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ScienceShot: Underground Rain Gauge?

Cave formations known as stalactites are often pretty, but they also may provide clues about past climates. In some instances, the surfaces of these icicle-like accumulations of minerals sport series of rings, ripples, and ridges (image) which are typically spaced between 5 and 10 millimeters apart. A new analysis suggests that the ridges develop when the steady flow of mineral-rich water is disturbed as it runs over tiny imperfections in a stalactite's surface. Small eddies in the flow lead to increased deposition of calcium carbonate at certain regularly spaced locations, researchers reported online this month in Physical Review Letters. Cross sections through stalactites reveal that even though water is flowing downward, these regular ridges, or crenulations, migrate upward as years pass because deposition of minerals tends to take place on the upstream side of the ridges. According to the team's model, the rate at which the crenulations migrate upward doesn't depend on the temperature in the cave or the concentration of dissolved minerals in the water. However, the migration rate does depend on the flow rate of the water across a stalactite's surface—a hint that scientists might be able to use crenulated stalactites as rain gauges for past eras, since the amount of water dripping down a stalactite's surface typically depends on the precipitation falling on the surface above the cave.

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