New insights into a long-standing climate question
Earth’s climate has long swung between cold glacial states and warmer interglacial periods. Among these, the so-called lukewarm interglacials—warm phases that occurred roughly between 800,000 and 430,000 years ago—stand out for their relatively cool temperatures and modest atmospheric CO2 levels (about 240–260 ppm). Even the later interglacials, which followed after the Mid-Brunhes Event around 430,000 years ago, peaked at 280–300 ppm. Today’s CO2 concentration has surpassed 420 ppm, driven by human activities. For decades, scientists puzzled over why these ancient warm periods did not reach the atmospheric CO2 and temperature highs seen in more recent interglacials. A new study places the Southern Ocean at the heart of this mystery.
What the new study found
Lead author Dr. Huang Huang (PhD GEOMAR, now at the Laoshan Laboratory in Qingdao, China) explains that stronger stratification of the Southern Ocean was a key driver of the comparatively cooler interglacials before the Mid-Brunhes Event. Stratification means less mixing between surface and deep waters, which in turn affects how much carbon stays in the deep ocean versus how much escapes to the atmosphere. The finding helps explain why, during these earlier warm periods, the climate was milder than in later interglacials with higher CO2 levels.
“With our new methodological approach, we were able to detect shorter-term variations in the ocean,” Huang notes, highlighting a more detailed view of Southern Ocean dynamics than previously possible. This refinement is essential for understanding how oceanic processes influence global climate and atmospheric composition over hundreds of thousands of years.
How the scientists unlocked the past
The research team analyzed a ferromanganese crust collected from the Antarctic continental margin at roughly 1,600 meters depth. These crusts accumulate extremely slowly and preserve a chemical record of seawater across vast timescales. To read this record with unprecedented precision, the researchers employed a novel laser-based 2D laser ablation technique. Tiny samples are vaporized and then analyzed to reveal the isotopic composition of lead preserved in the crust. Lead isotopes serve as markers for how vigorously water layers mixed in the past, offering clues about ocean stratification and carbon cycling.
A critical advance was the ability to date the layers of the same crust with high accuracy, providing a detailed, high-temporal-resolution reconstruction of past climate changes. “This new laser method opens up completely new possibilities for climate reconstruction,” says Dr. Jan Fietzke, head of GEOMAR’s LA-ICP-MS laboratory. The technique not only clarifies the role of the Southern Ocean in the global carbon cycle but also informs potential future climate scenarios by improving how we predict ocean-atmosphere interactions.
Why stratification mattered for CO2 and climate
In the lukewarm interglacials, the strengthened stratification of the Southern Ocean hindered the upwelling of carbon-rich deep waters to the surface. As a result, more carbon remained sequestered in the deep ocean, limiting the amount of CO2 that reached the atmosphere. This reduced greenhouse forcing and helped keep Antarctic temperatures lower, likely supporting a larger Antarctic ice sheet during those periods. The study therefore underscores a fundamental point: ocean processes—especially how the Southern Ocean stores and releases carbon—played a decisive role in modulating the climate system long before modern anthropogenic emissions emerged.
Implications for our understanding of climate sensitivity
By revealing a mechanism that dampened atmospheric CO2 in the past, the research highlights the Southern Ocean’s central place in determining the Earth’s climate sensitivity. While today’s CO2 rise is steep and rapid, the study emphasizes that ocean stratification and carbon cycling can produce substantial climate responses over long timescales. These findings offer a nuanced perspective on how future changes in ocean structure—driven by warming, wind patterns, and salinity—could influence atmospheric CO2 and global temperatures.
In sum
The lukewarm interglacials were cooler not only because of surface temperature dynamics but also because the Southern Ocean trapped more carbon in the deep ocean. By combining advanced laser techniques with meticulous geochemical analysis, scientists are painting a more complete picture of how the planet’s biggest ocean helped steer the climate history that shapes our present and future.
