Blue holes show hurricane activity in the Bahamas is at an all-time low |  Science

Blue holes show hurricane activity in the Bahamas is at an all-time low | Science

Blue holes show hurricane activity in the Bahamas is at an all-time low |  Science

Blue Hole in Bahamas

Blue holes, like the Great Blue Hole in Belize, are huge caverns that descend into the seafloor. Sediment accumulates at the bottom of a blue hole, allowing researchers to measure historical hurricane activity.
Ian Fles

The 2021 Atlantic hurricane season was the third most active hurricane season in 171 years of record. The 2020 season was even worse: There were so many tropical cyclones that meteorologists went through their list of possible storm names and needed the Greek alphabet to keep track of the flooding. But a new study shows that even this wave of activity could be a kind of lull in the century-long record of Atlantic hurricanes.

Evidence that hurricane activity is at an all-time low hides on the Caribbean seafloor, tucked into strange geological features called blue holes. These open pits form in limestone, often above collapsed caves. Prolonged erosion weathers the edges in an eerily rounded shape.

Blue holes are similar to sinkholes, but on a much larger scale. They can be 300 meters deep, like the Dragon Hole in the South China Sea, or 300 meters wide, like the aptly named Great Blue Hole in Belize. The Bahamas is home to the world’s largest concentration of blue holes, making it an attractive destination for paleotempestologists — scientists who study historic tropical cyclone activity.

The seafloor at the base of a blue hole acts as a calendar of past storms. Just as an ice core or tree ring grows season after season, the sediment at the bottom of a blue hole builds up over time. Natural currents lure a sugary litter of tiny grains of sand into the hole, while violent hurricanes throw larger grains into the pit. By comparing layers of coarse and fine grains in this sedimentary lasagna, researchers can count how many hurricanes have been in the area. What makes a blue hole a valuable long-term record is that once this sediment settles, there is very little activity in the well to disrupt it.

Hine’s Hole, a 340-foot-wide hole that penetrates the seafloor in the western Bahamas, provides a prime example of a blue hole hurricane record. Located halfway between Cuba and the Florida Keys, it’s far from any landform, so it can describe weaker storms coming from any direction. The bottom of the hole is also low in oxygen, so there are no animals living to disturb the delicate sediment. A constant surface current blasting over the hole sends two to three inches of sand into Hine’s Hole every year.

Hine’s Hole, says Tyler Winkler, a geologist at the Woods Hole Oceanographic Institution in Massachusetts who led the study, has the highest sedimentation rate of any blue hole he and his colleagues have seen.

For the new study, Winkler and his team drilled 18 meters into the sediment at the bottom of Hine’s Hole, finding cores that represent the past 540 years of deposition. After comparing the upper layers with the modern hurricane record, the researchers are very confident that Hine’s Hole recorded every Category 2 or higher hurricane within 75 kilometers. Analyzing the cores even further, it turns out that the number of tropical cyclones raging in this corner of the Bahamas is at an historic halt.

“Over the past 170 years, we’ve had an average of about five hurricanes per century,” Winkler says of the local record of Hine’s Hole. “But in the past, the speed was almost eight times higher.”

Blue Holes Charts

By analyzing the sediment records from the bottoms of blue holes, researchers have reconstructed historical hurricane activity. The first chart shows the trend in hurricanes larger than Category 2 hitting within 75 kilometers of Hine’s Hole in the Bahamas. The second chart shows an average record drawn from blue holes across the region and shows a similar, albeit less dramatic, trend.

Illustration by Winkler et al.

According to the research, the area around Hine’s Hole has experienced wide swings in storm frequency since 1480. The team compared their findings to other Bahamian blue holes, as well as tree rings and even ship captains’ logs. The study shows that hurricane activity has declined since the 1860s, shortly after modern recording began. That means every hurricane since Alfred Nobel invented dynamite has occurred during a historically calm period in the area.

Sediment cores extracted from blue holes are important, says Hali Kilbourne, a climate researcher at the University of Maryland Center for Environmental Science who was not involved in the study, because they can provide a glimpse of the pre-industrial era. “That’s something the instrumental records can’t do,” she says. As a bonus, the Bahamas limestone shelf is rich in blue holes. “They’re in the perfect spot,” she says.

Winkler and his team plan to return to Hine’s Hole to dig even deeper to get more context for the current silence. There is at least 60 meters of accumulation at the bottom of Hine’s Hole, which could illuminate more than 2,000 years of hurricane history.

“That’s why this particular site is so important,” notes Winkler. “There’s nowhere else we’ve encountered so far with the potential lifespan and resolution for storm activity.”

Hine’s Hole shows that in the pre-industrial era before registration began, the Bahamas experienced many more hurricanes each year than it does today. If that increased activity returns, even the Greek alphabet may not be enough to keep up with them all.

This article is from Hakai Magazine, an online publication about science and society in coastal ecosystems. Read more such stories at hakaimagazine.com.

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