Iron Isotope Systematics in Rivers and Estuaries: Processes Controlling the Fractionation between Continental Run-Off and the Oceans

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Iron (Fe) isotopes may provide a new approach to constrain the relative contribution of Fe sources to the ocean and to improve our understanding of how Fe is mobilized from source regions (i.e., weathering regimes) and transported into the ocean (i.e., dissolved or particulate species). However, the application of Fe isotopes as indicators of provenance and as biogeochemical tracers in aqueous systems remains largely unexplored. For this reason, researchers from Woods Hole Oceanographic Institution will carry out a field and laboratory study to determine the processes controlling the fractionation of Fe isotopes between continental run-off and the oceans. Specifically, the scientists plan to test the following three hypotheses: (1) chemical weathering yields soluble and colloidal Fe pools in rivers that are isotopically distinct from particulate Fe pools; (2) seasonal variability of Fe isotope composition of the dissolved pool is significant, reflecting possible redox reactions due to hydrological or temperature changes; and (3) the large scale removal of river-borne dissolved Fe can significantly modify the Fe isotopic signature of terrestrial Fe reaching coastal waters. The Fe isotope composition of dissolved, colloidal, and particulate matter will be determined from two contrasting environments along the northeast of the United States, an organic-rich and Fe-rich system (Mullica River and its estuary) and a large river carrying lithogenic particles (Connecticut River and its estuary). The possibility of seasonal variations of Fe isotope composition in a river system of global significance (Mississippi River) where redox reactions have been suggested to control dissolved Fe concentrations will also be addressed. Laboratory experiments will simulate the coagulation of naturally occurring humic-Fe-colloids by seawater, and results will be compared with natural samples from the Mullica and Connecticut estuaries. Ultrafiltration techniques will be used to measure the Fe isotope composition of different sizes of colloids and determine if the Fe isotope signatures of rivers are inherited from weathering or from in situ cycling of Fe associated with colloids. As regards broader impacts, this study will improve our understanding of the iron geochemical cycle in the marine environment. One undergraduate student will participate in the field and laboratory components of the study during the summer months.