INVESTIGATING THE USE OF POLYCYCLIC AROMATIC HYDROCARBONS (PAHS) AS A PROXY FOR HOLOCENE FOREST FIRES ON THE COLORADO FRONT RANGE

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental contaminants generated during the incomplete combustion of organic carbon material. PAHs are hydrophobic in nature and can adsorb to clays and organic materials in the soil. These compounds are highly resistant to biodegradation, which allows PAHs to persist in the soil at potentially measurable quantities after a forest fire event. This study examined the distribution of PAHs in modern and ancient soils within the Boulder Creek Critical Zone Observatory (BcCZO) and Fourmile Canyon in the Colorado Front Range to assess whether the spatial and temporal pattern of PAH accumulation preserves a record of Holocene fire frequency. The semi-arid climate of the Colorado Front Range produces conditions conducive to regular forest fires, and there are numerous documented fires throughout the Holocene. We analyzed PAH distributions in soil and sediments from the BcCZO from Gordon Gulch and Betasso Gulch, as well as from Fourmile Canyon, which in 2010 experienced an intense, 16.5 km2 forest fire as well as severe flooding in 2013. Depth profiles of PAHs show a strong correlation between PAH concentration and incidence of fire, with an observed increase in concentration corresponding to each burned and/or ashy buried layer relative to samples gathered above or directly below each layer. Depth profiles from recent (2010) fires show maximum total PAH concentrations occur at a depth of 5 cm, suggesting influence of volatilization at the surface PAHs, as well as translocation of PAHs downward through the soil column. Lighter PAHs (less than four rings) were preserved in greatest quantities and display a clearer trend among recently burned areas. Over longer timescales, it appears that total PAH concentrations as well as heavier molecular weight PAHs (four rings or more) offer a more accurate proxy for reconstructing fire occurrence.