Nature Today | Historical climate extremes along the northeast coast of South America

Just like today, before the last ice age, about 125 thousand years ago, the coastal waters of North America were home to mangrove forests that supported a rich diversity of fish and other marine animals. Tens of thousands of years later, during an ice age, the sea level receded and the coastline moved seaward. Dry savannahs extended there later, as evidenced by fossil finds made at Kouro, French Guiana. Little is known about the coastal fauna and flora of the equatorial Atlantic Ocean from this period until now.

Two climatic extremes

It is Research It is a collaboration between paleontologists, biologists and geologists from universities in 13 different countries, including the University of Amsterdam (UvA). They made their findings during a mission to the European Space Agency's launch pad, where the new Ariane 6 rocket will launch later this year. Scientists found fossils of 270 species of plants, animals and microbes. The remains include bivalve molluscs, bony fishes, crustaceans, sharks, foraminifera (calcareous plankton), a marine group called bryozoans, and large amounts of plants.

Paleoecologist and geologist Karina Hoorn from the UvA's Institute for Biodiversity and Ecosystem Dynamics (IBED) and her team reconstructed the flora of both periods. “In this short period of time, we see evidence for two climate extremes in this geographic segment,” says Hoorn. “Sediments and fossils dating back 125,000 years are unique to the equatorial coast and are among the best examples of northern South America. In addition, there are deposits from the last Ice Age, approximately 115,000 years ago.

Treaty with the current Caribbean

The contribution of the Amsterdam researchers was to analyze the fossil pollen. Horne: “In the warmer period we find a magnificent flora. And there appear to be many similarities between the fossils and the fauna of the present-day Caribbean, northwest of South America. The coasts of French Guiana, Surinam, and Guiana lie between the mouths of the Orinoco and Amazon rivers. Now that connection with the Caribbean is this great island.” Largely capped by the sediment supply of rivers, very little sediment reached the ocean before the last ice age.

Saddles and crosses

During the next period, after the sea level fell and the mangrove system moved seaward, a dry savanna landscape appeared. Horne: “There is evidence of arid conditions between 110 and 50 thousand years ago. Also liked our colleague–The authors note that charcoal indicates the presence of occasional natural fires.

By analyzing phytoliths, the glassy silica particles that strengthen the leaves of grasses, the researchers were able to get a more detailed picture of plant species from the Ice Age. PhD candidate Nina Wittwein: “Phytoliths are particles that allow you to cut into grass leaves. They have their own characteristic shape: under the microscope, phytoliths of grasses look like small saddles and crosses, and palms look like circles with spines. This way we can use phytoliths to detect what has grown.”

Phytoliths also provide a good picture of local flora, says Wittwein. “Unlike pollen grains, phytoliths are not dispersed by wind. After a plant dies, phytoliths end up in the soil where the plant was.

A tip of the veil for Suriname

Between 128,000 and 116,000 years ago, between the last two ice ages, global sea levels were about 4 to 6 meters higher than today and average temperatures were 2 degrees higher than today. Conditions are also predicted for the start of the 22nd^e century So along the coasts of French Guiana and neighboring Suriname there is a comparable risk of sea level rise, causing flooding.

According to Hoor, the results of the research could actually be extended to plants in Suriname during the same periods studied in French Guiana. “Fossil bones of large animals were once found in French Guiana. That is probably only one end of the veil. The work in French Guiana will be a great motivation for scientists in Suriname to further explore this fascinating part of geological history.

Text: IBED
Photo: Daniel Guerrera; Matteo Sciambatta