Madrid, 4 (European Press)
In the heart of the Yucatan Peninsula, scientists have discovered a hidden ancient mangrove forest that has revealed ancient sea levels from the last Ice Age.
The UCSD team and Mexican researchers are investigating this ancient coastal ecosystem found more than 200 kilometers from the nearest ocean. This is unusual because mangroves — salt-tolerant trees, shrubs and palms — are often found along tropical and subtropical coasts, they note in the Proceedings of the National Academy of Sciences.
This “lost world” is located far from the coast, along the banks of the San Pedro Martir River, which stretches from the El Petén forest in Guatemala to the Balancan region of Tabasco (Mexico).
Since red mangroves (‘Rhizophora mangle’) and other species in this unique ecosystem are known to only grow in brackish or somewhat brackish water, the national duo team set out to discover how coastal mangroves were created so far inland, in fresh water and completely cut off from the ocean.
The study, which integrates genetic, geological and botanical data with sea-level modeling, provides a first glimpse into the ancient coastal ecosystem. The researchers found that the mangrove forests in San Pedro reached their current location during the last ice period, about 125,000 years ago, and persisted there in isolation as the oceans receded during the last ice age.
The study provides a glimpse into the global environment during the last glacial period, when the Earth became extremely hot and the polar ice caps completely melted, bringing global sea level much higher than it is today.
Study co-author Octavio Aporto Uropza, a marine ecologist at the Scripps Institution of Oceanography from the University of California, San Diego and a PEW Marine Fellow, said.
“There is undoubtedly more to discover about how many species in this ecosystem have adapted through different environmental conditions over the past 100,000 years – he adds -. Studying these past adaptations will be very important to better understand the conditions. The future is in climate change.”
By combining multiple lines of evidence, the study shows that the rare and unique mangrove ecosystem of the San Pedro River is the remnant – that is, organisms that survived from an earlier period – of a warmer world in the past, when the relative level of the sea was six to nine meters from the day, high enough to flood the Tabasco lowland in Mexico and reach what is today the tropical forests on the banks of the San Pedro River.
The study highlights the extensive landscape effects of past climate change on the world’s coasts, and shows that during the last interglacial, most of the coastal lowlands of the Gulf of Mexico were submerged.
In addition to providing important insight into the past and revealing the changes experienced by the Mexican tropics during glaciers, these findings also open opportunities to better understand future scenarios for relative sea level rise as climate change progresses in a human-dominated world.
Carlos Borillo, a botanist from the Autonomous University of Tabasco and a native of the area, drew the attention of the rest of the team to the existence of this ecosystem in 2016. “When I was a kid I used to hunt here and play in these mangroves, but we never knew” how they got there. That was the driving question that brought the team together.”
Field work and studies of Burelo’s biodiversity in the region laid a solid foundation for the study. His remarkable discovery of an ancient ecosystem is documented in Memories of the Future: The Modern Discovery of an Ecosystem of Archeology, an award-winning short film produced by Scripps graduate Ben Fiscella Meissner (MAS MBC ’17).
Felipe Zapata and Claudia Henriquez, of the University of California, led the genetic work to estimate the origin and age of the painted forest. By sequencing parts of red mangrove genomes, they were able to establish that this ecosystem migrated from the shores of the Gulf of Mexico to the San Pedro River more than 100,000 years ago and remained isolated there after the ocean receded as temperatures cooled.
Although mangroves are the most well-known species in the forest, they found nearly 100 other, smaller species of lineage from the ocean.
“This discovery is extraordinary – confirms Zapata -. Not only are the red mangroves here with their origins imprinted in their DNA, but the entire coastal lagoon ecosystem of the last glaciers found refuge here.”
Sea-level modeling has been implemented by Paula Ezcura, Director of the Science Program of the Alliance for Climate Sciences, noting that the coastal plains of the southern Gulf of Mexico are so low that a relatively small change in sea level can produce dramatic effects inland. He adds that one of the fascinating aspects of this study is that it highlights the benefits of collaboration between scientists from different disciplines.
“Every bit of history alone is not enough, but when put together, genetics, geology, botany and field observations tell an incredible story. Each of the participating researchers brought their expertise, which allowed us to unravel the mystery of the 100,000-year forest,” adds Izcora, a former student In Scripps Oceanography (MAS CSP ’17).
The fieldwork was led by the team’s ecologists — Octavio Aburto-Urupeza, Paola Ezcura and Exquiel Ezcura from UC Riverside, and Sola Vanderblanc from Prontura Norwest. They visited study sites several times starting in 2016, collecting rocks, sediments and fossils for laboratory analysis, helping them find evidence from the past that matches the marine environment.
The authors note that the area around the study sites was systematically deforested in the 1970s through a faulty development plan. Only the banks of the San Pedro River were saved because bulldozers could not reach them. The area is still threatened by human activities, so the researchers stressed the need to protect this biologically important area in the future.
“We hope that our results will convince the Tabasco government and the Environmental Administration of Mexico of the need to protect this ecosystem – the Swabian -. The history of the Pleistocene Ice Age cycles written in the DNA of your plants awaits scientists has deciphered, but more importantly, the San Pedro mangrove forests warn us of the impact the dramatic climate change on the coastal plains of the Gulf of Mexico if we do not take urgent action to stop greenhouse gas emissions.”