Differential manganese and iron recycling and transport in continental margin sediments of the Northern Gulf of Mexico

Early Diagenesis in the Hypoxic and Acidified Zone of the Northern Gulf of Mexico: Is Organic Matter Recycling in Sediments Disconnected From the Water Column?

“…Indeed, the reported hypoxia frequency for these three stations are 50-100% during the hypoxic season (Rabalais and Turner, 2019). Four other stations were investigated during the same cruise on the slope and are presented in a companion paper (Owings et al, 2021). At each station, CTD profiles were measured, bottom water was sampled with Niskin bottles, one microprofiler deployment was conducted and three sediment cores were collected using a MC-800 multicorer (Ocean Instruments) and immediately treated on board: one for porosity, one for voltammetry followed by porewater extraction, and one for DET measurements.…”

“…A replicate core collected from the same MC800 deployment was profiled in millimeter increments for dissolved oxygen [O 2(aq) ], thiosulfate (S 2 O 3 2− ), total dissolved sulfide ( H 2 S = H 2 S + HS − + S 0 + S x 2− ), dissolved iron(II) (Fe 2+ ), and dissolved manganese(II) (Mn 2+ ) with 100 µm gold/mercury (Au/Hg) voltammetric microelectrodes (Owings et al, 2021). The electrodes were prepared daily and deployed on a computer-controlled micromanipulator with a DLK 70 electrochemical analyzer (Analytical Instrument Systems, Inc.) in a three-electrode system (Ag/AgCl reference, Pt counter, and Au/Hg working electrodes) as reported previously (Brendel and Luther, 1995;Luther et al, 2008).…”

“…The electrodes were prepared daily and deployed on a computer-controlled micromanipulator with a DLK 70 electrochemical analyzer (Analytical Instrument Systems, Inc.) in a three-electrode system (Ag/AgCl reference, Pt counter, and Au/Hg working electrodes) as reported previously (Brendel and Luther, 1995;Luther et al, 2008). All measurements were performed according to Owings et al (2021). Except for O 2(aq) which was calibrated using the Winkler-determined overlying water measurements, five-point external calibrations in 0.54 M NaCl were performed with Mn 2+ as pilot ion, and S 2 O 3 2− , Fe 2+ , and H 2 S were quantified from the pilot ion method (Brendel and Luther, 1995) with the following minimum detection limits (MDLs): ∼15 µM for Mn 2+ , ∼25 µM for Fe 2+ , and ∼0.2 µM for H 2 S. The electrochemical system also allows detection of organic complexes of Fe(III) [org-Fe(III)] (Taillefert et al, 2000) and aqueous iron sulfide clusters [FeS (aq) ] as intermediates in the precipitation of FeS (s) (Theberge and Luther, 1997).…”

“…After voltammetric profiles were completed, typically 3 h after core recovery, the same core was transferred to a glove bag (Sigma Aldrich) with a N 2 atmosphere for sectioning in 7-10 mm increments down to 12.5 cm and then in 20 mm increments down to the bottom of the core (Owings et al, 2021). For station 2b, the increment was increased to 50 mm after 20 cm depth.…”

“…DIC was determined by flow injection analysis, if necessary after 5 mM ZnCl 2 amendment to avoid dissolved sulfide interference (Hall and Aller, 1992) with an uncertainty of 2-3%. Total dissolved Mn (Mn d ) was determined in duplicate using the porphyrin kinetic spectrophotometric method (Madison et al, 2011) modified to account for dissolved Fe 2+ interferences (Owings et al, 2021). Mn d was quantified by converting the final absorbance to concentration using the molar absorptivity (ε = 91.2 ± 0.4 × 10 −3 µM cm −1 ) determined from external calibration curves with Mn 2+ (MDL: 0.11 µM, 3σ of blanks, n = 24).…”