Southern Ocean CO₂ release can explain the warmest period of the last 65 million years

Fossil plankton shells reveal that during the warmest period of the Eocene, the ocean around Antarctica released CO₂ from its deep waters into the atmosphere, explaining the high CO₂ levels and thus extreme temperatures on Earth at that time.

Between about 53 and 49 million years ago, an epoch known as the early Eocene, the Earth went through its warmest stretch since the dinosaurs died out 65 million years ago, with atmospheric CO₂ reaching roughly 1,600 parts per million, and temperatures more than 10 degrees warmer than present. The causes of these elevated CO₂ values remain a puzzle as volcanic sources alone do not appear to explain it. An international team led by the Max Planck Institute for Chemistry now reports that the Southern Ocean helped in keeping CO₂ levels that high. As the climate warmed, the ocean around Antarctica stored less carbon in the deep ocean and vented more of it to the atmosphere. 

Fossil shells as an indirect recorder of CO₂ storage in the ocean

Although geochemists have been able to estimate CO₂ levels during the early Eocene yet the contribution of ocean processes to explain those levels have remained elusive. To determine the ocean's contribution to the early Eocene CO₂ levels, researchers measured nitrogen isotopes on the organic matter preserved for millions of years inside the shells of foraminifera from sediment core taken at four locations in the Southern Ocean by the Ocean Drilling Program, an international research project to explore and study the composition and structure of Earth's oceanic basins. Foraminifera are tiny, single cell marine organisms whose shells are preserved in the sediment for millions of years.

The isotope signal of the shells reveals how algae used up nutrients in surface waters. Unused nutrients at the surface mean a weaker “biological pump”, the process by which marine organisms take up carbon at the surface and transport it into the ocean interior, where it can remain stored for long periods. With a less efficient biological pump, less carbon was retained in the ocean interior, allowing more CO₂ to accumulate in the atmosphere. 

Significantly reduced nutrient consumption by algae and its effect on atmospheric CO₂ 

At the peak of the Eocene warmth, Southern Ocean algae consumed about 40% less of the available nutrients than before or after, according to the study now published in the journal Nature Geoscience. 
The Southern Ocean plays a central role in regulating Earth’s climate by connecting the ocean interior, a huge reservoir of carbon, to the surface ocean and, ultimately, the atmosphere. Unused nutrients in the surface water mean that the ocean carbon cycle was weaker at that time. With a less efficient biological pump, less carbon was retained in the ocean interior, allowing more CO₂ to be released into the atmosphere. 

Impacts of ocean biology on 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. 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. 

“It is known how changes in the biological carbon pump in the Southern Ocean influenced atmospheric CO₂ during the relatively short ice age cycles of the Pleistocene, the epoch that ended about 12,000 years ago. Yet whether this mechanism also shaped Earth's climate over much longer geological timescales such as the four million years of the early Eocene, and under radically different climatic conditions, has remained poorly understood. Our study shows that the Southern Ocean was also a major player in regulating atmospheric CO₂ in a much warmer world.”, says Simone Moretti, researcher at the Max Planck Institute for Chemistry and lead author of the new study.

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

Persistent high CO₂ level due to less effective rock weathering

However, the 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 by rock weathering. Before the rise of major mountain ranges such as the Alps and Himalayas 50 to 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 recent 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 Antarcitc 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 mechanim during other periods of Earth’s history of sustained global warming. 

Distinction to current CO₂ levels 

The main difference between the current rise in CO₂ and earlier warm periods lies in the speed, and the human cause: Today’s rise in CO₂ is occurring nearly ten to hundred times faster than the most rapid natural increases in CO₂ in Earth’s history. While earlier climate fluctuations were triggered by natural orbital cycles or slow geological processes amplified by CO₂, the current rise is anthropogenic (human-caused) and driven by coal, oil, and gas.     

About the study

The 14-member team led by the Max Planck Institute for Chemistry includes researchers from Université Libre de Bruxelles, UC Santa Cruz, University of Ferrara, Utrecht University, Johannes Gutenberg University Mainz, Heidelberg University, Vassar College, ETH Zurich and Princeton University.