Sediments deposited in ocean caves are not all the same. Sediments deposited when a storm passes are different from those deposited in normal weather.

This difference is helping scientists unravel when hurricanes blew by various points in the Atlantic Ocean, going back about 1,500 years. Figuring out the timelines of old cyclones may help validate computer models and provide insights into future hurricanes.

Records of old hurricanes go back only about 150 years, and such intense storms are relatively rare, making it difficult to obtain statistically significant data. “The real value of using geological records to go back further in time [is that it] allows us to look at a lot more storms,” said Amy Frappier, a paleoclimatologist at Skidmore College in Saratoga Springs, N.Y., who was not involved in the study.

“It’s sort of the perfect sediment trap for the sorts of records we are after. Hurricane sediments can get in but can’t get out.” Sediments deposited in lakes or seas make up one such record. During normal weather, the sediments deposited are soft, with almost cold cream–like consistency. But when a big storm passes by, it rakes up and deposits coarse sediments that normal tides can’t move. Thus, if one digs up sediments without disturbing the layers, by looking where the coarse material was dumped, one can figure out when large storms passed by.

Go Blue

In a new study published in Paleoceanography and Paleoclimatology, Jeffrey Donnelly of the Woods Hole Oceanographic Institution in Massachusetts targeted blue holes. Blue holes are ocean caves that formed when sea levels were much lower. The roofs of the caves collapsed, and when the sea level rose much higher, they formed big holes at the bottom of the ocean.

“It’s sort of the perfect sediment trap for the sorts of records we are after,” said Donnelly. “Hurricane sediments can get in but can’t get out.”

So Donnelly and his team dug about 20 meters into blue holes off South Andros Island, Bahamas, to bring up sediment cores comprising a record going back to about 1,500 years. (Radiocarbon dating of organic matter like leaves trapped in the sediments indicated the age of the sediments.) The team also collected shorter sediment cores from two other blue holes to corroborate the results. The cores were analyzed in the lab for coarse grains, indicating hurricane activity.

The analysis revealed that the frequency of hurricane landfall on South Andros in the past 1,500 years varied between quiet periods and periods of intense storms. Periods of powerful storms occurred when the Intertropical Convergence Zone, a low-pressure belt near the equator, was far north, suggesting it influenced tropical storms. Researchers also noted the early part of the 13th century was uncharacteristically quiet. They think the reason may be unusually high volcanic activity.

It’s likely that storm tracks have been moving northward in the past millennium. The team also compared the data to similar data from the Gulf of Mexico and the East Coast of the United States. Increased storm activity in South Andros corresponded to increased landfalls in the Gulf of Mexico. But surprisingly, the East Coast saw more hurricanes when South Andros was relatively quiet. It’s likely that storm tracks have been moving northward in the past millennium, the authors suggest.

“If we can determine there are natural cycles in hurricane occurrences and strength, this gives researchers some skill in predicting future storms,” said Kristine DeLong, a paleoclimatologist at Louisiana State University in Baton Rouge who was not involved in the study. Computer models can then be refined using all these past data to make better predictions.

When such reconstructions from different parts of the world are put together, they may help create a map of ancient tropical storms and help us be better prepared for future storms.

—Lakshmi Supriya, Science Writer