Sediment and pore water collection:

Short sediment cores were collected using a Bowers & Connelly megacorer, a multiple coring device that can collect ~20-40 cm long sediment cores with undisturbed sediment surfaces. At two sites (stations 41 and 64) longer cores (up to ~2 m) were also collected with a Kasten corer.

Megacorer cores were either sectioned for solid phase analysis, profiled with polarographic microelectrodes to determine dissolved O2 concentrations, or sectioned in a cold van under N2 for pore water sample extraction (for details see, Komada et al., 2016). Kasten cores were brought into a large cold room on-board ship, laid on their side and one side of the core box removed to expose the sediment in the core. A plastic block was placed against the top of the core to prevent slumping of the sediment during processing, and pore waters were collected from these cores using Rhizon samplers (Seeberg-Elverfeldt et al., 2005) inserted directly into the cores at measured intervals.

Pore water samples collected from both types of cores were filtered through 0.45 µm polycarbonate filters and processed as follows. Samples for alkalinity determinations were stored without a headspace in 3-ml plastic syringes sealed with 3-way stopcocks. Titrated alkalinity samples (acidified to pH ~4 after titration) were stored in plastic snap cap vials, refrigerated and returned to ODU for the analysis of dissolved sulfate. Pore water samples collected for the analysis of total dissolved Fe and Mn were acidified to pH <2 on-board ship with trace metal grade HCl, and store refrigerated until analyzed back at ODU. Samples for pore water silicate analyses were analyzed on board the research vessel. Additional samples for the analysis of other dissolved nutrients (nitrate, nitrite, ammonium, phosphate) were filtered into tightly capped sample vials and frozen for return to NEOL for analysis. Selected pore water samples (collected as described above) were also used for the determination of dissolved organic carbon (DOC). These samples were filtered directly into acid-cleaned and muffled (550 °C for at least 4 h) glass ampules and were then acidified to pH < 2 with 6 N trace metal grade HCl and flame-sealed under a stream of UHP N2 gas. The sealed ampules were stored refrigerated and returned to ODU for analysis.

While it is possible to recover cores with intact sediment-water interfaces using a megacorer, loss of surface sediments is typical during Kasten coring, making it not possible to directly determine absolute depths below the sediment-water interface in a Kasten core. We therefore determined the absolute depths of pore water and solid phase sample intervals from Kasten cores by aligning Kasten core profiles of pore water alkalinity to megacore alkalinity profiles from the same site (Berelson et al., 2005; Komada et al., 2016).

Bottom water collection:

Bottom waters were collected by GO-Flo Bottles ~5-10 m off the seafloor. They were filtered through 0.45 µm polycarbonate filters and processed as described above for pore water samples.

Pore water analyses: Sampled collected for alkalinity determination were titrated aboard ship within 12 hours of collection by automated Gran titration (Hu and Burdige, 2008). Dissolved sulfate was determined on titrated alkalinity samples returned to ODU by ion chromatography and conductivity detection (Thermo-Scientific Dionex ICS-5000; Burdige and Komada, 2011; Komada et al., 2016). Concentrations of DOC were determined at ODU by high temperature combustion using a Shimadzu TOC-V total carbon analyzer (Komada et al., 2013; Komada et al., 2016). Frozen samples for the determination of dissolved nutrients were returned to NEOL and analyzed by autoanalyzer for nitrate and nitrite (Armstrong et al., 1967; Pavlou, 1972), ammonium (Koroleff, 1970; Slawyk and MacIsaac, 1972) and dissolved inorganic phosphate (Drummond and Maher, 1995). Pore water silicate was determined on board the research vessel used fresh pore water samples and a manual colorimetric method following Armstrong et al. (1967).

Pore water dissolved iron was determined colorimetrically at ODU using the ferrozine technique (Stookey, 1970; Viollier et al., 2000). Hydroxylamine-HCl (0.2% final concentration) was added to the samples before analysis, to reduce any dissolved Fe3+ in the samples to Fe2+. The pore water iron results reported here therefore represent total dissolved iron (i.e., Fe2+ plus any Fe3+ in the samples). This was done largely as a precaution against any iron oxidation that may have occurred during sample storage, since it is assumed that virtually all of the dissolved iron in these pore waters exists in situ as Fe2+ (e.g., Viollier et al., 2000).

Samples for the analysis of dissolved manganese were determined with a modification of the colorimetric formaldoxime method (Armstrong et al., 1979; Goto et al., 1962). These modifications were made based on the observation that the amount of EDTA typically added to destroy the Fe-formaldoxime complexes that interfere with the colorimetric determination of the Mn-formaldoxime complexes was insufficient because of the complexation (and presumed competition) of this EDTA by the much higher levels of dissolved Ca2+ and Mg2+ in our pore water samples ( 60 mM assuming a pore water salinity of ~35). Thus it was necessary to increase the amount of EDTA added to the samples so that it exceeded these Ca2+ plus Mg2+ levels.

In our method we made the formaldoxime mixed reagent by dissolving 8 g of hydroxylamine hydrochloride and 4ml formaldehyde (37%) in 200 ml of distilled deionized water. Next we combined 0.5 ml of either a pore water sample or Mn2+ standard with 0.5 ml of distilled deionized water and: 50 µl of the formaldoxime mixed reagent, 50 µl of concentrated (50%) NH4OH, 50 µl of a 20% hydroxylamine hydrochloride solution, and 0.2 ml of a 250 mM EDTA solution. The color of the solution was allowed to develop for 20 min. and then analyzed at 450 nm.

porewater.csv