Abstract
BECAUSE their molecular signatures are often prominent in extracts of sediments, bacteria are thought to be important contributors to petroleum source beds super(1). It has been shown recently super(2,3), however, that abundances of biomarkers do not always reflect relative contributions to sedimentary organic carbon (C sub(org>)). The contribution of photosynthetic green sulphur bacteria to sediments can be assessed effectively because the diagenetic products of distinctive carotenoids from these organisms occur widely super(4-11) and their biomass is isotopically labelled, being enriched in super(13)C (refs 12,13). We show here that, although sediments and oils from the Western Canada and Williston basins contain prominent biomarkers of photosynthetic bacteria, the absence of super(13)C enrichment in the total C sub(org) requires that the bacterial contribution is in fact minimal. Although the importance of bacterial reworking of sedimentary debris cannot be doubted super(14), we argue that our findings, when considered in conjunction with those from other settings, suggest that bacterial biomass may commonly represent only a minor component of total C sub(org) in carbonaceous rocks.