Whole seawater samples for obtaining the concentration of total organic sulfur (TOS) were collected directly from the Niskin bottles into combusted 40 mL glass vials. The vials were stored frozen (-20°C) until analysis on land.

TOS was determined as described in Cutter et al. In brief, a sample is pumped through three ion exchange cartridges connected in series, a 1 mL Ba (Dionex OnGuard II #057093) to remove most of the sulfate, then through a 2.5 mL Ag (Dionex OnGuard II Ag #057090) to remove chloride, and finally through a 1 mL cartridge packed with BioRad AG 4x4 resin (#143-3341) to remove the remaining sulfate. The resulting sample is then analyzed via ion chromatography to quantify remaining sulfate. Total sulfur is quantitatively converted to H2S by via reductive pyrolysis in pure hydrogen gas and quantified by gas chromatography/flame photometric detection using the trapping /detection system of Radford-Knoery and Cutter. Total organic sulfur is calculated as the difference between total sulfur and residual sulfate. Each step of this analysis has an error attached. The units for TOS are nM; when provided, TOS_sd is the standard deviation of the average for replicate samples. Precision was >10% RSD at/above 200 nM S and the detection limit was 26 nM S.