Today, Barro Colorado Island (BCI) is the biggest island in the Panama Canal waterway, a mountaintop isolated during the final stages of the Canal construction in 1914, when the Chagres River was dammed to create what was the largest artificial lake of its time, Lake Gatun.

But 22 million years ago, during the Early Miocene epoch, Barro Colorado was a mangrove forest, which was covered by a volcanic lahar and rediscovered recently by researchers at Smithsonian Tropical Research Institute (STRI).

This discovery is detailed in the scientific paper “An early Miocene (Aquitanian) mangrove fossil forest buried by a volcanic lahar at Barro Colorado Island, Panama”, published online in the journal Palaeogeography, Palaeoclimatology, Palaeoecology in December 2023. Samples of fossilized mangrove were found during a 2018 geological exploration of the island, suggesting that there was a mangrove forest growing along the coast of the volcanic chain of central Panama.

“It is difficult to know the extension of the mangrove during the Miocene, or of the lahar, although the volcanic source probably came from the Las Cascadas volcanic complex,” explains STRI research associate and EAFIT University professor, Camila Martínez, the lead author of the paper.

At the time, Panama was part of a long and narrow peninsula which became connected to North America as the geological activity in the region increased, and which was separated from South America by the Central American seaway.

The marine and coastal settings allowed for this mangrove forest to extend around the region, until an intense volcanic eruption triggered a lahar—a flow of water, mud, ash and rocks—that buried the forest, trapping it without oxygen needed to decompose the wood, and thus preserving it intact. Researchers believe that this lahar was a one-time event, one single layer of volcanic material that effectively covered the entire forest.

Using maps generated by LiDAR (Light Detection and Ranging) overflights, the 121 fossil samples discovered, were located and analyzed by the authors. The analysis determined the presence of one single species of mangrove, which they named as a new species Sonneratioxylon barrocoloradoensis Pérez-Lara sp. nov.

This species has only one distant living relative, Sonneratia, found in Southeast Asia. Since there are few plant species in mangrove forests that exist both in Southeast Asia and the Neotropics, this suggests that this mangrove species might have had a been more widely distributed around the tropics in Prehistoric eras before it became extinct in the Americas.

“Although there are many fossilized woods described from Panama, this is the first record for this genus. This might be because mangroves occupy very reduced and specific spaces and conditions,” Martínez explained. “It’s likelier to find fossil records from other types of forests, like the rainforest, for which we do have evidence from the Miocene in Panama. However, there is documented evidence (pollen) of the presence of mangroves.”

Based on the analysis of the wood samples, the BCI mangrove forest was taller than present day mangroves (including the modern Southeast Asian Sonneratia), reaching an average height of 25 meters (82 feet), with some specimens growing as tall as 40 meters (131 feet). Because of the high atmospheric concentration of carbon dioxide at the time, the mangrove forest wood had a concentration of up to 500 ppmv (parts per million by volume), compared to current mangroves concentration average of 400 ppmv.

“This discovery helps us understand the type of vegetation that existed in areas that had just emerged from the ocean,” Martínez adds. These findings also highlight the dynamic history and adaptability of mangrove forests, given that the survival of dominant mangrove species depends on their ability to adapt to saltwater conditions.

“The BCI Natural Monument, as well as its surroundings, are full of exceptionally well-preserved wood fossils. Every time we talk about this project, more new researchers and park rangers tell us about new records in the area. There is especially one location on the Gigante peninsula, which we are currently investigating and that is also going to provide a lot of clues about the history of BCI’s vegetation,” Martínez adds. “Studying the fossil wood in all the Monument is one of our priorities, however, we have to continue finding methods to allow us to overcome the big challenge of having only small outcrops to interpret the age and geological context of these samples.”

The discoveries of this study will appear in the March 2024 issue of Palaeogeography, Palaeoclimatology, Palaeoecology.

The Smithsonian Tropical Research Institute, headquartered in Panama City, Panama, is a unit of the Smithsonian Institution. The institute furthers the understanding of tropical biodiversity and its importance to human welfare, trains students to conduct research in the tropics and promotes conservation by increasing public awareness of the beauty and importance of tropical ecosystems.

Reference: Martínez, C., Pérez-Lara, D. K., Avellaneda-Jiménez, D. S., Caballero-Rodríguez, D., Rodríguez-Reyes, O., Crowley, J. L., Jaramillo, C. (2023) An early Miocene (Aquitanian) mangrove fossil forest buried by a volcanic lahar at Barro Colorado Island, Panama. Palaeogeography, Palaeoclimatology, Palaeoecology. doi: 10.1016/j.palaeo.2023.112006