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.