How is global warming and the associated acidification of the oceans affecting the plankton in the world’s oceans? So far, the researchers assumed that certain types of unicellular plankton that form calcareous shells would decline under these conditions – especially since no fossils of these species have been found from earlier warm phases of the Earth’s history. Now, however, a research team has discovered fossil traces of just those plankton species: Instead of the calcareous shells themselves, only their imprints on other fossils were preserved. The results suggest that the plankton can withstand global warming better than previously assumed.
As the CO2 content of the atmosphere increases, so does the proportion of dissolved carbon dioxide in the oceans. As a result, the seawater is becoming more and more acidic – a problem for organisms that build their shells out of lime, because this is broken down by acid. Calcifying organisms also include certain types of unicellular plankton. The so-called coccolithophores 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 conditions of acidic oceans.
discovery by chance
Now a new discovery challenges this view: a team led by Sam Slater from the Swedish Museum of Natural History in Stockholm has found imprints of the calcareous shells of coccolithophores on other fossils from prehistoric warm phases. Since these are not the calcareous shells themselves, but only their imprints, the researchers refer to them as “ghost fossils”. The finds indicate that, despite the actually unfavorable conditions, there were also numerous calcifying coccolithophores during earlier warm phases – they apparently at least coped better with the global warming of the time than previously assumed.
Slater and his team owe the discovery to a happy coincidence. They actually wanted to examine fossil rock samples for pollen and plankton species without calcareous shells. So they dissolved their samples in acid, leaving only the fossil remnants of the organic materials – a method considered unsuitable for searching for coccolithophore fossils because the acid breaks down lime. But on the surface of the pollen fossils prepared in this way, Slater’s team found the imprints of those same coccolithophores.
More resilient than expected
“The discovery of these beautiful ghost fossils was completely unexpected,” says Slater. “We first found them on the surface of fossilized pollen and it quickly became apparent that they were abundant at intervals where normal coccolithophore fossils were rare or absent – that was an absolute surprise!” For three major Jurassic warming events and the Cretaceous period, 94, 120 and 183 million years ago, the researchers proved in this way that numerous coccolithophores also occurred in these phases. “The preservation of these ghost nanfossils is truly remarkable,” says co-author Paul Bown of University College London. “The ghost fossils are extremely small – their length is about five-thousandths of a millimeter, that’s 15 times narrower than the width of a human hair – but the details of the original plates are still perfectly visible, albeit pressed into the surfaces of ancient organic matter the plates themselves have dissolved.”
According to the researchers, the fact that no coccolithophore fossils from the prehistoric warm phases have been found so far is due to the fact that the increased acidity of the surrounding water dissolved the limestone plates so that only their imprints remained. “Typically, paleontologists only look for the fossil coccoliths themselves, and when they don’t find any, 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 nannoplankton that need to be considered when trying to understand past climate change responses.”
In view of the new findings, the researchers assume that the calcareous nannoplankton also thrived during the past warm phases and, despite the acidic environmental conditions, could continue to form calcareous shells at least during their flowering phases. “This shows that nannoplankton have been more resilient to past events than the traditional fossil evidence suggests,” the authors write. In view of this, it is plausible that the current global warming is affecting the plankton less than feared. However, the authors emphasize that predictions are difficult given the speed of today’s climate changes.
Source: Sam Slater (Swedish Museum of Natural History, Stockholm) et al., Science, doi: 10.1126/science.abm7330