A 22,000-Year Rainfall Record

Desert conditions have seemingly always dominated, with only temporary periods that proved less harsh as a result of global climate changes. For instance, a recent report in Science presented a detailed account of rainfall in the Atacama going back 22,000 years. The researchers from the U.S. Geological Survey, the University of Arizona, and the Universidad de Chile analyzed pack rat nests and samples taken from dry lakebeds. Several Atacama rodents closely resemble the big-eyed, inquisitive pack rats of the American Southwest. Like their North American counterparts, the pack rats build nests from sticks, cactus, and debris, which their urine cements and preserves. As a result, scientists can analyze a pack rat midden and come up with a precise estimate of all the plants the rodent gathered up nearby when he lived there. This enables scientists to reconstruct rainfall patterns from the contents of the ancient nests, virtually fossilized by the urine of their makers.

In addition, the scientists analyzed layers of mineral deposits and sediment at a handful of springs, both those still active and those that dried up thousands of years ago. The flow of those springs affects the expansion and contraction of surrounding wetlands, which is reflected in the layers of ancient mud and the types of often-microscopic creatures that lived in the wetland. As a result, the springs can record fluctuations in rainfall. The study demonstrated that although the Atacama has remained a hard-core desert all that time, global and local climate shifts did result in periods when the rain increased five- or 10-fold. For instance, between 16,000 and 10,000 years ago, rains increased and some desert areas shifted over to grasslands. Interestingly enough, the same thing was happening in areas of the Sahara Desert half a world away. During this period, higher elevation shrubs moved down off the mountains and out into what biologists today call the Absolute Desert, where virtually nothing grows.

The region dried out again 10,000 years ago and remained barren and waterless, except for one odd and unexplained period from 8,000 to about 3,000 years ago. During that time, the evidence from the springs suggested that rainfall had roughly tripled. However, the levels of dried lakes in the area did not change. So it is possible that the increased rainfall only hit the Andes, since runoff from the Andes feeds the springs in the Atacama.

The Atacama hides all sorts of odd little secrets, some of which have led to war and death. For instance, during the 19th century, the early explorers of this seemingly worthless region were astonished to find rich deposits of nitrates in the soil. Nitrates are vital in fertilizer, gunpowder, and plant growth, but at that time no one knew how to manufacture them. As a result, such natural sources of nitrates as bat and bird droppings were valuable. In the Atacama, the soil contained high concentrations of nitrates from some mysterious source.

The discovery triggered conflict among Chile, Bolivia, and Peru. When explorers discovered the nitrates, the area was controlled by Bolivia and Peru. However, the American-based mining companies extracting copper from Chile immediately saw the value of mining those nitrate deposits. One thing led to another and the three countries fought the War of the Pacific between 1879 and 1883. Chile won and laid claim to the area, a bitter bit of history that still riles Peru and Bolivia, which lost their access to the sea as a result of the defeat. Chile's economy depended heavily on mining the nitrate deposits until World War I, as the mining companies invaded the desert and extracted some 3 million tons per year. At one point, Chile had a near world monopoly on nitrate. However, the bonanza ended when other countries learned how to manufacture nitrate artificially.

Much later, scientists figured out where the nitrate came from. Turns out, the nitrates came from microscopic plankton living in the nearby ocean. Plankton use the sun's energy to take nitrogen out of the atmosphere and convert it into forms other creatures can use. When the microscopic plankton die, they sink down into the depths of the Atacama Trench. Eventually, winds blowing off the shore of Chile drive surface currents that cause upwelling and bring this deep, cold, nitrogen-rich water to the surface. That plankton-produced nitrogen then gets caught up in the fogs that form regularly along the coast. In the course of millions of years, winds have carried traces of that nitrogen-rich fog up and into the desert high in the Andes. Since the desert has existed for 150 million years, the soil never developed the rich layering of plants, bacteria, fungus, and other organisms that might use the nitrate, so it just accumulated.

As it turns out, the same trench and ocean currents that produced the nitrates have played a crucial role in creating the desert itself. The connection between the geography of the seafloor and the presence of the desert illustrates the importance of vast, global forces like the shifting of crustal plates in determining the boundary between deserts, forests, jungles, and grasslands. The roots of the world's oldest, driest desert go deep down into the Atacama Trench, which runs 100 miles (160 km) off the coast of South America for 3,666 miles (5,900 km). The trench is up to 26,460 feet deep (8,800 m) and 40 miles wide (65 km), with an area of 228,000 square miles (365,000 sq km).