What do “ghost fossils” reveal about past climatic effects?

How does global warming and related ocean acidification affect plankton in the world’s oceans? Until now, scientists had assumed that certain types of single-celled plankton that make up the calcareous shells would degrade under such conditions – especially since no fossils of these species have been found from earlier, warm phases of Earth’s history. Now, however, the research team has found fossil traces of only these species of plankton: instead of the calcareous shells themselves, only their imprints on other fossils have been preserved. The results suggest that plankton may be better able to withstand global warming than previously assumed.

As the content of CO2 in the atmosphere increases, the proportion of dissolved carbon dioxide in the oceans increases. As a result, seawater becomes increasingly acidic – a problem for organisms that build their shells out of lime as it is broken down by acid. Certain types of unicellular plankton are also included in the calcification of organisms. So-called cocolithophores form tiny scales that can remain as fossils for millions of years. However, there seemed to be a lack of such fossils from past interglacial periods – evidence for researchers that these organisms could not grow under the acidic conditions of the oceans.

discovery by accident

Now a new discovery challenges this view: a team led by Sam Slater of the Swedish Natural History Museum in Stockholm found impressions of calcareous cocolithophore shells on other fossils from prehistoric warm phases. Since these are not the limestone shells themselves, but only their imprints, researchers refer to them as “ghostly fossils.” The findings indicate that despite the actually unfavorable conditions, the earlier warm phases also had numerous calcifying cocololitophores – apparently they coped better with global warming at the time than previously assumed.

Slater and his team owe their discovery to a lucky coincidence. In fact, they wanted to test fossil rock samples for pollen species and plankton without limestone shells. So they dissolved their samples in acid, leaving only fossilized remnants of organic materials – a method considered unsuitable for searching for cocolithophore fossils because the acid breaks down the lime. However, on the surface of the pollen fossils prepared in this way, Slater’s team found prints of the same cocolithophores.

More resistant than expected

“The discovery of these beautiful ghost fossils was completely unexpected,” says Slater. “We first found them on the surface of fossil pollen and quickly found that they were abundant at intervals where normal cocolithophore fossils were rare or absent – it was an absolute surprise!” For the three main events related to Jurassic warming and the Cretaceous period 94, 120 and 183 million years ago, scientists proved that numerous cocolithophores were also present in these phases. “Preserving these spiritual nanofossils is truly amazing,” says co-author Paul Bown of University College London. “Ghost fossils are remarkably 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 perfectly visible, although when pressed into the surfaces of ancient organic matter, the plates have dissolved themselves.”

The “ghost fossils” of cocolithophores prove their presence even in the interglacial periods. © SM Slater, P. Bown et al. / Science

According to the researchers, the fact that no fossils of cocolithophores from prehistoric warm phases have been found so far is due to the fact that the increased acidity of the surrounding water has dissolved the limestone plates, so that only their imprints remain. “Usually, paleontologists look for the fossil cocoliths themselves, and when they do not find them, they often assume that these ancient plankton communities have collapsed,” explains Slater’s colleague Vivi Vajda. “These ghost fossils show us that the fossil record sometimes plays tricks on us, and that there are other ways of preserving this calcareous nanoplankton that need to be considered when trying to understand past climate responses.”

Due to the new findings, scientists assume that calcareous nanoplankton also developed in the previous warm phases and was able to continue to form calcareous shells at least during the flowering phase despite acidic environmental conditions. “This shows that nanoplankton were more resistant to past events than traditional fossil evidence suggests,” the authors write. Therefore, it is likely that the current global warming is having less of an impact on plankton than previously feared. However, the authors emphasize that the forecasts are difficult given the pace of climate change today.

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

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