Oxygen isotope analysis measures the ratio of ¹⁸O/¹⁶O and provides a record of ancient seawater temperatures. Water evaporating from seawater is preferentially enriched in the lighter 16O isotope. Precipitation over low latitudes during cold climatic episodes results in the lighter isotope being locked up in ice. The oceans consequently become relatively enriched in the heavier isotope (¹⁸O). During warmer periods the ¹⁶O is returned to the sea as freshwater run-off and the ratio of ¹⁶O/¹⁸O in seawater is maintained.

A higher abundance of ¹⁸O is therefore indicative of cooler seawater temperatures, and vice versa. Thus measuring δ¹⁸O is indicative of past climates and can differentiate between glacial and interglacial periods. In this way a number of age calibrated Marine Isotope Stages (MIS) have been recognised. Analysis of a sequence of closely spaced samples from a core can therefore theoretically be used to match against a reference standard or to provide a correlatable signal for a particular region.

Calcium carbonate in marine shelled organisms contains oxygen from the surrounding seawater and provides a proxy for ¹⁶O/¹⁸O. The organisms most frequently studied for this purpose are foraminifera, in particular certain planktonic species but it is essential to analyse the same species in all samples in a suite to avoid any inter-species variability. It is also important to ensure that the sampled sediments are preferably laminated and not bioturbated as this can blur the results.

Each sample is prepared in the same way as for a micropalaeontological analysis, i.e. washed through a 125μm sieve and the residue dried at 100^oC. The dried residue is then examined for microfossils by picking through the residue for a few suitable well-preserved specimens. Measurements of the isotope concentrations are then made using an isotope ratio mass spectrometer (IRMS). The method of analysis is detailed on the Beta Analytic website.