Award: OCE-1649435

Iron is an important micronutrient for all living organisms. Within the oceans, low concentrations of iron often limit the growth of algae, which are the base of the marine food chain. Iron is made up primarily of two isotopes, Fe-54 and Fe-56, which differ in the number of neutrons contained within the nucleus. It has recently become possible to measure the ratio of these two isotopes with great accuracy in many Earth materials. There are now thousands of measurements of iron isotopes (denoted δ56Fe) in rocks, seawater, and living organisms. This project aimed to measure a small number of key Fe isotope fractionation factors experimentally, in order to better understand the processes which lead to natural variations in δ56Fe. Our work included a study which showed that iron isotope ratios remained constant for dissolved in seawater, colloids (extraordinarily small particles), and larger particles in the South Pacific, helping us to understand that iron does not easily move between these particles and the dissolved iron pool. Similarly, in a glacial river we found that iron isotopes did not change much during a nearly 1000-fold decrease in iron concentrations, showing that iron was lost by a physical mechanism such as small colloidal particles coagulating to form larger particles which sank, rather than by chemical reaction. Additionally, we have completed the largest-ever set of experiments on the fractionation of Fe isotopes during reduction of Fe(III) to Fe(II), finding that the isotopic fractionation depends on the pH of the solution, the reductant used, and the organic complexation of the iron atoms. With this information we are building models of how Fe redox chemistry affects the availability of Fe in the oceans, and how that affects the growth of algae and other organisms higher up the food chain. Last Modified: 11/29/2018 Submitted by: Seth G John