Preservation of carbohydrates through sulfurization in a Jurassic euxinic shelf sea: Examination of the Blackstone Band TOC cycle in the Kimmeridge Clay Formation, UK

Abstract

A complete total organic carbon (TOC) cycle in the Upper Jurassic Kimmeridge Clay Formation (KCF) comprising the extremely TOC-rich (34%) Blackstone Band was studied to investigate the controlling factors on TOC accumulation. Compared with the under- and overlying strata, TOC in the Blackstone Band was enriched by a factor of six and, concomitantly, the δ 13C TOC shows a ∼4‰ enrichment. Al-normalized TOC values indicated that the enhanced TOC values were probably caused by increased TOC accumulation and not by a decreased dilution with inorganic matter. The amounts of short chain alkylated thiophenes and the sulfur-rich unresolved complex mixture (UCM) in the kerogen pyrolysates, as well as the TOC/Al ratios, correlated linearly with δ 13C TOC for sediments with TOC/Al ratios >1.7. The alkylated thiophenes and sulfur-rich UCM both originate from sulfurized carbohydrate carbon (C carb), suggesting that the primary cause of the TOC maximum is the enhanced contribution of C carb to TOC. Since carbohydrates can be substantially 13C-enriched over lipids in biomass, the enhanced contribution of C carb explains the enriched δ 13C TOC values. Compound specific isotope data showed that primary productivity during deposition of the KCF TOC cycle varied little, while a two member isotopic mixing model showed that the preservation of C carb relative to that of the lipid carbon may have increased by a factor of >10 in the Blackstone Band. The enhanced preservation of C carb was most likely caused by more frequent or longer lasting events of photic zone euxinia, as revealed by the concentration of isorenieratene derivatives. Enhanced contributions of C carb have also been observed in other KCF cycles, suggesting that enhanced preservation of C carb, rather than an increase in primary production, exerted direct control on the TOC cycles of the KCF.