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Foto: Daniel Guerra

What deep layers in the Earth tell us about a changing climate

Jip Koene,
4 april 2024 - 15:53

The biodiverse mangrove forests on the northern coast of South America changed into savannahs in a short period of time due to the changing climate during the last Ice Age. This was discovered by UvA researchers using fossil remains. “Mangrove forests play a key role in protecting coastal areas in low-lying tropical regions.”

A recent fossil discovery in French Guiana, South America, shows a remarkable ecological transformation around the start of the last Ice Age, some 116,000 years ago. The species-rich coastal marine ecosystem surrounded by mangrove forests gave way to a vast savannah.
Like today, mangrove forests stretched across the northeastern coasts of South America between the Amazon rainforest and the ocean thousands of years ago. These forests grow in the tidal zones of the coastal landscape and consist of trees whose prominent roots curve above the seawater. Besides being home to rich biodiversity, including various bird and fish species, mangrove forests are important coastal protectors against storm surges or floods, for example.
With the dawn of the last Ice Age some 116,000 years ago, the climate changed and sea levels dropped, causing the mangrove forest of French Guiana to shift toward the sea and give way to savannah over thousands of years. An international research team that included the UvA’s Institute for Biodiversity and Ecosystem Dynamics (IBED) reconstructed the landscape just before and during the last Ice Age. The researchers published the results in the scientific journal PNAS.

“It remains to be seen whether the mangrove forests can cope with the speed at which the sea water is rising”

Fossil discoveries
The reconstruction was done using fossil material taken from the European Space Agency (ESA) site in French Guiana, which was discovered during the construction of a rocket launch platform. The scientists found the remains and fossils of 270 species of plants, shellfish, mollusks, crustaceans, and sharks, among others. These fossils offer insights into the coastal ecosystems of thousands of years ago and the vegetation that grew nearby. UvA researcher Carina Hoorn and PhD student Nina Witteveen examined fossil pollen and phytoliths, glassy silica particles produced mainly by grasses.
“For the study, we examined soil samples from the excavation,” Witteveen explains. “The fossils from strata from the period just before and during the Ice Age differed in composition. For example, the fossil pollen from mangrove trees gave way to pollen from grasses, indicating the transition from mangrove forest to savannah.”
Climate change
That change did not come out of nowhere, Witteveen says. “When the mangrove forest was present, some 125,000 years ago, the sea level was four to six meters higher and on average two degrees warmer than today. This is comparable to the worst climate scenarios for the end of the 21st century. In the following period, with the onset of the Ice Age, it became significantly drier and sea levels dropped, allowing savannah to develop. The mangrove forest remained but shifted with the sea level. After the Ice Age, sea levels rose again to current levels, and only a small savannah area remained between the Amazon rainforest and the coastline. In that sense, mangrove forests can adapt to a changing climate.”
But whether mangrove forests can cope with the upcoming sea level rise remains to be seen. “Mangrove forests have a key role in protecting coastal areas such as in French Guiana, but also in neighboring Suriname, for example. Due to urbanization, overfishing, and other industrial activities, these natural coastal protections are losing their resilience and are in danger of disappearing. It also remains to be seen whether the mangrove forests can cope with the speed at which the sea water is rising. Either way, the rich biodiversity and local populations are at great risk.”