A rich 125,000-year-old coastal ecosystem discovered under Ariane 6 in French Guiana

An international consortium of paleontologists, geologists and biologists, coordinated by scientists from the Montpellier Institute of Evolutionary Sciences (University of Montpellier/CNRS/IRD) and the University of Guyana/Géosciences Montpellier, describes the discovery of fossil assemblages spanning the last 130,000 years, beneath the Ariane 6 rocket launch site in Kourou, French Guiana. These coastal assemblages, rich in over 270 species of animals, plants and microorganisms, are the first to have been unearthed in the equatorial Atlantic. They tell us about the environmental and climatic conditions that prevailed in the region and its populations, both during a very warm period (the last interglacial, from -128,000 to -116,000 years ago) and the cold period that followed (the last glacial stage, from -100,000 to -15,000 years ago).

This discovery, published in the Proceedings of the National Academy of Sciences of the USA, provides valuable information on unanthropized equatorial Atlantic tropical coastal ecosystems and the potential long-term impact of climate change on endangered species.

Fossil oyster dating from around 125,000 years ago, discovered near the Ariane 6 launch pad in Kourou, French Guiana, during the 2019 field campaign. (Credit: P.-O. Antoine)

French Guiana: a favourable research environment

Tens of thousands of fossils were unearthed on the Guiana Space Center (CSG) site between 2017 and 2021, thanks to an authorization from CNES, the owner of the site. By pooling their expertise, paleontologists, geologists and biologists have for the first time been able to accurately reconstruct Guiana's past ecosystems. According to the authors, the results provide a better understanding of the ecology and biogeography of equatorial Atlantic tropical coastal ecosystems, prior to any human influence, as well as the potential long-term impact of ongoing climate change on current and endangered species. Indeed, climate oscillations over the last 2.5 million years have led to radical changes in the distribution of organisms across the globe. In particular, the last interglacial interval (128,000-116,000 years ago) was characterized by higher sea levels and warmer temperatures than today, making this period a plausible analogue of the global conditions predicted for 2100. However, nothing was known about the equatorial coastal fauna and flora of this period around the Atlantic, until the discovery of fossiliferous clays during work on the Ariane 6 launch site.

More than surprising discoveries

Firstly, scientists were able to characterize a coastal fossil assemblage dating from around 130,000 to 115,000 years ago. According to Pierre-Olivier Antoine (Université de Montpellier) and Arnauld Heuret (Université de Guyane), who led the study, "the reconstructed marine and terrestrial fossil communities included over 270 species, including foraminifera, molluscs, bony fish, bryozoans, decapods, sharks and numerous plants. All the fossil specimens identified at Kourou belong to present-day species, some of which are now on the brink of extinction". The results document a short interval of high sea level during the last interglacial, when the area was home to a mangrove ecosystem and shallow water habitats, including highly diverse mollusc assemblages. According to Austin Hendy (Natural History Museum, Los Angeles), "this diversity shows that there were very strong affinities between the coastal waters of the Caribbean and the Guianas at the time, probably with less influence from megacurrents of Amazonian origin." After the retreat of the ocean around 110,000 years ago, linked to the global cooling of the last glaciation, "plant fossils discovered on site (pollen, phytoliths and charcoal) suggest the existence of savannahs and drier local conditions, with episodes of natural fires, until around 50,000 years ago", details Carina Hoorn (University of Amsterdam). Finally, Pierre-Olivier Antoine hopes "that this work will contribute to a better understanding of the major environmental upheavals that the region could experience between now and 2100."

This work has been funded by the French National Research Agency (ANR) as part of the LabEx CEBA (ANR-10-LABX-25-01), through the Source, NeotroPhyl, Timespan and Emergence projects, the GAARAnti project (ANR-17-CE31-0009) and a future investment project (ANR-18-MPGA-0006). The team has also received funding from the Brazilian Council for Science and Technological Development (CNPq 304693/2021-9), the European Research Council (ERC 2019 StG 853394), and the Australian Research Council (ARC DP220100195 and LE200100022). None of the discoveries would have been possible without the authorizations issued by the Centre national d'études spatiales (CNES) and the logistical support provided by Eiffage and Sodexo.