Southern Ocean Weakening Boosted Eocene CO2 Levels

Researchers led by the Max Planck Institute for Chemistry have identified a significant mechanism that helped sustain Earth’s warmest climate in the past 65 million years. The study, published in Nature Geoscience, demonstrates that the Southern Ocean played a critical role in amplifying global warming during the early Eocene period, approximately 50 million years ago. During this epoch, global temperatures were more than 10°C higher than current levels, and atmospheric carbon dioxide concentrations reached around 1,600 parts per million. The international team found that the ocean surrounding Antarctica became less efficient at storing carbon in the deep ocean, a change that substantially increased atmospheric CO2 and reinforced the greenhouse effect.

The central finding of the research is that the biological carbon pump in the Southern Ocean was significantly weaker during the peak of Eocene warmth. This natural process involves microscopic marine organisms, known as phytoplankton, taking 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 sequestration helps regulate the amount of CO2 in the atmosphere. The study reveals that this mechanism was substantially less efficient during the early Eocene, reducing the amount of carbon stored in the deep ocean and allowing more carbon to accumulate in the atmosphere.

To investigate how efficiently this mechanism operated, the team analyzed nitrogen isotopes preserved in the fossil shells of foraminifera, tiny marine organisms. These 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 analysis covered the early Eocene, between approximately 53 and 49 million years ago, a period characterized by exceptionally warm conditions.

The results revealed a striking difference in nutrient utilization between the warmest period and the cooler periods before and after. At the peak of Eocene warmth, Southern Ocean phytoplankton utilized approximately 40% less of the available nutrients than during the cooler intervals. This indicates that the biological carbon pump was substantially less efficient, reducing the amount of carbon stored in the deep ocean. This 40% decline in nutrient utilization is a key piece of the puzzle explaining why CO2 levels remained extraordinarily high for millions of years. 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.

To determine the extent to which the reduced consumption affected the CO2 concentration in the atmosphere, the researchers used a numerical ocean-atmosphere model. This model simulates the physical, chemical, and biological interactions between the Earth’s atmosphere and the oceans. According to the study, the reduction of 40 percent alone raises CO2 levels from 700 to about 900 parts per million. Once warming and seawater chemistry feedbacks are included into the model, the CO2 levels rise to about 1,100 ppm. This accounts for at least half of the roughly 800 ppm rise during this period. The model results provide a quantitative estimate of the impact of the weakened biological pump on atmospheric CO2 concentrations.

François Fripiat, co-author of the study, explains that Eocene seawater had a much lower capacity to buffer changes in CO2 than today’s ocean. This means that a weakening of the biological carbon pump could have had a far greater impact on atmospheric CO2 than under present-day conditions. The lower buffering capacity of the ancient ocean amplified the effect of the reduced carbon sequestration.

Simone Moretti, researcher at the Max Planck Institute for Chemistry and lead author of the study, notes that while changes in the Southern Ocean’s biological carbon pump are known to have contributed to atmospheric CO2 variations during the ice ages, it has remained uncertain whether the same mechanism also operated in a much warmer world. The 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.

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.

Alfredo Martínez-García, Group Leader at MPIC and last author of the study, explains that 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.

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

The study, led by the Max Planck Institute for Chemistry and involving an international research team, was published in the journal Nature Geoscience. The research specifically focused on the early Eocene period, spanning approximately 53 to 49 million years ago, a time when Earth experienced its warmest climate since the extinction of the dinosaurs. During this epoch, global temperatures exceeded current levels by more than 10°C, and atmospheric carbon dioxide concentrations reached approximately 1,600 parts per million.

The investigation relied on chemical signatures preserved in microscopic fossil shells of foraminifera, which were recovered from sediment cores collected at four distinct sites in the Southern Ocean by the Ocean Drilling Program. By analyzing nitrogen isotopes within these shells, the researchers were able to reconstruct the efficiency of the biological carbon pump. This natural process involves phytoplankton taking up CO2 in surface waters; as these organisms die, some of the carbon sinks into the deep ocean, where it can remain stored for hundreds to thousands of years. The nitrogen isotope signatures provide a proxy for how completely phytoplankton consumed available nutrients, allowing scientists to quantify changes in the ocean’s capacity to transfer carbon into its interior.

The model simulations indicated that the 40% reduction in nutrient utilization alone would raise CO2 levels from 700 to about 900 parts per million. When the model incorporated feedbacks related to warming and seawater chemistry, the estimated CO2 concentration increased to approximately 1,100 ppm. This modeled increase accounts for at least half of the roughly 800 ppm rise observed during the period. The results suggest that the weakening of the biological carbon pump was a substantial driver of the elevated atmospheric CO2, amplifying the greenhouse effect during the early Eocene.

Alfredo Martínez-García, a Group Leader at the Max Planck Institute for Chemistry and the last author of the study, noted that the consistency between the Southern Ocean’s behavior during warm interglacials of the Pleistocene ice ages and the early Eocene warming reinforces the understanding of this mechanism. He explained that seeing the same response across such different timescales suggests a robust feedback mechanism, where warming brings carbon-rich deep water to the Antarctic surface, releasing CO2 to the atmosphere and driving further warming.

The persistence of high CO2 levels for millions of years is attributed in part to the limited effectiveness of Earth’s natural climate thermostat, which removes atmospheric CO2 through rock weathering. Before the formation of major mountain ranges like the Alps and Himalayas between 50 and 30 million years ago, continents were relatively flat and deeply weathered, offering little fresh rock to facilitate this process. With this thermostat operating less efficiently, the additional CO2 released from the ocean could remain in the atmosphere for much longer. This allowed the Southern Ocean to influence climate over millions of years, rather than the thousands of years typical of ice age variations.

Subject of Research: The role of the Southern Ocean biological carbon pump in regulating atmospheric CO2 during the early Eocene.

Article Title: Southern Ocean helped fuel Earth's warmest climate in 65 million years

Article References: Southern Ocean helped fuel Earth's warmest climate in 65 million years. Original publication (Bibliographic details unavailable.)

Image Credits: AI Generated

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Keywords: Southern Ocean, Biological Carbon Pump, Early Eocene, Atmospheric CO2, Climate Change, Paleoclimate, Foraminifera, Nitrogen Isotopes, Ocean Circulation, Greenhouse Effect, Southern, Ocean