Combustion controls on sedimentary charcoal: Experimental insights for Quaternary paleofire interpretations

Sedimentary charcoal is a widely used fire proxy in Quaternary science, forming the basis of fire variability reconstructions, and fire-climate-vegetation-human relationships across the Late Pleistocene and Holocene. But many uncertainties regarding its proxy systematics persist. These uncertainties include ambiguity regarding how combustion parameters (e.g. temperature and oxygen availability) influence charcoal production rates and charcoal morphometry, the relationship between sedimentary charcoal and finer sized atmospheric wildfire products (e.g., <1 μm), and the modes of charcoal dispersal and deposition from a source fire. To address these uncertainties, we (1) designed a novel furnace apparatus to conduct controlled experimental burns (manipulating temperature and air flow as variables), (2) collected atmospherically-mobile charcoal on filters (AMCF) and residual ash charcoal (RAC) fractions as an analog for atmospheric and geological transport modes, and (3) collected fine-mode particle size distributions using a scanning electrical mobility spectrometer (SEMS) to compare charcoal to finer scale atmospheric wildfire aerosols. We found that the bulk of charcoal produced consistently partitions into the residual ash charcoal (RAC) fraction relative to the atmospherically mobile charcoal on filter (AMCF) fraction, regardless of fuel type and combustion conditions. We saw little variation of charcoal morphometrics as a function of combustion temperature and oxygen conditions, but observed that low temperature and high oxygen conditions produce the most charcoal, which is dominantly in the RAC fraction. Additionally, our comparison of charcoal and SEMS data suggests that differing formation processes (e.g., flaming or smoldering combustion) may inhibit the direct comparability of charcoal and finer particulates. Overall, our data provide the first experimental perspective on the relatability of charcoal and finer-sized (e.g., <1 μm) atmospheric aerosols, opening the door for future analyses. Further, these experiments provide a mechanistic view of how Quaternary paleofire reconstructions reflect combustion processes.