An aerial view of the San Pedro Mártir River in Tabasco, Mexico. (Photo: Ben Meissner)

By UCLA Life Sciences

October 4, 2021

Much of the Gulf of Mexico’s coastal lowlands were underwater some 125,000 years ago, new research reveals, highlighting the extensive impact of past climate change on the world’s coastline.

The new findings, published Oct. 4 in the journal Proceedings of the National Academy of Sciences, may provide insights to understand future scenarios of relative sea level rise as global warming progresses, said co-author Felipe Zapata, UCLA assistant professor of ecology and evolutionary biology.

The San Pedro Mártir River, lies to the west of the Yucatan Peninsula, stretching from the El Petén rainforests in Guatemala to the Balancán region in Tabasco, Mexico. Surprisingly, a luxuriant red mangrove ecosystem grows along its banks. While more than 60 miles from the nearest ocean, the plant composition and appearance there resemble a typical coastal mangrove ecosystem.

Because the red mangrove (Rhizophora mangle) and other species present in this unique ecosystem are necessarily dispersed by ocean water, their inland presence in the El Petén rainforests is perplexing and difficult to explain, Zapata said. A group of researchers from Mexico and the United States set out to answer the question: how did coastal mangroves establish so deep inland in the rainforests? Their results were surprising.

Integrating genetic, geologic and vegetation data with sea-level modeling, they found that the San Pedro mangrove forests reached their current location during the Last Interglacial period, some 125,000 years ago, and have persisted there in isolation as the oceans receded during the last glaciation. The study provides a snapshot of the global environments during the Last Interglacial period, when the earth became very warm and the polar ice caps melted entirely, making global sea levels much higher than they are today, Zapata said.

Combining multiple lines of evidence, the researchers demonstrate that the rare and unique mangroves of the San Pedro River are a relict — organisms that have survived from an earlier period — from a past warmer world when relative sea levels were about 20-30 feet higher than at present, high enough to flood the Tabasco Lowlands and reach what today are the tropical rainforests on the banks of the San Pedro River.

The new findings providing an important glimpse on the past, revealing the changes suffered by the Mexican tropics during the ice ages.

Carlos Burelo, a botanist at the Universidad Juárez Autónoma de Tabasco and a native of the region, drew the attention of the rest of the research team toward the existence of this relict ecosystem. “I used to fish here and play on these mangroves as a kid,” he said, “but we never knew precisely how they got there. That was the driving question that brought the team together.” His field work, and his biodiversity surveys in the region, established the solid foundation of the study.

Zapata and his collaborator, UCLA research associate Claudia Henriquez, led the genetic work to estimate the origin and age of the relict forest. Sequencing segments of the genomes of the red mangroves trees, they were able to establish that this ecosystem migrated from the coasts of the Gulf of Mexico into the San Pedro River more than 100,000 years ago and stayed there in isolation after the ocean receded when temperatures dropped.

“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,” Zapata said. “We consider this discovery to be extraordinary.”

Paula Ezcurra, a scientist at the Climate Science Alliance in San Diego, carried out the sea-level modeling. “The coastal plains of the southern Gulf of Mexico lie so low,” she explained, “that a relatively small change in sea levels can produce dramatic effects inland.”

The field work was led by the ecologists in the team: Paula Ezcurra, Sula Vanderplank, Octavio Aburto-Oropeza and Exequiel Ezcurra. “This region was systematically deforested in the 1970s by a misguided development plan,” Ezcurra explained. “The banks of the San Pedro River were spared because the bulldozers could not get in. We hope our results convince the government of Tabasco and Mexico’s environmental administration of the need to protect this ecosystem.”

“The story of Pleistocene glacial cycles is written in the DNA of its plants, waiting for scientists to decipher it. More importantly, the San Pedro mangroves are warning us about the dramatic impact that climate change could have on the coastal plains of the Gulf of Mexico if we do not take urgent action to stop the emission of greenhouse gasses,” Ezcurra cautioned.