Os Isotope Record in a Cenozoic Deep-Sea Core: Its Relation to Global Tectonics and Climate

Abstract

The oceanic variation of ⁸⁷Sr/⁸⁶Sr over at least the last 100 million years has been established through the study of calcareous biogenic deposits found principally in deep-sea sediments. The large picture is that in sediments from about 100 million years ago until about 38 million years ago ⁸⁷Sr/⁸⁶Sr shows only small variations but from that time forward there has been an increase. This variation has been ascribed to the changing relative importance with time of two mixing components: Sr with relatively low ⁸⁷Sr/⁸⁶Sr derived primarily by the weathering of ridge basalts and added to the oceans at hydrothermal vents and more radiogenic Sr derived from the weathering of continental crust. The approximately 4-million-year residence time of Sr is about a thousand times the mixing time of the oceans, so a marine ⁸⁷Sr/⁸⁶Sr record anywhere in the deep ocean basins speaks to global changes. The fine details of the sources and types of the actual continental rocks undergoing weathering continue to be the subject of active discussion.