'Ghost' fossils reveal how oceans could be affected by climate change

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.

However, a group of researchers has now shown that the plankton were not only surviving at the time, but thriving. The scientists found the lack of body fossils was a result of them being dissolved after death, leaving imprints in sediment where the remains used to be.

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.

'These fossils are rewriting our understanding of how the calcareous nannoplankton respond to warming events.'

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?

Coccolithophores live across all the oceans of the world but are primarily found in the north and south of the Atlantic and Pacific. While they measure less than the width of a human hair, they collectively exert powerful impacts, such as producing much of the oxygen we breathe, as well as dimethyl sulphide which helps form clouds above the ocean.

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.

When the plankton die, lose or replace their scales, the calcite sinks to the bottom of the ocean where it is locked away in deep sea sediment.

Over time, this means that carbon dioxide is taken out of the atmosphere and into the deep ocean, mitigating the impacts of global warming.

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.

However, the new research reveals that understanding these past events is not always straightforward, and the fossil record may sometimes present a false impression of just what was happening at the time.

Co-author Prof Vivi Vajda says, '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.

'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.'

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.

Not all species will be affected equally, with previous research suggesting that some coccolithophore species do worse than others, which is likely to be the same with modern climate change.

While these findings provide some reassurance that plankton are likely to persist in some form as the world warms, further research is needed to assess how these organisms were affected by similar events in the past.

Prof Silvia Danise, another co-author, says, 'These 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 fossilization will be useful in the future, particularly when studying geological intervals where the original coccoliths are missing from the fossil record.'