In some apatite crystal ages, wildfire occurrence is recorded rather than landscape evolution, at least in mountain regions in the western United States.

Characteristics of the mineral apatite record the ages of surface rocks and grains, which are often used to reconstruct topographic and erosional histories of mountain ranges and river systems. However, these properties can be reset by wildfires. A new study shows just how far-ranging these altered grains are in the environment.

Peter W. Reiners at the University of Arizona, USA, and colleagues1 found a unique signature in the radioactive isotopes in apatite crystals caused by wildfire alteration. The team collected a range of samples from the Cascade and Sierra Nevada ranges in the western United States, a region whose wildfire history is well known. The samples collected from the upper three centimetres of bedrock, detrital grains in soils, and stream-bed sediments all had measured ages of near-zero and diagnostic age reversals of two different radioisotope systems — the wildfire heating signature. Most river sediments, in contrast, had apatite-derived ages close to that of unaltered regional bedrock and are probably derived from landslides.

These results suggest that apatite grains from low-slope areas within mountain regions — especially detrital grains in soils and streams — are more likely to be reset by wildfires and should not be used for tectonic studies. They may, however, be a useful proxy for ancient wildfires.