Systems Biogeochemistry of Major Marine Biomes. Группа авторов

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sulfur, if preserved) provide a detailed record of sediment diagenesis. Important supplementary information includes detailed petrographic examination of pyrite morphology, distinguishing early diagenetic framboidal pyrite from late diagenetic overgrowth or late‐stage euhedral crystals (Figure 2.3), as well as additional sediment geochemical data (e.g. organic carbon content, iron speciation).

Schematic illustration of different generations of sedimentary pyrite with early diagenetic framboidal pyrite overgrown by late(r) diagenetic pyrite (SEM image courtesy of Zhiyong Lin).

      (SEM image courtesy of Zhiyong Lin).

Image described by caption.

      (Modified from Lin et al., 2016.)

      Studies such as those by Lin et al. (2016, 2017) clearly indicate the sometimes substantial complexity in sediment diagenesis that is recorded by sulfur isotopes. In terms of its application, the high variability in δ34S and additional information obtained from Δ33S offers a strong potential for unraveling this complexity. Yet, how detailed a respective (hi)story unveils strongly depends on the detail of the analytical approach.

      Seawater sulfate is our prime recorder of secular changes in global sulfur cycling. Abundance and sulfur isotopic composition of oceanic sulfate reflect the delicate balance between inputs and outputs. Principal input parameters are the riverine delivery of sulfate from oxidative continental weathering of sulfide and the dissolution of sulfate as well as the interaction of seawater and ocean crustal rocks, most prominently exhibited at submarine hydrothermal vent sites along mid‐ocean ridges, island arcs and back‐arc settings. Dissolved sulfate leaves the ocean via precipitation of sulfate minerals or through microbially driven sulfate reduction and subsequent precipitation of the resulting hydrogen sulfide as iron sulfide or its incorporation into organic matter. In a simplified isotope mass balance

delta Superscript 34 Baseline normal upper S Subscript i n p u t Baseline equals normal f Subscript s u l f i d e Baseline times delta Superscript 34 Baseline normal upper S Subscript s u l f i d e Baseline plus left-parenthesis 1 en-dash normal f right-parenthesis Subscript s u l f i d e Baseline times delta Superscript 34 Baseline normal upper S Subscript s u l f a t e

      these two principal output functions (i.e. sulfate precipitation and biogenic sulfide formation) are balanced against an overall input function that supposedly reflects average crustal sulfur and that is generally considered to be invariable through time (Holser et al., 1988; for recent discussions, see Halevy et al., 2012 and Canfield, 2013).

Schematic illustration of differentiation of different forms of microbial sulfur cycling as demonstrated by multiple sulfur isotopes.

      Early accounts of the sulfur isotopic composition of oceanic sulfate through time were published by Nielsen (1965) and Holser and Kaplan (1966), followed by the seminal paper from Claypool et al. (1980). These studies used evaporitic sulfate minerals (generally gypsum or anhydrite) for reconstructing the temporal changes in δ34Ssulfate, thereby acknowledging the absence of a substantial isotope effect during mineral precipitation. Two observations were most apparent from these studies: the sulfur isotopic composition of oceanic sulfate was not homogeneous through time and the temporal record was not continuous, with gaps that reflect the nondeposition of evaporites or their loss during weathering


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