Sulphur cycling in organic-rich marine sediments from a Scottish fjord

Simon H Bottrell, Robert J G Mortimer, Ian M Davies, S Martyn Harvey, Michael D Krom

Research output: Contribution to journalArticle

39 Citations (Scopus)

Abstract

In this study, the biogeochemical transformations of sulphur in organic-rich marine sediments in a Scottish fjord are investigated by a combination of pore water and sediment geochemistry with sulphide diffusive gradient thin-film probes and sulphate isotopic data (delta S-34 and delta O-18). Particular attention is paid to sulphur cycling in the upper sediment profile where sulphate reduction occurs but free sulphide is below the detection limits of conventional pore water geochemical analysis but quantifiable by sulphide diffusive gradient thin film. In the uppermost part of the sediment core, delta O-18 sulphate decreased from near-sea water values to +7 parts per thousand, indicating that anoxic sulphide oxidation dominated during this interval. Sulphate delta S-34 remained unchanged as there was no net sulphate reduction (i.e. reduction was balanced by re-oxidation). Below 4 cm depth, there was a slight increase in sulphate delta S-34 from 20 parts per thousand to 23 parts per thousand associated with minor accumulation of iron sulphide. The delta O-18 of the sulphate also increased, to around +10 parts per thousand at 10 cm depth, as a result of the isotopic exchange of sulphate-oxygen with pore water and/or sulphur disproportionation reactions mediated during sulphur cycling. These processes continued to increase the delta O-18 of the sulphate to 14 parts per thousand at 20 cm depth with no further change in the delta S-34 of the sulphate. Below 20 cm depth, free sulphide is detectable in pore waters and both the delta S-34 of the sulphate and sulphide increase with depth with an offset controlled by kinetic fractionation during bacterial sulphate reduction. The delta S-34 of the sedimentary organic fraction shifted towards lower, more bacteriogenic, values with depth in the profile, without any increase in the size of this sulphur pool. Thus, the organic sulphur fraction was open to interaction with bacteriogenic sulphide without the occurrence of net addition. Therefore, caution should be exercised when using sulphur isotopic compositions to infer simple net addition of bacteriogenic sulphide to the organic sulphur fraction.
Original languageEnglish
Pages (from-to)1159-1173
Number of pages15
JournalSedimentology
Volume56
Issue number4
DOIs
Publication statusPublished - 2008

Keywords

  • REACTIVE IRON
  • EARLY DIAGENESIS
  • PYRITE
  • IRON REDUCTION
  • IN-SITU
  • BACTERIAL SULFATE REDUCTION
  • Geology
  • OXYGEN
  • ISOTOPE FRACTIONATION
  • ELEMENTAL SULFUR
  • REDUCING BACTERIA

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