From the Dust Bin of History to a Future of Integrated Natural Science

By Dale Griffin and Gene Shinn

Each year, about two billion tons of dust moves some distance in the Earth's atmosphere. Associated with that dust is an estimated quintillion microorganisms -- that's a 1 followed by 18 zeros -- or enough to form a microbial bridge between Earth and Jupiter. A significant source of this dust is the world's great deserts.

For more than 30 years, North Africa, home of the Earth's largest desert, has been gripped by weather conditions that are drier than usual for the region. The Sahara and its surrounding area is, even by its own arid standards, in a drought that is increasing its capacity to generate huge dust storms with consequences that reach for thousands of miles.

Although African dust had long been recognized in distant places, by the early 1970s something had changed. Boaters in the Caribbean noticed they were spending more time cleaning the red-brown silt from decks and sails. A prolonged drought had struck northern Africa, and by 1984 dust reaching the Caribbean had increased dramatically.

By the 1990's, the composition of African dust had changed. The North African population had exploded. More goats trampled the Sahelean grasses, burning plastics replaced wood smoke, and with no waste management or sewer systems the byproducts ended up on the ground where they were lifted into the air. Scientists realized that the dense dust storms from Africa sometimes carried DDT, mercury, arsenic, smoke, and camel and goat dung. They began to wonder what effect African dust was having on marine organisms and people.

The city of Bamako, Mali, during a dust event. The red tint of the dust is the classic color of windborne soils originating from the Sahara and Sahel.

During a "dust event" in the West African nation of Mali, the concentration of airborne particles can be more than 10 times the international health standard. Great clouds of dust ride the trade winds off the African continent west across the Atlantic. During the Northern Hemisphere summer, most of the dust is deposited throughout the Caribbean basin, where the clouds are sometimes still thick enough to obscure visibility and coat windshields. Of the quantity that reaches the United States, 50 percent is deposited in Florida. Scientists believe that the rest may reach west to Colorado and north to Maine.

Mali, in western Africa, on a clear day. Note the red soil.

Making the 5- to 7-day trip along with the dust is an assortment of organisms, generally microbes such as bacteria, fungi, and viruses. Occasionally, other visitors arrive. In 1988, North Africa had a plague of locusts. Thousands of them, some 3 inches long, survived the trip but were too weak to take up residence in the Caribbean. Hundreds of microbes per gram of dust do survive the transatlantic trip. About 30 percent of the dust-borne bacteria that scientists have isolated are pathogenic to plants, animals, or humans and may be contributing to a host of environmental problems, including the demise of Caribbean coral reefs, red tides, crop failures, and human health concerns like increasing rates of asthma.

A massive sandstorm blowing off the northwest African desert has blanketed hundreds of thousands of square miles of the eastern Atlantic Ocean with a dense cloud of Saharan sand. The massive nature of this particular storm was first seen in this SeaWiFS image acquired on Saturday, 26 February 2000 when it reached over 1000 miles into the Atlantic. These storms and the rising warm air can lift dust 15,000 feet or so above the African deserts and then out across the Atlantic, many times reaching as far as the Caribbean where they often require the local weather services to issue air pollution alerts as was recently the case in San Juan, Puerto Rico. Recent studies by the U.S.G.S have linked the decline of the coral reefs in the Caribbean to the increasing frequency and intensity of Saharan Dust events. Additionally, other studies suggest that Sahelian Dust may play a role in determining the frequency and intensity of hurricanes formed in the eastern Atlantic Ocean. Provided by the SeaWiFS Project, NASA/GSFC and ORBIMAGE.

Inorganic nutrients in dust, such as iron, are believed to influence microbial populations in surface waters, and dust events impacting Florida Bay have been correlated with harmful algal blooms. There is some good news about dust--many terrestrial plants derive nutrients from dust fallout in rainforests in South America and the Hawaiian Islands (from African and Asian desert dust storms, respectively.)

Through its Global Desert Dust Program, the USGS has been defining what types of microorganisms are capable of surviving long-range dust-borne atmospheric transport and causing disease in plants, terrestrial and marine animals, and people. Scientists are collecting samples at shore-based sites around the globe, culturing them, and identifying microbes. Researchers are also analyzing atmospheric dust samples for toxins such as metals, herbicides, pesticides and a suite of radioactive elements. They are observing pulses of dust, which are clearly shown on satellite imagery, and noting coincident historical trends in the environment along the African dust corridor. Geologists, chemists, biologists, and microbiologists at the USGS and many partner organizations are working together to address the human and environmental health issues in this emerging scientific field.