Water-column samples for trace metal measurements were collected from the Bermuda Atlantic Time-Series (BATS) site (31°40'N, 64°10'W) and adjacent BATS spatial stations during cruises in March (spring), May (early summer), August (late summer) and November (fall) 2019 aboard R/V Atlantic Explorer and R/V Endeavor.
Seawater samples and hydrographic data were collected using a trace-metal clean conductivity-temperature-depth sensor (SBE 19 plus, SeaBird Electronics) mounted on a custom-built trace-metal clean carousel (SeaBird Electronics) fitted with custom-modified 5-liter (L) Teflon-lined external-closure Niskin-X samplers (General Oceanics) and deployed on an Amsteel non-metallic line. On the August cruise, we also collected near-surface samples (small boat) (~0.3 meters (m) depth) in a Niskin-X sampler deployed from an inflatable dinghy ~500 m upwind of the research vessel, to avoid contamination from the ship. After recovery, the seawater samples were filtered through pre-cleaned 0.2-micrometer (μm) pore AcroPak Supor filter capsules (Pall) using filtered nitrogen gas inside a shipboard clean laboratory (Sedwick et al., 2020; Tagliabue et al., 2023) into acid-cleaned 1L LDPE bottles.
Samples were acidified back on shore at the University of South Florida (USF) using 1.2 milliliter (mL) 10 M Teflon-distilled HCl per liter of sample and stored for at least 12 months before processing. Seawater samples were then processed for dissolved iron (Fe) isotope and concentration analysis at the University of South Florida following Sieber et al. (2019), modified from Conway et al. (2013). Briefly, an Fe double-spike was added prior to batch extraction using Nobias PA-1 chelating resin, followed by purification by anion-exchange chromatography using AG-MP1 resin. Concentration and isotope analyses were performed on a Thermo Neptune Plus MC-ICPMS in the Tampa Bay Plasma Facility at the University of South Florida using the double spike technique via a ~100 microliters per minute (uL min-1) PFA nebulizer and Apex Ω introduction introduction system, Pt Jet Sampler cone and an Al X skimmer cone.
Fe stable isotope ratios are expressed in delta notation (δ56Fe) relative to the IRMM-014 standard. A secondary Fe standard, NIST-3126, was analyzed over 44 sessions to provide an estimate of long-term instrumental precision. We obtain a value of +0.36 ± 0.05‰ (2SD, n = 524; runs = 37), in agreement with consensus values (Hunt et al. 2022; Conway et al., 2013). Therefore, we consider a 2SD uncertainty of 0.05‰ as an estimate of analytical precision, and have applied it to all samples, except for low concentration samples where the larger internal error is considered a more conservative estimate of uncertainty. Concentrations were calculated using the isotope dilution technique based on on-peak blank, interference and mass-bias corrected 57Fe/56Fe ratios measured simultaneously with isotope analysis. We express uncertainty (1SD) on Fe concentrations as 2%, based on Conway et al. (2013).
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