Dataset: Dissolved Organic Matter Sulfur and Carbon Analysis

The following dataset of SPE (solid phase extracted) DOM (dissolved organic matter) accompanies Phillips et al. 2022 (doi: 10.1073/pnas.2209152119). Our project sought to address the question of where long-lived organic molecules that are dissolved in the oceans come from, in particular molecules containing sulfur (S). Our approach was to measure the relative abundance of two stable sulfur isotopes (S-32 and S-34) in these molecules, which is technically very difficult due to the presence of million-fold higher sulfate ions in seawater. We developed a new preparatory chemistry to adequately isolate these organic molecules, and a new elemental analyzer/mass spectrometry method to measure their isotope abundances with high precision at trace levels. We conducted these S isotope measurements on 100 samples of dissolved organic matter (DOM) that had been previously collected by our collaborators from around the world (Northeast Pacific oxygen minimum zone, Northeast Pacific Shelf, North Pacific Gyre, San Pedro Basin, Caeté Estuary, South Pacific Gyre, and the North Sea). We also collected 2 dozen new samples from oceanographic stations in the North Pacific Gyre (Hawaii Ocean Timeseries) and North Atlantic Gyre (Bermuda Atlantic Time Series). The dataset includes 1) sample information such as sample ID, sample location, station name, collection depth (ranging from 0 to 4800 meters), latitude and longitude, month and year sampled (ranging from 2010 to 2021); 2) elemental analysis such as sulfur isotope values (δ34S), carbon (C) isotope values (δ13C), and C:S molar ratios; 3) physical parameters from collaborators' CTD analysis, such as oxygen, salinity, fluorescence, and temperature; and 4) chemical data from collaborators such as dissolved nitrate, phosphate, silicate, and calculated DOC (dissolved organic carbon) and DOS (dissolved organic sulfur) concentrations. Our data show that DOM molecules have (34S/32S) isotope ratios that are entirely consistent with being formed from ocean sulfate, and inconsistent with being formed by reactions of hydrogen sulfide in anoxic porewaters. This result negates one of the leading hypotheses for how long-lived DOM forms, i.e. by reactions in anoxic sediments. Instead, this sharpens our focus on understanding how relatively short-lived biomolecules in the surface ocean are transformed into long-lived DOM molecules.