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The white beaches of the Bahamas might come from a surprising place: Saharan dust storms

Winds blow dust from the Sahara across the Atlantic, and scientists are discovering the effects of what happens when it lands.

Water turbidity in the Bahamas. Image: SeaWiFS Project, NASA/Goddard Space Flight Center, and ORBIMAGE
Water turbidity in the Bahamas. Image: SeaWiFS Project, NASA/Goddard Space Flight Center, and ORBIMAGE

A fascinating discovery came to light last week, which I've just seen thanks to Russ George - the white sands of the Bahamas (and other parts of the Caribbean) are a side product of a vast transcontinental cycle of dust storms.

To explain, it helps to see an image of the region via satellite - and thanks to Nasa, we've got one above. It shows the sea between the Bahamas, Florida, Cuba and the other islands of the area in September 2004, immediately after Hurricane Frances passed over the area (which you can see the edge of to the top-left). Those patches of light-blue water - especially the patches that are almost white - are that colour because they're now frothing with chalk. Carbonates, resting on the sea bed, have been stirred up by the storm and brought to the surface. The Bahamas rests on the Great Bahama Bank, the largest carbonate slab in the world, and storms like Frances regularly stir up big undersea carbonate storms to go with the winds above. 

The new paper, published in Geology, details an examination of the geological structure of the Bahamas, and a puzzle. Received wisdom has it that for a hundred million years corals sequestered carbon dioxide out of the water to grow, die and produce further corals - and, in the process, birthing the Bahamas islands themselves. But there's a problem with this. There shouldn't be anything like enough nutrients in the water to feed that much coral over that extreme length of time.

Marine geoscientist Peter Swart and his team propose that what the corals were feeding on was dust, from the Sahara. Every year something in the order of 600 million tonnes of dust blows from the Sahara across the Atlantic as part of the Saharan Air Layer, forming the majority of the natural dust found in the atmosphere. It's a phenomenon that we're only recently coming to understand - most astonishingly, it appears that the entire, vast Amazon rainforest entirely depends upon being fed by nutrient-rich dust from the Bodélé depression in Chad. That's a patch of desert only a few thousand miles wide, or less than 0.5 per cent the size of the rainforest that lives off it.

In the Bahamas, the iron from that dust feeds photosynthesising algae, which "bloom". They simultaneously sequester carbon dioxide from water, and when they die they keep it inside them as they fall to the ocean floor as a rain of what is essentially carbonate. That's the white stuff that Hurricane Frances stirred up in that image - countless memento mori from innumerable bacterial orgies, going back millions of years. (And, even better, they sequester nitrogen too - they're a natural fertiliser.)

It appears that the white sand that washes up on the beaches of the Bahamas is the byproduct of an ancient, natural carbon sequesteration process, one of what is surely hundreds of interacting systems that keep the climate in check. What's worrying, then, is that as the climate warms up the amount of dust in the atmosphere is decreasing. Are they related? If so, what can we do?

I mentioned that I read this on Russ George's blog. He's infamous for being responsible for a massive, unauthorised experiment in geoengineering - he dumped a hundred tonnes of iron filings into the Pacific, off the coast of Canada, to deliberately cause a plankton bloom. The idea is that, as political solutions to climate change fail, and our emissions of greenhouse gasses continue to increase, we can try to sequester the carbon dioxide we've produced by imitating processes like that seen in the Bahamas.

Some scientists think iron filings and forced plankton blooms could offset as much as a whole one per cent of global emissions, but fundamentally there's a difference between a natural process and a manmade one. This most-recent study has shown us that such processes are part of an overall ecosystem, as one cog in a wider sequence of events that the makes up what we think of as "the environment". Trying to mimic one single stage might look good on paper, but we can't know the consequences in advance. A plankton bloom could kill off other wildlife; it could upset feeding patterns; it could be offset by plankton elsewhere not blooming as it regularly would.

Fundamentally, it's a depressing response to a man-made disaster - instead of choosing to stop hurting our world, we choose to try and compensate for our poor behaviour.