Sediments were collected and profiled immediately with voltammetric Hg/Au microelectrodes deployed on a computer-controlled micromanipulator (Beckler et al., 2016). Sediment pore waters were then extracted from the same core and either preserved until analysis or analyzed immediately onboard ship. Sampling and analyses were conducted immediately after sediment collection to minimize artifacts from exposure to the atmosphere.

Sediment cores were obtained by a MC-800 multi-corer and profiled within 30 minutes with voltammetric Hg/Au microelectrodes deployed on a computer-controlled micromanipulator. After profiling, sediments were immediately sliced under N₂ atmosphere and pore waters extracted by centrifugation at 3000 rpm under N₂ atmosphere. Finally, pore waters were immediately filtered (0.2 µM PSE Puradisc syringe filters, Whatman) under N₂ atmosphere and either preserved at -20C until analysis (Br⁻, NO₂⁻, NO₃⁻, SO₄²⁻), preserved at 4C after acidification (NH₄⁺), dispensed directly into reagents for analysis (Fe(II), Fed, Mnd, SPO₄³⁻), or analyzed immediately (DIC, TA). Br⁻, NO₂⁻, NO₃⁻, and SO₄²⁻ were measured by non-suppressed HPLC with UV detection (Beckler et al., 2014). NH₄⁺ was measured spectrophopotmetrically by the indophenol blue method (Strickland and Parsons, 1972), Fed and Fe(II) were measured by the ferrozine method after addition or not of hydroxylamine (Stookey, 1970). Mnd was measured by the porphyrin kinetic spectrophotometric method (Madison et al., 2011) modified to account for dissolved Fe(II) interferences (Owings et al., 2020). SPO₄³⁻ 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 mM ZnCl₂ to prevent dissolved sulfide interferences (Hall and Aller, 1992). Finally, TA was measured by acid titration in an open-cell with continuous pH measurements (Dickson et al., 2007; Rassmann et al., 2016). All errors reported for the electrochemical measurements represent the standard deviation of at least triplicate measurements. Errors of all other parameters represent the analytical error propagated from calibration curves, dilution, and instrumental drift.

Problem report: Some of the data are missing because pore water volumes were too low to make all the measurements.