Tree Rings and Seismic Swarms

[Image: An otherwise unrelated print of tree rings from Yellowstone National Park, by LintonArt; buy prints here].

The previous post reminded me of an article published in the December 2010 issue of Geology, explaining that spikes in carbon dioxide released by subterranean magma flows beneath Yellowstone National Park have been physically recorded in the rings of trees growing on the ground above.

What’s more, those pulses of carbon dioxide corresponded to seismic events, as the Earth moves and gases are released, with the effect that the trees themselves can thus be studied as archives of ancient seismic activity.

“Plants that grow in areas of strong magmatic CO2 emissions fix carbon that is depleted in [Carbon-14] relative to normal atmosphere, and annual records of emission strength can be preserved in tree rings,” we read. “Yellowstone is a logical target” for a study such as this, the authors continue, “because its swarm seismicity and deformation are often ascribed to buildup and escape of high-pressure magmatic fluids.” The release of gases affects tree growth, which is then reflected in those trees’ rings.

I’ve written before about how tree rings are also archives of solar activity. See this quotation from the book Earth’s Magnetism in the Age of Sail, by A.R.T. Jonkers, for example:

In 1904 a young American named Andrew Ellicott Douglass started to collect tree specimens. He was not seeking a pastime to fill his hours of leisure; his motivation was purely professional. Yet he was not employed by any forestry department or timber company, and he was neither a gardener not a botanist. For decades he continued to amass chunks of wood, all because of a lingering suspicion that a tree’s bark was shielding more than sap and cellulose. He was not interested in termites, or fungal parasites, or extracting new medicine from plants. Douglass was an astronomer, and he was searching for evidence of sunspots.

Slicing open trees, searching for evidence of sunspots. This is a very peculiar—and awesomely poetic—form of astronomy, one locked inside objects all around us.

In the case of the Yellowstone study, a particular seismic swarm, one that hit the region back in 1978, apparently left measurable traces in the wood rhythms of local tree ring growth—in other words, surface-dwelling organisms in the Park were found to bear witness, in their very structure, to shifts occurring much deeper in the planet they live upon. They are measuring sticks of subterranea.

Combine this, then, with Andrew Ellicott Douglass’s work, and you’ve got tree rings as strange indicators of worlds hidden both below and far away: scarred by subterranean plumes of asphyxiating gas and marked by the variable burning of nearby stars. They are telescopes and seismometers in one, tools through which shifts in the sun and in the Earth’s own structure can be painstakingly divined.

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