Scientists at Northwestern University in the United States have announced the solution to a scientific enigma that has persisted for decades: the factor responsible for one of the largest extinctions in the history of planktonic foraminifera, microscopic marine organisms crucial to the global carbon cycle.
Cause of the Ancient Extinction
A study published in the journal Science indicates that the cause of this extinction was ocean acidification. This phenomenon was triggered by intense volcanic eruptions that occurred approximately 113 million years ago, during the Lower Cretaceous period.
The team reached this conclusion by analyzing chemical traces preserved in microscopic fossils. The findings confirm the link between major volcanic eruptions, the increase in atmospheric carbon dioxide (CO2) concentration, the subsequent ocean acidification, and the mass extinction events. This work represents the fifth led by Northwestern to identify such a pattern over more than 60 million years of Earth's history.
Current Impacts and Chemical Mechanism
In addition to clarifying a past event, the research provides vital information about the effects of the current increase in CO2 emissions from human activities, given that the oceans continue to absorb some of this carbon and undergo a gradual process of acidification.
Volcanic Activity and Marine Chemistry
The scientists attribute the acidification to the activity of the Kerguelen Plateau, a vast volcanic province located in the southern Indian Ocean. During the Lower Cretaceous, eruptions released large volumes of CO2 into the atmosphere. Subsequently, this gas was absorbed by the oceans, resulting in a decrease in water pH and hindering the ability of marine organisms to build and maintain their calcium carbonate shells.
Jonathan Chen, the study leader and a recent graduate of Northwestern University, explained to Phys.org that upon observing the fossils, it had already been noted that surface plankton was decreasing and forming thinner shells, indicating stress on the organisms. However, the responsibility for the acidification was only confirmed by measuring the calcium isotopes in the fossils, revealing a large increase in these proportions during the extinction, which proved that calcification was occurring much more slowly.
Foraminifera and the Carbon Cycle
Foraminifera are microscopic organisms that form calcium carbonate shells. They play an essential role in the natural sequestration of carbon by storing it in these structures, aiding in the stability of the global element's cycle. Chen emphasized that these organisms function as a fundamental stabilizing force in the carbon cycle, warning that the absence of this sink would alter the cycle in unpredictable ways.
The extinction that occurred during the transition between the Aptian and Albian periods was the second largest recorded for these organisms, surpassed only by the asteroid impact event that marked the end of the dinosaurs. After this massive loss, the remaining species became smaller and developed more delicate shells.
Detailed Fossil Analysis
To investigate this event, researchers examined hundreds of fossils retrieved from sediments collected in the 1980s during a mission of the Deep Sea Drilling Project, on the Falkland Plateau, located in the South Atlantic. The samples, preserved by the Smithsonian Institution, were meticulously separated by Chen using a fine brush, given the required precision, as each fossil is approximately the size of a grain of sand.
Next, the team measured the calcium isotopes contained in the fossilized shells. While under ideal conditions, foraminifera exhibit a specific isotopic signature, difficulty in calcification due to ocean acidification slows growth and modifies this chemical signature. The results showed a sharp increase in isotopic values during the extinction period, signaling a strong reduction in the rate of shell formation.
Andrew D. Jacobson, an expert in isotopic geochemistry and co-author of the study, commented that planktonic foraminifera not only decreased in number but also in size. He concluded that the most plausible explanation is slower growth, as calcium isotopes are sensitive to this, changing in a manner consistent with slower formation in unfavorable environments.
Survival of Benthic Organisms
For decades, an argument against the acidification theory was the relatively good survival of foraminifera inhabiting the ocean floor. The new study explains that the difference lies in the fact that CO2 was first released into the atmosphere before reaching the ocean depths.
Consequently, the acidification initially affected the surface waters where plankton live. Only later did the chemical change reach the deeper regions, and this occurred less intensely. The researchers detail that after the initial acidification, the excess alkalinity circulated through the water column, restoring the availability of carbonates for benthic organisms on the seafloor.
Consistency of Geological Pattern
This is the fifth study conducted by the Northwestern team correlating large volcanic provinces with ocean acidification and mass extinction events. Previous research, which analyzed mollusk and nanoplankton fossils from different geological eras, converged on the same pattern: massive volcanic eruptions raised CO2, modified ocean chemistry, and triggered extinctions.
Brad Sageman, another co-author, stated that the data show a very consistent pattern. He added that this is the first work to conclusively prove that the chemical signal recorded in individual shells corresponds to what is observed in the sediments as a whole, eliminating doubts about the link to a mass extinction.
Jacobson emphasized that the results represent a significant advance, as they brought calcium isotopes to a level where they can serve as a geochemical indicator of mortality in the geological record.
Implications for Climate Future
Although the studied event occurred in an environmental context distinct from the present, the researchers believe the findings help in understanding the risks posed by the modern increase in CO2 emissions. Sageman points out that human-caused ocean acidification is already measurable today, and studying past episodes allows for estimating the scale of future impacts.
He concluded that by studying these ancient events, it is possible to assess the magnitude of what may happen next, given that ocean acidification is already occurring as a result of humanity's CO2.