Samples From HMS Challenger’s 1870s Expedition Reveal How Carbon Emissions Are Changing The Ocean

Samples from one of history's greatest scientific expeditions are being used to see how plankton are responding to human-induced changes in ocean chemistry. They're revealing a complex picture, with some species barely affected while relatives struggle, but the work reinforces the importance of both emissions reductions and basic science.

Today the 1872-1876 expedition of the HMS Challenger is remembered primarily for giving its name to the deepest part of the ocean. In its day, however, it transformed humanity's knowledge of the ocean, surveying large areas of the seafloor and discovering more than 4,000 previously unrecorded animal and plant species.

Almost 150 years later the expedition is still proving a boon for science, with samples of marine plankton collected at 360 sites around the world on the voyage available for comparison to those from the same locations today.

Carbon dioxide forms an acid when dissolved in water. With the oceans currently absorbing a third of the carbon emitted from burning fossil fuels and felling forests, marine chemistry is changing. The process is known as ocean acidification, although it is in fact making slightly alkaline waters more neutral.

Many corals and plankton depend on alkaline environments to form calcium carbonate skeletons and shells. The situation is nowhere near bad enough to make these dissolve, as some people imagine, but laboratory tests show these life forms struggle as carbon dioxide concentrations rise. Changes in the wild are harder to measure without a baseline more than a few decades old.

Fortunately, however, the Challenger Expedition not only collected seawater samples, but had them carefully labeled by place and time and archived in London's Natural History Museum.

Shell-forming plankton known as foraminifera are among scientists' most valuable tools for assessing past marine conditions, with the mix of species changing to match the environment. However, timing the deposition of foraminifera samples on the sea floor, is somewhat imprecise, affecting any conclusions.

"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,” the Museum's Dr Stephen Stukins said in a statement.

Scientific Reports has published a comparison made between foraminifera from a Challenger sample and those collected at the same site and time of year today. Stukins told IFLScience the paper is a proof-of-concept with the team applying for funding to make the comparison global.

“We were surprised to see such dramatic reductions in shell thickness in some species of foraminifera, but much less in others,” colleague Dr Lyndsey Fox added.

If some species are able to cope with the new environment it may mitigate a feared feedback loop, where foraminifera become unable to remove carbon from their environment, accelerating both warming and acidification. On the other hand, a rapid shift in species dominance could have large and unpredictable effects on ocean food-webs.