Relationships between charcoal particles in air and sediments in west-central Siberia

Abstract

Production and size of charred particles determine transport and deposition in lakes. Lack of such data is a principal obstacle to interpretation of past fire from charcoal profiles. Our two-part analysis includes a calibration study, to assess charred-particle production and transport during fire, and a study of charred particles in sediment. The calibration step establishes the magnitude and size distribution of particle accumulation from traps during a controlled burn of Pinus sylvewtris forest in west-central Siberia. This high-intensity fire consumed 3.71 kg m-2 of fuels and produced 0.0729 kg m-2 of airborne particles, for an emission factor of 0.02 kg kg-1. Particle flux to the ground was 1 to 3 mm2cm-2 yr-1 inside the burn; it declined sharply within 5 m of the burn edge, and it was variable but without trend to a distance of 60 m. Particle-size distributions were conservative, with a slope of 2 on plots of log frequency versus log diameter, and sediment data suggest this slope may steepen as sources bcome more remote and as large particles are progressively lost due to settling. Deposition from the plume is similar to accumulation rates in sediment, with apparent upward bias in sediments as expected from broad geographic patterns in charcoal distributions. During the mid-Holocene charred-particle accumulation in lake sediments (101 mm2 cm-2 yr-1 was greater than observed in particle traps within the experimental burn. Particles were larger, suggesting nearby sources. Rates decreased by 3800 BP to values lower than average rates in particle traps, and samples were depleted in large particles. Low rates and infrequent large particles indicate sources were distant. Accumulation rates and particle sizes were again high from 3400 to 2800 and from 1400 to 700 BP. Close correspondence between the accumulation rates during the experimental burn and in sediments and particle evidence for source area, as well as their agreement with particle-trap data from the experimental burn, suggest that, in this region, fire may have been more frequent and closer in the mid-Holocene than today. We cannot rule out the possibility, however, that changes in charred particle accumulation also reflect changes in supply of sediment to the core site.