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Caves hold stories of climate change
As you read this, water is dripping onto the floors of caves, seeping through tiny rock fissures and beds of pebbles and sand into the absolute dark of the underground. It’s a commonplace, yet each drop carries clues to the local climate in which it formed. Over millennia, the drops have picked up carbon dioxide and minerals to create stalagmites that, like tree rings, tell how climate has changed.
Trees rings rarely go back more than 1,000 years. And other climate proxies—ice cores (absent in warm climates), marine sediments (absent on land), and pollen deposits (not very precise)—likewise have limitations. So do stalagmites; they exist only in caves formed in carbonate terrains (limestone bedrock, for instance). But, says Corinne Wong, an assistant professor of earth and environmental sciences, stalagmites are “the go-to proxies for reconstructing terrestrial climate on long timescales.” Wong has studied stalagmites in central Texas in an effort to unravel, as she puts it, “the climate processes governing precipitation variability” in a region plagued by drought. And, supported by a mid-six-figure grant from the National Science Foundation, she is engaged in a three-year research project to tease out the local climates around five caves across Brazil’s Amazon rainforest during the Holocene epoch (the 11,700 years from the end of the most recent ice age to the present day). With research colleague Lucas Silva, an environmental scientist at the University of California, Davis, and several Brazilian colleagues, she ventured into her first four Brazilian caves last summer, hunting as far down as 230 feet for what she calls “pristine” stalagmites—little towers of calcium carbonate (CaCO3) devoid of gunk, such as mud washed in from an ancient flood, that might corrupt lab tests. “The ideal sample,” says Wong, “is translucent. You can shine a light through it.”
A brownish gray sample that passes the test sits on a shelf in Wong’s Devlin Hall lab. It weighs about 20 pounds and is the size of a large zucchini. The team found it dislodged from the floor of a cave near Tamboril in far eastern Brazil.
Wong saws the rock samples in half and extracts tiny specimens roughly the size of a grain of salt, at one-millimeter (.04-inch) intervals. She uranium-dates each fleck, grinds it into powder, dissolves the powder in phosphoric acid, and then, with an isotope ratio mass spectrometer, analyzes the carbon dioxide released. The spectrometer uses a magnet to separate molecules of different weights, enabling Wong to find the ratio of a specimen’s “heavy” oxygen (O-18) to typical oxygen (O-16). Heavy oxygen is composed of eight protons and 10 neutrons instead of the typical eight/eight arrangement, and it predominates in wet climates. The more O-18, the rainier the time period of the sample.
While oxygen records are excellent for decoding regional climate, they do not as precisely capture the nuances of local climates, so Wong also tests the ratio of strontium-87 to strontium-86 in each specimen. Strontium-87 is more prevalent in soil, as opposed to bedrock; in a wet climate, where water percolates faster through the saturated bedrock, the 87/86 ratio is higher.
Wong is analyzing historical monsoon intensities at caves shaped, climatologically, by the South American Monsoon System, a massive circulation that broods over most of Brazil, all of Bolivia, and parts of Uruguay, Argentina, and Paraguay. The caves are far apart, spanning the breadth of Brazil. Over the last 10,000 years, a wobble of the earth’s rotational axis has caused solar radiation to increase in the Southern Hemisphere. With land heating more than ocean, the stronger land-to-sea temperature gradient has increased monsoon intensity over the Amazon. But what about distinct local responses? Wong expects the skies above each cave to have reacted uniquely to global and regional warming as clouds drifted in from the South Atlantic, and she will weigh any such variations alongside data accumulated by Silva delineating fluctuations in vegetation outside the caves (e.g., between savanna and forest).
As she explains, the important questions are: “What is the sensitivity of local ecosystems to climate change?” Is there a “feedback loop”—such that, as vegetation adapts to local climate change, the climate itself is affected? “We’re looking at climates reacting to global warming,” says Wong, who is also analyzing stalagmites collected by scientists from the North American Arctic to assess the response of permafrost to climate change.
Bill Donahue is a writer based in New Hampshire.