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Novel sulfur isotope analyses constrain sulfurized porewater fluxes as a minor component of marine dissolved organic matter
Marine dissolved organic matter (DOM) is a major reservoir that links global carbon, nitrogen, and phosphorus. DOM is also important for marine sulfur biogeochemistry as the largest water column reservoir of organic sulfur. Dissolved organic sulfur (DOS) can originate from phytoplankton-derived biomolecules in the surface ocean or from abiotically āsulfurizedā organic matter diffusing from sulfidic sediments. These sources differ in 34S/32S isotope ratios (Ī“34S values), with phytoplankton-produced DOS tracking marine sulfate (21ā°) and sulfurized DOS mirroring sedimentary porewater sulfide (ā¼0 to ā10ā°). We measured the Ī“34S values of solid-phase extracted (SPE) DOM from marine water columns and porewater from sulfidic sediments. Marine DOMSPE Ī“34S values ranged from 14.9ā° to 19.9ā° and C:S ratios from 153 to 303, with lower Ī“34S values corresponding to higher C:S ratios. Marine DOMSPE samples showed consistent trends with depth: Ī“34S values decreased, C:S ratios increased, and Ī“13C values were constant. Porewater DOMSPE was 34S-depleted (ā¼-0.6ā°) and sulfur-rich (C:S ā¼37) compared with water column samples. We interpret these trends as reflecting at most 20% (and on average ā¼8%) contribution of abiotic sulfurized sources to marine DOSSPE and conclude that sulfurized porewater is not a main component of oceanic DOS and DOM. We hypothesize that heterogeneity in Ī“34S values and C:S ratios reflects the combination of sulfurized porewater inputs and preferential microbial scavenging of sulfur relative to carbon without isotope fractionation. Our findings strengthen links between oceanic sulfur and carbon cycling, supporting a realization that organic sulfur, not just sulfate, is important to marine biogeochemistry.ISSN:0027-8424ISSN:1091-649