POLYCYCLIC AROMATIC HYDROCARBONS PRESERVE SEDIMENTARY RECORDS OF WILDFIRE IN DIVERSE ECOSYSTEMS

Polycyclic aromatic hydrocarbons (PAHs) are produced from the incomplete combustion of organic matter and may be preserved in the sedimentary record as an indicator of wildfire. The distribution of PAH compounds can relate to the intensity of wildfire and type of vegetation that burned, as well as to secondary transport, preservation or processes that lead to degradation. Researchers often reconstruct fire history from sedimentary records in floodplains and alluvial fans. However, this approach generally requires preservation of measureable charcoal, posing a challenge for reconstruction where charcoal is not well preserved. PAHs are hydrophobic in nature, readily adsorb to clays and organic materials in soils, and stay attached to hillslope material during downslope and downstream transport. We measured the distribution of PAHs in sedimentary sequences from both the semi-arid Colorado Front Range (within the Boulder Creek CZO) and the cool-temperate hardwood forest of the NE United States to examine the record of these fire indicators in diverse landscapes. Data from sedimentary archives that date from the past 10,000 years at these sites show the high concentrations of PAHs at times of increased fire. In Colorado, colluvial deposits preserve sediment following upslope wildfires and show a clear correlation between PAHs and discrete events throughout the record. Sedimentary cores from the NE USA represent wetland settings with local hillslope and stream inputs, and show peak levels prior to 8000 years kyr and in shallow sediments associated with anthropogenic activity. Comparison of PAH concentration between these locations is complicated by the grain size and organic content of the deposits and by the physical transport processes and mixing of the organic-bearing component at these sites.