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

Creators: Phillips, Alexandra A.¹; White, Margot E.; Seidel, Michael; Wu, Fenfang¹; Pavia, Frank F.¹; Kemeny, Preston C.¹; Ma, Audrey C.; Aluwihare, Lihini I.; Dittmar, Thorsten; Sessions, Alex L.¹

1. California Institute of Technology

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 DOM_SPE 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 DOM_SPE samples showed consistent trends with depth: δ³⁴S values decreased, C:S ratios increased, and δ¹³C values were constant. Porewater DOM_SPE 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 DOS_SPE 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.

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Data Availability

Samples spanned basins and water depths, including the North Atlantic Gyre, Northeast Pacific oxygen minimum zone, Northeast Pacific Shelf, North Pacific Gyre, San Pedro Basin, Caeté Estuary, South Pacific Gyre, and the North Sea. Samples spanned depths of 0 to 4800 meters. Exact location data, including latitude, longitude, and collection depth are provided in the dataset. Most data in this dataset represent measurements of aliquots of archived samples from collaborators; these were collected on many different cruises, on many different dates, around the world. New samples represented in this dataset include: 12 samples collected at station HOT on the R/V Kilo Moana in November 2020 (KM2013) and 12 samples collected at station BATS in September 2020 (AE2014). 10.26008/1912/bco-dmo.927046.1

Acknowledgement

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.

Conflict of Interest

The authors declare no competing interest.

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