Measuring isotopes preserved in the shells of ancient single-celled foraminifera—specifically, tracking two sets of isotope ratios, carbon-13 to carbon-12 and oxygen-18 to oxygen-16—is a key way that paleoceanographers reconstruct the temperature of the ancient ocean and past global carbon cycling. Foraminifera build their shells from the carbon and oxygen in the seawater, and the relative uptakes of these isotopes change with temperature.
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