What “ghost fossils” reveal about past climate influences

How does global warming and associated ocean acidification affect plankton in the world’s oceans? So far, researchers have hypothesized that certain types of single-celled plankton that form calcareous shells would decline under these conditions—particularly since no fossils of these types have been found from earlier warm phases of Earth’s history. Now, however, a research team has discovered fossil traces of only those types of plankton: instead of the calcareous shells themselves, only their imprints have been preserved on other fossils. The results indicate that plankton can withstand global warming better than previously thought.

The higher the atmospheric carbon dioxide content, the greater the dissolved carbon dioxide in the oceans. As a result, sea water has become more acidic – a problem for organisms that build their shells from lime, as this decomposes with acid. Calcifying organisms also include certain types of unicellular plankton. The so-called coccolithophores form small scales that can remain as fossils for millions of years. However, there appears to be a lack of such fossils from past interglacial periods – evidence for researchers that these organisms cannot thrive under acidic ocean conditions.

Accidental discovery

Now a new discovery challenges that view: a team led by Sam Slater of the Swedish Museum of Natural History in Stockholm has found imprints of calcareous shells of coccolithophores on other fossils from warm prehistoric times. Since these are not the calcareous shells themselves, but only their fingerprints, researchers refer to them as “ghost fossils.” The finds indicate that despite the actually unfavorable conditions, there were also many calcified coccolithophores during earlier warm phases—apparently at least better adapted to global warming at that time than previously assumed.

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Slater and his team owe this discovery to a happy coincidence. They actually wanted to examine the fossil rock samples for pollen and plankton species without calcareous shells. So they dissolved their samples in acid, leaving only the fossil remains of organic matter – a method considered unsuitable for searching for coccolithophores because acid breaks down lime. But on the surface of pollen fossils prepared in this way, Slater’s team found the imprints of those same microscopic grains.

More flexible than expected

“The discovery of these beautiful ghost fossils was totally unexpected,” says Slater. “We first found them on the surface of fossilized pollen and it soon became clear that they were abundant at periods when natural coccolithophores were scarce or absent – that was an absolute surprise!” For three major warming events in the Jurassic and Cretaceous periods, 94, 120 and 183 million years ago, the researchers demonstrated in this way that many staghorn coccolithophores also occurred in these phases. “The preservation of these ghostly nanofossils is really amazing,” says co-author Paul Bowen of University College London. “Ghost fossils are very small – about five thousandths of a millimeter long, 15 times narrower than the width of a human hair – but the details of the original plates are still quite visible, albeit compressed into the surfaces of antiquity. The organic material of the plates themselves has been dissolved.”

“Ghost fossils” of ice carriers prove their existence even in the interglacial periods. © S.M. Slater, P. Bowen et al / Science

According to the researchers, the fact that no fossils of coccolithophores from the warm prehistoric phases have been found so far is due to the fact that the increased acidity of the surrounding water dissolved the limestone slabs so that only their imprints remained. “Palaeontologists usually look only for fossils themselves, and when they find none, they often assume that these ancient plankton communities have collapsed,” explains Slater’s colleague Fifi Vajda. “Ghost fossils show us that the fossil record sometimes plays tricks on us and that there are other ways to preserve these calcareous nanoplankton that must be considered when trying to understand past climate change responses.”

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In light of the new findings, the researchers hypothesize that calcareous nanoplankton also thrived during past warm phases and, despite acidic environmental conditions, could continue to form calcareous shells at least during their flowering phases. “This shows that the nanoplankton were more resistant to past events than traditional fossil evidence suggests,” the authors wrote. In light of this, it is plausible that current global warming is affecting plankton less than feared. However, the authors stress that predictions are difficult given the speed of climate changes today.

Source: Sam Slater (Swedish Museum of Natural History, Stockholm) et al., Science, doi: 10.1126/science. abm7330

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