Oxygen isotopes The natural isotopes of oxygen are listed in table 15. 3 most of the oxygen in the oceans occurs as oxygen-16. Oxygen-16is much rarer (about 0.2 percent of total oxygen) but it is present in measurable amounts . The ratio of 18O/16O in the ocean at any given time in the past is built into contemporaneous marine carbonate minerals and the calcium carbonate shells of marine organisms as a permanent record of the isotopic composition of the ocean at those times fluctuations in oxygen isotopic ratios in the ocean with time thus show up in the geologic record as fluctuations in the isotopic ration of these marine carbonates and fossils . classification of deep-sea sediments on the basis of oxygen isotope ratios in the shell of calcareous marine organism, particularly, has given rise to a new stratigraphy for quaternary sediments. This stratigraphic method is commonly referred to as oxygen isotope stratigraphy . it was first used by Emiliani (1995), who studied the isotopic composition of foraminifers in deep-sea cores and used oxygen isotope ratios to subdivide the core sediments. Oxygen isotope stratigraphy has now developed in to a major tool for correlating quaternary and late tertiary marine succession, as explained below.The 18O/16O ratio in biogenic marine carbonates reflects both the temperature and the 18O/16O ratio of the water in which these carbonates formed. The relationships of ocean paleotemperature (T) to oxygen isotopic composition has been show by shackleton (1967) to be

FORMULA 15.1Where dc= the equilibrium oxygen isotope composition of calcite and ; dw=oxygen isotope composition of the water from which the calcite was precipitated. The dc and dw notations refer not to the actual oxygen isotopic abundances in calcite and water but to the per mil (parts per thousand ) deviation of the 18O/16O ratio in calcite and water from that of an arbitrary standard. A commonly used standard for oxygen isotopes in the past was the university of Chicago PDB standard, where PDB refers to a particular fossil belemnite from the Pee Dee Formation of South Carolina. More commonly now, the isotope composition of ocean water (Standard Mean Ocean Water, or SMOW )