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Thundahcaxzd t1_ja6yajf wrote

The lighter oxygen isotope O16 evaporates more readily than the heavier O18. Thus, when temperatures are higher and there is more evaporation, O18 gets concentrated in liquid water and O16 gets concentrated in the atmosphere. the water in the atmosphere falls as precipitation and some of it falls on ice caps and stays there. In the ocean, organisms such as foraminifera use water to help build their shells. These shells settle into the sediment after they die. Scientists drill cores into the ice caps and ocean sediments. Using a mass spectrometer, they measure the ratio of oxygen isotopes in the ice and foraminifera shells which gives us a proxy of how much evaporation was taking place, and therefore the temperature.

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Skarr87 t1_ja7wzw7 wrote

This is the correct answer. I would also like to add that O18 will precipitate out of the atmosphere faster as the temperature cools so as the temperature gradient decreases the ratio favors O16 more and more. So taking samples from all over we can get gradients for the temperature around the world at a particular timeframe.

Corals and animals with shells in the ocean make it out of calcium carbonate or silicon dioxide. The concentration of O18 to O16 in shells is dependent on the temperature of the water do to biological and chemical processes. So this is another check to corroborate ice core values.

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Undercover_in_SF t1_ja8gno5 wrote

Since O16/O18 ratio is stable, does this let you go back farther than C14 carbon dating?

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Skarr87 t1_ja8kwp4 wrote

Oxygen isn’t used for dating the samples. The oxygen ratio is for temperature. The stability of this isotopes means that we can be confident they haven’t changed since being deposited. To date the cores we use layers, chemistry, and radiometric dating.

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