Analysis of carbon cycle during last glacial period can help monitor climate crisis

A study conducted in Brazil analyzed marine sediments to investigate gas exchanges between the ocean and the atmosphere in the recent past. The researchers concluded that a rise in the temperature of the South Atlantic caused a release of CO₂ trapped at the bottom of the Southern Ocean.

By José Tadeu Arantes  |  Agência FAPESP – Gas exchanges between the atmosphere and the ocean are a key part of the carbon cycle, playing a vital role in climate regulation and maintenance of the planet’s ecological equilibrium. The oceans are thought to absorb roughly a third of all the carbon dioxide (CO₂) emitted by humanity. Understanding the complex processes involved in this gas exchange is therefore extremely important, especially in the context of the global climate crisis.

A new study discusses the processes that governed gas flows between the atmosphere and the South Atlantic in the recent past, showing a noteworthy natural balance in CO₂ exchanges even in a scenario of abrupt climate change. The study was conducted in Brazil and funded by FAPESP. An article about it is published in the journal Global and Planetary Change.

“We investigated periods in the recent geological past when the global climate underwent abrupt changes caused by a reduction in the intensity of the Atlantic meridional overturning circulation [AMOC]. These events are named Heinrich Stadials (HS) after the German climatologist Hartmut Heinrich,” said Tainã Pinho, corresponding author of the article. The study was part of his master’s research at the University of São Paulo’s Institute of Geosciences (IGc-USP).

The AMOC transports a large amount of heat from the South to the North Atlantic, so the reduction in its intensity cooled the North Atlantic and warmed up the South Atlantic. A significant effect of the warming of the southern (meridional) part of the Atlantic was an increase in the deep-water upwelling in the Southern Ocean, which surrounds Antarctica. Upwelling is a process in which cold water rises from the depths toward the surface.

The increase in this upwelling led to a release into the atmosphere of large amounts of CO₂ that had long been trapped under the Southern Ocean. CO₂ from the seafloor has a “fingerprint” that distinguishes it from CO₂ originating elsewhere.

All this history is recorded in marine sediment to some extent. “Our study was based on analysis of micro shells from planktonic foraminifera [marine protozoa that live in the upper ocean] preserved in three marine sediment cores, two of which were collected on the Brazilian coast in Alagoas and Santa Catarina states, and the third off the coast of South Africa. Analysis of the micro shells enabled us to reconstitute and understand a key link in gas exchanges between the atmosphere and the South Atlantic during Heinrich events,” Pinho said. 

“Isotope analysis and mathematical modeling showed that carbon from the seafloor is transferred from the Southern Ocean to the atmosphere and then enters into equilibrium with the upper portion of the South Atlantic.” 

This conclusion was based on the composition of the stable carbon isotopes in the micro shells, which consist mainly of calcium carbonate (CaCO₃). Stable isotopes are non-radioactive forms of atoms in chemical elements that do not decay into other elements. Their stability is determined by the ratio of the number of neutrons to the number of protons in the nucleus. Naturally occurring stable isotopes of water and other substances are used to trace the origin, history, sources, sinks and interactions in water, carbon and nitrogen cycles. In the case of carbon, natural processes such as photosynthesis tend to favor the inclusion of one isotope rather than another in organic matter. When organisms that perform photosynthesis at or near the sea surface die, they sink down the water column and decompose at great depths, so that the isotope that was preferentially included is released back into the deep-sea pool of CO₂, producing the fingerprint mentioned above.

“We compared the isotopic composition of the micro shells obtained in the three marine sediment cores with the isotopic composition of atmospheric CO₂ records in Antarctic ice core samples, and concluded that levels of atmospheric CO₂ in the South Atlantic rose during Heinrich Stadials,” Pinho said.

The researchers observed a surplus of carbon-12 (light carbon, the most common isotope of carbon) during Heinrich events, and found this to be the fingerprint of deep-sea CO₂. The micro shells reflected CO₂ emissions with excess carbon-12 from the seafloor in the Southern Ocean to the atmosphere, followed by a balance in the surface waters of the South Atlantic.

“This balance wasn’t limited to the top of the water column. It reached depths of at least 300 meters. Through this balance, the fingerprint of the deep-sea CO₂ was transferred to the sea surface in the South Atlantic,” Pinho said. 

Evidence is growing that the AMOC is weakening and could even collapse by the end of the century. If so, it would lead to a rise in the temperature of the South Atlantic, which in addition to global warming would severely affect the climate of the entire planet. 

“The solubility of CO₂ in water decreases as the temperature of the water rises, so the ocean’s capacity to absorb CO₂ diminishes as it warms up, at least partially disconnecting the ocean from the atmosphere. This important imbalance can be traced by analyzing the isotopic composition of the CO₂ in seawater, analogously to the method use in the study reported here,” said Cristiano Chiessi, a researcher at IGc-USP and a co-author of the article. Chiessi was Pinho’s thesis advisor for his master’s research.

A deeper understanding of the carbon cycle between the atmosphere and the ocean in the past will enrich the climate change scenarios now being developed. Any decoupling of these key carbon reservoirs – the atmosphere and the ocean – must be monitored with maximum attention. “Interpreting the composition of stable carbon isotopes in planktonic foraminifera is a complex task but offers significant clues to an understanding of specific aspects of the carbon cycle,” Chiessi said.

In addition to its findings, the study represents a noteworthy basic science endeavor, identifying novel records from three different parts of the South Atlantic via more than 940 analyses, he added.

The study was funded by FAPESP via three projects (19/10642-6, 18/15123-4 and 19/24349-9), and was conducted under the aegis of the Biodiversity Characterization, Conservation, Restoration and Sustainable Use Program (BIOTA) and the Research Program on Global Climate Change Research Program (RPGCC).

The article “Thermodynamic air-sea equilibration controls carbon isotopic composition of the South Atlantic thermocline during the last glacial period” is at: www.sciencedirect.com/science/article/abs/pii/S0921818123001960?via%3Dihub.