Charcoal abundance measurements are affected by freeze-drying

Abstract

Fire has been a part of Earth system processes for millions of years, and its use has been accelerated once hominins and humans entered landscapes. Charcoal amounts, usually measured by the number of particles or surface area of particles per sample, are used to reconstruct aspects of fire history in sedimentary records. Freeze-drying is commonly performed on sediment cores and soils that will undergo geochemical analysis, and the process is known to affect outcomes. We compared charcoal abundances in paired freeze-dried and non-freeze-dried samples from lake sediment cores to test the hypothesis that freeze drying reduces the amount of recoverable charcoal in paleoecological reconstructions. Wilcoxon Rank Sum tests showed that particle counts and surface area measurements of charcoal are significantly higher in non-freeze-dried samples compared with freeze-dried samples. Our results indicate that sediments should not be freeze-dried prior to charcoal analysis, and if it necessary, that subsamples of fresh material should be collected first.