By Dr. Pete van Hengstum
Why would anyone care about the mud at the bottom of a bluehole? Sure, many people have heard about the brilliant tortoise and crocodile fossils preserved in some of Abaco’s blue holes, and the importance of those fossils to the natural history of The Bahamas. The secrets in the blueholes, however, extend beyond the fossils. It turns out that the sediment filling in the blueholes over time is a trove of information about climate and hurricane activity on Abaco Island.
Dr. Pete van Hengstum from Texas A&M University at Galveston and Dr. Jeff Donnelly from Woods Hole Oceanographic Institution have spent the last several weeks collecting evidence for prehistoric hurricane strikes on Abaco Island. “It is typically only small-sized silt and clay sediment particles that accumulate in the blueholes,” says Dr. van Hengstum, “such as classic Abaco marl in ocean holes or leaves and sticks in the terrestrial sinkholes”.
Yet when a hurricane strikes, permanent evidence of its passage is sometimes preserved at the bottom of nearby blueholes. Dr. van Hengstum is a Geological Oceanographer that uses sediments – or mud — to understand how marine environments change over thousands of years in response to changes in climate or oceanography.
He describes how “… more energy is required to transport larger sand-sized particles than smaller-sized materials like silt and clay. This means that during a hurricane, abundant volumes of sand are mobilized in the coastal zone by the storm surge from a hurricane and transported further than they would be under fair weather conditions”.
The end result is that some of Abaco’s blueholes could potentially preserve evidence for hurricanes that have hit the Abacos over thousands of years.
“The sediments that are preserved in the blueholes are like a barcode…” describes Dr. van Hengstum, “…we collect vertical sediment cores that we can date back in the laboratory with radiocarbon dating to provide a timeline. Then, we can read the barcode of hurricane events by analyzing alternating layers of sand and mud”. When a hurricane strikes, a tell-tale layer of sand often becomes deposited in the blueholes, but afterwards, only mud is again deposited.
The project really began in May 2011, when Dr. van Hengstum collected several preliminary samples with in collaboration with Mrs. Nancy Albury of the National Museum of The Bahamas / Antiquities, Monuments and Museums (AMMC). They also enlisted the support of Technical SCUBA diver Mr. Brian Kakuk of the Bahamas Underground. Their preliminary results from one particular site Thatchpoint Bluehole, proved particularly interesting.
“Thatchpoint Bluehole is also known as the bluehole near Shallow Water Point” describes Mrs. Nancy Albury, “but there are clear sand layers associated with several recent hurricanes, including hurricane Jeanne in 2004”. Hurricane Jeanne struck the Abaco’s and Grand Bahama on 25 September 2004, causing widespread flooding and damage locally, and resulting in the tragic loss of over 3000 souls in the Caribbean region. “Additionally,” Dr. van Hengstum continues, “Thatchpoint Bluehole provides evidence that more hurricanes struck the Abaco’s from 1350 to 1650 AD. Although, we need more samples to further analyze this time period”.
Why would the ocean and climate generate periods of time with higher versus lower hurricane activity in the northern Bahamas? This is a puzzle the scientists are hoping to solve, especially considering how there is little consensus among the global scientific community regarding how hurricane activity will respond to modern climate change.
Although the research project is jointly supported by several organizations, including the AMMC, Friends of the Environment, the National Science Foundation in the USA, and their respective research institutions, Dr. van Hengstum is quick to point out the role of Bahamians in making their project feasible.
“Without the support of local Bahamians this research would be impossible. The local fisherman and hunters know the Marls and the woods like the back of their hand, yet to me it often looks all the same. And, Friends of the Environment helps connect our team with people who are passionate about the environment and willing to help us complete our research objectives”.
Mrs. Nancy Albury laughs, “we always want to visit the most challenging sites to access, yet those sites are often very scientifically interesting!”
The blue holes on Abaco seem to continually reveal secrets of the islands’ history. Sometimes through through fossils, their geologic formations, or apparently, the mud and sand that has quietly accumulated on the bottom of the bluehole for thousands of years.