Museum collections used to show our oceans are more acidic than 140 years ago| Curator of Micropaleontology

This pre-lockdown publication from the Micropaleontology team at the Museum has received a lot of press and social media attention. CT scans of the calcareous shells of microscopic plankton called Foraminifera have shown that modern examples can be considerably thinner than their equivalents recovered by the ground breaking Challenger Expedition of the 1870s. We argue this thinning is due to increased levels of carbon dioxide in the atmosphere and subsequently more acidic oceans.

Read on to find out about the methods used and why this discovery is so significant for the future of our oceans and planet.

CT scans Foraminifera
CT scans of microscopic planktonic Foraminifera showing differences in wall thickness; historical specimens are on the bottom row and the warm colours indicate considerably thicker shells.

Planktonic Foraminifera

Tiny microscopic single celled organisms called Foraminifera are floating in the ocean as we speak. They are ideal for studying the past state of the ocean as they are very abundant and use calcium carbonate in the ocean water to create their shells. They also have a well established taxonomy so there are relatively few arguments about how to identify them. Their isotopic composition can be used to tell us about past climate conditions. This video features Dr Lyndsey Fox, now of Kingston University who was principal author on the new publication.

Historic Challenger vs modern Tara

The HMS Challenger Expedition is arguably the origin of the modern subject of Oceanography as it sailed the globe from 1872-1876 studying the state of the ocean and seafloor. The Challenger sediment collections are now housed at the Natural History Museum as part of the Ocean Botton Deposits Collection and includes plankton tows as well as bottom dredges. These tows are vital as they provide accurately dated Foraminifera from the oceans compared to the bottom sediments that possibly represent hundreds or thousands of years of accumulation of dead Foraminifera that found their way to the ocean floor.

Challenger map
Map showing the collecting locations of HMS Challenger from 1872-1876 and below, the location of the Challenger and Tara material studied .

To put the Challenger collections in a modern context, a plankton collection was obtained from the Tara Expeditions vessel from a similar area in the western equatorial Pacific that was collected at the same time of the year so that seasonal variation could be discounted as a reason for any differences observed. This provided 2011 collected specimens of planktonic Foraminifera to compare with the 1875 collection from the Challenger.

CT scanning

Various analytical methods such as scanning electron microscopy were used to assess the surface and internal condition of the Foraminifera to check that they had not undergone dissolution or recrystallisation at any stage. Specimens were then CT scanned to allow thicknesses of shells to be calculated for the final three chambers of the planktonic Foraminifera in the samples. The coloured renditions produced give an indication of the relative thicknesses of the shells and direct measurement of shell thickness was possible by virtually manipulating the CT scans.

Shell thickness variations
CT scans of a single specimens of the planktonic Foraminifera Neogloboquadrina dutertrei showing a large difference in shell thickness between the historical Challenger specimen (below) and the recently collected Tara specimen (above).

Thinner shells

Results showed that without exception all modern examples had thinner walls than their historical counterparts with some species showing up to 76% thinner shells. It was interesting to see that there was variability between the two species studied with some showing less dramatic reductions in size.

If you have a 3D printer, you can now download our scans from the NHM Data Portal and print your own hand sized copies of planktonic Foraminifera (see below).

Significance to our Oceans

The main conclusion of the paper is that these differences are due to anthropogenic increases in the concentration of carbon in the atmosphere which causes the oceans to become more acidic. This does not cause the Foraminifera to dissolve but it does make it harder for them to form their shells. This has potentially catastrophic consequences for the carbonate plankton in the ocean which could, if this trend continues, become extinct. As well as being a major food source for larger oceanic creatures, it also causes less carbon to become locked in the oceans, more in the air and the ongoing cycle results in the oceans becoming more acidic.

Altimetrics score for Fox et al 2020
The Altimetrics score for our paper as of 21st July 2020. These scores are calculated based on the amount of on-line attention an article receives.

Impact of research

Articles in The Telegraph, iNews, IFL Science and Science Magazine have underlined the significance of this initial research and that our work here can help advocate for our planet. Lyndsey has also featured on Radio 4 talking about this research.

The images from the publication have also inspired some beautiful art. These prints were sent to us by Marcus Badger.

Prints of cross sections of Foraminifera
Prints made by Marcus Badger inspired by the figures in our recent publication.

Next Steps

“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,” said Lynsey Fox, lead author on the paper.

However, the work is time consuming and costly and has only so far been possible due to an internal investment grant. The team are continuing to seek external funding to continue this important research that shows direct evidence, using our collections, for the consequences of us filling the atmosphere with additional carbon dioxide.

Post by Giles Miller, Steve Stukins, Tom Hill and Lyndsey Fox