An ancient example of modern global warming was too hot for tiny, important ocean creatures
In another time when Earth was warming rapidly in conjunction with a spike in atmospheric carbon similar to our modern climate, seawater temperature and chemical shifts decimated an important component of the tropical Pacific Ocean’s food web. , according to new research from the University of Wisconsin. –Madison.
Planktonic foraminifera are single-celled ocean organisms known to make complex shells that are not only the size of a grain of sand, they are often are grains of sand on the seabed and on the beaches. Planktonic foraminifera evolved about 180 million years ago, and they spent all that time evolving further, all the while dying and sinking to the ocean floor, where their distinctive shells pile up to form a layer after sediment layer.
Foraminifera existed 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 do today. today – and that global temperatures have warmed rapidly by 4 to 5 degrees Celsius.
“Foraminifera are quite sensitive environmental indicators. I think of them as canaries in the coal mine,” says Clay Kelly, professor of geosciences at UW-Madison and author of a new study on PETM published recently in Proceedings of the National Academy of Sciences. He also notes that “the PETM is arguably our best old analogue for future climate change.”
In the 1990s, Kelly published descriptions of foraminifera found in deep-sea sediments from the time of the PETM. They were recovered from the top of an extinct underwater volcano in the middle of the Pacific Ocean. What he found, by carefully separating and examining foraminifera shells, was a startling increase in the number of species living near the equator in an ocean that was becoming warmer, more acidic and less saturated with oxygen.
But the beginning of the PETM happened quickly, geologically speaking, in less than 5,000 years. During this period, ocean currents and the burrowing activity of bottom-dwelling organisms churned and mixed the ocean floor sediments so that the microscopic shells of foraminifera were pushed up or down in the mud, leaving their possibly fossilized remains sitting next to ancestors or descendants 10,000 years away.
“This reworking can leave 20 centimeters of a mixed sediment core, so that some foraminifera deposited in sediment much earlier appear to have lived thousands of years later,” says Brittany Hupp, a co-author who worked on the study while earning it. PhD in geosciences at UW-Madison and is now a postdoctoral fellow at Oregon State University.
Hupp used the abrupt change in atmospheric chemistry (and the upper layers of the ocean, where planktonic foraminifera live) to unravel the reworking of sediments and came to a much different conclusion about the response of foraminifera to PETM conditions.
The PETM is marked in the rock and fossil record by a global carbon isotope excursion, during which one carbon isotope became much more common in the atmosphere. Isotopes are atoms of the same element that have slightly different weights. The isotope studied by the researchers is called carbon-12.
“Carbon-12 is a stable isotope, which means that it does not undergo radioactive decay like its sister isotope carbon-14, and remains relatively concentrated in organic materials such as coal and hydrocarbons that have spent a long time buried in the ground,” says Kelly. “We are currently mining it as fossil fuels, burning it and releasing it at an unprecedented rate into the ocean-atmosphere system.”
Since carbon-12 also increased during PETM and foraminifera use carbon to make their shells, there is a clear difference between the amount of carbon-12 in the shells of foraminifera living before PETM and those living during the period of Warming.
“Foraminifera are quite sensitive environmental indicators. I consider them like canaries in the coal mine.
Hupp scoured the sediment samples again and discarded the shells whose carbon content marked them as from the wrong era, possibly pushed through the sediment by an ancient burrowing worm. She collected over 500 individual PETM foraminifera that represented a significant change from the pre-PETM foraminifera community.
“Diversity dropped markedly at the site of this sediment core,” Hupp says. “We go from nine major taxa – representing about 90% of foraminifera – to four. Thus, more than half of the species have disappeared from this site during this period of warming.
But they have not disappeared. They probably migrated out of the warm tropics to find cooler waters. Species that disappeared from the equator were still found in higher latitudes. In fact, other studies show that some of the equatorial species appear in the polar regions during the PETM.
“In the PETM tropics, their population dynamics change so drastically,” Kelly says. “We’ve seen a lot of that leave the tropics, and that would have dramatically changed the food chain during the PETM event, which has so much in common with the current warming.”
Because they are the primary consumers of single-celled plants near the bottom of the food chain, the absence of foraminifera from their normal home ranges would have been catastrophic. This may also be the case in our near future.
The researchers, which include UW-Madison co-author and geography professor John “Jack” Williams, also noticed changes in foraminifera shells likely due to acidification of ocean water as ‘it absorbs additional carbon from the atmosphere. Acidification makes it more difficult for the shells of foraminifera to grow.
“But they are by far not the only organisms living in the ocean to have shells built from calcium carbonate,” Hupp says. “So much of the pressures on planktonic foraminifera during the PETM will affect many other important organisms that build these shells today.”