Background and Local Charcoal in Sediments: Scales of Fire Evidence in the Paleorecord

Summary

Stratigraphic charcoal data are used to interpret combustion at a range of spatial and temporal scales. Interpretation is based on assumptions regarding how charred particles are transported to and deposited in sedimentary basins and how the variance structure of stratigraphie profiles is affected by transport. Here we examine evidence for “background” and “local” signals in stratigraphie data based on total abundances of particles and on distributions of particle sizes. Distributions of particle sizes in sediments and in the atmosphere show remarkable consistency, with a ~ 2% decrease in frequency for a 1% increase in particle diameter. Relatively large differences in source distance are required to produce differences in particle size distributions. Thus, stratigraphie samples with disproportionate representation of large particles are likely to represent a nearby source. Simple transport models suggest particles 10⁰ to 10¹ μm have substantially longer atmospheric residence times than do particles > 10² μm. Several factors can produce a “dichotomy” (background vs local) in total abundances of charred particles. Transport during a single experimental burn shows abrupt decline in accumulation at a burn edge and relatively constant values out to 10² m. Saltation and redistribution of particles in surface runoff following fire are expected to focus accumulation within a catchment (local scale). The amount of transport that occurs by these modes is unstudied. The stratigraphie record appears to support a distinction between local, catchment sources vs. fires burning at greater distance. Charcoal profiles often show distinct peaks when fires burn within a lake catchment but are unaffected by fires that occur at greater distance. Broad subcontinental scale patterns in particle accumulation indicate shifts in regional importance of fire. The low frequency variance can be extracted from charcoal profiles and used to differentiate local from background changes in burning. Although the empirical data are still far too few to permit good characterization of particle transport, the evidence suggests utility in the concepts of background and local signals in profiles.