title: Placing an upper limit on cryptic marine sulphur cycling.
abstract: A quantitative understanding of sources and sinks of fixed nitrogen in low-oxygen waters is required to explain the role of oxygen-minimum zones (OMZs) in controlling the fixed nitrogen inventory of the global ocean. Apparent imbalances in geochemical nitrogen budgets have spurred numerous studies to measure the contributions of heterotrophic and autotrophic N2-producing metabolisms (denitrification and anaerobic ammonia oxidation, respectively). Recently, 'cryptic' sulphur cycling was proposed as a partial solution to the fundamental biogeochemical problem of closing marine fixed-nitrogen budgets in intensely oxygen-deficient regions. The degree to which the cryptic sulphur cycle can fuel a loss of fixed nitrogen in the modern ocean requires the quantification of sulphur recycling in OMZ settings. Here we provide a new constraint for OMZ sulphate reduction based on isotopic profiles of oxygen ((18)O/(16)O) and sulphur ((33)S/(32)S, (34)S/(32)S) in seawater sulphate through oxygenated open-ocean and OMZ-bearing water columns. When coupled with observations and models of sulphate isotope dynamics and data-constrained model estimates of OMZ water-mass residence time, we find that previous estimates for sulphur-driven remineralization and loss of fixed nitrogen from the oceans are near the upper limit for what is possible given in situ sulphate isotope data.
authors.affiliation: Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, USA.
authors.forename: D T
authors.initials: DT
authors.lastname: Johnston
authors.affiliation: Department of Geosciences, Virginia Tech University, Blacksburg, Virginia 24061, USA.
authors.forename: B C
authors.initials: BC
authors.lastname: Gill
authors.affiliation: Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, USA.
authors.forename: A
authors.initials: A
authors.lastname: Masterson
authors.affiliation: Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, USA.
authors.forename: E
authors.initials: E
authors.lastname: Beirne
authors.affiliation: Department of Environmental Earth System Science, Stanford University, Stanford, California 94305, USA.
authors.forename: K L
authors.initials: KL
authors.lastname: Casciotti
authors.affiliation: Department of Earth, Ocean and Atmospheric Sciences, Florida State University, Tallahassee, Florida 32306, USA.
authors.forename: A N
authors.initials: AN
authors.lastname: Knapp
authors.affiliation: Department of Earth Sciences, University of Southern California, Los Angeles, California 90089, USA.
authors.forename: W
authors.initials: W
authors.lastname: Berelson
mesh_terms: D000641:Ammonia / Q000378:metabolism; D000693:Anaerobiosis; D059001:Aquatic Organisms / Q000378:metabolism; D009584:Nitrogen / Q000378:metabolism; D009586:Nitrogen Fixation; D010084:Oxidation-Reduction; D010100:Oxygen / Q000032:analysis / Q000378:metabolism; D010103:Oxygen Isotopes; D012623:Seawater / Q000737:chemistry*; D013455:Sulfur / Q000032:analysis* / Q000737:chemistry / Q000378:metabolism; D013460:Sulfur Isotopes
chemical_list: D010103:Oxygen Isotopes; D013460:Sulfur Isotopes; D013455:Sulfur; D000641:Ammonia; D009584:Nitrogen; D010100:Oxygen