The oxygen isotopic composition of sea water is determined by interactions with fresh, silicate rocks. Interactions at high temperature, principally hydrothermal interations on the sea floor, make sea water isotopically heavier. Interactions at low temperature, including sea floor weathering but principally weathering of fresh, crystalline rocks on the continents, make sea water isotopically lighter. Any change in the relative rates of these high and low temperature interactions will cause a change in the composition of sea water. Extreme limits might be -15 per mil relative to SMOW to +4 per mil. Possible rates of change appear to be quite slow, perhaps 1 per mil in 108 years, because of the large size of the oceanic reservoir. Evolution in tectonic style over the course of earth history may have caused a change in the oxygen isotopic composition of sea water as fresh crystalline rocks have been increasingly blanketed by weathered sediments and as deepening oceans have increasingly flooded the places where earth's internal heat is released at the surface. ¿ American Geophysical Union 1989