150-year-old Challenger expedition specimens reveal effect of climate change

Sediment samples collected on the Challenger Expedition © The Trustees of the Natural History Museum.

Samples collected 150 years ago during the HMS Challenger expedition are helping scientists measure how acidification from climate change is impacting the world’s oceans today.

Ocean acidification is a problem caused by the rapidly increasing concentration of carbon in the atmosphere due to human activity. As the concentration of carbon dioxide in the atmosphere continues to rise, more of the gas dissolves into the world’s oceans lowering the pH of the water and making it more acidic.

Now a team of scientists led by Dr Lyndsey Fox, a former post-doctoral researcher at the Natural History Museum and now a Lecturer in Climate Change at Kingston University, and including Dr Stephen Stukins, the Senior Curator of Micropalaeontology at the Natural History Museum, have been trying to assess how life is coping with these changes in ocean chemistry.

Dr Fox said of the study ‘Here we present a snapshot of how some of the smallest yet most important organisms in our ocean are beginning to struggle in response to recent changes in our climate and oceans.’

Using samples collected during the HMS Challenger expedition which took place between 1872 and 1876, researchers have been able to compare them with modern samples in order to track differences over a period of around 150 years.

These samples contain single-celled organisms known as foraminifera, a significant component of the plankton. Most of these small organisms build a shell from calcium carbonate. Laboratory experiments have shown that their ability to build these shells is dramatically hindered by increasingly acidic conditions.

Using nano-CT scans the team compared the thickness of the shells built by foraminifera from the HMS Challenger collection to those collected recently from similar localities at the same time of year.

Dr Stukins says, ‘We found that the HMS Challenger collections didn’t just contain samples of dredges from the ocean floor, but there were also plankton tow residues meaning that we can accurately date when these foraminifera were living.

‘We also have all of the papers and notebooks from the expedition, which are remarkable and give detailed accounts of exactly how they collected the samples, which was surprisingly thorough.’

This is significant because it means that the samples collected in the 1870s can then be directly compared with those collected in the 21st century. This gave the team an understanding of how the foraminifera have coped over time with an increasingly acidic environment.

Dr Stukins continues, ‘The data gathered from these samples show how the organisms are responding over multiple generations. It is not about the shells being dissolved, which is a common misconception, but that the foraminifera are struggling to build them.’

The result is something of a feedback loop. As the concentration of carbon in the atmosphere increases it causes the oceans to become more acidic, thinning the shells of the plankton and so resulting in less carbon being locked in the oceans and more remaining in the air, which makes the oceans more acidic.

Dr Fox said of the results, ‘We were surprised to see such dramatic reductions in shell thickness in some species of foraminifera, but much less in others. Our results show that there is still a lot of work to do, but the wealth of material housed at the Natural History Museum means that, in time, we can start building a picture of biological responses to climate change around the world.’

Dr Stukins added, ‘If the foraminifera are struggling, then that is going to cause a knock on effect to the larger creatures that consume the plankton and the predators that subsequently feed on them, this will inevitably lead to species extinctions.’

Due to the specimens that were collected by the HMS Challenger expedition, this study has provided the first direct evidence of how ocean acidification is impacting carbonate producing marine life over a long time period in modern oceans.

These tiny zooplankton have continued to highlight the importance of the Natural History Museum’s collections for future studies of anthropogenic climate change. The team now hope to study how this varies across the world’s oceans and the different species that live in them.

The paper is titled ‘Quantifying the Effect of Anthropogenic Climate Change on Calcifying Plankton and is available through Nature.

Natural History Museum, 31 January 2020. Article.

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