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Abstract

The geochemical composition of sediments (squeeze cake samples)from five drill sites (Ocean Drilling Program Sites 1257–1261) on theDemerara Rise in the tropical Atlantic was determined, with special re-gard to a sequence of Cretaceous black shales. Sediments were analyzedfor different iron (total, pyrite, Na dithionite, and HCl leachable) andsulfur (total, pyrite, acid volatile, and organic bound) fractions, in addi-tion to total organic carbon (TOC) and total inorganic carbon. The rela-tive abundance of highly reactive iron (FeHR/FeT) in the investigatedblack shale samples indicates that pyrite was formed both in the watercolumn and the sediment. This corresponds to euxinic paleoenviron-mental conditions, a situation similar to the modern deep Black Sea.This geochemical approach is independent of a possible minor contri-bution from ongoing sulfate reduction which is triggered by anaerobicmethane oxidation above the black shale sequence. Pyrite sulfur inblack shales makes up between 30% and 100% of total sulfur. In addi-tion to fixation of sulfide with iron, organic matter (OM) acted as animportant sulfur trap during early diagenesis, with organic sulfur com-posing between 5 and 10 atom% of TOC. The relative importance of OM sulfurization is increasing with its content.