ShareThisTUCSON, Ariz. - A 44,000-year climate history recorded in a stalagmite in a wet cave in the Santa Rita Mountains lends credence to what scientists have long suspected: When the climate warms globally, the Southwest dries out.
In a paper being published in February's Nature Geoscience, a team of University of Arizona researchers reports abrupt shifts between wet and dry periods that correspond to shifts in cold and warm periods recorded in Arctic ice samples.
Julia Cole, a UA professor of geosciences and atmospheric sciences and a co-author of the report, said the record is pretty clear: "When Greenland is warm, we're dry; when it's cold, we're wet."
Greenland is warming, and scientists predict it will warm more in the coming decades.
"Abrupt" is a relative term in geology. The shifts from wet to dry took hundreds of years.
The measurements made by Cole and her colleagues are from a foot-tall stalagmite found in Cave of the Bells. The stalagmite, a calcium carbonate deposit on the floor deep in the cave, began growing 55,000 years ago and stopped its growth about 10,000 years ago, Cole said.
The dates correspond to a global ice age that reached its peak about 21,000 years ago. During that time, the Southwest overall was wetter and cooler than it is today, but it continued to shift between relatively wetter and drier periods, a phenomenon that Cole believes is caused by the warming and cooling of the north Atlantic Ocean.
Changes in ocean-surface temperatures and circulation influence the storm tracks, Cole said. "That's the leading hypothesis for the cause of the changes."
Most research on drought in the Southwest has focused on surface-temperature variations in the Pacific Ocean -- the phenomenon of El Nino and La Nina -- but this cave record documents a longer-term influence from the North Atlantic, Cole said.
Combining data could lead to more accurate predictions of seasonal precipitation, she said.
The data also raise an alarm, the paper concludes. "The net effect ... on Southwest U.S. precipitation as climate warms under human influence will have profound consequences for water resources, and thus for human and natural systems, in this rapidly growing region."
To analyze the cave record, the stalagmite was removed at its base, and a 1-inch core sample was taken.
The core was sliced, and the slices were then sectioned in 100-micron stages to produce 1,400 samples of powder.
Each sample represents 20 to 100 years, Cole said.
Dates were determined by measuring the known rate of decay of naturally occurring uranium as it turns to thorium in the stalagmite. Relative readings of precipitation were determined by the presence of a stable isotope of oxygen, known as Oxygen-18.
Under a protocol worked out with the U.S. Forest Service, the stalagmite was epoxied back into place in the cave.
Stalagmites are a fairly new scientific tool, especially in this region, Cole said.
They aren't as chronologically precise as tree rings, which clearly delineate each year, but they peer much more deeply into geologic time, revealing long-term change.
This one stalagmite, Cole said, provided "the first continuous, high-resolution, well-documented record of that change."
Cole and her students are investigating at least eight caves in the region, hoping to build a database that will further illumine climate change in the Southwest.
The lead author of the study is Jennifer Wagner, a former graduate student of Cole's. Primary funding came from the National Science Foundation.
(Contact Tom Beal at tbeal(at)azstarnet.com)
(Distributed by Scripps Howard News Service, www.scrippsnews.com.)
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