An ancient mangrove forest in the heart of Mexico’s Yucatán Peninsula reveals ancient sea levels and serves as a warning about the “dramatic impact” climate change could have on the coastal plains of the Gulf of Mexico, according to researchers from Mexico and the United States.

Located 170 kilometres from the ocean on the banks of the San Pedro Martir River in the municipality of Balancán, Tabasco, the ecosystem is unusual because mangroves – which include salt-tolerant trees, shrubs, and palms – are typically found along tropical and subtropical coastlines.

Unlike mangroves elsewhere, these trees grow in freshwater. This means that many other species can grow with them: orchids, bromeliads and other air and land plants that cannot tolerate the saline conditions where red mangroves are normally found.

Because the red mangrove (Rhizophora mangle) and other species present in this unique ecosystem are only known to grow in salt water or somewhat salty water, a research team from Mexico and the University of California set out to discover how the coastal mangroves were established so deep inland in fresh water completely isolated from the ocean.

In a recent study released in the Proceedings of the National Academy of Sciences, researchers found that the mangroves of the river have been separated from coastal mangroves for around 120,000 years.

The study provides a snapshot of the global environment during the last interglacial period, when the Earth became very warm and polar ice caps melted entirely, making global sea levels much higher than they are today. It highlights the extensive landscape impacts of past climate change on the world’s coastlines and shows that during the last interglacial, much of the Gulf of Mexico coastal lowlands were under water.

Aside from providing an important glimpse of the past and revealing the changes suffered by the Mexican tropics during the ice ages, these findings also open opportunities to better understand future scenarios of relative sea-level rise as climate change progresses in a human-dominated world.

Carlos Burelo, a botanist at the Universidad Juárez Autónoma de Tabasco and a native of the region, drew attention to the ancient mangrove forest in 2016. He said: “I used to fish here and play on these mangroves as a kid, but we never knew precisely how they got there. That was the driving question that brought the [research] team together.” 

Felipe Zapata and Claudia Henriquez of UCLA led the genetic work to estimate the origin and age of the relict forest. Visiting the study sites several times starting in 2016, the research team collected rocks, sediments and fossils to analyse in the lab, helping them pinpoint evidence from the past that is consistent with a marine environment. 

Felipe Zapata said: “This discovery is extraordinary. Not only are the red mangroves here with their origins printed in their DNA, but the whole coastal lagoon ecosystem of the last interglacial has found refuge here.”

In a statement, the researchers said: “We hope our results convince the government of Tabasco and Mexico’s environmental administration of the need to protect this ecosystem.”

Image Credit: Octavio Aburto

Rosa Medea is Life & Soul Magazine’s Chief. She writes about lifestyle including sustainable and green living