The CO2 discharging from Yellowstone’s thermal areas is distinctive from CO2 typically found in air. The carbon atoms in CO2 emitted as a volcanic gas contain 6 protons and 6 neutrons, making it an isotope, or form, of carbon that we refer to as carbon-12. In contrast, the carbon in CO2 in the air is mostly carbon-12, but some atoms have 8 neutrons — an isotope called carbon-14. Trees growing in areas where magmatic CO2 is discharging take up a mixture of both types of CO2, and the ratio of carbon-14 to carbon-12 atoms in atmospheric CO2 is preserved in tree wood.
Looking at the ratio of carbon isotopes preserved in tree growth rings thus provides a tool for geoscientists to look back in time. By analyzing the carbon from individual tree rings it is possible to determine if there were year-to-year changes in the amount of carbon-14 taken up by that tree. Less carbon-14 indicates greater emissions of magmatic CO2. At Yellowstone a prerequisite to performing this kind of study is locating old trees that can provide a lengthy time line. This can be a challenge given the regularity of forest fires and the harsh growing conditions on the edges of thermal areas.
The first such study at Yellowstone took place in the Mud Volcano thermal area in the south part of Hayden Valley, at a site known as Cooking Hillside. The thermal area is located near the center of a 7-month-long seismic swarm that began in May 1978. By the end of the swarm new thermal features had developed, existing features increased in temperature and vigor, and rising soil temperatures started to kill trees. The anomalous heating event ceased during the winter of 1979. Analysis of individual growth rings from a lone surviving tree showed the amount of magmatic CO2 was stable through the mid-1970s but began to increase in 1978. By 1979 the magmatic CO2 output recorded by the growth rings was nearly 5 times greater than the previous year—a spike in emissions that persisted into 1980.