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Science - Associated Press - updated 5:36 AM ET Oct 5
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Friday October 5 5:36 AM ET

Stalagmites in Caves Show History

By PAUL RECER, AP Science Writer

WASHINGTON (AP) - Stalagmites created over thousands of years in New Mexico caves preserve a rainfall climate record of the arid Southwest and help explain why ancient Americans fled the high, dry mountains and settled in river valleys some 700 years ago, researchers say.

Victor J. Polyak of the University of New Mexico said that two-foot-long stalagmites taken from Carlsbad Caverns and from two other caves contain mineral deposition rings that correspond to levels of precipitation in the region.

Polyak, first author of a study appearing Friday in the journal Science, said the rings formed in the stone by the slow dripping of mineral-rich water are similar to growth rings found in tree trunks.

``We sliced the stalagmites and then made thin sections from the bottom to the top,'' said Polyak. Each section contained distinct bands of calcite deposition.

``The drier years will form thin bands (about .05 millimeter thick) and the wet years will form thicker bands (about .2 millimeter),'' said Polyak. ``During very dry seasons, the stalagmites stopped growing.''

The researchers found evidence for wet periods from about 4,000 years ago to about 800 years ago. They then compared this record with the known cultural history of the ancient Americans, such as the Pueblo, who lived in the Southwest.

``We found that the changes in the cultural history correspond to the climate changes,'' said Polyak. ``The correlation seems consistent from 4,000 years ago to 700 years ago when the stalagmites quit growing.''

He said that the stalagmite growth apparently stopped at the beginning of a dry period that continues to this day. It was about 700 years ago that the Pueblo abandoned their mountain and high plateau villages and moved into river valleys.

Polyak said a wet period, that started about 3,000 years ago, roughly corresponds to an era when the ancient peoples settled and started raising maize or corn.

A gradual drying set in about 1700 years ago, reflected in thinning mineral deposition rings, and ``there was probably a struggle to hang on'' before the people abandoned their homes and moved to places with dependable sources of water.

Although the climate became generally drier about 700 years ago, Polyak said the stalagmite record suggests there were wet eras that lasted for years before the drying trend resumed.

Jeffrey S. Dean, a professor at the University of Arizona, Tucson, and an expert on Southwest climate history as reflected in tree ring growth, said the Polyak study ``reinforces previous evidence to a great extent.''

However, Dean said that the stalagmite record has some unexplained inconsistencies in details when compared with other records.

For instance, Polyak reports evidence of a wet period from about 440 to about 290 years before the present.

``That was in the middle of one of the driest periods that we know about,'' said Dean.


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