: OCE-1829819

The ocean’s ability to support life and also take up carbon dioxide is often limited by the availability of nutrients. In about one-third of the ocean, the limiting nutrient is iron. Hence, there is significant interest in how iron behaves in the ocean. In contrast to other nutrients such as carbon, nitrogen, and phosphorus, iron is relatively insoluble and tends to precipitate and also attach to the surfaces of other particles in the ocean, causing iron to be removed from surface waters where growing phytoplankton need it. The factors that control the precipitation and loss of iron from the upper ocean are not well known, but dissolved organic molecules that bind to Fe are thought to play an important role. This project funded a team of ocean scientists to study the upper ocean iron cycle in the northwest Atlantic Ocean at the Bermuda Atlantic Timeseries Station over the four seasons of a year. Team members participated in research cruises in March, May, August, and November, 2019. Samples of all the different physical and chemical forms of iron were collected from the upper 1,700 meters of the ocean. Our team was responsible for measuring iron in various types of particulate matter, including phytoplankton cells, dust particles from continental minerals, and other minerals that likely precipitated or formed in situ. We discovered that particulate forms of iron do indeed play an important role in this part of the ocean. Notably, we found that dissolved organic molecules, created by microorganisms to attach to iron, are not able to keep iron from precipitating and attaching to particle surfaces. Below the surface waters of the ocean these processes occur, and iron is transformed from a form associated with cells to a precipitated mineral. This material is more likely to sink from surface waters and remove the nutrient iron from use by phytoplankton. One of our team has developed a quantitative computer model of the ocean system that is able to capture this behavior by assuming that iron precipitates rapidly into very small particles (termed ‘colloids’), which may combine into larger particles. Overall, this project has resulted in a significant improvement in our understanding of the ocean cycle, as well as our ability to model the ocean iron cycle. This project provided opportunities for training for several undergraduate and graduate students, as well as a postdoctoral scholar and an early-career faculty member. Last Modified: 12/31/2022 Submitted by: Benjamin S Twining