Scientists discover "ghost" fossils - "completely unexpected"

SciTech Daily
Fri, 20 May 2022 16:38 UTC

© Scientists Discover "Ghost" Fossils - "Completely Unexpected"

The images show the impressions of a collapsed cell-wall covering (a coccosphere) on the surface of a fragment of ancient organic matter (left) with the individual plates (coccoliths) enlarged to show the exquisite preservation of sub-micron-scale structures (right). The blue image is inverted to give a virtual fossil cast, i.e., to show the original three-dimensional form. The original plates have been removed from the sediment by dissolution, leaving behind only the ghost imprints.

Discovery of 'ghost' fossils reveals plankton resilience to past global warming events.

An international team of researchers has discovered a new type of fossilization.

An international team of scientists from University College London (UCL), the Swedish Museum of Natural History, Natural History Museum (London), and the University of Florence has found a remarkable type of fossilization that has remained almost entirely unnoticed 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 revolutionizing our understanding of how climate change affects plankton in the oceans.

Coccolithophores are important in today's oceans, providing much of the oxygen we breathe, supporting marine food webs, and locking carbon away in seafloor sediments. They are a type of microscopic plankton that surround their cells with hard calcareous plates, called coccoliths, and these are what normally fossilize in rocks.

© S.M. Slater et al.

Ghost nano-fossil from the Jurassic rocks of Yorkshire, UK.

Declines in the abundance of these fossils have been documented from multiple past global warming events, suggesting that these plankton were severely affected by climate change and ocean acidification. However, a study published today in the journal Science presents new global records of abundant ghost fossils from three JurassicThe Jurassic (from the Jura Mountains) is a geologic period and system that spanned 56 million years from the end of the Triassic Period 201.3 million years ago to the beginning of the Cretaceous Period 145 million years ago. It constitutes the middle period of the Mesozoic Era and is divided into three epochs: Early, Middle, and Late.Jurassic and Cretaceous. The Cretaceous is a geological period that lasted from about 145 to 66 million years ago. It is the third and final period of the Mesozoic Era. It ended with the Cretaceous-Paleogene extinction event. Cretaceous warming events (94, 120, and 183 million years ago), suggesting that coccolithophores were more resilient to past climate change than was previously thought.

© Nannotax

The individual plates are coccoliths.

"The discovery of these beautiful ghost fossils was completely unexpected," says Dr. Sam Slater from the Swedish Museum of Natural History. "We initially found them preserved on the surfaces of fossilized pollen, and it quickly became apparent that they were abundant during intervals where normal coccolithophore fossils were rare or absent - this was a total surprise!"

Despite their microscopic size, coccolithophores can be hugely abundant in the present ocean, being visible from space as cloud-like blooms. After death, their calcareous exoskeletons sink to the seafloor, accumulating in vast numbers, and forming rocks such as chalk.

© S.M. Slater, P. Bown, et al. / Science journal

The fossils are approximately 5 µm in length, 15 times narrower than the width of a human hair.

"The preservation of these ghost nanofossils is truly remarkable," says Professor Paul Bown (UCL). "The ghost fossils are extremely small ‒ their length is approximately five-thousandths of a millimeter, 15 times narrower than the width of a human hair! ‒ but the detail of the original plates is still perfectly visible, pressed into the surfaces of ancient organic matter, even though the plates themselves have dissolved away."

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. Later, acidic waters within spaces in the rock dissolved away the coccoliths, leaving behind just their impressions - the ghosts.

© S.M. Slater et al.

Ghost nannofossils were found globally, in rocks from three rapid warming events in Earth's history (the T-OAE, OAE1a, and OAE2).

"Normally, paleontologists 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 (Swedish Museum of Natural History). "These ghost fossils show us that sometimes the fossil record plays tricks on us and there are other ways that these calcareous nanoplankton may be preserved, which need to be taken into account when trying to understand responses to past climate change."

Professor Silvia Danise (University of Florence) says: "Ghost nanofossils 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 fossilization will be useful in the future, particularly when studying geological intervals where the original coccoliths are missing from the fossil record."

© S.M. Slater et al.

Ghost nannofossils were found globally, in rocks from three rapid warming events in Earth’s history (the T-OAE, OAE1a, and OAE2).

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) from Sweden, and Oceanic Anoxic Event 2 (94 million years ago) from Italy.

"The ghost fossils show that nanoplankton 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 (Natural History Museum, London). "These fossils are rewriting our understanding of how the calcareous nanoplankton respond to warming events."

Finally, Dr. Sam Slater explains: "Our study shows that algal plankton were abundant during these past warming events and contributed to the expansion of marine dead zones, where seafloor oxygen-levels were too low for most species to survive. These conditions, with plankton blooms and dead zones, may become more widespread across our globally warming oceans."

Reference: "Global record of "ghost" nannofossils reveals plankton resilience to high CO₂ and warming" by Sam M. Slater, Paul Bown, Richard J. Twitchett, Silvia Danise and Vivi Vajda, 19 May 2022, Science. DOI: 10.1126/science.abm7330 Text