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.

Sulfur isotope fractionation in modern euxinic systems: Implications for paleoenvironmental reconstructions of paired sulfate–sulfide isotope records

“…The only site where this was not possible was Effingham Inlet. For this site, which receives inputs of sulfate from the Pacific Ocean, we used a modern marine d 34 S SO 4 ;surf value of 21.15& reported in a recent study on the isotopic characterization of modern seawater sulfate (Johnston et al, 2014). The compilation is further split into two subsets based on the available sulfide data: (1) sites with solid phase sulfide d 34 S data (i.e., sedimentary pyrite or acid-volatile sulfide, abbreviated as d 34 S SPS ) and (2) sites with water column sulfide d 34 S data.…”

Subglacial Lake Whillans microbial biogeochemistry: a synthesis of current knowledge

“…As a result, these reduction reactions, or the presence of pyrite or greigite framboids, could be a source of reduced sulfur or iron that can be coupled to the reduction of oxygen or nitrate to provide energy for species such as Sideroxydans or Thiobacillus at the sediment-water interface (figure 4a). The extraction of energy through a cryptic sulfur cycle, similar to that described in Blood Falls [8] and marine sediments [85,86], appears to be a widespread phenomenon in aquatic environments given the appropriate redox conditions [103]. Above the redox minimum zone, any resulting reduced nitrogen compounds could then be used by species such as Candidatus Nitrotoga or Candidatus Nitrosoarchaeum along with any processes involving the reduction of oxygen or nitrate that are observed higher in the water column.…”

Impact of Aeolian Dry Deposition of Reactive Iron Minerals on Sulfur Cycling in Sediments of the Gulf of Aqaba

“…The observed 18 O isotope enrichment over the isotopic composition of water (magnitude 22 - 30‰) reflects intracellular oxygen isotope exchange (Böttcher et al, 1998, 1999; Brunner et al, 2005, 2012; Turchyn et al, 2006, 2010; Antler et al, 2013). Therefore, if a significant fraction of sulfate is reduced and reoxidized to sulfate, exchange of oxygen isotopes between sulfate and water can be detected, and thus the oxygen isotopic composition of sulfate may be a proxy for any quantitative cycling of sulfur such as during a cryptic sulfur cycle (Bishop et al, 2013; Johnston et al, 2014). …”