Controls on the molecular and carbon isotopic composition of organic matter deposited in a Kimmeridgian euxinic shelf sea: Evidence for preservation of carbohydrates through sulfurisation

Thirteen samples from the Kimmeridge Clay Formation (KCF) in Dorset, coveri ng all different lithologies, were studied using bulk and molecular geochem ical and microscopical techniques. Our data show that the positive correlat ion between TOC and delta(13)C(TOC) reported for shales (Huc et al., 1992) also holds for other lithologies (e.g., limestones) if we correct for dilut ion by carbonate (TOC*). Despite the wide range of delta(13)C(TOC) values ( -26.7 to -20.7 parts per thousand), the delta(13)C values of individual bio markers of algal and green sulfur bacterial origin and of kerogen pyrolysis products (i.e., n-alkanes) show in general only small changes (<2 parts pe r thousand). This indicates that changes in the concentration of dissolved inorganic carbon (DIC) or delta(13)C of DIC (delta(13)C(DIC)) in the palaeo water column cannot account for the 6 parts per thousand difference in delt a(13)C(TOC). Kerogen pyrolysates indicated that with increasing TOC*, and thus increasin g delta(13)C(TOC), carbon isotopically heavy C-1-C-3 alkylated thiophenes w ith a linear carbon skeleton become increasingly abundant; in the case of t he Blackstone Band kerogen (TOC* = 63%) they dominate the pyrolysate. These thiophenes are probably derived from sulfur-bound carbohydrates in the ker ogen. Algal carbohydrates are typically 5-10 parts per thousand heavier tha n algal lipids and differences in preservation of labile carbohydrate carbo n through sulfurisation may thus explain the range in delta(13)C(TOC) value s without the need to invoke any change in water column conditions. The inc reasing dominance of thiophenes in the kerogen pyrolysate with increasing T OC* is consistent with the increasing Sulfur Index (mg S org/g TOC), the de creasing S-PYRITE/S-TOT ratio, and the increasing dominance of orange amorp hous organic matter produced by natural sulfurisation. The organic matter of all sediments was deposited under euxinic conditions as revealed by the occurrence of isorenieratene derivatives indicating (per iodic) photic zone euxinia. At times of reduced run-off from the hinterland , represented by so-called condensed sections, the flux of reactive iron wa s relatively small compared to the flux of reactive organic matter, which r esulted in the formation of relatively small amounts of pyrite and an exces s of hydrogen sulfide capable of reacting with fresh organic matter. Within the condensed sections, variations in the degree of sulfurisation of organ ic matter are probably due to both differences in primary production and di fferences in the supply of reactive iron. These findings demonstrate that c limatic changes, probably driven by Milankovitch cycles, can have a large i mpact on the molecular and carbon isotopic compositions of the sedimentary organic matter in an otherwise relatively stable stratified basin. They als o show that large amounts of labile carbohydrate carbon may be preserved th rough sulfurisation. Copyright (C) 1998 Elsevier Science Ltd.