Sea-level reconstruction offers new interpretation of Pleistocene climate

At the onset of the Pleistocene Epoch about 2.6 million years ago, global temperature fell, polar ice sheets grew, and sea level dropped. Another big switch occurred 1.2 million years ago in the middle of the Pleistocene: Sea-level fluctuations that had been recurring every 41 000 years became larger and started happening every 100 000 years. Since the Mid-Pleistocene Transition was first identified more than 40 years ago, researchers have predominantly interpreted the decreased frequency as a response to the ice sheets’ increasingly large variations in size.

A new reconstruction of sea level challenges that interpretation. Peter Clark of Oregon State University and colleagues show, using geological archives spanning the past 4.5 million years, that global ice sheets experienced large changes in size throughout the Pleistocene. ¹ The results, which are contrary to those from previous studies, indicate that the Mid-Pleistocene Transition was a switch only in the frequency of sea-level changes.

Many records of ocean temperature and sea level come from the relative amounts of oxygen-18 and oxygen-16 that are found in the fossil shells of foraminifera, microscopic organisms that have lived in the ocean, often at the bottom, for hundreds of millions of years. The isotope ratios reflect the environmental conditions in which the organisms lived, specifically ocean temperature and ice volume—and thus sea level.

That relatively simple picture, which has been used in many previous reconstructions of sea level, assumes that the relationship between sea level and the oxygen isotope composition of seawater is constant over time. But that’s not entirely accurate. The ocean had more ¹⁸O relative to ¹⁶O during periods when the poles were warmer than during periods when the poles were cooler.

Clark and colleagues developed their picture of the climate by first reconstructing ocean temperature over the past 4.5 million years—a result they published in a paper earlier this year. To yield the oxygen isotope composition of seawater and thus the sea level, they combined the temperature results, which were inferred from oxygen isotope measurements, with the foraminifera isotope measurements.

Clark and colleagues discovered that in the early Pleistocene, when the temperature was higher than it was in the middle to late Pleistocene, Earth experienced sea-level changes that were similar in magnitude to those in the middle to late Pleistocene. The researchers hypothesize that the long-term cooling , which they reported in 2024, caused the Southern Ocean to become more stratified, which would have resulted in atmospheric carbon being stored in deep water for long periods of time. As glaciation intensified, sea level dropped precipitously and reached as low as 150 m below today’s sea level multiple times throughout the Pleistocene, including about 21 000 years ago, during the Last Glacial Maximum.