Final Report Summary - ISOMAR (Sources and Biogeochemical Cycling of Iron Isotopes in Marine Environments) Iron (Fe) is the most important metal for marine ecosystems. It is essential for the growth of marine phytoplankton and plays an important role in many biochemical reactions such as photosynthesis and nitrate reduction. However, there is a continuing debate over the sources of dissolved Fe to the deep ocean. In this project, the principal investigator (PI) aimed to apply iron isotope systematics in seawater in order to evaluate if the iron flux in the deep ocean derived from benthic diagenesis and seafloor hydrothermal systems has characteristic isotope signatures that are distinct from riverine and atmospheric iron sources.A fundamental goal of this project was the development of a new analytical method in the PI's new laboratory to measure Fe isotopes in seawater. During the first four months of the project, the PI and research team successfully developed the analytical methods for the measurement of Fe isotopes in seawater. In particular, we employed a chelating resin with nitrilotriacetic acid (NTA) functional groups to separate Fe from seawater. We successfully measured iron isotopes in the water column using 1L-size seawater sample with low blanks (about 5 ng) and overall precision of 0.10 ‰. Those blanks / precisions are good enough to measure iron isotopes in deep seawater, hydrothermal plumes and coastal area. However, to measure iron isotopes in same volumes of open surface seawater samples, blanks and precision need to be improved.As originally planned, the PI participated to a research cruise at the Loihi Seamount (Hawaii, United States (US)) during the first year of the project and collected seawater samples affected by hydrothermal input (cruise FeMO2009 cruise, R/V Kilo Moana). Deep-sea hydrothermal systems such as the Loihi Seamount hydrothermal field are important examples of environments where both chemical and biological oxidation of Fe can occur simultaneously, and provide an ideal system to study the speciation and distribution of redox-sensitive bio-reactive elements such as Fe, Mn and S. Among important results, a novel hydrothermal field has been discovered at the base of Loihi at 5000 mbsl. Geochemical analyses demonstrate that 'FeMO deep', while only 0.2 °C above ambient seawater temperature, derives from a distal, ultra-diffuse hydrothermal source. A PhD student has been hired during the first year to work on the analytical and scientific aspect of this proposal. An important part of this PhD work consisted in analysing coupled iron and nickel isotopic composition of hydrothermal and hydrogeneous deposits, as well as rivers and seawater by MC-ICP-MS for investigating metal sources and biogeochemical cycling in seawater. The PI successfully obtained additional funding from IFREMER (Brest, France) to cover the remaining funding for the graduate student.During years 2 and 3 of the project, the research team investigated seawater samples from contrasted oceanic environments: the oxygen minimum zone of the Pacific ocean affected by:1) hydrothermal input from the Loihi Seamount (samples collected during 3 cruises in 2007, 2008 and 2009) and 2) shelf input from the Peru Margin (samples collected in February 2009). Analysis of samples from those two studies gave interesting and novel results. At Loihi Seamount, our first priority was to investigate Fe isotope fractionation processes occuring as Fe enters the ocean, possibly resulting in modification of original source compositions due to Fe-oxide and/or Fe-sulfide precipitation. We collected both hydrothermal vent fluids taken at the seafloor using ROV, and rising buoyant plume samples collected directly above the hydrothermal field. Our analysis of these samples reveals that, for the particulate Fe species within the buoyant plume, about 25 % of the Fe is precipitated as Fe-oxide. The isotope fractionation caused by the formation of these Fe-oxides is up to 2.7 ‰. From mass balance calculations, we have been able to calculate the isotope composition of the dissolved Fe fraction in non-buoyant, and hypothesise that the isotope composition of any stabilised dissolved Fe species exported from volcanic seamount to the surrounding ocean may be lighter than the original vent fluid. Such species would be expected to travel some distance from areas of hydrothermal venting and, hence, contribute to not only the dissolved Fe budget of the deep ocean but also it's dissolved Fe isotope signature.At the Peru Margin, samples collected close to the sediment exhibited negative isotope composition in the dissolved pool, adding support to the idea that sedimentary iron reduction fractionates iron isotopes and produces an isotopically light iron pool transferred to the ocean water column. Vertical distributions of Fe concentrations and isotopes were determined in the total dissolvable and dissolved pools in the water column at three coastal stations located along the Peru margin. Fe concentrations exhibit a maximum in surface layer in both dissolved and total dissolvable pools at the two southerly stations. Fe isotopic composition systematically showed a fractionation toward negative values both in the dissolved and total dissolvable pool. The demonstration that both Fe speciation and isotopic composition along the Peru margin is affected by sedimentary iron reduction and redox cycling support the idea that shelf-derived benthic iron represent an important source of bioreactive Fe in seawater.Additionally to those two study areas, international collaborations were initiated in order to measure isotopic composition of riverine iron in the Gulf of Alaska and to investigate Fe isotope systematics in porewaters and sediments of the subterranean estuary in Indian River Lagoon, Florida. Those additional results further confirm our initial hypothesis that iron in coastal seawater, derived from benthic diagenesis and/or groundwater, has a unique (very light) isotope signature that is distinct from other iron sources, such as rivers. Together with colleagues at Caltech (US) and LEGOS (France), we also reported Fe-isotope composition in seawater from the Geotraces intercalibration site in the North Atlantic (Western North Atlantic, BATS Station, 32° N, 65° W). In summary, this project has generated important new data suggesting that Fe isotopes are valuable tracers for shelf-derived iron and hydrothermal input, both of which have been hypothesised to be important components of global oceanic iron cycle. By developing such new geochemical proxies, this proposal will help to better assess how climate-driven changes in weathering and coastal hypoxia might impact future iron delivery to the global ocean.
