Dataset: Electrochemical data from sediment cores collected on the Louisiana Shelf of the Northern Gulf of Mexico during November 2020 on R/V Savannah cruise SAV-20-07

Intact sediments were collected and brought inside the ship's laboratory for profiling within 30 minutes after sampling. Each sediment core was profiled with a single non-invasive mercury/gold (Hg/Au) voltammetric microelectrode deployed on a computer-controlled micromanipulator along with an M-414 combination pH mini-electrode (Microelectrodes, Inc.). Dissolved oxygen (O2(aq)), manganese (Mn2+), iron (Fe2+), organic-Fe(III) complexes (Org-Fe(III)), aqueous clusters of FeS (FeS(aq)), thiosulfate (S2O32-), and total dissolved sulfide (SH2S = H2S + HS- + S(0) + Sx2-) were detected voltammetrically using either linear sweep voltammetry (LSV) for O2(aq) or cathodic square-wave voltammetry (CSWV) for the other species (Luther et al., 2008). Each voltammetric microelectrode was fabricated in-house and prepared by polishing the 100-micrometer (µm) gold tip to a mirror-like surface with diamond pastes of different sizes (15, 6, 1, and ¼ µm, Buehler Inc.), electroplating a liquid mercury layer on the Au surface at -0.2 volts (V) for 4 minutes, and calibrating for Mn2+ in degassed seawater according to previously published methods (Brendel and Luther, 1995). Voltammetric microelectrode quality was determined before calibration by measuring O2(aq) in a fully aerated 0.54 molar (M) NaCl solution by LSV. Dissolved oxygen is reduced to water at the electrode surface in two steps of two-electron transfer with the formation of hydrogen peroxide (H2O2) as intermediate species. As a result, good electrodes should produce two linear sweep waves of the same shape and current intensity. Electrodes that passed this quality test were calibrated for deployment in the sediment cores. Mn2+ calibrations were obtained by CSWV using at least five standard additions of a 0.1 M Mn2+ stock to a 0.54 M NaCl solution degassed with UHP N2 before calibration. Dissolved oxygen measured by LSV was calibrated ex situ in the overlying waters of sediment cores collected during the same cast by Winkler titration. Dissolved Fe2+ and SH2S were quantified by the pilot ion method using Mn2+ as the pilot ion (Luther et al., 2008; Tercier-Weber and Taillefert, 2009). Org-Fe(III) and FeS(aq) cannot be quantified by these methods and are reported in current intensities. Finally, the pH on the total scale was determined by measuring the temperature and electrode potential of both samples and a TRIS buffer in NaCl 0.54 M, then using the Nernst equation to calculate the pH of the samples (Skoog and Leary, 1996). Errors of all reported concentrations and current intensities represent the standard deviation of at least three replicate measurements at each depth in the sediment core.

Instruments:
Sediment cores were collected with an MC-800 multi-corer (Ocean Instruments). Core barrels of 10-centimeter (cm) inner diameter and 75 cm long were used to collect sediments. A DLK-70 Electrochemical analyzer with electrode stand (STAND-1) and EX-MAN-1 micromanipulator (Analytical Instrument Systems, Inc.) was used for all electrochemical analyses, including voltammetric scans and potential measurements at the pH minielectrode.

Known Issues or Problems:
"nd" in the data set is provided when the chemical species were not determined, typically because of electrochemical noise issues on the ship, or because the electrochemical technique was not applied (e.g., the CSWV technique was not applied in the overlying waters). All data sets provided have been analytically validated using the procedures described above.