Other
Scientific paper
Oct 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998gecoa..62.3259v&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 62, Issue 19-20, pp.3259-3283
Other
8
Scientific paper
Thirteen samples from the Kimmeridge Clay Formation (KCF) in Dorset, covering all different lithologies, were studied using bulk and molecular geochemical and microscopical techniques. Our data show that the positive correlation between TOC and 13 C TOC reported for shales (Huc et al., 1992 ) also holds for other lithologies (e.g., limestones) if we correct for dilution by carbonate (TOC*). Despite the wide range of 13 C TOC values (-26.7 to -20.7 ), the 13 C values of individual biomarkers of algal and green sulfur bacterial origin and of kerogen pyrolysis products (i.e., n -alkanes) show in general only small changes (<2 ). This indicates that changes in the concentration of dissolved inorganic carbon (DIC) or 13 C of DIC ( 13 C DIC ) in the palaeowater column cannot account for the 6 difference in 13 C TOC . Kerogen pyrolysates indicated that with increasing TOC*, and thus increasing 13 C TOC , carbon isotopically heavy C 1 -C 3 alkylated thiophenes with a linear carbon skeleton become increasingly abundant; in the case of the Blackstone Band kerogen (TOC* = 63%) they dominate the pyrolysate. These thiophenes are probably derived from sulfur-bound carbohydrates in the kerogen. Algal carbohydrates are typically 5-10 heavier than algal lipids and differences in preservation of labile carbohydrate carbon through sulfurisation may thus explain the range in 13 C TOC values without the need to invoke any change in water column conditions. The increasing dominance of thiophenes in the kerogen pyrolysate with increasing TOC* is consistent with the increasing Sulfur Index (mg S org/g TOC), the decreasing S PYRITE /S TOT ratio, and the increasing dominance of orange amorphous 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 (periodic) photic zone euxinia. At times of reduced run-off from the hinterland, represented by so-called condensed sections, the flux of reactive iron was relatively small compared to the flux of reactive organic matter, which resulted in the formation of relatively small amounts of pyrite and an excess of hydrogen sulfide capable of reacting with fresh organic matter. Within the condensed sections, variations in the degree of sulfurisation of organic matter are probably due to both differences in primary production and differences in the supply of reactive iron. These findings demonstrate that climatic changes, probably driven by Milankovitch cycles, can have a large impact on the molecular and carbon isotopic compositions of the sedimentary organic matter in an otherwise relatively stable stratified basin. They also show that large amounts of labile carbohydrate carbon may be preserved through sulfurisation.
Köster Jörgen
Schouten Stefan
Sinninghe Damst'e Jaap S.
van Kaam-Peters Heidy M. E.
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