Estimating the mass flux of charcoal from sedimentary records: effects of particle size, morphology, and orientation

Reconstructing past fire requires ways to compare concentration estimates obtained by different charcoal analysis methods. These methods estimate concentrations from area measurements or particle counts on pollen slides, on sieves, and on thin sections, and they do so at different magnifications. Depending on particle properties, each method might give substantially different results. Image analysis of charred particles in thin sections of lake sediments is used to parameterize relationships among different concentration estimators. We determined distributions of particle size, shape, and orientation in undisturbed sediments and assessed how those distributions affect concentration estimates. Particle-diameter distributions were conservative across different magnifications. Particle shape was also conservative, with major:minor axis ratios near 2.5. Our derived relationships among estimators could be used to improve comparability for all but the smallest particle classes, which tended to be unpredictable in orientation. Simple equations with nomograms for necessary parameter values are provided. We derive a settling velocity index for composite samples that can be used to estimate relative differences in source region and dispersion. Comparisons of estimates from lakes in eastern North America showed geographic coherency, with highest values in the midwest and low values in the nor theast. A single exception to the eastwardly decline came from a sand plain site in New Hampshire, where fires were expected to be more common than in the prevailing northern hardwoods of the region. Application of our method to previous estimates of mass fluxes in eastern North America show values closer to those expected based on current understanding of fire behaviour than were previous, uncorrected estimates.