DUST FERTILIZATION OF THE WESTERN ATLANTIC : A BIOCHEMICAL MODEL

Ever since Darwin recognized African dust wafting across the HMS Beagle while anchored in the Azores, its fate and effect on the chemistry of the ocean has been of great interest. Every year an estimated 170 to 700 million tons of dust leaves the African Sahara. An estimated 50 million tons of this material reaches the Western Atlantic. The dust is composed of quartz sand, clay, and a mixture of quartz and clay particles agglutinated with ferruginous cement. It has been documented in Pleistocene and Holocene strata from Bermuda to the Florida Keys and is fertilizer for biota in the canopy of the Amazon rain forest. Over the last decade, its chemical and mineralogical signature has also been detected in corals and sediments of the southeastern United States. However, whether it is friend or foe to biochemical systems is a matter of conjecture.

Corals are ideal recorders of changing conditions. The age of layers can be independently determined and codified so that the record of chemical changes within the photic zone is easily assessed. There is extensive shallow-and deep water coral development off the Southeastern portion of the United States and in the Florida Straits. The changes in trace element chemistry within these corals show a positive relationship with the African dust record and topical cyclone intensity.

Recently, it has been demonstrated that many of the metals contained within the dust are necessary micronutrients in the fertilization of plankton. Using the results of these studies, a biochemical model has been constructed. This model suggests a path from inorganic dust through microbial transformation to micronutrient enzymes (i.e. Cd-enriched carbonic anahydrase) and carbonate precipitation on the Bahamian Banks. It is estimated that more than ten million metric tons of this fine, metal-rich sediment is formed each year. However, for much of this sediment, its deposition is temporary, as it is transported into the Florida Straits yearly by tropical cyclones. This metal-enriched fine carbonate becomes nutrients for phytoplankton forming the base of the food chain for the extensive coral growth. Thus fertilization by African dust has had a positive effect on the growth of biota in the Western Atlantic region.