An international team of scientists from the Natural History Museum, UCL (University College London), the University of Florence and the Swedish Museum of Natural History have found a remarkable type of fossilisation that has remained almost entirely overlooked until now. The fossils are microscopic imprints, or ‘ghosts’, of single-celled plankton, called coccolithophores, that lived in the seas millions of years ago, and their discovery is changing our understanding of how plankton in the oceans are affected by climate change.
Coccolithophores are important in today’s oceans because they provide a large portion of the oxygen we breathe, support marine food webs, and store carbon in seafloor sediments. They are a type of nannoplankton with intricate plates made of calcium carbonate that surround their cells and are what normally fossilize in rocks.
Declines in the abundance of normal coccolithophore fossils have been documented during multiple past global warming events, suggesting that these plankton were severely affected by climate change and ocean acidification. However, this new study presents records of abundant ‘ghost’ fossils from these warming intervals, suggesting that coccolithophores were more resilient to past climate change than was previously thought.
The researchers discovered the ‘ghost’ fossils, using powerful microscopes, in rocks from three past warming events that took place during the Jurassic and Cretaceous periods.
The international team of researchers included Dr Sam Slater and Professor Vivi Vajda (Swedish Museum of Natural History), Professor Paul Bown (UCL), Professor Silvia Danise (University of Florence) and Professor Richard Twitchett (Natural History Museum, London).
‘The discovery of these beautiful ‘ghost’ fossils was completely unexpected’, says Dr Sam Slater. ‘We initially found them preserved on the surfaces of fossilised pollen, and it quickly became apparent that they were extremely abundant during intervals where normal coccolithophore fossils were rare or absent. This was a total surprise!’
Despite their microscopic size, when coccolithophores multiply rapidly, they can become so abundant that they form blooms in the ocean that be seen from space. After the plankton die, their exoskeletons sink to the seafloor, accumulating in vast numbers, forming rocks such as chalk.
‘The preservation of these ‘ghost’ nannofossils is truly remarkable,’ says Professor Paul Bown. ‘The ‘ghost’ fossils are extremely small ‒ their length is approximately five thousandths of a millimetre, 15 times smaller than the width of a human hair! ‒ but the detail of the original plates is still perfectly visible, even though the plates themselves have dissolved away, so we can easily tell which species were present.’
The ghost fossils formed while the sediments at the seafloor were being buried and turned into rock. As more mud was gradually deposited on top, the resulting pressure squashed the coccolith plates and other organic remains together, and the hard coccoliths were pressed into the surfaces of pollen, spores and other soft organic matter in the sediments. Later, acidic waters inside the pore spaces of the rock dissolved away the actual coccoliths, leaving behind just their impressions – the ‘ghosts’.
‘Normally, palaeontologists only search for the fossil coccoliths themselves, and if they don’t find any then they often assume that these ancient plankton communities collapsed,’ explains Professor Vivi Vajda. ‘These ‘ghost’ fossils show us that sometimes the fossil record plays tricks on us and there are other ways that these calcareous nannoplankton may be preserved, which need to be taken into account when trying to understand responses to past climate change.’
Professor Silvia Danise says, ‘‘Ghost’ nannofossils are likely common in the fossil record, but they have been overlooked due to their tiny size and cryptic mode of preservation. We think that this peculiar type of fossilisation will be useful in the future, particularly when studying geological intervals where the original coccoliths are missing from the fossil record.’
The study focused on the Toarcian Oceanic Anoxic Event (T-OAE), an interval of rapid global warming in the Early Jurassic (~183 million years ago), caused by an increase in CO2-levels in the atmosphere from massive volcanism in the Southern Hemisphere. The researchers found ‘ghost’ nannofossils associated with the T-OAE from the UK, Germany, Japan and New Zealand, but also from two similar global warming events in the Cretaceous: Oceanic Anoxic Event 1a (~120 million years ago) of Sweden, and Oceanic Anoxic Event 2 (~94 million years ago) of Italy.
‘The ‘ghost’ fossils show that nannoplankton were abundant, diverse and thriving during past warming events in the Jurassic and Cretaceous, where previous records have assumed that plankton collapsed due to ocean acidification,’ explains Professor Richard Twitchett. ‘These fossils are rewriting our understanding of how the calcareous nannoplankton respond to warming events’.
Photos and article source: Natural History Museum