Dataset: Dissolved iron and iron isotopes for SGD well, river, and estuary samples collected between November 2022 and March 2024 along the West Florida Shelf

Sample Collection

SGD Wells

Submarine groundwater samples were collected by U.S. Geological survey (USGS) small boat from three offshore well transects, each with three well location. Samples were taken from well head (GWd) and from surface (SB) and bottom waters (BW). Divers, using acid-cleaned Teflon tubing attached to a trace metal clean diaphragm pump. First, bottom waters were collected, then the well uncapped, adequately purged, and sampled. All samples were filtered through 0.2 um Acropak Supor Cartridges (Pall, USA) into acid-cleaned LDPE bottles. NC5, NC10, NC15 = Nature Coast Transect, IRB25, IRB30, IRB35 = Indian Rocks Transect, VH45, VH50, VH55 = Venice Headlands Transect. More details are provided in Hunt (2025).

Rivers and Estuaries

River Sample sites were located on land and samples were collected from five rivers (Hillsborough, Alafia, Manatee, Peace, and Caloosahatchee), using trace metal clean protocols. Estuary samples were collected in collaboration with Mote Marine Laboratory via small boat (RV Eugenie Clark), also using trace metal clean protocols. All water samples for dissolved Fe and d56Fe were syringe filtered using acid-cleaned 50 mL Henke Jet HDPE syringes and 0.2 um PVDF membrane Titan3 syringe filters into acid-cleaned LDPE bottles. AR1-2 = Alafia River, HB1-2 = Hillsborough, MR1-2 = Manatee, PR1 = Peace River, CR1 = Caloosahatchee, CS and E= Estuary sample locations. More details are provided in Hunt (2025).

Sample Processing

Samples were acidified back on shore at the University of South Florida (USF) using 2.4 mL 10 M Teflon-distilled HCl per liter of sample and stored for at least 6 months before processing. Samples were then processed for dissolved 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 (Multicollecter Inductively Coupled Plasma Mass Spectrometer) in the Tampa Bay Plasma Facility at the University of South Florida using the double spike technique via a ~100 uL min^-1 PFA nebulizer and Apex Ω introduction system, Pt Jet Sampler cone and an Al X skimmer cone.

Sample Analysis

Dissolved 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 56 sessions to provide an estimate of long-term instrumental precision. We obtain a value of +0.36 ± 0.06‰ (2SD, n = 772; runs = 56), in agreement with consensus values (Hunt 2025; Conway et al., 2013). Therefore, we consider a 2SD uncertainty of 0.06‰ 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. Dissolved Fe 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).