Ancient example of modern global warming was too hot for tiny, important ocean creatures

The carbon composition of sand grain-sized shells left behind 56 million years ago by single-cell ocean organisms called foraminifera—like this one from foram species Morozovella allisonensis—helped UW-Madison researchers describe a likely catastrophic collapse of the marine food web during a global warming very similar to the one we are embarking on now. Credit: Brittany Hupp

During another time in which Earth warmed rapidly in conjunction with a spike in atmospheric carbon similar to our modern climate, seawater temperature and chemical changes decimated an important piece of the food web in the tropical Pacific Ocean, according to new research from the University of Wisconsin-Madison.

Planktonic foraminifera are single-celled ocean organisms known for making intricate shells that aren’t just the size of a grain of sand, they often are grains of sand on ocean floors and beaches. Planktonic foraminifera evolved approximately 180 million years ago, and they’ve spent all that time evolving further, while dying off and sinking to the bottom of the ocean, where their distinctive shells pile up to form layer after layer of sediment.

Foraminifera were around 56 million years ago during an event called the Paleocene-Eocene Thermal Maximum (the PETM), when atmospheric chemistry and carbon dioxide levels changed abruptly—as they are doing today—and global temperatures warmed rapidly by 4 to 5 degrees Celsius.

“Foraminifera are pretty sensitive environmental indicators. I think of them as canaries in the coal mine,” says Clay Kelly, a UW-Madison geosciences professor and one author of a new study of the PETM published recently in Proceedings of the National Academy of Sciences. He also notes that “the PETM is arguably our best ancient analog for future climate change.”

In the 1990s, Kelly published descriptions of the foraminifera found in deep-sea sediments dating to the time of the PETM. They were recovered from atop an extinct submarine volcano in the middle of the Pacific Ocean. What he found, by meticulously separating and examining foraminifera shells, was a surprising expansion of the number of species living near the equator in an ocean that was getting hotter, more acidic and less oxygen-saturated.

But the onset of the PETM happened fast, geologically speaking, over less than 5,000 years. Over that time, ocean currents and the burrowing activity of bottom-dwelling organisms churned and blended ocean floor sediments so that the microscopic shells of foraminifera were pushed up or down in the muck, leaving their eventually fossilized remains sitting next to ancestors or descendants 10,000 years distant.

“That reworking can leave 20 centimeters of a sediment core all mixed up, so that some foraminifera deposited in the sediment much earlier appear to have been living thousands of years later,” says Brittany Hupp, a co-author who worked on the study while earning her doctorate in geosciences at UW-Madison and is now a postdoctoral researcher at Oregon State University.

Hupp used the sharp change in atmospheric chemistry (and the upper layers of the ocean, where planktonic foraminifera live) to untangle the reworking of the sediment and came to a much different conclusion about the response of foraminifera to PETM conditions.

The PETM is marked in the rock and fossil records by a global carbon isotope excursion, in which an isotope of carbon became much more common in the atmosphere. Isotopes are atoms of a single element that have slightly different weights. The isotope the researchers studied is called carbon 12.

More information: Brittany N. Hupp et al, Isotopic filtering reveals high sensitivity of planktic calcifiers to Paleocene–Eocene thermal maximum warming and acidification, Proceedings of the National Academy of Sciences (2022). DOI: 10.1073/pnas.2115561119

University of Wisconsin-Madison (via Phys.Org), 9 March 2022. Press release.

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