Eolian deposition of forest-fire charcoal above tree limit, Colorado Front Range, USA: Potential contamination of AMS radiocarbon samples

Shallow soil cores from 56 localities along the crest of the Colorado Front Range were processed by water flotation and wet sieving, then examined. for wood charcoal and charred conifer-needle fragments. Charred particles were largest and most numerous in samples from the subalpine forest. Particle size and abundance decreased abruptly in a narrow zone just above timberline, but showed only slight further decrease at higher elevations. Seventy-one percent of 31 alpine-tundra samples contained megascopic charcoal. Charred particles with geometric mean diameters of I to 2 mm were found on rocky summits above 4000 m and in windswept fellfields as Much as 1.6 km from the nearest forest outlier. Long-distance eolian transport of charred material is attributed to raging crown fires in the subalpine forest, and to the region's characteristically high wind velocities. Accelerator Mass Spectrometry (AMS) radiocarbon dates for charcoal from archaeological features above tree limit in the Front Range should be attributed to human occupation only if particle diameters exceed 3 mm, or independent age evidence strongly supports a cultural origin.