Stalagmites Show Drying of the Great Basin 8,200 Years Ago

This article was originally published in The Midden – Great Basin National Park: Vol. 17, No. 1, Summer 2017.

Photo of a stalagmite that has had samples taken from it for geochemical analysis in order to study changes in climate.

Photo by Elena Steponaitis

by Elena Steponaitis, Postdoctoral Fellow, Tulane University

The Great Basin has experienced dramatic hydroclimate changes over the past 30,000 years and beyond. Today’s dry Great Basin is very different from the much wetter conditions that prevailed over most of the last 30,000 years.

Stalagmites from caves are rich archives of past climate information. Geochemical analyses of stalagmite calcite can provide information about past conditions in and around a cave. Importantly, stalagmites can be precisely dated using a radiometric method that exploits the decay of uranium to thorium within the sample over time.

With the help of the staff at Great Basin National Park, our research group at MIT used stalagmite samples from Lehman Caves to study the timing of this drying event in the region. By studying the timing of past climate changes in the Great Basin relative to global-scale climate events, we can begin to understand what kinds of changes might be in store in the future.

Lehman Caves was an ideal location for this type of study because so many of the cave’s stalagmites had been previously broken off and piled up inside the cave by late 19th and early 20th century occupants, so our work did not involve doing any additional damage to cave formations.

Over the course of several trips into the cave, our group took small samples for dating from the tops and bottoms of the already broken stalagmites in order to find samples in the age range of interest. Additionally, with the help of the park staff, we were able to conduct a cave monitoring study that helped us interpret the geochemical results we obtained from the stalagmite.

The selected stalagmites were slabbed and polished and then finely sampled down their growth axes for geochemical analyses that can be used to study changes in climate. Together, these analyses produce a time series of data points that span the amount of time between when the stalagmite started growing and when it stopped growing. Stalagmite records are very useful in part because we can sample the stalagmite for dating at multiple points along the growth axis, so we can get a very good idea of exactly when climate changes occurred in the record.

Our record from Lehman Caves suggests that the region dried rapidly after about 8,200 years ago. This is broadly consistent with existing records from many parts of the Great Basin, but the dating precision of this stalagmite record makes it particularly useful for trying to understand how the Great Basin “reacted” to known globalscale climate events. Interestingly, the timing of rapid drying of the Great Basin shown in this record is approximately coincident with the timing of the collapse of the Laurentide Ice Sheet, a large event which is visible in paleoclimate records from around the globe. Though more work needs to be done to understand the exact climactic mechanisms that caused this abrupt drying in the Great Basin, this study in Lehman Caves is an important step towards understanding when and how this drying happened.