NASA Earth Observatory has a good summary on the Sahara Dust and its importance to the Amazon and the harm scientists believe it does to the Caribbean coral reefs. Basically the dust fallout replenishes some nutrients lost in the Amazon rainforest but the dust also carries pathogens which are believed to cause some organism die-off in the Caribbean. An amazing Earth system. What is so strange about the dust is that even though we can see it, it doesn't seem to settle on anything. We don't notice a film of sediment on our horizontal surfaces or on the windshields of our car.