Scientists call them “ghost fossils” because they are more remaining footprints than fossils as we imagine them. These are precisely fossils of coccolithophores, a species of phytoplankton that still live today at the bottom of the oceans. These microscopic algae protect their single cell under a layer of hard limestone slabs (called coccoliths); it is the latter that petrifies in the rock over time.
But specialists have noted that the abundance of these fossils tends to decrease after certain episodes of global warming that our planet has experienced. Coccolithophores were therefore considered to be particularly vulnerable to climate change. Because acidification of the oceans can damage the calcification process. A new study published in the journal Science shows that they are actually much more resilient.
Ghost fossils hidden in marine sediments
These single-celled organisms are the basis of marine food chains. They capture carbon in the sediments of the ocean floor. At the same time, they contribute greatly to the production of oxygen. When coccolithophores die, their limestone exoskeletons sink to the bottom of the ocean. They accumulate there to form limestone sedimentary rocks. The latter is composed of calcium carbonate (CaCO)3) – where the chalk is subtracted from.
Exoskeletons of modern (left) and Jurassic (right) coccolithophores. Credit: Nannotax
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How will these organisms respond to current global warming? The Earth has undergone several global warming events throughout its history. Researchers are therefore studying the fossil record for nanoplankton. However, they noticed a decrease in the abundance of calcium carbonate and nanofossils after these key periods. This finding led them to believe that ocean acidification and other related factors had caused “biocalcification crises”.
A study led by researchers from Sweden, Italy and the UK today presents new global data on the incidence of coccolithophores during three warming episodes, which took place in the Jura and then in the Cretaceous. These data show that despite these major climate changes, the species continued to thrive through the alleged biocalcification crisis.
As evidence is the discovery of a large amount of ghost fossils of coccolots in marine sediments. With a focus on the search for “normal” coccolit fossils, paleontologists had never thought of this other form of fossilization. Therefore, their interpretation of the fossil record was simply wrong!
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Traces “printed” in organic fabric
The ghost fossils were formed when sediments on the seabed turned to rock. As more mud settled on top, the pressure eventually crushed the coccoliths and other organic residues together. The coccoliths, relatively hard, were compressed on the soft organic matter (composed of pollen, spores and others). So, when the acidic water dissolved the coke’s limestone, only their footprints remained – what scientists call “ghost fossils”.
The ghost fossils (left) and their virtual castings (right). The fossils are about 5 langem long. Credit: S. Slater et al., Science (2022)
” We first found them preserved on the surface of fossil pollen, and it quickly turned out that they were abundant at intervals where normal coccolithophore fossils were rare or absent – that was a complete surprise! says Dr. Sam Slater, paleobiologist at the Swedish Museum of Natural History.
One of the other co-authors of the study, Professor Paul Bown, from University College London, evokes a “remarkable” conservation of these nanofossils. Even though they are extremely small – about five thousandths of a millimeter, or 15 times narrower than a human hair! – and the limestone slabs were dissolved, the structural details were still clearly visible.
The researchers have thus highlighted that the fossils of this calcareous phytoplankton could appear in a completely different form. And this, of course, needs to be taken into account in order to analyze the response of these organisms to past climate change.
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A species that has survived for millions of years
The study focused on three oceanic anoxic events. The first, which took place in Toarcian about 183 million years ago, corresponds to a period of global warming. It was probably caused by an increase in CO levels2 in the atmosphere after intense volcanic activity in the southern hemisphere. Several ghost nanofossils associated with this period have been found in several places around the world (UK, Germany, Japan and New Zealand).
The footprints related to two other oceanic anoxic events in the Cretaceous, which took place for approx. 120 and then 93 million years ago, also confirms that phytoplankton remained abundant during these events. ” These fossils rewrite our understanding of how calcareous nanoplankton respond to warming events concludes Professor Richard Twitchett of the Natural History Museum in London. Therefore, despite their microscopic size, these organisms are particularly resistant.
Not only has this species of phytoplankton survived previous warming, but it has contributed to the expansion of marine dead (or hypoxic) zones – areas where oxygen levels are too low for most species to survive. The comparison with the current situation is not so simple: the rapid climate change that we are experiencing actually exceeds the temporal resolution of most fossil records. But scientists believe that plankton blooms and dead zones may unfortunately become more prevalent in our garden in the coming years.