Thursday, March 31, 2016

Of Ice Cores & Isotopes

Earth’s atmosphere has a history — not just in terms of temperature and composition, but also in the dynamics of its motions and chemistry. By tracing ultra-rare molecules in the present-day atmosphere and back into the past, Laurence Yeung, Assistant Professor of Earth Science at Rice University and recent Clarke Award recipient, is setting out to trace that dynamic history. “In the same way you'd tag a shark to figure out what its migration patterns are,” he explains, “you can exploit the natural tags that Mother Nature gives us in these stable isotopes.” Stable isotopes are atoms of the same element that differ slightly in mass thanks to an extra neutron or two, and they act as passive tracers in the atmosphere. A heavy isotope’s extra bit of mass can affect the physical and chemical processes it undergoes, concentrating or diluting it with respect to its lighter siblings. As a result, the ratio of these isotopes, like typical oxygen-16 (which has 8 protons and 8 neutrons) to heavier oxygen-17 (8 protons, 9 neutrons) or oxygen-18 (8 protons, 10 neutrons) can hold clues to a given reservoir’s past.

Yeung is at the front lines of the field, pioneering a technique that takes oxygen isotope analysis one step further. Atoms of oxygen-18 (18O) make up only 0.2% of all of the oxygen atoms on Earth. Even more rare are oxygen molecules (O2) that contain more than one of these heavy isotopes, and it’s these 18O-18O molecules that Yeung and his group are after. By taking measurements of the present-day atmosphere at different altitudes above the surface, and comparing those values to a detailed model of atmospheric chemistry and circulation, the amount of mixing going on between different parts of the atmosphere can be sorted out. “There's a little bit less 18O-18O in the troposphere than there is in the stratosphere,” says Yeung. “So when those two reservoirs mix...then you establish some characteristic proportion of stratospheric air vs. tropospheric air.”

These measurements and calculations can be compared against many other methods for getting at the same question, calibrating the 18O-18O technique before using it to look at similar quantities in samples of past atmosphere. Where does one come across these secret stashes of ancient air? One key place to look is in the ice deep under the glaciated regions of the Earth, where layers of snow that accumulated and compressed into ice hundreds or even thousands of years ago trapped atmospheric gases in tiny bubbles. “These bubbles end up trapping gases from the ancient atmosphere, and it tells you something about the recent past, up to something like a million years at this point.”

Yeung’s search for ultra-rare oxygen molecules brought him to the National Ice Core Laboratory in Denver, CO, which houses 17 kilometers of precious ice collected from Greenland and Antarctica. Measurements of the oxygen isotopes in the layers of ice themselves, as well as the gases contained in the bubbles, have established a detailed record of past climate, revealing quantities like global temperature and atmospheric composition over the past several hundred thousand years. By extending this research into the 18O-18O realm, Yeung hopes to shed light on some of Earth’s dynamical history as well — a subject of great interest today. “There are thermodynamic aspects of climate, things like temperature, chemical composition,” Yeung explains, “but then there are all these dynamical aspects of climate in Earth's atmosphere that aren't nearly as well constrained…How often do the lower and the upper atmosphere overturn, how often do they talk to each other, how rapidly or vigorously do they mix? How strong are storm systems in the past?” These are questions get to the heart of how the atmosphere works as an Earth system, and thanks to these ultra-rare molecules, we have a new promising way of getting answers.
—Podcast & post by Meg Rosenburg

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