Deep in the Yucatan Peninsula's rainforests is a system that has swampy red mangroves. It's far from home.
The closest seashore is 170 km away (105 miles), but these salt-loving shrubs did not just walk off the coast to get a drink on the banks the San Pedro river.
Combining genetic, geologic and botanical research has confirmed what many locals have suspected: This was once a saltwater mangrove ecosystem that was left stranded in the last ice age when the oceans receded.
We see the freshwater remnant of a coastal lagoon ecosystem that is 125,000 years old today.
Felipe Zapata, University of California Los Angeles biologist, says that "this discovery is extraordinary."
"The red mangroves are here with their genetic origins, and the entire coastal lagoon ecosystem from the last interglacial have found refuge here."
Red mangroves (Rhizophora magle) are usually found in the tropics' brackish or salty waters, but they can also be found in freshwater in very rare cases.
Mangroves can be established if they have sufficient calcium deposits.
Numerous other studies have been done along the Mexican Caribbean coast to find possible fossil lagoons that can be separated from the ocean. However, they are not as far as the one on the San Pedro limestone banks.
Researchers analyzed the genomes from 79 trees at 11 locations around the Yucatan and found that river populations of red-mangroves were different than coastal populations.
The mangrove at Trminos lagoon, on the Gulf of Mexico side of Yucatan, was the closest to the freshwater trees. This suggests that the two mangroves may be sisters and shared a common ancestor.
On the other hand, Mexico's other inland lagoons appear to be linked to Caribbean mangroves. This suggests that there are two distinct R. mangle lineages: one from the north and one of south.
The authors speculate that the population genetic analysis confirmed that San Pedro River mangroves were a remnant of a coastal ecosystem, which may have colonized the river’s tufa lake, possibly during the Last Interglacial. They also remained along the riverbanks following the Wisconsin glaciation.
Researchers used sea level data from the southern Gulf of Mexico to create models that identified a low coastal plateau that could easily become a bathtub, if the sea level rose even slightly.
All the polar ice caps had already melted, and the sea level was approximately 6-9 meters (20-30 feet) higher than it is today.
The models show that this is enough water to flood the Tabasco lowlands in Mexico and submerge tropical rainforests bordering the San Pedro. Researchers claim that the 'tub' was filled within centuries with saltwater.
This means that red mangroves could establish themselves along shifting coastlines in just a few generations.
These new environments were also colonized by smaller species of vegetation. Nearly 100 species are still present today.
This unique ecosystem can be used to trace back the Earth's last heating event and could provide valuable information about our current situation.
While sea levels won't rise by 9 meters due to human-caused climate changes, models suggest that the seas could rise five metres by 2300.
It remains to be seen how ecosystems will adapt to that change, but resilient natural habitats such as the red mangrove may give us some clues about what's to come.
"The best part of this study was that we were able examine a mangrove eco-system that had been trapped in time over 100,000 years," said Octavio Aburto Oropeza, a marine ecologist at the University of California San Diego.
"There's a lot more to learn about how species have adapted to different environments over the past 100,000 year." It will be crucial to study these adaptations in order to better understand the future climate.
To fully understand San Pedro's mangrove system, you must first exist. It is currently under serious threat. The rainforests around this river were severely deforested in the 1970s. Mangroves were difficult to reach and they didn't survive.
According to the study's authors, we must preserve and conserve ancient ecosystems in order to understand where and how we got there.
The study was published by PNAS.
