TRENDS IN SEDIMENTARY CHARCOAL SHAPES AND PAH ASSEMBLAGES CORRESPOND WITH BROAD-SCALE LAND USE CHANGES: INSIGHTS GAINED FROM A 300-YEAR LAKE SEDIMENT RECORD FROM EASTERN VIRGINIA, USA

Fires leave material evidence behind in the stratigraphic record; these products range from visible, ‘macroscopic’ pieces of charcoal to single molecules. Sedimentary charcoal is a useful and widely used fire proxy, and Polycyclic Aromatic Hydrocarbons (PAHs) are growing in use in the field. However, many uncertainties exist regarding the linkages of charcoal characteristics and PAH assemblage to fuel type burned. In this study, we leverage a 300-year sediment record from Lake Matoaka in Williamsburg, USA to (1) make inferences about the fuel sourcing of certain charcoal morphotypes and morphometries, (2) compare PAH assemblages throughout the record in order to characterize the chemical fire history of the core, and (3) to explore the relationship between the two proxies. Fortuitously, historical events and land use changes are well constrained in Williamsburg, enabling us to link the history of the region to changes observed in our record. We found that the morphology and morphometry of particles in the Matoaka sediments changed in response to broad scale shifts of historical land use and population variations, whereas total PAHs increased through time. Prior to the American Revolutionary War (ca. 1780 CE), charcoal morphologies indicative of agricultural burning coincided with agricultural expansion and development, and after the fall of the Confederacy (ca. 1865 CE), changing charcoal morphologies reflected increased population and urban development. Comparison of these historical variations with established keys for charcoal morphology shows that certain irregular and structured charcoal particles are likely sourced from burning herbaceous fuels. We also found evidence that charcoal particle circularity is modulated by physical weathering. Our array of analyzed PAHs demonstrates a relationship between PAHs representative of gymnosperm burns with PAHs representative of more particulate-phase burn products. Lastly, we propose modifications to an existing charcoal morphological key to create the first key for application in the SE USA. Overall, this research underscores the need for more work establishing paleofire methodologies in the SE USA, as well as the consideration of limiting factors in each proxy not yet explored in existing literature.