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Volcanologists have a variety of ways of measuring present-day gas emissions from volcanoes. But what about gas emissions that happened in years past, before measurements were possible? For these periods, it turns out that you can actually read the signature of gas emissions in tree rings!
Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Deborah Bergfeld, research geologist with the U.S. Geological Survey.
The Mud Volcano thermal area is one of the more exciting places in Yellowstone because gas that discharges there has the most magmatic character of any thermal area in the region. In addition to fumaroles, the area contains a variety of other degassing features such as mud pots, steaming ground, and pools of bubbling water. At some pools the bubbling action is simply the result of boiling as liquid water transforms into steam. Other pools are cooler and exhibit vigorous degassing, and the water surface churns and splashes. This roiling action is due to gas forcefully discharging from vents at the bottom of the pool. The gas is a mixture of many components but primarily contains carbon dioxide (CO2).
During the growing season trees take up CO2 from the air and use it along with water and light energy to create new wood. In winter months at Yellowstone the trees go dormant and wood production stops. The process begins anew in the spring when a new layer of wood starts to form. The character of this early wood is different from the wood in the later part of the growing season and makes a distinctive band called a growth ring. Since each ring generally indicates a single year of growth (although sometimes 2 rings, or none, are possible!), it is possible to use tree rings to see how trees grew over time, and how they were affected by environmental conditions like fire, drought, and even volcanic gases.
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.
Scientists used this tree-ring record to show that large spikes in magmatic CO2 emission do occur at Yellowstone, and they further proposed that the pressure pulse of the rising CO2 could have actually caused the seismic swarm in 1978. So even without the ability to actually measure CO2 emissions at the time, we now know that those emissions did fluctuate thanks to the tree-ring record!
Tree rings are also useful for establishing growth histories for whole regions, and how vegetation might have been impacted by changes in hydrothermal activity at Yellowstone. Future editions of Caldera Chronicles will explore how tree rings might even be able to help us go back in time to see how geysers were behaving many centuries ago!
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