Reading stalagmites
Victor Polyak and Yemane Asmerom, scientists at the University of New Mexico, used uranium-thorium dating to determine the age of the bands in stalagmites taken from caves in the Carlsbad Caverns National Park and Hidden Cave in the Guadalupe Mountains National Park, both in southwestern New Mexico. What they found was a clear link between changes in the local climate and the way of life of the people living there at the time. They found evidence of climate changes that had changed the course of history.
Water that falls as rain and then percolates through the soil is naturally slightly acid because of the carbon dioxide that dissolves in it to form carbonic acid (H2CO3). If the underlying rock is limestone, the acid may dissolve it away. Gradually the flow of water washes out enough rock to form a cave below ground. If the water table falls below the level of the cave floor, the cave will dry out, but water may continue to drip from the ceiling. This water contains dissolved minerals. As it evaporates into the dry air inside the cave, some of the minerals are left behind as deposits growing from the ceiling. They grow like icicles hanging downward. These are stalactites. Some of the water trickling down a stalactite drips to the floor of the cave and evaporates there. This leaves a mineral deposit that grows upward from the floor, as a conical mound called a stalagmite.
Stalagmites are fed by water dripping onto them. If the amount of water feeding them varies from time to time, that variation will be preserved as identifiable bands in the stalagmites—a thick band representing a rainy period and a thin band representing a dry period. Dr. Polyak and Professor Asmerom dated the bands in the stalagmites they studied and used the thickness of the bands—representing their rate of growth—to determine whether the weather was wet or dry in each year. In this way they compiled a year-by-year history of the climate of that part of New Mexico over the past 4,000 years.
They found that the climate became wetter around 4,000 years ago. Until about 3,000 years ago it was as wet as the present climate, or possibly somewhat wetter. Then it became wetter still and remained wetter than today's climate until around 800 years ago. After that the weather grew drier.
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