Abstract: Oxygen isotope ratios of <0.1-µm smectite in bottom sediments of the Mississippi River and the Gulf of Mexico near the mouth of the river have been determined to investigate diagenesis of land-derived clay minerals during sedimentation in the sea. No difference was detected in δ18O (SMOW) between the river and the Gulf samples indicating that no smectite alteration or addition of neoformed smectite to the river samples took place during sedimentation. Thus, authigenic minerals in the river sediments cannot make up more than a few tenths of a percent of the bulk sediments.
Similar results were obtained from 3 × 106-yr b.p. sediments buried to 80 to 600 m at Deep Sea Drilling Project site 323, Bellingshausen Abyssal Plain. No significant change with depth was noted in the δ18O of the <0.3-µm size fraction, mostly smectite, of these land-derived sediments. On the basis of the δ18O of the deepest sample, the maximum amount of authigenic minerals in the land-derived sediments during burial in the sea cannot be more than one or two percent of the bulk sediments. Hence, the alteration at seafloor temperatures of 25–45% of the <0.1-µm size clays in 3 × 106 yr b.p. sediments reported in a previous study is not substantiated. The data demonstrate that land-derived smectite is stable in the sea, and that oxygen isotopes can be used to investigate the modes and the temperatures of formation of authigenic smectites in marine sediments that are younger than 25 × 106 yr and that formed below 25°C.