The marvels of the Grand Canyon extend beyond its dramatic scenery. Underground, vast cave systems hold clues to the region’s future climate.
Over millennia groundwater percolating down through the rocks above dissolves calcium-rich minerals and redeposits them as long icicle-shaped mineral deposits known as stalactites and stalagmites. Somewhat like tree rings, the layers in these deposits contain a record of the region’s rainfall.
Water molecules naturally contain oxygen with eight protons and eight neutrons, as well as a heavier version or ‘isotope’ with two extra neutrons. The lighter isotope evaporates more quickly in warmer weather, allowing scientists to differentiate between winter and summer precipitation seeping down into the caves over time.
In 2017 project leader Matthew Lachniet from the University of Nevada Las Vegas rappelled into a cave on the South Rim and extracted a stalagmite from its floor. His research team studied the oxygen isotopes, along with uranium isotopes which showed the age of the stalagmite’s layers.
Results revealed that during rapid warming after the last Ice Age between 8,500 and 14,000 years ago, higher summer rainfall contributed to groundwater recharge much more than today. Nowadays almost all recharge comes from winter snowmelt.
The findings suggests that ongoing climate warming may lead to higher summer rainfall and summer groundwater recharge across the Colorado Plateau.
How much this will be offset by predicted decreases in winter precipitation and snowpack, along with higher evaporation rates in warming summers, remains to be seen.
This Earth Note was written by Diane Hope and produced by KNAU and the Sustainable Communities Program at Northern Arizona University.
