Arizona Caves: An underground archive of climate data


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Holding a faded red backpack out in front of her like an overstuffed life preserver, Sarah Truebe squeezed her 6-foot frame through a tight opening in the rocks. She inched her way through utter darkness, eventually emerging into a cavernous room in the underbelly of the Santa Rita Mountains southeast of Tucson.  

Small, calcium carbonate stalactites hang from a rock wall in the Cave of the Bells in the Santa Rita Mtns southeast of Tucson. The stalactites form over thousands of years when minerals are carried by rainwater through the earth and deposited in the cave [Credit: Green Valley News]

Truebe switched on her bright fluorescent headlamp, and the ghostly faces of four amateur cavers emerged from the gloom. One of the first rules of caving is never to go alone, so she brings friends, colleagues and anyone else brave enough to follow on her expeditions underground. 

Truebe is a University of Arizona graduate student who’s looking for clues about past climates inside undeveloped caves. “Our trips are especially badass because we explore wild caves,” she said. “There are no lights, paths or tour groups where we’re going.” 

After more than 30 expeditions through the Cave of the Bells, Truebe said she feels at ease in the pressing, muggy darkness of the ancient cavern. She said she makes the hour drive to the cave once a month. 

A graduate of Stanford University, Truebe is a doctoral student in the University of Arizona’s Department of Geosciences. She works alongside some of the world’s foremost researchers in paleoclimatology-Jonathan Overpeck, Julia Cole, Thomas Swetnam and other scientists who examine Earth’s ancient past to understand how the planet reacts to periods of abnormal warming and cooling. Local, national and international policymakers use the data collected by UA researchers to formulate responses to global climate change. 

Back in time 

Truebe’s research focuses on deciphering the ancient patterns of the Southwest’s monsoon rains. While studying rain inside a cave may seem like an oxymoron, Truebe said data on thousands of years of rainfall are available inside Arizona’s numerous limestone caves. The Cave of the Bells is one of many locations she visits to access this vast natural archive of data. 

Inside the cave, Truebe studies whitish rock formations known as speleothems. The most common types of speleothems are stalactites and stalagmites, which are created through a process that starts when rainwater hits the ground and seeps into the soil. The rainwater picks up carbon dioxide trapped in air pockets in the soil. The water reacts with the carbon dioxide to make a slightly acidic solution that slowly dissolves carbonate-based rocks, such as limestone. 

Now mineralized, this water solution continues through layers of the ground until it reaches a cave. When it encounters air inside a cave, the solution undergoes another chemical reaction that produces calcium carbonate. 

Truebe said. It deposits the calcium carbonate in discernible layers, or rings, inside the cave. Slowly, the rings build up, forming stalactites on the ceiling and stalagmites on the floor. 

By studying the different layers of speleothems that accumulate over thousands and in some cases hundreds of thousands of years, Truebe can determine how much rainwater the monsoon brought to the desert during a specific period of time. She uses a drill to extract core samples from stalactites and then analyzes the samples back in her lab. 

Truebe calculates the age of a core using a method called radiocarbon dating. Over thousands of years, the amount of carbon in a sample decays by half. By analyzing the amount of carbon left in a core, she can pinpoint the approximate time when the sample formed. She determines how much water fell by analyzing the different core layers from that period. 

“I’m mostly interested in uncovering rain patterns from 5,000 to 7,000 years ago,” she said. “During this time the world was experiencing a similar period of warming as to what we’re seeing now.” 

Truebe said it is of the utmost importance to understand how warmer weather affects monsoon rains. “The monsoon rains play a critical role in the desert ecosystem,” she said. “If we can understand how a warmer climate affected the monsoons in the past, we might be able to predict how a warmer climate will change them in the future.” 

One of the trickier parts of Truebe’s research is figuring out the source of the ancient precipitation. 

Rainfall in the Southwest comes from either the Pacific Ocean or the Gulf of Mexico. The rainwater that originally seeped through the ground retains enough characteristics to identify its source by analyzing the mineral composition of a core sample. Water from the Gulf of Mexico is carried by powerful monsoon storms. This water is “heavier,” or richer in hydrogen and trace metals, than water from the Pacific Ocean, which loses its heavy metals as it travels a longer distance over several mountain ranges. 

Collecting ancient climate data 

Truebe works directly under Julia Cole, a world-renowned expert in gathering data from cave stalagmites and coral reefs. “As paleoclimatologists, we try to extend the records of climate variation back in time,” said Cole, a professor in the UA Department of Geosciences. 

Cole said Truebe’s work is important because the climate record extends back only 50 to 100 years in many areas of the world. This record consists of precipitation patterns, temperature readings and other data observed or collected by humans. 

“For a lot of places, the climate record only goes back about a century, and then the data runs out,” she said. “In order for us to create an accurate climate model to make predictions and formulate policy, we need some kind of benchmark.” 

To create a benchmark model, paleoclimatologists use data from numerous sources “We try to piece together what the history was by using proxy records like tree rings, coral reefs and layers in cave formations,” said Tom Swetnam, director of the UA’s Laboratory of Tree-Ring Research. 

These proxy records are combined to form a model of how the Earth’s climate reacted to drastic changes in the past. “We can reconstruct all sorts of things from these natural archives,” Swetnam said. “In addition to common variables like precipitation, we can reconstruct forest fire histories, volcanic eruptions, ocean currents and much more.” 

Rings found in tree trunks give researchers a year-by-year account of what the climate was like as each ring was forming. “Our methods of analysis have become a lot more advanced than merely counting the rings,” Swetnam said. “We now have tools for extracting information on the cellular level.” By measuring levels of heavy metals embedded in wood, researchers can determine whether trees growing in certain places were subjected to different types of pollution. 

“We’ve been collecting wood for over a hundred years on this campus. We have a huge collection,” he said. “I believe we’ll learn a whole lot more in the future.” 

Swetnam said his lab can construct a continuous climate model back 9,000 years “based on really, really old trees and dead trees, particularly the bristlecone pines that grow at high elevations in Nevada, Utah and eastern California.” Many of the specimens found in the White Mountains of California are 4,000 to 5,000 years old. 

Applying the research 

Benchmark models of past climates can be used to make predictions about future weather and climate patterns. These models can also help officials formulate environmental policy. 

Swetnam said scientists in his lab worked with the U.S. Bureau of Land Management to develop a water management plan for the Colorado River, which provides water for numerous metropolitan areas in Arizona, Nevada and California. A decade-long drought has led to low water levels in the Colorado River Basin. 

Managing this dwindling resource is vital for future generations, Swetnam said. By sampling tree rings in the Colorado’s upper watershed, scientists can deduce flow patterns dating back more than a thousand years. 

“Our scientists have been working with water managers to use the long historical perspective for planning purposes,” he said. “They use the long record to interpret how frequently droughts occur in the past.” These data can then help predict when droughts might occur and how severe they might be. 

Swetnam said the UA is a world leader in providing data for environmental policymakers. “Research in paleoclimatology is one of the great strengths of the UA,” he said. “All around campus you could probably add up around 20 scientists who have national and international reputations in terms of understanding climate history.” 

Swetnam said the field of paleoclimatology has come a long way in the last few decades with the advent of powerful supercomputers and satellite mapping. But no matter how great sophisticated and detailed real-time observations might be, advanced computers can’t predict the future without long-term data. 

“Despite the fact that we have all these tools and so on, they can’t replicate what was going on in the past in the absence of data,” Swetnam said. “Our research here is critical as we move forward to face new and greater climate challenges.” 

Accurate predictions 

Back in the cave, Truebe perched on a precariously narrow ledge to collect a water sample from a small glass tray she had left behind on a previous visit. She crouched low under a large stalactite and, with a plastic pipette, suctioned a few drops of water into a small bottle. 

She collects samples to determine the mineral composition of the water carried from the surface to the cave. “I want to make sure the water coming through the rock is consistent with what I’m finding in other locations,” she said. “If the results aren’t consistent, then the data isn’t valid.” 

Truebe hopes to finish collecting data on monsoon rain patterns by the end of the summer. Then she’ll analyze the data and work with climate modelers to create a “what if” scenario for policymakers. 

“Until I collect enough data to make an accurate prediction, I don’t want to make any guesses,” she said. “Once we have enough data, we’ll be able to formulate a climate model that policymakers can use in the future.” 

Author: Will Ferguson | Source: Green Valley News [May 24, 2011]



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