Intercomparison of dissolved iron isotope profiles from reoccupation of three GEOTRACES stations in the Atlantic Ocean

Abstract

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 (δ 56 Fe; relative to IRMM-014) profiles in the oceans, from re-occupations of three locations in the Atlantic Ocean; (1) the Bermuda Atlantic Time Series Station (31.75°N 64.17°W) during the U.S. GEOTRACES IC1 cruise (June 2008) and the U.S. GEOTRACES GA03 cruise (Nov. 2011); (2) a station near Cape Verde (17.4°N 24.5°W) during the U.S GEOTRACES GA03 cruises (2010; 2011), and (3) two nearby stations 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 U.K. 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 δ 56 Fe at these locations on a 1-3 year timescale. A good agreement between data and profiles generated by different laboratories does allows assessment of temporal variation of δ 56 Fe in the water column, as well as spatial variability and synthesis of datasets from different regions of the ocean. Comparison of δ 56 Fe at the three locations in this study demonstrates a remarkable consistency between the shape of ocean δ 56 Fe 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 δ 56 Fe 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 δ 56 Fe signatures. In contrast to the stable δ 56 Fe at these locations, we observe some apparent variability in δ 56 Fe between cruises at other locations, both throughout the water column at Cape Verde, and in Agulhas-leakage influenced surface waters in the Cape Basin. Such variability may provide information about changes in internal Fe cycling or external Fe sources on these timescales. Overall, this study highlights the usefulness of repeat δ 56 Fe

Thorium distributions in high‐ and low‐dust regions and the significance for iron supply

“…While we acknowledge that both Fe and Th may have seasonal changes in distribution at both sites [ Fitzsimmons et al ., ; Hayes et al ., ; Middag et al ., ; Conway et al ., ], we find an insightful comparison between Fe and Th data from BATS and ALOHA. Here we compare the present data set (BATS and AE1410‐6 in June 2014) with Fe data collected at BATS in November 2011 [ Fitzsimmons et al ., ; Hatta et al ., ] and with Fe and Th data collected at ALOHA in September 2014 [ Fitzsimmons et al ., ; Hayes et al ., ].…”

Dissolved iron and iron isotopes in the southeastern Pacific Ocean

“…Regardless, the δ 56 Fe of −0.20 ± 0.24‰ is representative of relatively newly formed AAIW in the Southeast Pacific and is not significantly eroded on a zonal scale. Although direct δ 56 Fe data from AAIW Antarctic source regions are not yet published, the isotopically light δ 56 Fe in AAIW observed in this study (−0.20‰) corresponds precisely with δ 56 Fe measured in samples collected from the depths of AAIW at ~31°S in the Atlantic in both 2008 and 2010 (−0.17 ± 0.07‰ and −0.18 ± 0.09‰ [ Conway et al , ; Lacan et al , ]). Taken together, these observations suggest that AAIW is carrying an isotopically light preformed δ 56 Fe signature northwards or that similar internal biogeochemical processes occur as the water mass moves northward in both ocean basins.…”

Tracing and constraining anthropogenic aerosol iron fluxes to the North Atlantic Ocean using iron isotopes

“…However, data from the present study suggest that a second significant aerosol component should be considered, particularly in regions with high potential deposition fluxes of combustion-derived Fe. Indeed, a continuous background delivery of light δ 56 Fe to the Western North Atlantic could contribute to the observation that the δ 56 Fe of the water column at Bermuda appears to show little variability between season 54 , and is near +0.4‰ throughout the water column 6,55 , lighter than the proposed natural dust endmember. However, we note this might also be influenced by the recently described process of eddy-driven sediment Fe supply to this region with a near-crustal δ 56 Fe signature 56 .…”

“…Sample data in plots are expressed with 2σ internal uncertainty on δ 56 Fe ratios based on the internal analytical error of samples and bracketing standards following our previous work 60 . We estimate 2σ external precision to be 0.05‰ for the water and seawater-soluble aerosol leach samples based on replicate similar analyses of δ 56 Fe in 60 seawater samples 54 (with duplicates measured over several analytical sessions during a similar time period as these samples), and 0.05‰ for the total-digested aerosol samples (based on duplicate analyses of 22 total-digested Fe samples in this study (with internal errors 0.02–0.04‰) over three analytical sessions, excluding two lower concentration samples which had internal errors >0.1%). We therefore regard these 2σ external error values as a more conservative estimate of uncertainty when larger than 2σ internal error.…”