Sources of coal remains from the Jartai Pass Site in Nilka County, Xinjiang, China

The Jartai Pass site is a Late Bronze Age (3600-3000cal. a B.P.) settlement site in Nilka County, Xinjiang, China, containing the world's earliest coal-burning remains including coal and ash. To investigate possible sources of coal used at the Jartai Pass site, extractable polycyclic aromatic hydrocarbons (PAHs) of the coal remains and coals around the site were analyzed. The concentrations of total PAHs ( n-ary sumation PAHs), 16 US Environmental Protection Agency prioritized PAHs ( n-ary sumation 16PAHs), and alkylated PAHs ( n-ary sumation aPAHs) in 14 coal remains samples ranged from 1680-8579, 1090-4193, and 180-4066 ng/g, respectively. Four samples had higher PAH concentrations than the other 10 samples and were dominated by parent 4-6-ring PAHs and alkylated 2-3-ring PAHs. The other 10 samples were dominated by parent 2-3-ring PAHs, indicating different sources for the coal remains at the site. Total PAH concentrations in six coal samples around the site ranged from 2858-21582 ng/g. Three raw coal samples from mines were dominated by parent 2-3-ring PAHs, and the other three outcrop coals were dominated by parent 4-6-ring PAHs and alkylated 2-3-ring PAHs, revealing a significant difference in the PAH composition between raw coal and weathered outcrop coal. Based on the regional coalfield geology and development history of coal utilization in China, we suggest that the coals used at Jartai Pass site were from outcrop coal seams. The four high-PAH coal remains might have been from the coal line on the bank of the Kashgar River. Considering the amount used at the site over a long period, we speculate that the remaining coal may have been from the subsurface layer of the outcrop coal seam, indicating variation in mining depth. This study provides a basis for understanding coal remains unearthed at an archeological site in northwestern China and early coal utilization by humans.