ENHANCED PRESERVATION OF ORGANIC MATTER IN THE KIMMERIDGE CLAY FORMATION THROUGH NATURAL SULFURIZATION OF CARBOHYDRATES

The organic matter (OM)-rich sediments of the Jurassic Kimmeridge Clay Formation (KCF) have total organic carbon (TOC) contents of up to 60% and d¹³C_TOC values ranging from -21 to -27‰. It was hypothesized that this resulted from preservation of carbohydrates since the abundance of isotopically enriched alkylated thiophenes (presumably derived from carbohydrates) in kerogen pyrolysates correlated positively with d¹³C_TOC and TOC.

To substantiate this hypothesis, several lines of research were followed. Isotopic investigations of cultured algae showed that carbohydrates are significantly enriched (up to 16‰) compared to lipids and the total cell material (up to 10‰). Furthermore, simulation experiments of natural sulfurization showed that sulfurization of carbohydrates results in an S-rich polymer, which, upon pyrolysis, yields similar alkylated thiophenes as those found in KCF pyrolysates. A high resolution study of the upper KCF, around the Blackstone Band, showed that with increasing TOC and d¹³C_TOC values increased amounts of alkylated thiophenes and a S-rich unresolved complex mixture (UCM) were found in the pyrolysates, whilst the d¹³C values of extractable phototrophic biomarkers did not change. In addition, chemical degradation studies of the UCM showed that more than 60% consists of S- and O-bound short chain carbon skeletons, most likely derived from bound monosaccharides.

These results suggest that carbohydrate preservation through natural sulfurization is a feasible and likely mechanism for the enhanced preservation of sedimentary OM in the KCF and concomitant increased d¹³C_TOC values. S-rich UCM's and C₄-C₇ alkyl hiophenes have also been identified in a number of other Phanerozoic kerogen pyrolysates , which suggests that carbohydrate sulfurization can play an important role in OM preservation.