Wildfire thermochronology and the fate and transport of apatite in hillslope and fluvial environments

Wildfire heating of the outer few centimeters of exposed rock or soil generates short-duration, high-temperature thermal events that produce characteristic thermochronologic signatures in minerals. Contrasting activation energies of fission track annealing and He diffusion in apatite lead to a kinetic crossover whereby wildfire heating resets fission track (FT) ages much faster than (U-Th)/He ages, resulting in "inverted" FT-He ages in single grains. This can be used to trace wildfire-affected detritus at the Earth's surface. We show that in exposed bedrock, inverted apatite FT-He ages vary systematically with depth to ∼3 cm, and detrital clasts on hillslopes also show strong but heterogeneous wildfire-resetting signatures. In soils, colluvium, and low-order channel sediments, strongly wildfire-reset apatite grains are abundant, and in some cases dominate the population of detrital apatite, to depths at least as great as 10 cm. Wildfire-reset apatite is rare, however, in fluvial sediments sampled from larger basins, indicating a strong fractionation of apatite populations from hillslopes to rivers. Characteristic dissolution features in hillslope apatite and slower relative dissolution rates of other common minerals suggest that wildfire-reset apatite grains are rare or absent in rivers because they dissolve relatively rapidly in soil profiles. Apatite that does contribute to fluvial sediments is likely to be dominantly derived from bedrock landslides in steep regions or from large clasts containing grains protected from both wildfire heating and dissolution. This means that apatite in fluvial sediment is spatially fractionated with respect to its sources in the catchment, even if catchment erosion rates are spatially uniform.