Southern Ocean Weakened Carbon Storage and Fueled Eocene Heat

The reading showed the 40% drop at peak Eocene warmth. Less nutrient consumption meant less carbon sinking into the deep, leaving more of it free in the atmosphere.

How much CO₂ did the weaker pump add?

Numbers do most of the talking here. The team ran a numerical ocean-atmosphere model that simulates how air and sea exchange heat, chemistry and biology. The 40% reduction alone, they calculated, pushes atmospheric CO₂ from 700 to about 900 ppm. Add in feedbacks from warming and seawater chemistry, and the figure rises to roughly 1,100 ppm.

That covers at least half of the roughly 800 ppm increase during this stretch of Earth history — a substantial share of the greenhouse gas burden that kept the planet sweltering.

"Because Eocene seawater had a much lower capacity to buffer changes in CO₂ than today's ocean, a weakening of the biological carbon pump could have had a far greater impact on atmospheric CO₂ than under present-day conditions," said co-author François Fripiat.

In plain terms: the ancient ocean pushed back less against carbon buildup than our ocean would today, so the same biological slowdown packed a bigger punch.

Why did high CO₂ persist for millions of years?

The findings answer one question and raise another. Why didn't Earth's natural climate thermostat — the slow removal of atmospheric CO₂ through rock weathering — bring levels back down faster?

Part of the answer may be geography. Before major mountain ranges like the Alps and Himalayas rose between roughly 50 and 30 million years ago, the continents were relatively flat and already deeply weathered. They offered little fresh rock for weathering to chew on, potentially slowing the whole carbon-removal process.

"With Earth's natural thermostat operating less efficiently, the additional CO₂ released from the ocean could remain in the atmosphere for much longer," explained co-author Mathis Hain. "This would allow the Southern Ocean to influence climate not just over thousands of years, as during the ice ages, but over millions of years."

Is this a pattern we've seen before?

Yes, and that's what makes the result more than ancient trivia. During warm interglacials of the Pleistocene ice ages, the Southern Ocean also released CO₂. Seeing the same behavior across vastly different timescales and climate states suggests a robust feedback loop, according to Alfredo Martínez-García, group leader at MPIC and the study's last author.

"Warming brings carbon-rich deep water to the Antarctic surface, releasing CO₂ to the atmosphere that drives further warming," Martínez-García said. Lead author Moretti added that the study shows the Southern Ocean "played a major role in regulating atmospheric CO₂ during the early Eocene, millions of years before the onset of the recent ice ages."

The team is now checking whether the same feedback mechanism drove other prolonged warm spells in Earth's history. The study carries the usual caveats for paleoclimate reconstructions — it relies on proxies from a handful of drilling sites and on model simulations — but the alignment of shell chemistry, modeling and ice-age evidence makes the mechanism hard to dismiss.