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

Creators: 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.

Abstract

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 in ³⁴S/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 DOMSPE δ³⁴S 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 DOMSPE samples showed consistent trends with depth: δ³⁴S values decreased, C:S ratios increased, and δ13C values were constant. Porewater DOMSPE was ³⁴S-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 DOSSPE and 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.

Additional Information

© 2022 the Author(s). Published by PNAS. This article is distributed under Creative Commons Attribution-NonCommercial-NoDerivatives License 4.0 (CC BY-NC-ND). We thank the crew, administrative teams, and science parties of the R/V Atlantic Explorer and the R/V Kilo Moana for their assistance during the Bermuda Atlantic Time-Series and Hawaii Ocean Time-Series cruises, especially Rod Johnson and Carolina Funkey. We thank Daniela Osorio Rodriguez and Sijia Dong for their assistance in sample collection aboard the R/V Sally Ride andJess Adkins from Caltech. We thank Troy Gunderson from the San Pedro Ocean Time-Series and the crew of the R/V Yellowfin. We acknowledge Mike Beman for inviting our participation on cruises that provided the NE Pacific OMZ samples and Ken Smith for supporting the acquisition of the Station M NE Pacific samples. We are grateful to members and technicians in the Aluwihare Lab at Scripps Institution of Oceanography, especially Brandon Stephens, Irina Koester, and Tran Nguyen. We acknowledge Usha Lingappa for drafting Figure 1. We thank colleagues for early reviews and conversations about the manuscript, including Tony Wang, Hannah Dion-Kirschner, Morgan Raven, and Ted Present, and other members of the Adkins and Sessions Labs at Caltech. We thank University of California Santa Cruz Professor Matthew McCarthy for early conversations and support for the project. Color-blind-friendly palettes were generated from Paul Tol's online resource. Funding for this work was provided by NSF OCE (Division of Ocean Sciences) Grant 2023676 to A.L.S. and A.A.P.; M.S. and T.D. acknowledge funding by the DFG-FAPERJ (German Research Foundation) cooperative project (DI 842/6-1) and within the Cluster of Excellence EXC 2077 "The Ocean Floor - Earth's Uncharted Interface" (DFG Project number 390741603). P.C.K. is grateful for funding from the Cohan-Jacobs and Stein Families Fellowship of the Fannie and John Hertz Foundation. Portions of this work were developed from the 2021 doctoral dissertation of Alexandra A Phillips at Caltech. Data, Materials, and Software Availability. All study data are included in the article and/or supporting information. The authors declare no competing interest.

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