The ghosts of ancient hurricanes live in Caribbean blue holes

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[Many thanks to Michael O’Neal for bringing this item to our attention.] Robin George Andrews (National Geographic) reports on evidence of hurricanes discovered by researchers today in the blue holes close to South Andros Island in The Bahamas. He writes, “Core samples pulled from submarine sinkholes reveal a 1,500-year record of powerful hurricanes that passed through the Bahamas on the way to the U.S. East Coast.” See full article and spectacular photographs at National Geographic.

South Andros Island, part of the Bahamian archipelago, is a sandy slice of paradise whose shores conceal buried geological treasures: blue holes. Hiding in the depths of these ethereal submarine sinkholes lay ancient sediment sandwiches whose layers betray the bygone passages of powerful hurricanes. The isle is often a pitstop for hurricanes heading toward the Gulf of Mexico or North America’s east coast. If these lithic libraries could be accessed, scientists could travel back in time and compare the Atlantic hurricanes of today with the specters of storms past.

With some ad-hoc engineering ingenuity, researchers have extracted several of these towers of sediment from their blue hole homes. As reported earlier this month in the journal Paleoceanography and Paleoclimatology, one 59-foot-long continuous core chronicles the encounters the island has had with tropical cyclones going back a jaw-dropping 1,500 years. It has peaks and troughs in activity, but in general the sediments show that South Andros Island has been a hurricane highway for much of the last millennium and a half. And although the island has been visited by dozens of tropical cyclones during the past 150 years, just two powerful hurricanes have paid it a visit during that timeframe.

That likely won’t last, explains the study’s lead author Lizzie Wallace, a doctoral student in the Massachusetts Institute of Technology and Woods Hole Oceanographic Institution Joint Program in Oceanography. The island’s unearthed history indicates that calmer recent times are probably atypical, meaning the island has just got lucky lately.

As human-caused climate change continues to warm the world, hurricanes will get wetter, more intense and more capable of flooding coastlines. Along with the revelations buried in the ancient cores, it appears likely that the island, and the wider region, may be at a greater risk of intense hurricane strikes in the future than modern instrumentation records alone can show.

Climate scientists, perpetually ravenous for more data, are increasingly turning to a myriad of unusual sources. These including the weather logs taken by 19th century sailors, the changing flavors of French wine grapes, and even within the poop of Romanian bats.

The same creative thinking is starting to apply to long-gone hurricanes. The National Oceanic and Atmospheric Administration (NOAA) only has Atlantic hurricane records as far back as 1851, so researchers are hoping to find traces of older ones in antiquated sediments..

Hurricane-strength winds tear up the environment. Jeffrey Donnelly, Wallace’s supervisor, noticed that muddy sediments within marshes, lakes, and ponds in Massachusetts seemed to record the pandemonium. So do blue holes, sinkholes carved into erodible rock and flooded with water that can capture larger sand grains and shell fragments blasted in their direction. Several of these natural hurricane archives in a few Atlantic locales, from western Florida to multiple Caribbean Sea shorelines, have already been liberated by researchers. But South Andros Island, positioned along a well-known contemporary hurricane path and pockmarked with untapped blue holes, was especially enticing for Wallace and her team.

Those watery pits are deep, near sandy beaches and reefs, and are shielded from the wider sea, allowing them to take on centuries’ worth of sediment with minimal disruption and interruptions. They are also low in oxygen, discouraging life from burrowing in and inconveniently reordering the layers.

Visiting the island in late 2014, the team attempted to dig out continuous cores from several blue holes. That was easier said than done: these otherwise deep sinkholes were in shallow water, meaning most core drill-capable boats cannot access them. Instead, Wallace’s team had to build their own drill rigs made from aluminum tubes, wooden planks, and inflatable dinghies, which were then towed above their targets. [. . .]

For full article, see https://www.nationalgeographic.com/science/2019/11/ghosts-ancient-hurricanes-caribbean-blue-holes/?

[Photograph above by Jad Davenport (Nat Geo Image Collection): Blue holes like this one in the Bahamas are excellent stores of ancient sediments that can reveal strong storms from the past.]

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