Lauren Fuge
A 30m sediment core extracted from an underwater cave has given scientists a glimpse into the past, showing how tropical cyclones have changed in the Caribbean over 5,700 years.
The research team, led by Eberhard Gischler from Germany’s Goethe University Frankfurt, identified and dated 574 different storm events visible in the sediment core.
What they found in this time machine is key to understanding the future of cyclones in the region.
Drilling down
A sediment core is a tube of the Earth’s crust taken from the bottom of a body of water like a lake or ocean. Its layers preserve a record of the environment over many years, in a similar way to ice cores or peat cores.
This sediment core was taken in 2022 from the “Great Blue Hole”, a deep underwater sinkhole off the coast of Belize in South America.
The Great Blue Hole was originally a karst cave on a limestone island. The cave roof collapsed during the last ice age, and when the ice melted and the seas rose, the cave was flooded. By 7,200 years ago, it was inundated by the sea.
Today, the Great Blue Hole is a 125m-deep underwater sinkhole, 300m in diameter and surrounded by a ring of coral reef. Since its inundation, it has been accumulating layers of sediment washed in by the ocean. These layers have been preserved, becoming a long, paleoenvironmental archive.
“Due to the unique environmental conditions – including oxygen-free bottom water and several stratified water layers – fine marine sediments could settle largely undisturbed in the ‘Great Blue Hole’,” explains Dominik Schmitt, a researcher from Goethe University in Frankfurt, Germany, and the new study’s lead author.
“Inside the sediment core, they look a bit like tree rings, with the annual layers alternating in colour between grey-green and light green depending on organic content.”
This natural archive includes records of extreme weather events.
Storm waves and surges washed in especially coarse particles, which comprise clear layers in the sediment core that are distinctly different in grain size, composition and colour to the regular sediments.
These layers of storm deposits are called tempestites.
Storm signals
Gischler, Schmitt and team found 574 tempestites in the sediment core, indicating ancient storm events. Each event was precisely dated, spanning the last 5,700 years – reaching much further back than current human records for the region, which only date back 175 years. This is the longest available continuous tropical storm record.
It reveals the natural variability of storms, showing how tropical cyclones in the Caribbean have increased steadily over the past 6 millennia.
Schmitt says that this increase is partially due to how the equatorial low-pressure zone has shifted southward.
“Known as the Intertropical Convergence Zone, itinfluences the location of major storm formation areas in the Atlantic and determines how tropical storms and hurricanes move and where they make landfall in the Caribbean,” he explains.
It is also partially due to higher sea-surface temperatures; the team found a correlation between warmer seas and more storms.
“These shorter-term fluctuations align with five distinct warm and cold climate periods, which also impacted water temperatures in the tropical Atlantic,” Schmitt says.
On average, between 4 and 16 storms and hurricanes passed over the Great Blue Hole every century. But now 9 storms have passed over it in the past 20 years alone, indicating a change in regime where human-caused climate change is leading to more frequent and more intense tropical storms.
“Our results suggest that some 45 tropical storms and hurricanes could pass over this region in our century alone,” Gischler. “This would far exceed the natural variability of the past millennia.”
The study appears in the journal Science Advances.