Does 14C in Tree Core Record the History of CO2 Emission and Deformation at Yellowstone?

The 640-ka Yellowstone caldera represents the most recent of three overlapping caldera-forming events at the NE end of the Snake River Plain. Continued unrest is evident in cycles of uplift and subsidence of the caldera floor, possibly due to changing magmatic intrusion rates. An alternative hypothesis attributes uplift to partial trapping of magma-derived fluids beneath zones of mineral deposition, and subsidence to episodic rupture of the sealing layers. Rupture releases the trapped brines or gases, mainly CO2, into the overlying hydrothermal system. Because of the low solubility of CO2 in hot water, changes in CO2 input to the hydrothermal system should translate fairly rapidly into variations in CO2 efflux at the surface. Yellowstone hosts one of the largest hydrothermal systems in the world, with spatially separated thermal areas containing hot springs, geysers, and fumaroles. The total efflux of magmatic CO2 from all the thermal areas within and near the caldera boundary has recently been estimated at 45,000 tonnes/day, but no data are available on past CO2 emissions for comparison with episodes of uplift and subsidence. We are studying the 14C content of cores collected from trees at the edges of large thermal areas to reveal the long-term temporal record of magmatic CO2 discharge to the atmosphere. Magmatic CO2 contains no 14C, and photosynthetic uptake by trees of CO2 with some magmatic component produces wood that is depleted in 14C relative to normal atmospheric levels. The magnitude of the depletion in any tree should be a qualitative measure of the strength of magmatic CO2 emissions from the nearby thermal area. To date, measurements at three thermal areas have shown air-CO2 concentrations at canopy height that exceed normal atmospheric concentrations by 2-15% due to magmatic CO2 emissions. Seven trees were cored at these thermal areas. A background tree was also cored from an area well away from the influence of magmatic carbon. The 2005 ring from the 7 trees in the thermal areas shows 14C depletion ranging from 2.6 to 12.5%, in good agreement with the measured atmospheric concentrations. Analysis of several older rings in two of the cored trees from the thermal areas shows consistent depletion in 14C compared to correlative rings in the background tree. The 14C record in one tree shows a large depletion of 14C that may correspond with historically observed changes in local thermal activity. The high precision now available in 14C accelerator mass spectrometry (AMS) analysis should allow detection of temporal changes even when 14C depletion is in the 2% range. We plan to broaden the study to include more trees and thermal areas, and focus our analysis on years when the direction or style of deformation changed (e.g., 1984-1985). Results of the study should help determine whether CO2 buildup and release is a viable mechanism to explain uplift and subsidence of the Yellowstone caldera.