A Comparison of the Stable Oxygen and Carbon Isotope Composition of Early Cretaceous and Late Jurassic Carbonates from DSDP Sites 105 and 367

Abstract

Oxygen isotope data indicate that vein-filling carbonates begin to crystallize in approximate isotopic equilibrium with seawater. Pore waters in fracture systems which are partially isolated from seawater become isotopically lighter as the surrounding basalt weathers to clay minerals and zeolites. Vein formation continues at relatively low temperatures in water progressively depleted in O18. Basalts at DSDP Sites 105 and 367 are believed to be extrusive because the sediments immediately overlying them do not have isotopic compositions indicative of high-temperature recrystallization. Sediments at Site 367 are depleted in O18 relative to those from equivalent units at Site 105. This difference is due to the greater amounts of authigenic carbonate formed at higher temperatures, and possibly in the presence of lighter pore waters, at Site 367. The differences in isotopic composition and degree of recrystallization between the two sites are attributable to the thicker overburden at Site 367. Significant amounts of Ou must have been incorporated into authigenic silicate phases in order to explain the oxygen isotope composition at Site 367. The lowermost sediments at Site 367 are enriched in O18 relative to the sediments overlying them. Part of this enrichment is due to less recrystallization in these sediments. Oxygen isotope data suggest that recrystallization of these basal carbonates occurred early in the burial history of the sediments. Nodular chert at Site 367 is associated with carbonates depleted in O18. Present theories of chert formation do not account for this depletion. Clay content also exercises a large effect on the oxygen isotopic composition of carbonates. Carbonate-rich regions are depleted in O18 relative to associated clay-rich zones. This relationship suggests that carbonate-rich regions contain more authigenic carbonate formed at in situ temperatures than do clayrich zones. The carbon isotopic composition of carbonates from Site 367 varies systematically with depth, reflecting variations in the isotopic composition of dissolved inorganic carbon in the surface ocean. These variations are believed to reflect either changes in the amount of organic matter buried in sediments or changes in the ratio of C/P in organisms to that in deep ocean water.