Novel sulfur isotope analyses constrain sulfurized porewater fluxes as a minor component of marine dissolved organic matter

• Main Authors: Phillips, Alexandra A., White, Margot E., Seidel, Michael, Wu, Fenfang, Pavia, Frank F., Kemeny, Preston C., Ma, Audrey C., Aluwihare, Lihini I., Dittmar, Thorsten, Sessions, Alex L.
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: National Academy of Sciences 2022
• Subjects: Multidisciplinary; Pacific; North Atlantic
• Online Access: https://doi.org/10.1073/pnas.2209152119; https://www.ncbi.nlm.nih.gov/pmc/PMC9565371

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 “sulfurized” organic matter diffusing from sulfidic sediments. These sources differ in34S/32S isotope ratios (δ34S values), with phytoplankton-produced DOS tracking marine sulfate (21‰) and sulfurized DOS mirroring sedimentary porewater sulfide (∼0 to –10‰). We measured the δ34S values of solid-phase extracted (SPE) DOM from marine water columns and porewater from sulfidic sediments. Marine DOMSPEδ34S values ranged from 14.9‰ to 19.9‰ and C:S ratios from 153 to 303, with lower δ34S values corresponding to higher C:S ratios. Marine DOMSPEsamples showed consistent trends with depth: δ34S values decreased, C:S ratios increased, and δ13C values were constant. Porewater DOMSPEwas34S-depleted (∼-0.6‰) and sulfur-rich (C:S ∼37) compared with water column samples. We interpret these trends as reflecting at most 20% (and on average ∼8%) contribution of abiotic sulfurized sources to marine DOSSPEand conclude that sulfurized porewater is not a main component of oceanic DOS and DOM. We hypothesize that heterogeneity in δ34S 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. © 2022 the Author(s). Published by PNAS. This article is distributed under Creative Commons Attribution-NonCommercial-NoDerivatives License 4.0 (CC BY-NC-ND). Samples spanned basins and water depths, including the North Atlantic Gyre, Northeast Pacific oxygen minimum zone, Northeast Pacific Shelf, North Pacific ...