The first method used to extract previous temperatures from ice cores is the oxygen isotope ratio. Atoms of the exact same element that contain differing numbers of neutrons are known as isotopes. For at least the past 1 million years, Earth has gone through warm (interglacials) and cold (ice ages) periods on around 100,000-year cycles.
Oxygen isotope ratio phases are periodic changes in the proportion of the abundance of oxygen having atomic mass 18 to that of oxygen with atomic mass 16 found in various materials, such as calcite in sea core samples or polar ice. The number of protons, electrons, and neutrons in the nuclei of an element's isotopes are all the same, but the number of neutrons varies.
The Huronian (2.4 to 2.1 billion years ago), Cryogenian (850 to 635 million years ago), Andean–Saharan (460 to 430 mya), Karoo (360 to 260 mya), and Quaternary ice ages have all been noted by geologists (2.6 mya-present). The Eocene, which lasted for 53 to 49 million years, was the planet's warmest time in 100 million years.
Although pedogenesis processes can change the ratios, foraminifera and other species may be able to retain their initial isotopic ratios for many thousands of years. The orbital theory of the ice ages was discovered thanks in large part to research on the oxygen isotope ratios of foraminifera, which are still extensively employed in the study of rapid climatic change. The extensive record of global climate changes so over previous 100 million years has been revealed by compilations of oxygen isotopes from deep sea benthic foraminifers. In order to determine the extent of historical "greenhouse" warming and global ocean surface temperatures, planktonic foraminifer oxygen isotopes are employed to study the records of previous sea surface temperatures.
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