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 18-metre-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.
Raiders of the lost bark
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 flavours 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.
An approaching storm turns into a waterspout on Andros Island.
PHOTOGRAPH BY WES C. SKILES, NAT GEO IMAGE COLLECTION
It didn’t always work, but they ultimately succeeded in extracting cores from three blue holes, including the 312-foot-deep portal responsible for that 18-metre-long core. Within tell-tale hurricane layers were fragments of mangrove trees, which were radiocarbon dated to provide dates for each hurricane.
The century of silence
The cores show that there have been plenty more hurricanes impacting the island in the past 1,500 years than the modern observational record suggested. Other studies, looking at different locations, have drawn similar conclusions.
Tom Knutson, the Weather and Climate Dynamics Division leader at NOAA, who wasn’t involved with the study, says he is constantly surprised by this. This curiosity alone, he adds, demonstrates the value of blue hole expeditions.
There are also multiple peaks and troughs in the hurricane frequency, something seen in cores taken elsewhere in the Atlantic. Comparisons to other cores show that the low-pressure belt encompassing the equator–the Intertropical Convergence Zone, or ITCZ – plays a major role.
The zone moves to whichever hemisphere, north or south, is warmest at the time. Thanks to a long-term cooling trend in the Northern Hemisphere, the zone has slowly been moving south for the last 8,000 years. But superimposed on that drift are abrupt, shorter-term changes, up and down, which have a variety of possible causes.
When the zone shifts south, it leads to lower sea-surface temperatures and disrupting winds in the Atlantic Ocean’s hurricane-making factory, which stifles their production. Indeed, the team’s new data indicate that during periods of hurricane inactivity at South Andros Island the belt is in a more southerly position.
The 13th century, though, stood out for its steadfast silence. From 1204 to 1273, there were no major hurricanes at all. After 1273, as many as eight major cyclones then came along and pummeled it, says Wallace.
The team speculates that the near-century of hushed hurricanes is perhaps attributable to an as-of-yet unidentified burst of explosive volcanic activity. That can create airborne clumps of sulfuric acid, which can partially block out the sun for a handful of years and briefly make it far more difficult for the sea to heat up and fuel tropical cyclones.
Enough sulfate aerosols can also cause the ITCZ to migrate, says Matthew Watson, a volcanologist at the University of Bristol who wasn’t involved with the study. If, say, a major volcanic paroxysm happened in the Northern Hemisphere, it could force the ITCZ south into problematic terrain for hurricane formation.
Promisingly, 13th century ice cores from Greenland and Antarctica do show an increased load of these aerosols. But a different period of plentiful aerosols matches up with a time of high hurricane activity at the island. Unless the volcano responsible can be pinpointed–a notoriously difficult task–it’s too early to say what caused the silence in the 1200s.
Never-ending ghost hunt
This is a “fantastic dataset,” and a blue hole core with 1,500 years’ worth of data is “pretty unprecedented,” says Alexander Farnsworth, a palaeoclimatologist at the University of Bristol who wasn’t involved in the study. It does, however, leave the story of Atlantic hurricanes still largely incomplete.
The core doesn’t capture every hurricane. Modern observations of local hurricanes, and the deposits they did or were expected to leave behind, revealed that only Category 3 hurricanes—those with windspe of no less than 178 kilometres per hour—created identifiable layers. Any weaker hurricanes, even those that directly hit the island, aren’t documented.
That record also only provides a hurricane count and a minimum wind strength; it cannot reveal any more detailed characteristics of the cyclones. Plus, it’s just one core of a small total number. Plenty more cores, up and down the Atlantic, will be needed to see which data points and patterns are repeatable or not, says Farnsworth—a point Wallace agrees with.
What is important right now, says Watson, is that this extraordinary dataset can be opened up to the wider scientific community. Some may be able to hunt down that 13th century volcanic culprit, while others could identify what else may be moving the zone.