Around 50 million years ago, Earth experienced its warmest climate since the extinction of the dinosaurs. Global temperatures were more than 10°C higher than today, and atmospheric CO₂ concentrations reached around 1,600 parts per million. But what kept CO₂ levels so extraordinarily high for millions of years? An international research team led by the Max Planck Institute for Chemistry has identified an important piece of the puzzle: the Southern Ocean.

Using chemical signatures preserved in microscopic fossil shells, the researchers found that the ocean surrounding Antarctica became less efficient at storing carbon in the deep ocean during this exceptionally warm period. Their findings, published in Nature Geoscience, suggest that this weakening of the ocean's biological carbon pump substantially increased atmospheric CO₂, amplifying global warming.

Fossil shells reveal a 40% decline in nutrient utilization

The researchers reconstructed changes in the Southern Ocean's biological carbon pump, a natural process through which microscopic marine organisms take up CO₂ in surface waters. As these organisms grow and eventually die, some of the carbon they contain sinks into the deep ocean, where it can remain stored for hundreds to thousands of years. This process helps regulate the amount of CO₂ in the atmosphere.

To investigate how efficiently this mechanism operated during the early Eocene, between approximately 53 and 49 million years ago, the team analyzed nitrogen isotopes preserved in the fossil shells of foraminifera, tiny marine organisms. The shells were recovered from sediment cores collected at four sites in the Southern Ocean by the Ocean Drilling Program.

The nitrogen isotope signatures provide information about how completely phytoplankton consumed the nutrients available in surface waters. Because nutrient utilization is closely linked to the biological carbon pump, these measurements allow scientists to reconstruct changes in the ocean's capacity to transfer carbon into its interior.

The results revealed a striking difference: at the peak of Eocene warmth, Southern Ocean phytoplankton utilized approximately 40% less of the available nutrients than during the cooler periods before and after. This indicates that the biological carbon pump was substantially less efficient, reducing the amount of carbon stored in the deep ocean.

Weaker biological carbon pump increased atmospheric CO₂

To determine the extent to which the reduced consumption affected the CO₂ concentration in the atmosphere, the researchers used a numerical ocean-atmosphere model, that simulates the physical, chemical, and biological interactions between the Earth's atmosphere and the oceans. According to the study now published in the journal Nature Geoscience the reduction of 40 percent alone raises CO₂ levels from 700 to about 900 parts per million (ppm), and to about 1,100 ppm once warming and seawater chemistry feedbacks are included into the model. That accounts for at least half of the roughly 800 ppm rise during this period.

"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," explains François Fripiat, co-author of the study.

The findings suggest that changes in the Southern Ocean's biological activity could have amplified an already warm climate by allowing more carbon to accumulate in the atmosphere, reinforcing the greenhouse effect.

“While we know that changes in the Southern Ocean's biological carbon pump contributed to atmospheric CO₂ variations during the ice ages, it has remained uncertain whether the same mechanism also operated in a much warmer world. Our study shows that 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, says Simone Moretti, researcher at the Max Planck Institute for Chemistry and lead author of the study.

Why did high CO2 levels persist for millions of years?

One question remains: Why could these elevated CO₂ levels persist for millions of years? According to the scientists, part of the answer may lie in the limited effectiveness of Earth's natural climate thermostat: the gradual removal of atmospheric CO₂ through rock weathering. Before the rise of major mountain ranges such as the Alps and Himalayas between approximately 50 and 30 million years ago, relatively flat and deeply weathered continents offered little fresh rock, potentially slowing this process.

“With Earth's natural thermostat operating less efficiently, the additional CO₂ released from the ocean could remain in the atmosphere for much longer. 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," explains Mathis Hain, co-author of the study.

“The consistency between warm interglacials of the Pleistocene ice ages, when the Southern Ocean also released CO₂, and an episode of warming that lasted for millions of years, reinforces our understanding of how the Southern Ocean leverages the biological pump across timescales and boundary conditions. Seeing the same response across very different timescales suggests that this is a robust feedback mechanism, in which warming brings carbon-rich deep water to the Antarctic surface releasing CO₂ to the atmosphere that drives further warming” explains Alfredo Martínez-García, Group Leader at MPIC and last author of the study.

Researchers are now exploring the role of this feedback mechanism during other periods of sustained global warming in Earth's history.