Biomarker evidence for wildfire activity in surface soils from Mt. Yulong on the southeastern Tibetan Plateau: Sources and controls

Forest fires have become more frequent in many regions throughout the world over recent decades, principally as a result of global warming. Polycyclic aromatic hydrocarbons (PAH) have become recognized as a byproduct of wildfires and an important indicator of ancient wildfires. However, the factors controlling wildfires remain elusive. This study presents analyses of PAH compounds found in 22 surface soil samples from the snow-capped Mount Yulong (Mt. Yulong), on the southeastern Tibetan Plateau (TP; 1906 similar to 4132 m above sea level (asl)), as well as the total organic carbon (TOC) and n-alkanes values for these soils. This area is climatically influenced by alternating Westerlies as well as the Southwest Asian Monsoon and the Southeast Asian Monsoon. PAH ratios, including the indeno[1,2,3-cd]pyrene/(indeno[1,2,3-cd]pyrene + benzo[g,h,i]perylene) and fluoranthene/ (fluoranthene + pyrene) ratios, as well as the ratios of low molecular weight (LMW) PAH to high molecular weight (HMW) PAH (LMW/HMW) and the LMW/& sum;PAH ratio, can reliably confirm that these PAH were primarily produced by biomass combustion. Further principal component analysis (PCA) also supported this conclusion. The highest & sum;PAH concentration (Conc(PAH) 89.9 ng/g) from sites >3800 m asl that exhibit significant wildfire remains further confirming that PAH can be taken as a reliable indicator of the occurrence of ancient wildfires. Analysis of variance confirmed that the composition of local vegetation type has a minimal contribution to Conc(PAH) across the study area, but soil type played an obvious effect on Conc(PAH) as the result of change in absorb ability of PAH. When analyzing samples that had no wildfire remains, there was no correlation between Conc(PAH) and mean annual temperature and mean annual precipitation (MAP). The relatively lower prevalence of wildfire activity (indicated by a low mean Conc(PAH) value of 21.29 +/- 9.80 ng/g) along the Mt. Yulong transect between 2200 and 3500 m asl may be due to a wetter climate (with MAP between 800 and 1800 mm in the rainy season), a climate which would in turn have reduced the combustibility of vegetation fuel and thus limited wildfire activity. Indeed, the highest standardized dimethylphenanthrene ratio (0.76 +/- 1.16), Retene/3-ring (0.81 +/- 0.66) and Retene (0.63 +/- 1.03 ng/g) values all presented between 2200 and 3500 m asl, confirming their reliability as indicators of conifer combustion.