Dionex ion chromatograph

Hydrogen sulfide in the porewater was measured by the methylene blue method (24 (link)). The concentration of nitrate (NO₃⁻) was analyzed colorimetrically with a Quaatro continuous flow analyzer (SEAL Analytical Ltd, Southampton, UK). Sulfate concentration was determined using a Dionex Ion Chromatograph (Thermo Fisher Scientific, Waltham, MA, USA). The ammonium concentration in the porewater was measured onboard via indophenol blue spectrophotometry (25 (link)).

Water samples were used to quantify salinity as well as pH and nutrients ([NO₃⁻ + NO₂⁻], [PO₄⁻], [NH₄⁺], [Cl⁻], and [SO₄⁻]). The additional sediment sample was used to measure gravimetric moisture content, % C, and % N by weight. In the lab, salinity of water samples was measured by determining specific conductivity using a CDM 210-m radiometer analytical probe (platinum electrode, 2 poles). The pH of each water sample was determined with an Orion 520A meter with a Ross combination pH electrode. The PO₄ concentration in the water was measured using the spectrophotometric method, as described by Murphy and Riley (22 (link)). The combined concentration of nitrate and nitrite in the water was determined using the cadmium reduction method on a flow injection analyzer (Lachat QuikChem 8000). The ammonium concentration in the water was measured by the phenol hypochlorite method, as described by Solorzano (23 (link)). Chorine and sulfate concentrations were measured using a Dionex ion chromatograph (ThermoFisher Scientific, Pittsburgh, PA).