CM – Hidden mangrove forest on the Yucatan Peninsula reveals ancient sea levels


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October 4, 2021

from University of California – San Diego

Deep in the heart of the Yucatan Peninsula, an ancient mangrove ecosystem flourishes more than 200 kilometers from the nearest ocean. This is unusual as mangroves – salt-tolerant trees, shrubs, and palms – are typically found on tropical and subtropical coastlines.

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A new study led by researchers from the University of California system in the United States and researchers in Mexico focuses on this lush red mangrove forest. This “lost world” lies far off the coast on the banks of the San Pedro Martir River, which stretches from the El Peten rainforests in Guatemala to the Balancán region in Tabasco, Mexico.

As the red mangrove (Rhizophora mangle) and others Species that occur in this unique ecosystem only grow in salt water or a little salty water, the binational team set out to find out how the coastal mangroves were formed so deep inland in completely isolated fresh water from the ocean. Their results were published in the Proceedings of the National Academy of Sciences on October 4th.

By integrating genetic, geological and vegetation data with sea level modeling, the study offers a first glimpse into an ancient coastal ecosystem. The researchers found that the San Pedro mangrove forests reached their current location during the last interglacial approximately 125,000 years ago and remained isolated there as the oceans receded during the last ice age.

The study provides a snapshot of the global environment during the last interglacial period when the earth became very warm and the polar ice caps melted completely, making global sea levels much higher than today.

« The most amazing thing about this study is that we were able to examine a mangrove ecosystem that has been trapped in time for more than 100,000 years, « said study co-author Octavio Aburto-Oropeza, a marine ecologist at the Scripps Institution of Oceanography at. UC San Diego and a PEW Marine Fellow. « There is certainly more to be discovered about how the many species in this ecosystem have adapted to different environmental conditions over the past 100,000 years. Examining these past adaptations will be very important for us to assess future conditions in a changing climate better to understand. « 

By combining several lines of evidence, the study shows that the rare and unique mangrove ecosystem of the San Pedro River is a relic – organisms that have survived from an earlier time – from a warmer one World when the relative sea level was six to nine meters (20 to 30 feet) higher than it is today, high enough to flood the Tabasco lowlands of Mexico and reach today’s tropical rainforests on the banks of the San Pedro River.

The study highlights the far-reaching landscape effects of climate change in the past on the world’s coasts and shows that during d he last interglacial period a large part of the coastal lowlands of the Gulf of Mexico was under water. In addition to providing an important insight into the past and the changes that the Mexican tropics suffered during the Ice Ages, these results also open up opportunities to better understand future scenarios of relative sea level rise in the course of climate change in a human-dominated world.

Carlos Burelo, a botanist at Universidad Juárez Autónoma de Tabasco and a native of the region, made the rest of the team aware of the existence of this relic ecosystem in 2016, but we never knew exactly how they got there, « Burelo said. “That was the driving question that brought the team together.”

Burelo’s fieldwork and biodiversity surveys in the area formed the solid foundation of the study. His remarkable discovery of the ancient ecosystem is in “Memories of the Future : the Modern Discovery of a Relict Ecosystem « , an award-winning short film by Scripps alumnus Ben Fiscel la Meissner (MAS MBC ’17).

Felipe Zapata and Claudia Henriquez from UCLA directed the genetic work to estimate the origin and age of the relic forest. They sequenced segments of the genomes of the red mangrove trees and found that this ecosystem migrated from the coasts of the Gulf of Mexico to the San Pedro River over 100,000 years ago and remained isolated there after the ocean receded as temperatures fell. While mangroves are the most notable species in the forest, they found nearly 100 other smaller species that also have ocean ancestry.

« This discovery is extraordinary, » said Zapata. « Not only are the red mangroves and their origins inscribed in their DNA here, but the entire ecosystem of the coastal lagoon of the last interglacial period has found refuge here. » found that the coastal plains of the southern Gulf of Mexico are so deep that a relatively small change in sea level inland can have a dramatic impact. She said a fascinating part of this study is how it highlights the benefits of collaboration between scientists from different disciplines.

“Each part of the story alone is not enough, but taken together genetics, geology, botany and field observations tell an incredible story. Each researcher involved contributed their expertise that enabled us to unravel the mystery of a 100,000-year-old human. « Forest, » said Ezcurra, an alumna of Scripps Oceanography (MAS CSP ’17).

The Field work was led by the team’s ecologists – Octavio Aburto-Oropeza, Paula Ezcurra, Exequiel Ezcurra from UC Riverside and Sula Vanderplank from Pronatura Noroeste. From 2016, they visited the study sites several times, collecting rocks, sediments and fossils to analyze in the laboratory , and helped them find evidence from the past that is compatible with a marine environment.

The authors note that the area around the study areas was systematically deforested in the 1970s by a misguided development plan; the shores of the San Pedro River was only spared because the bulldozers couldn’t reach it, and the area is still threatened by human activity, so the researchers stressed the need to protect this biologically important area in the future.

« We hope our results convince the Tabasco government and the Mexican environmental agency of the need to protect this ecosystem, » they said. « The story of the Pleistocene glacial cycles is written in the DNA of their plants, waiting for scientists to decipher, but more importantly, the San Pedro mangroves warn us of the dramatic impact climate change could have on the Gulf’s coastal plains Mexico if we don’t take urgent action to stop greenhouse gas emissions.  »

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