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 δ13CTOC 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 δ13CTOC for sediments with TOC/Al ratios >1.7. The alkylated thiophenes and sulfur-rich UCM both originate from sulfurized carbohydrate carbon (Ccarb), suggesting that the primary cause of the TOC maximum is the enhanced contribution of Ccarb to TOC. Since carbohydrates can be substantially 13C-enriched over lipids in biomass, the enhanced contribution of Ccarb explains the enriched δ13CTOC 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 Ccarb relative to that of the lipid carbon may have increased by a factor of >10 in the Blackstone Band. The enhanced preservation of Ccarb 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 Ccarb have also been observed in other KCF cycles, suggesting that enhanced preservation of Ccarb, rather than an increase in primary production, exerted direct control on the TOC cycles of the KCF.