A MOLECULAR AND CARBON ISOTOPIC STUDY TOWARDS THE ORIGIN AND DIAGENETIC FATE OF DIAROMATIC CAROTENOIDS

Pyrolysates of high-molecular-weight sedimentary fractions of the Duve rnay 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 aggregat es. This was substantiated by desulphurization of sulphur-rich aggrega tes of the polar fraction, which released (partly) hydrogenated carote noids. Furthermore, these components were important constituents of th e aromatic hydrocarbon fractions and related oils. Apart from renierat ane and isorenieratane, H-1 NMR analysis established the aromatic subs titution pattern of the most abundant component present, which was ide ntified as a diaromatic compound with an unprecedented 2,3,6-/3,4,5-tr imethyl aromatic substitution pattern. Molecular and isotopic analyses of both soluble and insoluble fractions of organic matter revealed re lationships between diagenetically-derived carotenoids found in bitume n and related oils and their precursors incorporated into high-molecul ar-weight fractions. Aryl isoprenoids, important components in extract s and oils, were apparently derived from thermal cracking of bound dia romatic carotenoids rather than cleavage of free carotenoids as previo usly suggested. Furthermore, products derived from diaromatic caroteno ids were substantially enriched in C-13 relative to n-alkanes of algal origin. Together with the characteristic carotenoids, this isotopic e nrichment provides evidence of significant contributions from photosyn thetic green sulphur bacteria (Chlorobiaceae), which fix carbon via th e reversed tricarboxylic acid (TCA) cycle. In spite of the prominence of these molecular signals, the overall isotopic composition of the or ganic matter indicated that only a very small portion of the preserved organic carbon was derived from the biomass of photosynthetic green s ulphur bacteria.