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How many distinct glaciations may have occurred during the Pleistocene Epoch?

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Quaternary paleogeography — Quaternary Distribution of landmasses, mountainous regions, shallow seas, and deep ocean basins during the Quaternary Period. Included in the paleogeographic reconstruction are the locations of the interval's subduction zones.

The Pleistocene Epoch was characterized by repeated cycles of glacial and interglacial periods, rather than a fixed number of distinct glaciations. Because terrestrial records are often incomplete due to erosion, scientists rely on the marine oxygen isotope record as the standard for identifying these events.12

In this record, colder glacial periods are assigned even numbers and warmer interglacial periods are assigned odd numbers. Data from the last 730,000 years indicate that eight major glacial and interglacial cycles occurred, each lasting approximately 100,000 years. While the first major glaciation in the North Atlantic region is estimated to have occurred about 2,400,000 years ago, the first glaciation extensive enough to cover large areas of North America and Eurasia is believed to have taken place approximately 850,000 years ago.1

Ultimately, the Earth system has been in a state of continual flux between these regimes for more than two million years, with the duration and severity of glacial periods increasing over time.3