Sediment in the Great Blue Hole reveals an alarming storm record

In the heart of the Lighthouse Reef Atoll, off the coast of Belize in the Caribbean, lies a fascinating geological wonder – the Great Blue Hole. This vast, circular sinkhole bears an uncanny resemblance to a giant oceanic eye gazing up at the sky.

Nestled in the turquoise waters of the atoll, about 43 miles from the coast of Belize, this dark circular pit is surrounded by coral and contains sediments that tell a story about centuries of storms.

The Great Blue Hole began as a limestone cave on a dry island. During the last ice age, its roof collapsed, leaving it open to the elements.

Today, this underwater cave plunges to a depth of 410 feet, spans 980 feet, and is one of the most fascinating places to scuba dive.

As glaciers melted and sea levels rose, the cave filled with water, turning it into a natural vault of sediment that has quietly recorded the passage of time.

In the summer of 2022, a team of scientists from Goethe University Frankfurt led a mission to explore this hidden archive.

Funded by the German Research Foundation, the researchers brought a drilling platform out to sea and extracted a 30-meter-long sediment core from the bottom of the Great Blue Hole.

The analysis, carried out by researchers from the universities of Frankfurt, Cologne, Göttingen, Hamburg, and Bern, has given new insight into storm history and climate change in the southwestern Caribbean.

Storm record written in sediment

Roughly 7,200 years ago, the limestone island that once held the Great Blue Hole was overtaken by the sea. Since then, layers of sediment have quietly accumulated at the bottom of the underwater cave.

These layers act like a natural time capsule, offering a window into 5,700 years of storm activity. Each layer records the passing of time, just as is found in tree rings.

“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,” said Dr. Dominik Schmitt, a researcher in the Biosedimentology Research Group at Goethe University Frankfurt.

“Inside the sediment core, they look a bit like tree rings, with the annual layers alternating in color between gray-green and light green depending on organic content.”

But not all layers are the same. Some are made up of coarser, lighter-colored particles.

These event layers, called tempestites, formed when powerful waves from hurricanes and tropical storms swept coarse material from the atoll’s outer reef into the hole.

“The tempestites stand out from the fair-weather gray-green sediments in terms of grain size, composition, and color, which ranges from beige to white,” Schmitt added.

Long-term climate patterns

The team identified and dated 574 individual storm events that were preserved in the sediment core. This detailed storm record stretches far beyond the reach of written or instrumental history, which only covers about 175 years.

The results offer a unique look into long-term climate patterns and hurricane behavior in the region.

The data paints a clear picture: storm activity in the southwestern Caribbean has steadily risen over the past 6,000 years.

“A key factor has been the southward shift of the equatorial low-pressure zone. Known as the Intertropical Convergence Zone, this zone influences 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,” Schmitt said.

The researchers also discovered a connection between sea surface temperatures and storm frequency. Warmer waters matched up with periods of increased storm activity.

“These shorter-term fluctuations align with five distinct warm and cold climate periods, which also impacted water temperatures in the tropical Atlantic,” Schmitt noted.

A warning from the storm record

Historical averages suggest that the Great Blue Hole experienced between four and sixteen major storms per century. However, the sediment record shows that nine storms have already occurred in just the past 20 years.

“Our results suggest that some 45 tropical storms and hurricanes could pass over this region in our century alone. This would far exceed the natural variability of the past millennia,” said Prof. Eberhard Gischler of Goethe University Frankfurt.

The team emphasizes that natural climate changes alone cannot explain this dramatic increase. Instead, they point to human-driven global warming.

The rise in ocean temperatures and the growing strength of La Niña events are creating the perfect conditions for more frequent and more intense storms.

A sediment core with a message

The Great Blue Hole, once thought to be a quiet underwater marvel, is now telling a louder story – one of climate shifts and increasing storm risk.

As the sediment layers continue to build, so does the urgency to address the accelerating pace of extreme weather.

This research reminds us that history, especially the kind written in mud and sand, still has much to teach us.

The full study was published in the journal Science Advances.

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