Sulfur isotopic variations in nodular and disseminated pyrite at DSDP Hole 93-603B

Analyses of modern marine sediments have suggested that availability and type of organic matter, sedimentation rate, and openness of the sulfate system influence the degree of isotopic fractionation between seawater sulfate and sedimentary iron sulfides. Isotopic studies of ancient sulfides should, therefore, provide insights into conditions of deposition and early diagenesis. Analysis of d34S of disseminated pyrite from Cretaceous sediments of Hole 603B yielded fractionations relative to coeval seawater sulfate ranging from 40 to 55 per mil, which are within the range for modern oxic marine sediments reported by others. Sulfur/carbon ratios are similar to those found from modern marine sediments and suggest that disseminated pyrite formation was dependent upon available organic carbon. These results imply that depositional and early diagenetic conditions during the Cretaceous in Hole 603B were similar to those occurring in initially oxic marine environments today. Macroscopic (nodular) pyrite from Hole 603B is isotopically variable (d34S values = - 48 to + 33 per mil), but generally more positive than disseminated pyrite. The isotopic evidence suggests that macroscopic pyrite formed during late stages of sulfate reduction in a system closed with respect to sulfate. However, detailed analyses of large pyrite nodules did not yield a consistent pattern of isotopic variation from center to rim.