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

Abstract

ABSTRACT. 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 O 18 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.

O18 and C13 Isotopes in Leg 44 Carbonates: A Comparison with the Alpine Series

“…We detect some apparent concordant trends between the Soureliou Series and the Hole 391C sequence (shown schematically on Figure 4) Brennecke (1977) has found even lower values in Leg 11 samples. The Alpine deposits were never deeply buried, as were the Hole 391C sediments, and apparently were not subjected to volcanic influences but have undergone the tectonic stress of alpine movement.…”

“…All the other values reflect diagenesis and isotope exchange. Stylolitization and replacement of radiolarians by calcispheres give further evidence for such a process, as does the gross inverse correlation between δ O 18 and carbonate content, noted in Leg 11 samples by Brennecke (1977), who discussed it thoroughly following the reasoning given below.…”

“…There is much evidence of silica recrystallization in the Hole 391C samples, but the original material from this hole appears to have been high δ O 18 radiolarians. Consequently, the explanation given by Brennecke (1977) is harder to apply in the case of Hole 391C.…”

Oxygen 18 and Carbon 13 Isotopes from Bulk Carbonate Samples, Leg 47B

“…However, oxygen isotope values differ from those observed in contemporary beds from Legs 11 and 44 in the western Atlantic (Brenneke, 1977;Létolle et al, 1978), where much lower O 18 values are interpreted as being due to recrystallization in a high geothermal gradient setting and/or hydrothermal influences not evident at Hole 398D.…”

Stable-Isotope Evidence for the Origin of Secondary Carbonate Veins in Deep Sea Drilling Project Leg 58 Basalts

“…Because carbonates are among the most abundant secondary phases in oceanic volcanic rocks, information derived from their isotopic composition is useful in determining the environment(s) of seafloor alteration. Isotopic analyses of secondary carbonates in basalt recovered from numerous DSDP sites have been reported previously Brenneke, 1977;Lawrence et al, 1977;Seyfried et al, 1976;among others). These results are consistent with the formation of most secondary carbonates with sea water at low temperatures.…”