Dataset: Depth profiles of pore water constituents 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

Sediments were collected and pore waters were extracted within 30 minutes after sampling. Pore waters were obtained by centrifugation at 3000 rotations per minute (rpm) for less than 10 minutes after slicing the cores in a sealed anaerobic two-hands bag flushed three times with UHP N2 gas. Pore waters were immediately filtered onto 0.22-micrometer (µm) Whatman 25-millimeter (mm) Acrodisc syringe filters (PES membrane) and either preserved until analysis or analyzed immediately onboard the ship. Samples were preserved at 4° Celsius after acidification (dissolved Ca, dissolved Mn, NH4+) or addition of HgCl2 (δ13C-DIC, TA), dispensed directly into reagents for analysis (Fe(II), total dissolved Fe, ΣPO43-), or analyzed immediately (DIC). Sealed borosilicate glass vials were used for TA and δ13C-DIC as recommended (Dickson et al., 2007; Wang et al., 2018).

NH4+ was measured spectrophotometrically by the indophenol blue method (Strickland and Parsons, 1972), total dissolved Fe and Fe(II) were measured spectrophotometrically by the ferrozine method after the addition or not of hydroxylamine (Stookey, 1970). ΣPO43- was measured spectrophotometrically using the molybdate-blue method after natural color correction to avoid interferences from dissolved silica and sulfides (Murphy and Riley, 1962). DIC was measured by flow injection analysis with conductivity detection after spiking samples with 10 millimoles (mM) ZnCl2 to prevent dissolved sulfide interferences (Hall and Aller, 1992). TA was measured by acid titration in an open-cell with continuous pH measurements (Dickson et al., 2007; Rassmann et al., 2016). δ13C-DIC was measured using an isotope ratio mass spectrometer with high-performance liquid chromatography preparation module for gas samples (Brandes, 2009; Wang et al., 2018). Finally, dissolved Ca and dissolved Mn were measured by ICP-MS with collision cell to prevent argon interferences. Internal standards were used to correct for the drift of the instrument, quality control blanks, standard checks, and certified seawater references were run several times during each run to control accuracy and reproducibility. All calibrations were conducted with at least five standards prepared in a 0.54 molar (M) NaCl matrix prior to each series of measurements. Blanks and quality control checks were run routinely during each analysis. Finally, calibrations sensitivities were compared routinely to ensure accuracy of the methods. For DIC analyses, Dickson DIC certified seawater samples were run routinely during analyses to validate the accuracy of the method. Errors of all reported concentrations represent the analytical error propagated from calibration curves, dilution, and instrumental drift.

Instruments:
Sediment cores were collected with a MC-800 multi-corer (Ocean Instruments). Core barrels of 10 cm inner diameter and 75 cm long were used to collect sediments. Aldrich two-glove atmospheric bags were used to control the atmosphere during pore water extraction and centrifugation with an ELMI CM-7S clinical centrifuge. All spectrophotometric measurements were conducted with a Beckman Coulter DU 720 UV-vis spectrophotometer. DIC measurements were conducted with an in-house system consisting of a Dynamax peristaltic pump, Rheodyne injection valve, Amber Science conductivity detector, and Analytical Instruments, Inc. LCC-100 integrator with computer control. TA measurements were obtained with a computer-controlled Metrohm 877 Titrino plus. δ13C-DIC measurements were conducted with a Thermo Electron SurveyorLite autosampler, a Surveyor MS HPLC pump, and a LC Isolink interface coupled to a Thermo Scientific Delta V plus stable isotope mass spectrometer. Finally, Finally, Cad and Mnd measurements were conducted with an Agilent 7900e ICP-MS with SPS4 Autosampler.

Known Issues or Problems:
"nd" in the dataset is provided when the chemical species was not determined, typically because pore water volumes were too small to be able to conduct all analyses in the same sample. All data sets provided have been analytically validated via the procedures described above.