The 2021 Atlantic hurricane season was third in 171 years worth of records. The 2020 season was worse. There were so many tropical storms that meteorologists had to go through their entire list to find the right name for each one. A new study has shown that this activity is not a typical occurrence in the long-standing record of Atlantic hurricanes.
Blue holes, odd geological features that hide on the Caribbean's seafloor, are evidence of hurricane activity at an historic low. These open pits are formed in limestone and often over collapsed caverns. The edges become eerie circular shapes from prolonged erosion.
Blue holes look similar to sinkholes, but they are much larger. They can reach 300 meters deep (like the Dragon Hole in South China Sea) or 300 meters wide (like the Belize Great Blue Hole). The Bahamas is home to the world's greatest concentration of blue holes, making it an appealing destination for paleotempestologists--scientists who study historical tropical cyclone activity.
The blue hole's seafloor acts as a record of past storms. The sediment at the bottom builds up over time, much like an ice core and tree ring that grows season after season. Natural currents pull a few small grains of sand into the hole. However, hurricanes push larger grains into it. Researchers can determine how many hurricanes have passed by comparing the layers of fine and coarse grains within this sedimentary lasagna. A blue hole is a long-term record that records how many hurricanes passed close to it. This is because once the sediment settles, there is very little activity in its pit.
Hine's Hole is a 340-meter wide hole that cuts through the western Bahamas' seafloor. It is a prime example for a blue hole hurricane record. It is located halfway between Cuba, Florida Keys, and far from any landform. This allows it to record weaker storms from all directions. It is also very low in oxygen so there are no animals that could cause damage to the fragile sediment. Each year, Hine's Hole is flooded by a steady surface current that blows over the hole. It sends about two to three centimeters worth of sand into the hole.
Tyler Winkler, a geologist from the Woods Hole Oceanographic Institution, Massachusetts, who conducted the study, said Hine's Hole has the highest sedimentation rate among all blue holes he and his colleagues had seen.
Winkler and his colleagues drilled 18m into the sediment below Hine's Hole to recover cores that represent the past 540 year of deposition. The researchers are confident that Hine's Hole has recorded every Category 2 and higher hurricane within 75 km of the current hurricane record by comparing the top layers. Further analysis of the cores shows that this area is experiencing a historic low in the number of tropical storms.
Winkler explains that the local record from Hine’s Hole shows that there have been five hurricanes in the past 170 years. "But, the rate has been nearly eight times as high in the past."
The research shows that the Hine's Hole area has seen major swings in storm frequency ever since 1480. Their findings were compared with other blue holes in Bahamian, tree rings, and ship captain logs. The study found that hurricane activity has declined since the 1860s, just after modern record-keeping began. This means that every hurricane since Alfred Nobel invented Dynamite has occurred in a historically quiet area.
According to Hali Kilbourne (a climate researcher at University of Maryland Center for Environmental Science), sediment cores from blue holes are crucial because they can give a glimpse into the preindustrial era. She says that instrumental records cannot do this. The limestone shelf in the Bahamas is full of blue holes, which is a bonus. She says, "They are in the perfect spot."
Winkler and his crew plan to return to Hine's Hole in order to dig deeper to find more context for the current lull. At least 60 meters of accumulation is found at Hine's Hole. This could provide more context for the current lull and illuminate more than 2,000 years worth of hurricane history.
Winkler says, "That's why the particular site is so essential." "Nowhere else have we seen storm activity that is as long-lasting and resolved as this site."
Hine's Hole reveals that the Bahamas had more hurricanes in the preindustrial period before records were kept. If this heightened activity returns to the Bahamas, it is possible that even the Greek alphabet will not be sufficient to keep track.
This article comes from Hakai Magazine. It is an online publication that covers science and society in coastal ecosystems. You can find more stories like these at hakaimagazine.com.
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