Is there evidence for a substantial contribution of prokaryotic biomass to organic carbon in Phanerozoic carbonaceous sediments?

The role of prokaryotes (bacteria and archaea) in producing biomass and reworking settling and buried organic matter is of paramount importance since it exerts a major impact on the composition of sedimentary organic matter and, ultimately, petroleum. This key role of prokaryotes, in combination with a biomarker fingerprint often dominated by prokaryotic molecular fossils (e.g. hopanoids), has led to the assumption that prokaryotic biomass is a substantial source for organic carbon in carbonaceous sediments. However, biosynthesising and reworking organic matter does not necessarily imply sourcing organic matter by the prokaryotes, and the abundance of biomarkers does not always reflect relative contributions to sedimentary organic matter. In this paper the existing evidence for a substantial prokaryotic biomass contribution to sedimentary organic carbon (OC) is critically evaluated. This shows that despite the often prominent presence of prokaryotic biomarkers in sediments, evidence for a substantial prokaryotic contribution to sedimentary OC is generally lacking. This is demonstrated by: (i) the application of the carbon isotopic mass balance principle in various sedimentary settings; (ii) an alternative explanation for the proposed dominant role of heterotrophic reworking in the Oxford Clay; (iii) a summary of the effects of bacterial oxidation in organic matter-rich turbidites; and (iv) recent data showing that the evidence for recalcitrant carbon in prokaryotes is scant. Although these observations do not rule out substantial contributions of prokaryotic carbon to OC in Phanerozoic carbonaceous sediments, this remains to be demonstrated and the final answer awaits the development of new technologies to full characterise sedimentary OC.