Sample preparation methods and replicability in macroscopic charcoal analysis

Abstract

We explored the use of hydrogen peroxide (H₂O₂) of different strengths (1, 3, 6, and 9%) in the preparation of lake-sediment samples for macroscopic charcoal determination. In previous work we found that soaking sediment samples in 3% H₂O₂ for 24 h disaggregates samples and removes or bleaches some non-charred organic material, making it easier to distinguish macroscopic charcoal particles from dark, non-charred organic matter. To determine whether this procedure alters charcoal particle abundances and size distributions, we tested the procedure on wood and grass samples containing a known number of laboratory-produced charcoal particles from different size classes (250–500, 500–1,000, and 1,000–2,000 µm). For both sample types, we found that post-treatment numbers were not statistically different from pre-treatment numbers. However, application of hydrogen peroxide solutions of 3, 6, and 9% to fossil charcoal assemblages in a sediment core from northwestern Costa Rica reduced charcoal concentrations markedly, by bleaching or removing particles that were not fully charred. We also tested the replicability of charcoal counts by conducting enumerations on multiple samples from the same stratigraphic depths in a core. Horizontally adjacent samples from the same core may vary in charcoal concentration. This finding has implications for establishing sample volume in macroscopic charcoal analysis.