Wildfire influences soils, vegetation, erosion, sediment transport, and atmospheric chemistry, and has done so since the advent of land plants on Earth several hundred million years ago. Currently available proxies for tracing the effects of wildfire in both modern and paleoenvironments are largely limited to combusted organic matter, however, which limits our ability to understand wildfire on both short and long-timescales. We have developed several new wildfire proxies in order 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 wildfire proxies are geochronologic, mineralogic, and geochemical. The first relies on radioisotopic dating of detrital apatite grains in soils and exposed bedrock. High-temperature, short-duration heating associated with burning results produces characteristic signatures in thermochronometric ages of fission-track (FT) and (U-Th)/He systems, due to contrasting kinetics of fission-track annealing and He diffusion. Using “double-dating” methods on single detrital grains in burned regions we have identified regions of severe to moderate heating, and inverted these signatures for the temperatures and durations of wildfire heating. Spatial gradients in these thermal histories in soils and exposed rock provide well-constrained models of wildfire dynamics on local scales.
On larger scales, we have identified wildfire-affected detrital grains in modern landscapes, documenting extremely common and widespread wildfire effects in forested regions of the western U.S. This work has also led to the recognition of rapid apatite dissolution in soils in temperate environments, and the conclusion that most detrital apatite in large rivers derives from landslides or large blocks that enter rivers. We have also identified detrital grains from East Antartica that we tentatively interpret as representing wildfire at about 40-45 million years ago, a time of relatively warm temperatures and widespread vegetation in Antarctica.
Mineralogically, we have examined the effects of wildfire heating, through both laboratory experiments and natural samples, on transformations of Fe-oxides and clays, and changes in dissolution rates of apatite and other soil phosphates. Our laboratory experiments showing transformations of goethite and magnetite to hematite and maghemite during heating are supported by findings of increased magnetic susceptibility and higher abundances of ferric-Fe phases in lake sediments containing charcoal.
Finally, we are examining the use of depletions in volatile elements such as mercury in soil profiles, and enrichment of the same elements in distal sedimentary deposits, as indicators of wildfire. This work has been done in conjunction with Postdoctoral Fellow Abir Biswas, and complementary funding from the Canadian Institute for Advanced Research (CIFAR).
