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Earth's climate has fluctuated between cold and warm periods for millions of years. During the so-called "lukewarm interglacials"—warm phases between 800,000 and 430,000 years ago—atmospheric CO₂ concentrations were only around 240 to 260 ppm (parts per million, i.e., molecules per 1 million molecules of air). Later interglacials reached values of 280 to 300 ppm.

By comparison, today's concentration has already exceeded 420 ppm due to human emissions. Why these earlier warm periods were cooler remained unclear until now. A new study in Nature Communications now highlights the Southern Ocean, the ocean surrounding the South Pole, as a decisive factor.

"Our data show for the first time that stronger stratification of the Southern Ocean was crucial for the comparatively cool interglacials before the Mid-Brunhes Event," says Dr. Huang Huang, the study's lead author. He completed his Ph.D. at GEOMAR in 2019 and now works at the Laoshan Laboratory in Qingdao (China).

The Mid-Brunhes Event refers to a significant climate change that occurred around 430,000 years ago. Following this event, the interglacial periods became warmer, longer and had higher CO₂ levels in the atmosphere. "With our new methodological approach, we were even able to detect shorter-term variations in the ocean—providing us with a much more detailed view of Southern Ocean dynamics."

A look into the past with innovative laser technology

To address their research question, the team analyzed a ferromanganese crust collected from the Antarctic continental margin at a depth of about 1,600 meters. These crusts grow extremely slowly and record the chemical signature of seawater over hundreds of thousands of years.

Using a novel laser-based technique—known as 2D laser ablation technique, in which tiny samples of material are precisely vaporized and then analyzed—the researchers investigated the isotopic composition of lead preserved in the crust. Lead isotopes reveal how strongly the water layers in the ocean were mixed in the past. A new method also enables absolute dating of the layers of the same crust sample. In this way, past climate changes can be reconstructed at very high temporal resolution.

"This new laser method opens up completely new possibilities for climate reconstruction," says Dr. Jan Fietzke, a physicist and the head of the LA-ICP-MS (laser ablation inductively coupled plasma mass spectrometry) laboratory at GEOMAR. "It enables us to gain a better understanding of the role of the Southern Ocean in the global carbon cycle, which is also relevant for predicting future climate developments."

Stronger stratification: Ocean processes determine the climate

The data show that during the lukewarm interglacials, the Southern Ocean was more strongly stratified—the upper and lower water layers mixed less. This meant that more carbon remained stored in the deep ocean instead of reaching the atmosphere. Less atmospheric CO₂ in turn led to a weaker greenhouse effect, cooler Antarctic temperatures and probably also a larger Antarctic ice sheet.

The results highlight the crucial role of ocean changes in the sensitivity of Earth's climate system.