A ROCKY ROAD TO THE PAST BY KEVIN BIJU ART BY EMMA HEALY TWO HUNDRED AND FIFTY TWO MILLION YEARS AGO, THE WORLD WAS ENGULFED IN A NIGHTMARISH SCENARIO AKIN TO WHAT WE FEAR TODAY.
environmental science FOCUS Surging carbon dioxide levels, combined with increased radiation from the Sun, dramatically increased global temperatures. Ocean surface temperatures reached upwards of 104 degrees Fahrenheit. Consequently, mass extinctions occured, with marine and terrestrial life suffering huge losses in biodiversity. Until now, the scientific community has struggled to determine the relative importance of the two forces that drove the Permian-Triassic mass extinction event: volcanic activity and erosion. A study led by Ryan McKenzie, a postdoctoral associate at the Yale Department of Geology and Geophysics, has now proposed a solution. The study argues that on the time scale of the past several hundred million years, volcanoes have been the principal driver of climate change. These revolutionary findings are key to understanding long-term climate change, and thus, may prove informative in our present-day combat against global climate change. The greenhouse effect Carbon dioxide (CO 2 ) is a double-edged sword—both vital to life yet potentially harmful. On one hand, much of Earth’s plant life depends on CO 2 to produce food for itself and consumers, like us. At the same time, increasing CO 2 levels since the industrial revolution have led to rising global temperatures, which pose a risk to the global ecosystem. How does this happen? The Earth’s atmosphere normally reflects much of the Sun’s invisible infrared radiation back into space, thereby preventing surface temperatures from becoming too high. However, when sufficiently concentrated in the atmosphere, CO 2 can form a blanket of sorts, which traps some of this radiation and prevents it from leaking back to space. Thus, CO 2 is aptly termed a greenhouse gas. Various processes regulate the levels of CO 2 in the atmosphere. Volcanic eruptions, which release gases from the Earth’s interior, contribute to atmospheric greenhouse gases and raise global temperature levels. Chemical weathering, on the other hand, has the opposite effect. When CO 2 reacts with water vapor, carbonic acid is formed. This weak acid then eats away at rocks and other surfaces. Other forms of chemical weathering include burial of carbonate minerals, along with burial of organic carbon. Thus, chemical weathering is a CO 2 sink and has the ultimate impact of decreasing global temperatures. The scientific community recognizes these two forces—volcanism and weathering—as the principal drivers of long-term climate change. Due to these two processes oscillating and changing pace over time, the content of CO 2 in the atmosphere is in constant flux. Thus, the Earth’s temperature has risen and fallen multiple times within its history, creating various periods of global warming followed by global cooling in the form of ice ages. The unearthing begins The study began with McKenzie’s fascination with the links between climate change and biodiversity. “I became interested in the anomalies characteristic of the Cambrian period,” McKenzie said. “A lot of species extinction occurred, which many people attribute to the Cambrian having one of the highest atmospheric carbon dioxide levels of the past six hundred million years.” McKenzie set out to discover the root cause of this carbon dioxide flux. First, McKenzie and team needed to obtain a record of Earth’s volcanic history. The Earth is made up of multiple tectonic plates, which are large pieces of the Earth’s crust. When an oceanic plate collides with a continental plate, a subduction zone is formed. The oceanic plate sinks deeper into the earth, liberating water in the process. This water gradually seeps upward, melting the hot mantle rocks and forming magma in the process. This magma finally rises to the surface and forms a chain of active volcanoes. Unfortunately, however, it is often difficult to track the formation of these volcanic emissions through Earth’s history because erosion and destruction of volcanoes obscures the important data. In addition, the commonly used sea-level approach to track volcanic rates through time relies on too many vague assumptions. This is where zircons come in. Zircons, otherwise known as zirconium silicate, are grains of sedimentary rocks that crystallize from magma. Young zircon is especially prevalent in the subduction zones of continental volcanoes, such as the Andes and the Cascade volcanoes. Zircon grains are able to withstand high degrees of erosion, so they represent untampered records of volcanic activity. Fortunately, due to zircon’s uranium impurities, the age of zircon samples can be determined very precisely through radioactive isotope analysis. Thus, if one can trace an abundance of young zircon to a specific period, this period likely experienced massive continental volcanic activity. McKenzie and his team used this property of zircon to contribute to a precise record of continental volcanic activity throughout the geologic timeline. The team could now accurately map the relationships between carbon dioxide levels and volcanic activity. IMAGE COURTESY OF RYAN MCKENZIE ►Dr. Ryan McKenzie stands with a fuming Mt. Bromo in Indonesia. McKenzie analyzed sedimentary rock to more closely link volcanic emissions to long-term climate change driven by carbon dioxide concentration. www.yalescientific.org December 2016 Yale Scientific Magazine 13
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