Oceans were stressed preceding abrupt, prehistoric global warming

Shelled organisms helped buffer ocean acidification by consuming less alkalinity from seawater.

Microscopic fossilized shells are helping geologists reconstruct Earth’s climate during the Paleocene-Eocene Thermal Maximum (PETM), a period of abrupt global warming and ocean acidification that occurred 56 million years ago. Clues from these ancient shells can help scientists better predict future warming and ocean acidification driven by human-caused carbon dioxide emissions.

Led by Northwestern University, the researchers analyzed shells from foraminifera, an ocean-dwelling unicellular organism with an external shell made of calcium carbonate. After analyzing the calcium isotope composition of the fossils, the researchers concluded that massive volcanic activity injected large amounts of carbon dioxide into the Earth system, causing global warming and ocean acidification.

They also found that global warming and ocean acidification did not just passively affect foraminifera. The organisms also actively responded by reducing calcification rates when building their shells. As calcification slowed, the foraminifera consumed less alkalinity from seawater, which helped buffer increasing ocean acidity.

“The formation and dissolution of calcium carbonate help regulate the acidity and alkalinity of seawater,” said Northwestern’s Andrew Jacobson, a senior author of the study. “Our calcium isotope data indicate that reduced foraminiferal calcification worked to dampen ocean acidification before and across the PETM.”

“This is a pretty new concept in the field,” added Gabriella Kitch, the study’s first author. “Previously, people thought that only the dissolution of carbonates at the sea floor could increase alkalinity of the ocean and buffer the effects of ocean acidification. But we are adding to existing studies that show decreased carbonate production has the same buffering effect.”

The research was published online last week (March 4) in the journal Geology. This is the first study to examine the calcium isotope composition of foraminifera to reconstruct conditions before and across the PETM and the third recent Northwestern study to find that ocean acidification — due to volcanic carbon dioxide emissions — preceded major prehistoric environmental catastrophes, such as mass extinctions, oceanic anoxic events and periods of intense global warming.

Consistent pattern emerges

This is the third study led by Jacobson to find that ocean acidification precedes major environmental catastrophes that correlate with large igneous province eruptions. Last month, Jacobson’s team published results finding that volcanic activity triggered a biocalcification crisis prior to an ocean anoxic event that occurred 120 million years ago. Just over a year ago, Jacobson’s team published another study finding ocean acidification preceded the asteroid impact leading to the Cretaceous-Paleogene mass extinction event 66 million years ago, which included the demise of dinosaurs.

In all three studies, Jacobson’s team used sophisticated tools in his laboratory to analyze the calcium isotope composition of calcium carbonate fossils and sediment. Jacobson said a clear pattern is emerging. Influxes of carbon dioxide led to global warming and ocean acidification and, ultimately, to massive environmental changes.

Predictor for future ocean stress

Many researchers study the PETM because it provides the best analog for current-day, human-caused global warming. The carbon influx during the PETM is similar to the amount of carbon released during the past two centuries. The timescales, however, differ significantly. Temperatures during the PETM increased by 5 to 8 degrees Celsius over 170,000 years. With human-caused climate change, the same level of warming is projected to occur in less than 200 years, if carbon dioxide emissions remain unabated.

Frighteningly, terrestrial and ocean stress, including a major decrease in foraminiferal calcification, accompanied the PETM.

“The PETM is a model for what happens during major large carbon cycle perturbations,” Jacobson said. “A lot of predictions for Earth’s future climate rely on understanding what happened during the PETM.”

 Northwestern University (via ScienceDaily), 8 March 2021. Press release.

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