Effects of particle size and pretreatment methods on the morphometry of grass charcoal particles: Implications for morphometric analysis of microcharcoal particles

The morphometry of sedimentary charcoal particles illuminates the source fuel types. This study explores the variation in morphometry (specifically, the length-to-width (L/W) ratio) of charcoal particles of different sizes, using charcoal collected from Miscanthus sinensis (Japanese pampas grass) fields following controlled burning. We also investigated the impact of pretreatment processes on sedimentary microcharcoals. The results indicate a general decrease in the L/W ratio with decreasing particle size, with the microcharcoal exhibiting a substantial decrease. The mean L/W ratios for size categories of 250 µm–1 mm, 125–250 µm, 63–125 µm, and <63 μm in size are 6.42, 5.54, 4.94, and 3.45, respectively. Furthermore, pretreatment processes for microcharcoal decrease the L/W ratio. Consequently, the L/W ratio of grass microcharcoal (<125 μm) likely falls within 50–80% of the ratios observed in grass macrocharcoal (>125 μm), necessitating caution in interpreting microcharcoal data. Our findings emphasize the importance of considering particle size and pretreatment effects when interpreting paleofire records and highlight the need for further research to establish robust cutoff values for fuel type inference based on charcoal morphometry, particularly for microcharcoal.