<Tr> <Td> </Td> <Td> This article includes a list of references, but its sources remain unclear because it has insufficient inline citations . Please help to improve this article by introducing more precise citations . (June 2008) (Learn how and when to remove this template message) </Td> </Tr> <P> Oxygen isotope ratio cycles are cyclical variations in the ratio of the abundance of oxygen with an atomic mass of 18 to the abundance of oxygen with an atomic mass of 16 present in some substances, such as polar ice or calcite in ocean core samples, measured with the isotope fractionation . The ratio is linked to water temperature of ancient oceans, which in turn reflects ancient climates . Cycles in the ratio mirror climate changes in geologic history . </P> <P> Oxygen (chemical symbol O) has three naturally occurring isotopes: O, O, and O, where the 16, 17 and 18 refer to the atomic mass . The most abundant is O, with a small percentage of O and an even smaller percentage of O. Oxygen isotope analysis considers only the ratio of O to O present in a sample . </P> <P> The calculated ratio of the masses of each present in the sample is then compared to a standard, which can yield information about the temperature at which the sample was formed - see Proxy (climate) for details . </P>

Where are the highest concentrations of the isotopes h-2 and o-18 found in the u.s