(Table 1) Carbon and oxygen isotope composition of carbonate sediments at DSDP Hole 93-603B

Compositional, textural, and oxygen and carbon stable isotopic analysis of Lower Cretaceous pelagic cyclic sediment is necessary in order to understand the mechanisms causing isotopic fluctuation. Pelagic cycles consist of dark, laminated marl, containing 75-80% CaCO3 and typically about 0.5% organic carbon, alternating with lighter, bioturbated limestone (about 90% CaCO3 and 0.1% organic carbon). The dark marl consistently contains heavier d18O and d13C than the light limestone. Scannning electron microscope observations indicate significant dissolution and diagenetic precipitation of carbonate in the limestone but only minor precipitation of carbonate in the dark marl. This suggests that lighter oxygen in the limestone has its origin in diagenetic precipitation at elevated burial temperatures. The shift toward heavier carbon in the marl end member may have more basis in changing paleoceanographic conditions.