The δ13C of biogenic methane in marine sediments: The influence of C(org) deposition rate

Neal Blair*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

32 Scopus citations

Abstract

The δ13C of biogenic methane produced in marine sediments ranges from - 110 to - 55‰. The isotopic composition of the methane (δ13C(CH)4) is constrained by the fraction of metabolized organic carbon converted to CH4. The flux of labile organic carbon into the seabed (J(MOC)) and the availability of oxidants (A(OX)), such as O2 and SO4(=), dictate that fraction, i.e., proportionately more methane should be produced as the ratio J(MOC)/A(OX) increases. Chemical, physical (e.g., sediment resuspension) and biological (bioturbation and bioirrigation) processes determine A(OX). Given that δ13C(CH)4 is always less than δ13C of the metabolizable organic carbon, δ13C(CH)4 should increase when J(MOC) and the portion of metabolized carbon converted to methane increase. A positive linear correlation (r2 = 0.92) is observed between δ13C(CH)4 and J(MOC) for a database containing four continental margin sites. When the pore water sulfate gradient (ΔSO4=/Δdepth) is used as a surrogate for J(MOC), the data set is extended to 15 locations spanning all latitudes. A linear relationship between the sulfate gradient and δ13C(CH)4 (r2 = 0.98) for shelf/slope environments suggests that either J(MOC) or J(MOC)/A(OX) is the master variable that controls the 13C/12C content of the biogenic methane. Carbonate precipitation and/or a methanogenic back reaction may obscure the correlation in deep-sea sediments. Evidence for the relationship between δ13C(CH)4 and J(MOC) appears to be preserved in Miocene-age dolomitic deposits.

Original languageEnglish (US)
Pages (from-to)139-150
Number of pages12
JournalChemical Geology
Volume152
Issue number1-2
DOIs
StatePublished - Oct 26 1998

Keywords

  • Biogenic methane
  • Marine sediments
  • δC

ASJC Scopus subject areas

  • Geology
  • Geochemistry and Petrology

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