Many of Yellowstone’s lakes have been in existence for thousands of years, steadily accumulating sediments that capture different climatic periods, geologic events, and biological conditions. But how is this valuable record sampled when lakes are located far in the backcountry, and how do researchers use the record?
Stories in Mud: How and why lake cores are collected from Yellowstone’s small backcountry lakes
Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Lauren Harrison, a postdoctoral researcher with the U.S. Geological Survey, and Cathy Whitlock, an emeritus professor at Montana State University.
Yellowstone Lake is the largest lake in Yellowstone National Park, and has been studied using submersible vehicle, acoustic mapping, geophysical (seismic) reflection surveying, and lake coring. The record of sediments in the bottom of the lake is especially valuable, as these sediments contain evidence of everything from past hydrothermal explosions to climate shifts over time. Although still logistically challenging, the large amount of scientific equipment for this work was delivered to Yellowstone Lake relatively efficiently using existing roads and cars. However, many small lakes are far from roads. How do researchers access the sedimentary records held in these small lakes? The answer comes with the help of multiple species and a lot of work!
In the summer of 2022, scientists with a National Park Service research permit collected cores from a small lake located four miles into the backcountry of Lower Geyser Basin. This distance was much too far to carry on foot the multiple boats and lake coring supplies required. It was also located in an area treated as a wilderness, so no motorized vehicles were allowed. Instead, the services of pack mules were employed to carry the equipment in! Four pack mules carried roughly 350 pounds of gear, making the trip to the lake in about an hour. While gear was unloaded at the destination, the mules happily snacked on the grass before heading back to the corral, their work completed.
Then it was time for the researchers to get to work. Dr. Cathy Whitlock of Montana State University designed a lake coring platform that can be assembled from pieces of wood that are no longer than 5 feet (the maximum length that can be packed on a mule) and still has the strength and stability to support the efforts of multiple people pushing the lake corer into the soft sediment and lifting it back out (sometimes a huge effort!). One team assembled the coring platform, while another group inflated the boats—two to support the lake coring platform and serve as the main working area, and another for ferrying people to and from shore and serving as the “core description/wrapping boat.”
Once the platform was secured to the boats and launched into the lake, it was anchored with gunny sacks full of rocks in an optimal place for coring, usually the deepest portion of the lake or near a feature of interest. Then it was time for lake coring! First, the depth of the water was measured, and the appropriate amount of core rods were allocated and labeled to lower the corer to the mud-water interface. The corer used was a Livingstone-type piston corer, which recovers lake cores that are 5 cm (2 in) in diameter and 1 meter (3 feet) in length. In successive one-meter drives, the Livingstone was lowered into the water and sediment to the depth that the last core drive ended. Then, the square rod was locked and the piston secured so that the core barrel could be pushed a meter deeper into the mud. Each new core was brought back up to the coring platform to be extruded, described, and measured before being wrapped and packed into a core box for travel back to the lab. This process was repeated until the corer encountered sediments that were too stiff to penetrate—at Twin Buttes Lake, this allowed for recovery of nearly 6 meters (almost 20 feet) of sediment! After coring was finished, the entire setup was disassembled and re-packed to be carried out by the mule team. Everything, that is, except the lake cores, which are precious cargo that were refrigerated as soon as possible after recovery to discourage mold growth.
Back in the lab, lake cores can be scanned for multiple types of data, including magnetic susceptibility, P-wave velocity and amplitude (which are measures of how seismic waves pass through the sediment), natural gamma radiation, and electrical resistivity—these measures provide information about the composition and physical properties of the sediment. Cores were split along their lengths into two equal halves, exposing the layering of the core (the long awaited “ah-ha!” moment). One half of the core is further studied, while the other half is put into storage for archival. High-resolution photo scans were taken of the cores, along with measurements of composition using a handheld x-ray fluorescence instrument. Cores were then carefully described, subsampled, and interesting layers were studied under a microscope. In the months ahead, samples will be taken for radiocarbon dating, studies of any ash layers, pollen analysis, charcoal analysis, and further chemical or grain-size analysis.
All of these data help researchers determine the age of Twin Buttes Lake and the history and timing of events recorded in the core stratigraphy. Changes in the sediment type, grain size, or chemistry will reveal changes in the basin catchment from past tectonic events and hydrothermal activity. The organic material records variations in lake productivity as a result of past climate change. Analysis of the pollen in the core will be used to reconstruct the vegetation history of the surrounding area, and the frequency and intensity of fires through time will be studied from changes in the abundance of charcoal. The process of collecting cores from small backcountry lakes is logistically challenging, but the information gained from the study of these cores provides a wealth of information related to both local and regional changes over time, each telling their own unique, muddy story.