A molecular and carbon isotopic study towards the origin and diagenetic fate of diaromatic carotenoids.

Pyrolysates of high-molecular-weight sedimentary fractions of the Duvernay Formation (Western Canada Basin) are dominated by 1,2,3,4- and 1,2,3,5-tetramethylbenzene, which, generated via beta-cleavage, indicate the presence of diaromatic carotenoids in the macromolecular aggregates. This was substantiated by desulphurization of sulphur-rich aggregates of the polar fraction, which released (partly) hydrogenated carotenoids. Furthermore, these components were important constituents of the aromatic hydrocarbon fractions and related oils. Apart from renieratane and isorenieratane, 1H NMR analysis established the aromatic substitution pattern of the most abundant component present, which was identified as a diaromatic compound with an unprecedented 2,3,6-/3,4,5-trimethyl aromatic substitution pattern. Molecular and isotopic analyses of both soluble and insoluble fractions of organic matter revealed relationships between diagenetically-derived carotenoids found in bitumen and related oils and their precursors incorporated into high-molecular-weight fractions. Aryl isoprenoids, important components in extracts and oils, were apparently derived from thermal cracking of bound diaromatic carotenoids rather than cleavage of free carotenoids as previously suggested. Furthermore, products derived from diaromatic carotenoids were substantially enriched in 13C relative to n-alkanes of algal origin. Together with the characteristic carotenoids, this isotopic enrichment provides evidence of significant contributions from photosynthetic green sulphur bacteria (Chlorobiaceae), which fix carbon via the reversed tricarboxylic acid (TCA) cycle. In spite of the prominence of these molecular signals, the overall isotopic composition of the organic matter indicated that only a very small portion of the preserved organic carbon was derived from the biomass of photosynthetic green sulphur bacteria.

Evidence for only minor contributions from bacteria to sedimentary organic carbon.

Because their molecular signatures are often prominent in extracts of sediments, bacteria are thought to be important contributors to petroleum source beds. It has been shown recently, however, that abundances of biomarkers do not always reflect relative contributions to sedimentary organic carbon (Corg). 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 and their biomass is isotopically labelled, being enriched in 13C. We show here that, although sediments and oils from the Western Canada and Williston basins contain prominent biomarkers of photosynthetic bacteria, the absence of 13C enrichment in the total Corg requires that the bacterial contribution is in fact minimal. Although the importance of bacterial reworking of sedimentary debris cannot be doubted, 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 Corg in carbonaceous rocks.

Observation of a subsurface oil-rich layer in the open ocean.

A layer of water at a depth of 200 meters containing 3 to 12 milligrams per liter of oil was found during February and March 1978 over a distance of 800 nautical miles in the southwest North Atlantic and the eastern Caribbean. The geochemistry and carbon-14 activity of the oil shows it to be a weathered crude, probably from a submarine seep. Although the dimensions of the oily layer were not determined, conservative estimates indicate that more than I megaton could have been present.