Reconstruction of changes in global sulfur cycling from marine sulfate isotopes

Abstract

Stable isotope ratios of sulfur ( δ 34S) and oxygen ( δ 18O) in marine sulfate respond to changes in fluxes and processes in the global sulfur cycle. The two isotope systems respond on different time scales to different factors affecting the global sulfur cycle. Sulfur isotopes respond primarily to the relative fluxes of weathering-derived sulfate to the oceans versus the net flux of sulfur exported from the oceans as pyrite in marine sediments. At the present day, this response is relatively slow (∼ 20 Ma) because of the large size of the marine sulfate reservoir; ancient oceans had lower sulfate concentrations and thus sulfate residence times may have been shorter. The δ 34S of marine sulfate is also sensitive to the development of a significant reservoir of H 2S in ancient stratified oceans. Sulfate–oxygen isotopes respond primarily to changes in the cycling of sulfur in the oceans and marine sediments. Sulfur disproportionation, oxic and anaerobic sulfide oxidation all impart different δ 18O signatures to marine sulfate; the relative importance of these different processes is in turn controlled by the distribution of redox conditions and microbial ecology of sea-floor sediments. The mutual interpretation of changes in marine sulfate δ 34S and δ 18O can thus give rather precise information on the nature of changes in global sulfur cycling. Recently, records of isotope compositions of ancient marine sulfate have become available through the analysis of carbonate-associated sulfate (CAS) in limestones and marine biogenic barite, both of which provide far more continuous records than previous data based primarily on marine evaporite sulfate minerals. We review a number of case studies which use sulfur or combined sulfur and oxygen isotopic records derived from CAS and biogenic barite. Proterozoic CAS-sulfur isotope datasets often show marked variations over very small stratigraphic intervals (e.g. 20‰ over 300 m). When examined in conjunction with estimates of deposition rates, they are used to infer a new evolutionary curve of marine sulfate concentrations and therefore a protracted oxidation of the Earth's biosphere. Through the Cretaceous–Tertiary interval, the marine sulfate–sulfur record is characterised by significant periods of relative stability punctuated by phases of more rapid change (limited to 7‰ variation over the last 130 Ma). The Carboniferous was characterized essentially by a gradual change of 9‰ over 60 Ma. By contrast, at the Permian–Triassic boundary, extreme changes occur over very short geological time scales (14‰ over < 1 Ma); simple mass balance considerations suggest the development of a large, relatively short-lived, reservoir of H 2S in the oceanic water column at this time. In this case, it is the short time scale of the response of the sulfate–oxygen record that enables the interpretation of rapid changes in oceanic redox state. Over the last 10 Ma, sulfate–oxygen δ 18O in biogenic barite exhibits large isotope changes (∼ 7‰) that occur independently of those in sulfate–sulfur. These changes must reflect changes in depositional/redox environment at the ocean margins and/or changes in microbial cycling of sulfur, but the exact cause remains controversial. Our limited understanding of the influence of redox environment and sediment microbial ecology on sulfur cycling currently restricts the level to which we are able to interpret such changes in marine sulfate δ 18O.