Systems Biogeochemistry of Major Marine Biomes. Группа авторов

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Systems Biogeochemistry of Major Marine Biomes - Группа авторов


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AND TEMPORAL CHANGES IN SULFUR CYCLING

      Sulfur has four stable isotopes, 32S, 33S, 34S, and 36S, with average natural abundances of 94.93%, 0.76%, 4.29%, 0.02%, respectively (Coplen et al., 2002). Results are commonly expressed in the delta notation, placing the ratio of the two major isotopes 32S and 34S in a sample in relation to this ratio in a reference material and normalizing it to the Vienna Canon Diablo Troilite standard (VCDT; Krouse and Coplen, 1997):

delta Superscript 34 Baseline normal upper S left-parenthesis per-mille right-parenthesis equals left-bracket left-parenthesis Superscript 34 Baseline normal upper S slash Superscript 32 Baseline normal upper S Subscript upper S a Baseline slash Superscript 34 Baseline normal upper S slash Superscript 32 Baseline normal upper S Subscript upper S t Baseline right-parenthesis en-dash 1 right-bracket times 1000

      Despite early studies recording all four stable sulfur isotopes (Hulston and Thode, 1965), only the last 20 years has there been an increasing number of multiple sulfur isotope studies that also recorded the minor 33S and 36S isotopes. Acknowledging the fact that modern day physical, chemical and biologically mediated reactions are generally associated with a mass‐dependent fractionation in sulfur isotopes, these publications express their results as deviation from the calculated mass‐dependent isotope fractionation (Farquhar et al., 2000):

upper Delta Superscript 33 Baseline normal upper S left-parenthesis per-mille right-parenthesis equals delta Superscript 33 Baseline normal upper S en-dash 1000 times left-bracket left-parenthesis 1 plus delta Superscript 34 Baseline normal upper S slash 1000 right-parenthesis Superscript 0.51 5 Baseline en-dash 1 right-bracket upper Delta Superscript 36 Baseline normal upper S left-parenthesis per-mille right-parenthesis equals delta Superscript 36 Baseline normal upper S en-dash 1000 times left-bracket left-parenthesis 1 plus delta Superscript 34 Baseline normal upper S slash 1000 right-parenthesis Superscript 1.90 Baseline en-dash 1 right-bracket period

      Early sulfur isotope measurements in marine sediments were published by, for example, Thode et al. (1949, 1953) and Szabo et al. (1950) and provided the basis for the application of sulfur isotopes in earth and life sciences.

Schematic illustration of homogeneous sulfur isotopic composition of modern seawater sulfate as compiled by Rees et al.

      Modern marine sediments, i.e. sediments deposited from a bottom water containing dissolved oxygen, contain on average 0.6 weight per cent of sulfur (Goldhaber, 2003), generally present as sedimentary pyrite and attributed to microbial sulfate reduction and subsequent precipitation as iron sulfide (Canfield, 2001a; Rickard and Luther, 2007). Microbial sulfate reduction, more specifically organoclastic sulfate reduction, is associated with a distinct isotopic fractionation of up to 70‰, generally displaying a δ34S value for the resulting sulfide that is 34S‐depleted compared with the parental sulfate. The magnitude in isotopic fractionation is determined by a multitude of factors including the availability and reactivity of sulfate and organic substrate as well as physicochemical boundary conditions, such as temperature. Milestones in our understanding in this respect were published by Kaplan and Rittenberg (1964), Canfield (2001b), Detmers et al. (2001), and more recently by Johnston et al. (2007), Sim et al. (2011), Leavitt et al. (2013), and Wing and Halevy (2014).

Schematic illustration of simplified depth distribution of changes in porewater sulfate concentration and sulfur isotopic composition of dissolved sulfate and sulfide as a consequence of progressive microbial sulfate reduction.
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