Method for estimating past ocean temperatures. The ratio of the stable oxygen isotopes, 18O and 16O, is temperature dependent in water, 18O increasing as temperature falls. Oxygen incorporated in the calcium-carbonate shells of marine organisms will reflect the prevailing 18O:16O ratio (see oxygen-isotope ratio). Acidification (to release oxygen) of fossils of these organisms under carefully controlled conditions can therefore be used, with appropriate calibration, to indicate the record of past ocean temperatures.