Dataset: Dissolved Organic Matter Sulfur and Carbon Analysis
Deployment: KM2013

Isotope data were recorded using Isodat v3.0 (Thermo Scientific). Isodat calibrates the integrated ion-current ratios against internal reference gas standards, and reports them as δ13C and δ34S values relative to the VPDB and VCDT isotopic scales, respectively. All δ values are reported as ‰ (permil, ppt) deviations from the relevant zero points. Subsequent data processing in spreadsheets included subtraction of capsule blanks, discarding outliers, linearity (peak size) correction, and further calibration to external standards. Sulfur isotope and concentration standards included a methionine working standard, seawater sulfate, and silver sulfide reference materials (IAEA S1, S2, S3). Additionally, a working standard of DOMSPE extracted in a large batch (∼200 L) from the Scripps Institution of Oceanography (SIO) pier was run in at least triplicate with each sample set. C:S ratios were obtained by dividing corrected carbon and sulfur amounts (molar ratio), as calculated from EA peak areas. DOS concentrations were calculated as the product of measured total DOC concentration and DOMSPE C:S ratio.

Extracts supplied by collaborators were often limited by sample size to single or duplicate analyses, so we could not always estimate precision directly for each sample. Instead, uncertainties in isotopic compositions and C:S ratios for samples are reported using the SD of our DOM SIO pier standard measurements, divided by the square root of number of sample replicates analyzed. These SEs (1σ) were ≤0.2‰ for δ34S values and δ13C values and ≤6 for C:S ratios, and we therefore refer to DOSSPE concentrations as apparent or calculated throughout.