Hidden mangrove forest in the Yucatan peninsula reveals ancient sea
levels

Date:
October 4, 2021
Source:
University of California - San Diego
Summary:
Deep in the heart of the Yucatan Peninsula, an ancient mangrove
ecosystem flourishes more than 200 kilometers (124 miles) from
the nearest ocean.

This is unusual because mangroves -- salt-tolerant trees, shrubs,
and palms -- are typically found along tropical and subtropical
coastlines.



FULL STORY ==========================================================================
Deep in the heart of the Yucatan Peninsula, an ancient mangrove
ecosystem flourishes more than 200 kilometers (124 miles) from the
nearest ocean. This is unusual because mangroves -- salt-tolerant trees, shrubs, and palms -- are typically found along tropical and subtropical coastlines.


==========================================================================
A new study led by researchers across the University of California system
in the United States and researchers in Mexico focuses on this luxuriant
red mangrove forest. This "lost world" is located far from the coast along
the banks of the San Pedro Martir River, which runs from the El Pete'n rainforests in Guatemala to the Balanca'n region in Tabasco, Mexico.

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, the binational team set out to discover how the coastal
mangroves were established so deep inland in fresh water completely
isolated from the ocean.

Their findings were published Oct. 4 in the Proceedings of the National
Academy of Sciences.

Integrating genetic, geologic, and vegetation data with sea-level
modeling, the study provides a first glimpse of an ancient coastal
ecosystem. The researchers 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 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.

"The most amazing part of 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 Scripps Institution of Oceanography at UC San Diego and a PEW Marine
Fellow. "There is certainly more to discover about how the many species
in this ecosystem adapted throughout different environmental conditions
over the past 100,000 years.

Studying these past adaptations will be very important for us to better understand future conditions in a changing climate." Combining multiple
lines of evidence, the study demonstrates that the rare and unique
mangrove ecosystem of the San Pedro River is a relict -- that is,
organisms that have survived from an earlier period -- from a past warmer
world when relative sea levels were six to nine meters (20 to 30 feet)
higher than at present, high enough to flood the Tabasco lowlands of
Mexico and reach what today are tropical rainforests on the banks of
the San Pedro River.



==========================================================================
The study 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 Jua'rez Auto'noma de Tabasco
and a native of the region, drew the attention of the rest of the team
towards the existence of this relict ecosystem in 2016. "I used to fish
here and play on these mangroves as a kid, but we never knew precisely how
they got there," said Burelo. "That was the driving question that brought
the team together." Burelo's field work and biodiversity surveys in the
region established the solid foundation of the study. His remarkable
discovery of the ancient ecosystem is documented in "Memories of the
Future: the modern discovery of a relict ecosystem," anaward-winning short
film produced by Scripps alumnus Ben Fiscella Meissner (MAS MBC '17).

Felipe Zapata and Claudia Henriquez of UCLA led the genetic work to
estimate the origin and age of the relict forest. Sequencing segments
of the genomes of the red mangrove trees, they were able to establish
that this ecosystem migrated from the coasts of the Gulf of Mexico into
the San Pedro River over 100,000 years ago and stayed there in isolation
after the ocean receded when temperatures dropped. While mangroves are
the most notable species in the forest, they found nearly 100 other
smaller species that also have a lineage from the ocean.

"This discovery is extraordinary," said Zapata. "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."
Paula Ezcurra, science program manager at the Climate Science Alliance,
carried out the sea-level modeling, noting that the coastal plains of the southern Gulf of Mexico lie so low that a relatively small change in sea
level can produce dramatic effects inland. She said a fascinating piece
of this study is how it highlights the benefits of working collaboratively among scientists from different disciplines.



========================================================================== "Each piece of the story alone is not sufficient, but when taken together,
the genetics, geology, botany, and field observations tell an incredible
story.

Each researcher involved lent their expertise that allowed us to uncover
the mystery of a 100,000+ year-old forest," said Ezcurra, an alumna of
Scripps Oceanography (MAS CSP '17).

The field work was led by the ecologists on the team -- Octavio
Aburto-Oropeza, Paula Ezcurra, Exequiel Ezcurra of UC Riverside, and
Sula Vanderplank of Pronatura Noroeste. Visiting the study sites several
times starting in 2016, they collected rocks, sediments and fossils to
analyze in the lab, helping them pinpoint evidence from the past that
is consistent with a marine environment.

The authors note that the region surrounding the study sites was
systematically deforested in the 1970s by a misguided development plan;
the banks of the San Pedro River were only spared because the bulldozers
could not reach it. 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 our results convince the government of Tabasco and Mexico's environmental administration of the need to protect this ecosystem,"
they said.

"The story of Pleistocene glacial cycles is written in the DNA of its
plants waiting for scientists to decipher it but, 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 gases." Funding was provided by the David and Lucile Packard Foundation, the Baum Foundation,
the National Geographic Society, and a donation by Gina Rogers.

========================================================================== Story Source: Materials provided by
University_of_California_-_San_Diego. Original written by Brittany
Hook. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Octavio Aburto-Oropeza, Carlos Manuel Burelo-Ramos, Exequiel
Ezcurra,
Paula Ezcurra, Claudia L. Henriquez, Sula E. Vanderplank, and Felipe
Zapata. Relict inland mangrove ecosystem reveals Last Interglacial
sea levels. PNAS, 2021 DOI: 10.1073/pnas.2024518118 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/10/211004153735.htm

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