Morphological Classification of Microscopic Charcoal as an Indicator of Fuel Type: Developing a System and Applying it to Records from Two Lakes in Southern Pacific Costa Rica

Charcoal fragments in lake sediments are proxies for past fire activity. Charcoal presence indicates fire occurrence, while the morphology of charcoal particles can reveal the types of plants that burned, or fuel types, and hint at whether fires were natural or set by people. I developed a classification system for charred microscopic particles using a reference collection developed from laboratory charcoalification of modern plant samples and limited prior literature. The classification includes three primary morphologies: graminoid leafy, dicotyledon (dicot) leafy, and woody along with multiple subtypes. I applied this classification to pollen slides spanning the last two millennia prepared from sediment cores from two nearby lakes in southern Pacific Costa Rica, Laguna Zoncho and Laguna Santa Elena. Previous studies documented prehistoric agriculture, forest recovery, historic agriculture, and changes in climate at both lakes that show broadly similar trends but with differences in the scale and possibly timing of prehistoric agricultural decline and in historic patterns of biomass burning. I compared charcoal morphologies in multiple cores from Laguna Zoncho, and between Lagunas Zoncho and Santa Elena, to explore whether the new classification system could improve understanding of site differences. The Laguna Zoncho 1997 core showed a positive relationship between grass pollen and Graminoid charcoal percentages and between tree pollen and Compact charcoal percentages, and supported previous findings of agricultural decline and forest recovery in the century leading into the Spanish Conquest. The Santa Elena core demonstrated a more negative relationship between grass pollen and Graminoid charcoal and between tree pollen and Compact charcoal. Charcoal morphology showed little change around the Conquest, with trends suggesting morphology may be more sensitive to the expansion of agriculture than its cessation. Higher Graminoid charcoal percentages corresponded with high charcoal concentration and influx at both lakes, indicating that fires during intervals of high fire activity were characteristically fueled by grasses and sedges. Percentages for Graminoid, Compact, and Leafy charcoal morphology were similar across the Laguna Zoncho cores, suggesting that charcoal morphology may not be strongly affected by the position of a core within a lake, though more research is needed.