Scoping Out the Gulf of Mexico’s Secret Submerged Forest
Understanding what happened to this ancient forest can help us know what is coming as the sea rises again.
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On a ship floating 13 kilometers off the Alabama coast, a group of researchers led by Kristine DeLong, a Louisiana State University paleoclimatologist, coaxed a slim pipe from the bottom of the Gulf of Mexico. It contained a column of sand, peat, ancient gunk, and, they hoped, information about the bald cypress forest submerged 18 meters below the surface. They peered at the bottom of the core. Splinters of wood poked through—a sure sign, says DeLong, that they were above the forest.
This past July, DeLong and her team had returned to Alabama’s sunken forest—a site few have visited, and the location of which is a guarded secret so that the submerged wood is not salvaged or destroyed. The expedition was their third to the site after having last visited in 2016. The forest offers a rare glimpse into the late Pleistocene, an epoch known for its megafauna, kilometer-thick ice sheets, and extreme climatic change. Since the forest was first revealed after a hurricane in 2004, it has faced similar storms, as well as the slower threats of erosion and decomposition, and DeLong and her colleagues were eager to see what had changed since their last visit.
The trees on the bottom of the Gulf of Mexico are bald cypresses. Known for its feathery bark and knobby knees, the bald cypress is a familiar tree in the southeastern United States. It’s long-lived and grows consistently, so is useful for tree ring analyses.
The trees submerged in the Gulf are thought to have lived and died between 42,000 and 74,000 years ago. This was a time of upheaval: as the ice sheet that covered North America grew and retreated, the sea level fell and swiftly rose, then fell again. These changes were rapid, with the sea level sometimes rising or falling by tens of meters in as little as 1,000 years. Rising seas carried sediment that buried the trees, but exactly how remains uncertain.
The team’s working hypothesis is that the trees experienced a mass die-off before sediment flooded the swamp where they grew. Analyses of previously collected cores and wood samples show the trees grew straggly rings—a sign of stress—around the same time. Then sediment-laden water, either from the rising sea or from glacial meltwater coursing down the Mississippi River, entombed the site.
The site’s preservation resulted from a perfect storm of conditions spanning 70,000 years. Most shorelines dating to the end of the last ice age have been eroded. That this forest was preserved intact implies “a rate of sea level rise that’s high enough that it allows for lots of sediment to bury that area really quickly,” says Emily Elliott, a University of Alabama coastal geologist not involved with the research.
The forest has not yielded its secrets easily. In fact, scientists did not know it existed at all until Hurricane Ivan tore overhead in 2004. The passing storm shoved off the heavy blanket of sand that had preserved the trees for so long. After the storm, a fisherman was surprised to find an abundance of fish. He asked some diver friends to investigate, and they reported tree stumps scattered across the seafloor. The fish had colonized the submerged forest like a coral reef. It would be nearly 10 years before DeLong and her colleagues first visited the site and obtained samples of this ancient forest.
On their expedition this past summer, DeLong’s group re-mapped the seafloor by sailing back and forth while scanning with sonar. “It’s a long day of mowing the yard on the ocean,” DeLong says. The team worried that Hurricane Sally, which had ripped through the Gulf in September 2020, had damaged or reburied the trees. Their mapping showed the storm had indeed displaced sand, thinning it in some places.
Using their map to identify where the sand was shallowest, and where they were most likely to be able to access Pleistocene sediment, the scientists began taking cores.
The new data these cores reveal will help the scientists tackle outstanding questions, such as what the Gulf Coast’s climate and ecology were like in this distant past, how the region responded to climate change, and how the site was buried. Understanding the forest’s burial offers insights into future climate change in the Gulf. Speaking of DeLong’s latest analysis, Elliott says, “one of the things that is amazing to me is how timely it is.” The site represents one of the best looks at coastal transitions during rapid sea level rise, she says. That can inform climate models predicting different future scenarios.
With climate change, “we’re very worried about Antarctica and Greenland losing their ice,” DeLong says. “What happened during the ice ages is a great example of how quickly an ice sheet can melt.”
The forest’s history can also help the researchers develop a model to locate other buried sites. More patches of ancient forests may be hidden in the Gulf of Mexico, waiting to be discovered. Identifying such sites is crucial for coastal managers, like the Bureau of Ocean Energy Management, which signs off on oil, gas, and wind activity and funds the team’s research.
While the researchers analyze their newly extracted cores, the drowned forest, no longer protected by sediment, will continue to erode. Anemones will bloom from the tree trunks; future hurricanes will brew in the Gulf. The forest is endangered by its own fame. Presumably interested in its novelty, one furniture company has already submitted requests to salvage the ancient wood. DeLong says onlookers tailed their scientific vessel when they undocked, hoping to find the hidden forest. To throw off curious boaters, they took a winding course at sea. A bill proposed in 2020 would establish the site as a national marine sanctuary, protecting it—and potential discoveries—for years to come.