Studying Stalagmites to Understand the Past and Predict the Future
Many, many hours east of Grinnell, Iowa, and an hour north of Lisbon, Portugal, is an olive farm with trees in “nice little rows,” says Diana Thatcher, assistant professor of chemistry at Grinnell College. There are large rocks behind the trees. And where the rocks meet the ground, there is a cave.
Much of Thatcher’s research begins in that cave. More specifically, it begins with the cave’s stalagmites, the long forms growing from the ground up.
Cornucopias of Climate History
Stalagmites contain information about the climate history in the area they grow. And because stalagmites can be old — the stalagmites in the main cave Thatcher studies are up to 200,000 years old — they even contain information about the paleoclimate, the climate before humans began directly recording it.
She also studies stalagmites from some caves in Colombia, over which the intertropical convergence zone (ITCZ) passes. The ITCZ is a “band of cloudiness and rain” that seasonally migrates north and south. Both the ITCZ and El Niño, a climate pattern in which the surface waters of the eastern Pacific Ocean warm, significantly affect Columbia’s weather. So, studying these stalagmites is also a great way to study how the ITCZ and El Niño have changed over time.
Thatcher’s research ultimately seeks to answer: “What was climate doing in the past, how does that compare to what it’s doing now, and what we expect it to do for the future?” She explains that understanding the paleoclimate improves our understanding of how the climate will change in the future because “climate models need information about the past in order to predict the future.”
A Perilous Journey
Collecting samples from these caves is not for the faint of heart. “One [cave] requires slithering up a rocky, narrow little tube,” says Thatcher. Local cavers help her team get in and out of the caves safely, including telling them which rocks they should not touch. She recalls them warning, “if you touch [that rock], all the ones on top of it might come crashing down on top of you.”
Inside the cave, it is dark and quiet, and smells like wet dirt. When she has samples in hand, she says she “feels a tiny bit guilty [that she] just took this out of the cave, and it took 10,000 years for it to grow there.” Stalagmites can grow as little as 10 micrometers a year, so even a small piece is the product of significant time.
Decrypting Stalagmites
Stalagmites grow as water finds its way into a cave, drips onto a stalagmite, releases carbon dioxide, and leaves calcium carbonate behind. The growth is brighter at the start of the wet season and darker at the end of the dry season. The difference between bright and dark and the resulting layers, however, are not always visible. Thatcher was not expecting to see them when she cut open a stalagmite from a cave in Portugal, but when she put it under a fluorescent light, “suddenly they were right there!” she says.
“You can see the annual layers much like you’d see the rings on a tree,” she says. And just as rings on a tree mark the tree’s age, layers in a stalagmite mark the stalagmite’s age. For the layers that were not visible and to get even better information about the layers that were visible, Thatcher measured the relative concentrations of uranium and thorium, which she used to estimate the age of each layer.
Since water is the basis for stalagmite growth, layer thickness is directly related to the amount of precipitation. “If there’s a thinner layer, it was drier that year than if there was a thicker layer,” Thatcher explains. As useful as that is, it also presents a problem: Stalagmites stop growing during extremely dry years. That makes it more difficult for her to understand the climate during dry years, which is especially important to understand considering the Mediterranean is dry and predicted to become drier.
To learn more from the layers that are there, she and her student collaborators, most recently Xander Wurtz ’22, go down to the molecular level. Back at the College, Thatcher and Wurtz cut and polish the stalagmites before sending them through an instrument that blasts them with lasers. They then identify the elements in each layer and measure their relative quantities. They are especially interested in what carbon and oxygen isotopes there are and how much magnesium there is relative to calcium. This information all contributes to a better understanding of what the climate was when a layer formed.
With thousands of years contained in one stalagmite and thousands of things to learn about each year, “I could probably spend the rest of my scientific career studying just this one little cave,” she says.
The Power of Climate History Paired with Human History
The Mediterranean is becoming drier with climate change. Thatcher’s findings have helped answer, “Has the drying that we’re already seeing in this region been replicated in the past?” It turns out it has, but it has not been this dry in at least 1,000 years.
That knowledge and an improved understanding of the paleoclimate more generally is especially important when paired with human history. Together, they can answer how humans have responded to climate changes in the past and predict what climates we must respond to in the future.
Stalagmites not only help predict the future climate, but they will record it as it happens. How society acts on the climate crisis, whether we resist change or embrace it, will be reflected in the stalagmites’ layers and the elements within.