January 4, 2018
When you hear someone talk about “caves,” you probably think of bats, darkness, claustrophobia, and deep passages below the very ground you are standing on. The last one is especially true if you’re in Texas! It turns out that the mineral properties of stalagmites from these environments can help unfold the history of past climates, in times before the existence of satellite and other human instrumental data (e.g. rain gauges and thermometers). Studying the climate patterns from before the advent of instrumental data is important because we can develop a process-based understanding of how climate has varied in the past, allowing us to contextualize present climate variability and future climate projections.
How can a stalagmite record what happened in the past? It’s not a time machine — or is it? Every time a water droplet falls from the cave ceiling, it carries with it the signal of the climate above or around it. Drip by drip, a stalagmite grows from the ground up, forming laminations (much like tree rings) through time. The geochemical signals that are locked within these laminations waits for researchers like myself to come about and unlock the mysteries stored within them. The geochemical signals/properties that are sensitive to environment variables (i.e., temperature and rainfall) and that are commonly measured include stable isotopic composition, trace element concentrations, and laminae thickness to name a few. Researchers have used these geochemical properties within stalagmites all over the world to study the rise and fall of the Hung Dynasty in China, the evolution of the East Asian Monsoon, and the history of fire in Australia.
For part of my dissertation work I’m investigating stalagmite formations in a shallow cave in New Mexico. Remember all the images that you associate with caves? Now, forget those! The shallow cave I study is atypical in that it formed above ground, sits behind a 120-foot water fall and is accessible to surface light from its deepest part (Figure 1 and Figure 2).
Sitting Bull Falls Cave lies in the Guadalupe Division of the Lincoln National Forest in New Mexico. A lot of the caves in the United States are either under private or public land. The reason why I can work at this particular site is because of the close working relationship we’ve built with the Forest Service officers. A strong working relationship between land managers and researchers is key to develop a successful scientific project.
Traditional studies of stalagmites have focused on deep caves, which record climate variability on decadal to centennial time scales, Now, recent studies conducted by the Banner Research group here at UT Austin have highlighted the importance of investigating the geochemistry of stalagmites that grow in shallow caves. The researchers found that the calcite sculptures found close to the outside environment grow very fast compared to in deeper caves, thus allowing for sub-annual variability to be recorded in their geochemistry. Motivated by this peaked interest in using atypical cave environments to establish high-resolution climate variability, I will be reconstructing sub-annually resolved time windows of the past climate that will then equip me with the data to answer interesting climate questions. Additionally, my site is in a region that has a rich history of Native American culture. In fact, it is commonly believed that the Native American community carved the steps in the rock face back in the 19th century, thereby making the cave easily accessible to researchers today.
I’m fortunate enough that my dissertation work has a strong field component to it which takes me out to my cave site every other month. The drive is 8 hours long but worth every minute once I am out in the field (Figure 3 and 4). Given that my site falls in the Lincoln National Forest, I get to camp under clear starry skies when the weather is nice.
During these field excursions, I deploy temperature loggers to collect hourly temperature data in the cave and surrounding area. Additionally, to study the modern cave environment I regularly measure carbon-dioxide levels and relative humidity of the cave (Figure 5).
In the lab, I analyze the geochemistry of stalagmite samples (Figure 6) to establish their geochemical variability through time. All of these methods will aid me to piece together the climate history of this understudied region that is sensitive to the changing climate.
Cover photo picture credit: Peter Carlson
