Intercomparison of Dissolved Iron Isotope Profiles from Reoccupation of three GEOTRACES Stations in the Atlantic Ocean

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Trace metals, GEOTRACES, Biogeochemistry, Intercomparison

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Intercomparison of trace metal data is a key aspect of the International GEOTRACES program, allowing data from multiple laboratories and countries to be combined to produce high-resolution datasets for the oceans. The use of crossover stations by the GEOTRACES program provides the opportunity both for comparison of analytical techniques and assessment of temporal variability in the cycling of trace metals such as iron (Fe). Here, we present the first comparison of dissolved Fe stable isotope ratio (δ56Fe; relative to IRMM-014) profiles in the oceans, from reoccupations of three locations in the Atlantic Ocean; (1) the Bermuda Atlantic Time Series station (31.75°N 64.17°W) during the US GEOTRACES IC1 cruise (June 2008) and the US GEOTRACES GA03 cruise (Nov. 2011); (2) the Tenatso Time Series station near Cape Verde (17.4°N 24.5°W) during the U.S GEOTRACES GA03 cruises (2010; 2011), and (3) a station in the Cape Basin close to South West Africa (31.1–31.4°S 36.5°W) during the French GEOTRACES GIPY4 Bonus Good Hope Cruise (Feb. 2008) and the UK GEOTRACES GA10 D357 cruise (Oct. 2010). These datasets provided us with the opportunity not only to compare sampling and analysis techniques by two different laboratories (USC and LEGOS), but also the temporal variability of δ56Fe at these locations on a 1–3 year timescale. We found that a good agreement between data and profiles generated by different laboratories does allow assessment of temporal variation of δ56Fe in the water column, as well as spatial variability and synthesis of datasets from different regions of the ocean. In fact, comparison of δ56Fe at the three locations in this study demonstrates a remarkable consistency between the shapes of ocean δ56Fe profiles measured 1–3 years apart, pointing to the overall stability of Fe cycling at all three locations on these timescales, despite the expected dynamic nature of the Fe cycle. This consistency is highlighted by strong agreement in δ56Fe throughout the whole water-column at Bermuda, and in waters deeper than 500 m in the Cape Basin, which suggests that different water masses may carry distinct δ56Fe signatures. In contrast to these stable δ56Fe, we did observe apparent temporal variability in δ56Fe between cruises at other locations and in surface waters, both throughout the water column at Cape Verde, and in Agulhas-leakage influenced surface waters in the Cape Basin. Such temporal variability may thus provide information about changes in internal Fe cycling or external Fe sources on these timescales. Overall, this study highlights the usefulness of repeat δ56Fe measurements to provide information on the variability of Fe cycling throughout the oceans.

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Marine Chemistry, v. 183, p. 50-61