Ghostly footprints of tiny plankton-like creatures have been found haunting the sediments of prehistoric oceans at a time when these organisms were thought to be extinct. The so-called nannofossil footprints reveal that the organisms survived acidic oceans caused by climate change and could offer a clue to how modern creatures can withstand rising ocean temperatures, researchers have said.
Nannofossils are the remains of marine plankton called coccolithophores (cox-oh-LITH’-oh-fours), which belong to the class Prymnesiophyceae and still exist today at the bottom of many ocean food chains. Each of these single-celled algae-like organisms is less than 0.001 inch (30 micrometers) wide and surrounded by a tough layer of geometric scales of calcium, according to the Faculty of Geosciences at the University of Bremen in Germany. And these nannofossils are incredibly abundant.
“There are many, many more nannofossils than any other type of fossil,” Paul Bown, a micropaleontologist at University College London, UK, and co-author of the new study, told Live Science. “That means we can really be statistically robust, because we see so much of it.”
When these tiny planktons die, they sink to the sea floor, where their calcium shells slowly build up. Over time, these piles of mineralized white scales, called coccoliths, are pressed together to form chalk walls. A classic example, according to Brown, is the famous White Cliffs of Dover in England. “White chalk cliffs are white because they’re almost 100% nannofossil,” Bown said.
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However, there are points in the fossil record where coccolithophores seem to suddenly disappear, only to mysteriously return millions of years later. “You get these abrupt changes in the sediment where you go from almost pure white sediment to black sediment,” Bown said. These points coincide with ancient ocean warming events, in which seawater became more acidic as it reacted with increased atmospheric carbon dioxide. As the ocean’s pH dropped during these events, it ate away the calcium shells of the coccolithophores, much like vinegar can dissolve an eggshell, according to a National Oceanic and Atmospheric Administration (NOAA).
Scientists once thought that most species of calcium-coated plankton in these acidic seas had been repeatedly wiped out en masse and replaced by shellless species, whose bodies decayed into dark, muddy slime and later hardened into rock.
Bown’s co-author, Sam Slater, a micropaleontologist at the Swedish Museum of Natural History in Stockholm, had previously concluded much the same. But then Slater noticed something odd while doing research for another study looking for traces of ancient pollen, while examining black sediment from a warming event during the Jurassic period (201 million ago). 145 million years ago). Under a powerful microscope, Slater detected tiny geometric imprints in the rock and realized that these imprints were exactly the shape of coccolithophores.
Slater contacted Bown and a handful of other specialists to help investigate. Indeed, the rock was stamped with coccolithophores. “These were spectacularly preserved impressions,” Bown said. “I could identify these things down to the species level.”
Intrigued by this discovery, the researchers then examined fossil sediments from other Jurassic sites around the world, as well as samples from two warming events during the Cretaceous period (145 to 66 million years ago). ). “And we found these impressions, these ghost fossils, everywhere we looked,” Bown said.
These results suggest that, contrary to previous research, some coccolithophores survived catastrophic ocean acidification and global warming mortality, even as other species disappeared. But the low pH of the ocean dissolved their shells posthumously, erasing them from the fossil record.
This information could help shed light on our current climate catastrophe, the researchers said, which is already eating away at calcium-rich coral reefs, according to Smithsonian. If coccolithophores can adapt to warmer, more acidic conditions, that may be good news for modern creatures higher up the food chain.
However, Bown cautions against equating ancient warming events too closely with modern climate change, which is occurring at about 10 times the rate of previous disasters, according to research published in 2019 in the journal Paleooceanography and paleoclimatology.
“It’s a cautionary tale,” Bown said, “And you have to be careful how you’re going to read the rocks.”
The new study was published May 19 in the journal Science.
Originally posted on Live Science.