The expeditions on Tara organized by the Tara Ocean Foundation these last ten years have collectively helped to generate a view of the state of our ocean at the beginning of the 21 st century.
Scientists working on board Tara have made numerous discoveries about plankton ecosystems in the global ocean, about the state of coral reefs throughout the Pacific Ocean, and about the alarming levels of microplastics everywhere. A question that remains is how much the ocean has already changed as a result of human activities because Tara was the first to analyse life in the ocean at global scale, generating a time zero from which we can compare changes in the future. But only with difficulty can we delve into the past.
The HMS Challenger expedition, a work that still is a reference a century and a half later
A new study by researchers at the Natural History Museum in London has addressed this issue by comparing samples collected during the Tara Oceans expedition with samples collected by the HMS Challenger expedition, before the industrial age went into its exponential phase. This expedition, over 5 years in the 1870s, is considered to be foundational for modern oceanographic studies and was aimed to test the idea current at the time that the dark ocean beneath where light can pass is azoic – devoid of life. They debunked the theory, and brought to light an astonishing variety of life forms whose representation by Ernst Haeckel in his Art Forms in Nature inspired the movement of Art Nouveau and organic design.
The current study aimed to examine whether ocean acidification is already impacting ocean life by focusing on a group of calcifying organisms called foraminifers, used as sentinels of climate change in many paleo-oceanographic studies. The authors compared the calcified shells of these organisms in samples collected by the Challenger with samples collected in the same location by Tara in the Central Pacific Ocean at approximately the same time more than 140 years later. Using advanced imaging techniques, they revealed dramatic changes in the thickness of the shells of these microscopic organisms, providing some of the first real evidence that human activities have already affected life in the ocean. Because most corals also build calcified structures, the results further suggest that they are also being impacted by rising ocean acidification.
Figure 1 Nano-X-ray computed tomography reconstructions of Neogloboquadrina dutertrei foraminifers from Tara (a–c), and Challenger (d–f); 2a and (d) Colour map illustrating shell thickness, warm colours representing areas of relatively thicker test wall; 2b and (e) Cross sections through rendered foraminifera tests; (c) and (f) slices through foraminifera tests. © Scientific Reports / Nature
The Tara sample collection as a legacy
While we do not know the precise depth at which the Challenger samples were collected, and we do not know whether longer multi-annual processes such as El Nino-La Nina cycles may confound the comparison, the study shows the value of historical expeditions and the importance of archiving samples as future techniques make them accessible in ways that the original scientists could never have imagined. Just like the Challenger samples being used today, who knows what we will be able to discover about our changing ocean in a hundred years time thanks to the legacy of the Tara sample collection.
From Chris Bowler, CNRS research director of the École Normale Supérieure and scientific coordinator of the Tara Oceans expedition
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