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Saharan dust explains Bahamas’ paradoxical existence

Windblown nutrients may fertilize island-building bacteria

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3:23pm, July 11, 2014
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The Bahamas owes its origins to windswept dust from Africa’s Sahara Desert, scientists propose June 30 in Geology.

The dust nourishes microbes that produce calcium carbonate, the core building block of the Bahama Islands, the researchers say. Over the last 100 million years this carbonate has assembled into the Great Bahama Bank, a 4.5-kilometer-thick platform. This underwater shelf the size of Iceland  forms the base of much of the Bahama Archipelago.

“Without this dust, the Bahamas might never be there,” says geochemist Peter Swart of the University of Miami in Florida. “We’d just have a big open ocean and maybe a little underwater bump on sonar.”

The Great Bahama Bank is still growing outward toward Miami as fresh carbonate sediment collects on its borders. Other carbonate platforms, such as the Great Barrier Reef, form from the skeletal remains of mollusks and reef-building corals. But these shelled and hard–bodied creatures are relatively rare in the Bahamas, so scientists have long puzzled over the Bahamas’ origin.

In the early 1990s, researchers discovered evidence of carbonate-secreting microbes called cyanobacteria in whitings, which are milky-white streaks of floating calcium carbonate that frequently blemish the Bahamas’ otherwise crystal-clear waters. Whitings range in size from a few meters across to larger than Manhattan. 

These cyanobacteria presented another enigma, Swart says. They appeared to be the most likely source of the region’s carbonate, yet the area lacks enough nutrients, especially iron, to support bacterial populations large enough to build up the islands.

“The Great Bahama Bank sits in a nutrient-poor environment,” he says. “So the question is: Why is it there?”

To unravel the mystery, Swart and colleagues navigated the length of the Great Bahama Bank, collecting 270 seafloor sediment samples over three years. Areas with more whitings contained sediments richer in iron, the team discovered, strengthening the argument that cyanobacteria are the region’s chief carbonate producers and responsible for the carbonate in whitings.

The researchers also found that the waters around Andros Island, the largest island in the Bahamas, not only has the largest concentration of whitings and the thickest carbonate sediments, but also the highest concentration of iron. Like many other islands in the Bahamas, Andros collects iron-rich dust blown across the Atlantic Ocean from the Sahara. Swart concluded that this dust drops into the Bahamas’ waters, either directly or in runoff from the islands, and fuels the cyanobacteria to reproduce and release carbonate.

To test the idea, Swart plans to look for chemical fingerprints in Saharan dust samples that can prove the Bahamian iron’s African origins. Dust-fertilized cyanobacteria could also explain the origin of carbonate rocks elsewhere in the ocean that formed over 400 million years ago, before mollusks and corals evolved, Swart says.

Oceanographer Lisa Robbins of the U.S. Geological Survey in St. Petersburg, Fla. says Swart’s explanation fits with the marine bacteria that she and other scientists have observed for decades. “It needs some testing, but it ties everything together neatly,” she says. “Anyone who has a biology background will say, ‘Oh yeah, this makes sense.’”

Editor's Note: This story was updated July 24, 2014 to correct the size of whitings.

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