January 6, 1997
New way to gauge ages of stalactites may yield precise climate-change tool
The slow but relentless drippings of calcium-rich water in caves may open a new window on earth's past climate, thanks to a precise dating technique under development at UCSC.
Preserved within the stark beauty of stalactites and stalagmites are two records of changes in the climate of the outside world. One such record is purely physical: When more water oozes from the roof of a cave, the rocky icicles grow faster. If researchers find similar growth spurts in caves over a wide region, they can infer that more rain fell during that period.
The second record relies on a well-known chemical phenomenon. As global temperatures warm up or cool down, and as precipitation patterns change, the ratio of "heavy" to "light" isotopes in rain or snow (defined by the weights of oxygen atoms in water molecules) changes in different regions. For instance, glaciers may lock up more light water, leaving heavier water in the sea. Paleoclimatologists can unmask past temperature fluctuations by detecting these subtle shifts in old layers of ice and in ocean sediments. The mineralized water that makes a stalactite also carries those clues, frozen in as the calcite deposit slowly lengthens.
It's no simple matter to retrieve those records in a useful way. Scientists must know when changes occurred in the growth rate or chemistry of a particular stalactite, but resolving those dates has proven devilishly difficult. The objects grow perhaps an inch every thousand years, making the sample sizes small. Further, traditional techniques for dating ancient geologic samples aren't precise enough to yield accurate ages for layers deposited within the last few thousand years.
That's the value of work by UCSC research specialist Peter Holden, postdoctoral researcher Craig Lundstrom, and undergraduate Andy Jacobson. The team adapted a technique used to date volcanic rocks with high precision. The method uses the radioactive decay of uranium, which stops at several elemental stations on its train ride of decay to lead. Two of those elements are thorium, which has a half-life of 75,000 years, and protactinium, with a half-life of 33,000 years. Others have used uranium-thorium analysis to try to measure the ages of cave deposits, but the addition of protactinium makes the technique much more sensitive.
The researchers analyzed a 10-inch stalactite that had fallen from the roof of a cave on California's central coast. They measured ages ranging from 8,500 years at the top of the stalactite to less than 800 years near its tip. The scientists estimate their technique is accurate to within 1 percent for the older end of the stalactite--corresponding to a time window of about 80 years.
Paleoclimatologist James Zachos of UCSC hopes his group will be able to apply the method to resolve the frequency of terrestrial climate changes during the last 20,000 years on timescales ranging from decades to millennia. For example, cave deposits in the western U.S. and Mexico may reveal the broad effects of El Nino events on regional temperatures and precipitation.
Return to the Currents home page.
Go to UCSC's home page.