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42354-AC2
Thermochronology of Paleowildfire
Wildfire has important influences on and feedbacks with climate, evolution, geomorphology, and atmospheric chemistry, but our ability to understand these linkages in deep time has been hampered by difficulties identifying evidence for paleowildfire and its frequency and intensity in the geologic record. We are developing ways of identifying heating by wildfire in detrital mineral grains at and near the Earth's surface. We then use these diagnostic wildfire indicators to: 1) map variations in the intensity and frequency of wildfire in the past; 2) trace wildfire-affected grains through soils, hillslopes, and river networks; 3) prospect for evidence of paleowildfire in the geologic record.
Our primary diagnostic tool for identifying paleowildfire thus far is a characteristic combination of thermochronologic ages in detrital apatite grains. The high-temperature, short-duration heating of wildfire results in an inversion of the normal thermochronometric age relationships between fission-track (FT) and (U-Th)/He radioisotopic systems, due to contrasting kinetics of fission-track annealing and He diffusion. Thermal histories for samples in the upper ~3 cm of exposed rock or soil typically involve temperatures of roughly 250-500 degrees C for seconds to hours, producing these characteristic age inversions.
We developed methods for measuring both He and FT ages (He-FT double-dating) in single grains of apatite from exposed bedrock, colluvial pebbles, and soils as deep as 10 cm, in numerous study sites on the flanks of the Sierra Nevada and Washington Cascades. Large proportions (typically ~25-90%) of detrital apatites in soil and colluvium from these areas are strongly reset by wildfire. Surprisingly, however, wildfire-affected apatites are quite rare (typically 3-10%) in fluvial sediment, even from rivers draining terrain with abundant wildfire signatures in the hillslopes. Many of the apatite grains in hillslopes also show evidence for strong dissolution, in the form of c-axis-parallel dissolution pits and discolored c-axis perpendicular fractures. These lines of evidence suggest that apatite grains in rivers are mostly derived from the steepest parts of the drainage basin, possibly via landsliding, where they enter the fluvial system as large clasts. In contrast, in low-relief, soil-mantled hillslopes, most apatite dissolves in soils. This implies that detrital apatite thermochronologic signals are biased by oversampling the steepest parts of drainages.
In the next phase of our project, we are turning our attention to using this technique, and developing others, to identify evidence for paleowildfire in the geologic record. Thus far our results indicate that Paleogene paleosols from several sites in Wyoming contain apatite with FT ages that are depositional age (and therefore zero at the time of deposition), but He ages that are much older, consistent with widespread wildfire effects at the time of formation of these paleosols. Through the Canadian Institute for Advanced Research (CIFAR) we have also leveraged a new postdoctoral scholar to work on this project. Our ongoing work in this project includes 1) reconstructing detrital evidence for wildfire through the Holocene, using lake cores and comparisons with tree-ring records, and 2) prospecting for paleowildfire in Neogene paleosols from high latitudes, to constrain climate change affecting wildfire intensity and frequency.
