‘Ghost’ fossils reveal how oceans could be affected by climate change

Coccolithophores produce individual coccolith plates using calcium carbonate, taking carbon dioxide out of the atmosphere. Image © Nannotax

Plankton which help feed the ocean, lock away carbon dioxide and even influence the weather may not be as vulnerable to climate change as feared.

Despite their fossils having been dissolved away by acidic sediment waters, new research has found that the organisms themselves were thriving during the Jurassic, providing hope that they can still act as a carbon sink in modern global warming. 

Though measuring smaller than the width of a human hair, the ‘ghost’ fossils of Jurassic plankton can help us understand how their modern relatives will respond to an increasingly acidic ocean.

Coccolithophores are a group of phytoplankton which form microscopic scales made of calcite, a type of calcium carbonate, in a case around themselves. With rising carbon dioxide levels making seawater more acidic, there were concerns coccolithophores may be left unable to form their exoskeleton.

This was supported by evidence from past warming events, where plankton body fossils are scarce in the record.

Prof Richard Twitchett, a Research Leader at the Museum and co-author of the paper, says, ‘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.

The findings of the study, conducted by an international group of researchers, were published in the journal Science.

How does calcium carbonate influence climate change?

Aside from affecting the weather, they are also one of the largest producers of marine calcite in the world. This alkaline mineral is produced when calcium reacts with bicarbonate ions, which form after carbon dioxide dissolves into seawater.

Rising levels of carbon dioxide may initially contribute to blooms of these phytoplankton, which use the carbon dioxide in photosynthesis, but beyond certain levels it is thought that they cannot survive. To better understand the impacts, researchers often look to past examples of climate change documented in the fossil record, where fossil plankton are notably absent during other ocean acidification events.

How does ocean acidification affect plankton?

The researchers examined rocks dated to a period known as the Toarcian Oceanic Anoxic Event, which took place 183 million years ago in the Jurassic Period. This event was rapid on a geological scale, with volcanism in the southern hemisphere leading to increased levels of carbon dioxide and ocean acidification.

These samples, including those from the Museum’s collection, were analysed using a scanning electron microscope to reveal the minute impressions of nannofossils left behind by the ancient plankton. Digitally inverting the images creates a ‘virtual cast’ that allowed the scientists to work out which species left the mark.

Instead of showing an absence of coccolithophores, their findings demonstrated that these phytoplankton communities were diverse and abundant throughout this period, with similar events in the Cretaceous reflecting the same.

The study shows that the dead plankton were buried after death in soft sediment at the bottom of the sea. While acidic waters subsequently dissolved the fossils themselves, their imprints were preserved in the surfaces of other organic matter, such as pollen, in the sediment.

Eventually, the living plankton would have helped to bring the ocean acidification events to an end as carbon was locked up in the sediments, leading to the deposition of plankton body fossils once more.

James Ashworth, Natural History Museum, 19 May 2022. Press release.

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