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

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.