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What is in a cave? Maybe you think immediately of bats, bears, bones, spiders, and ancient paintings on the walls. CEOAS climate scientist Katie Wendt finds something else in them, too: climate records.
Wendt is clambering into caves to take samples of the drippy, trippy formations collectively referred to as speleothems, the generic term that includes stalactites, stalagmites and other mineral formations found in underground caves. These natural sculptures, resembling icicles, ribbons or sand castles, can contain answers to wide variety of climate-related questions in the layers of minerals they deposit over time. Scientists can take a cross section of the speleothems to examine the constituents of these layers, similar to analyses carried out in sediment cores or tree rings.
Speleothems form as rain hits the ground above a cave and becomes slightly acidic as it travels down through the soil and reacts with carbon dioxide. The acidic water continues on its downward path, through cracks in the bedrock and new paths it etches, eventually emerging and dripping from the cave ceiling. Conditions inside the cave cause calcium carbonate to be left behind as the water evaporates, leaving layer after layer over time. The formations that hang from the ceiling are stalactites and the ones that form on the floor as water drips and falls are stalagmites (you can remember the difference because stalactites, with a t, are found at the top of the cave). In the calcium carbonate (CaCO3) formations, those three oxygen atoms carry critical information that climate scientists crave, as the ratio of the forms of oxygen called isotopes vary with climate conditions.
Wendt explains that scientists prefer to use stalagmites to answer research questions over other kinds of speleothems. “Stratigraphically, they’re just the simplest to understand: The bottom is oldest and the top is youngest,” she says. “We slice them open and you can literally see the layers.” Analyzing parameters in those layers can help scientists like Wendt reconstruct various kinds of histories over hundreds of thousands of years.
One thing that makes speleothems such good recorders of history is that their layers can be very accurately aged using a technique called uranium-thorium dating, which relies on the known rate at which uranium-234 atoms decay to become thorium-230 by shedding particles.
Along with a sample’s age, other parameters can be measured in order to construct regional histories. For example, by examining ratios of oxygen isotopes in speleothems collected in the tropics, scientists can reconstruct rainfall histories. In more temperate environments like Oregon, oxygen ratios reveal changes in temperature records. Other isotopic signatures can be used, too: Ratios of carbon isotopes are related to the kinds of vegetation that are present in the environment above the cave, so scientists can determine whether plant communities that thrive in varying kinds of climates grew above the cave over time. Wendt has also used cave records from Death Valley National Park to determine changes in the depth of the water table over time.
Wendt is in the early stages of planning a project in Oregon Caves National Monument in southern Oregon in which she hopes to study the history of wildfires in the area using trace metal concentrations in stalagmites.
“Depending on the makeup of the bedrock and other factors, we might expect to see a spike in concentrations of metals like mercury, iron and chromium during wildfires. While these metals are usually pretty immobile in the environment, processes that occur during and after a wildfire can mobilize them,” Wendt explains. “So I’ll look at metals in the layers in the cave’s stalagmites and see if there is a peak during known periods of wildfire, and then I can reconstruct the frequency of wildfires potentially for the entire Holocene [approximately the past 12,000 years].”
Because other factors – rainfall, temperature, vegetation type – can be determined from the same samples, Wendt can then infer relationships between these variables and occurrence of wildfires.
This project is in its very early stages; Wendt is hoping for some initial funding that will allow proof-of-concept work in Oregon Caves in the summer of 2024. She plans to try to compare her speleothem data with information collected via other methods, like tree ring analyses, to reconstruct a unique history of wildfire in a region that is likely to see significant fires in the coming decades. This approach has never been tried in the U.S.
Regardless of her findings, Wendt is simply excited to see what the caves can reveal generally. “It’s really cool to think about what Oregon was like during the last interglacial period,” she says. “There are a lot of unanswered questions that I’d like to dive deeper into.”
And what better place for deep dives than below the Earth’s surface?
By Nancy Steinberg, posted January 2, 2024
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