Polycyclic aromatic hydrocarbons (PAHs) in sediment cores from lakes Shinji and Nakaumi, SW Japan: A proxy of recent fire events in the watershed

Abstract

PAHs in sediment cores have been investigated as a useful proxy of historic fire events in the watershed of an estuary. Oxygen poor and anoxic lake bottom conditions in estuarine brackish lakes Shinji and Nakaumi could prevent the oxidization of polycyclic aromatic hydrocarbons (PAHs) and preserve a record of PAHs in sediments. PAHs emitted from wood and/or grass burning have been analyzed for composition and concentrations during the last ∼300 years. Carbonized plant material which remained after artificial agricultural fires and heating experiments have been used to estimate fire temperature. The equation relating maximum temperature and hydrogen to carbon (H/C) atomic ratio of the charcoal is as follows: Temp (°C) = 317 x (H/C)−0.708 (R2 = 0.924, 200–700 °C).These experiments and the gas chromatography - mass spectrometry analysis of PAHs on the charcoals suggest that PAHs were primarily produced at temperatures less than 500 °C in the burning of wild plants.The PAHs contents in the two sediment cores were not related to total organic carbon (TOC) contents and emission of industrial waste, and were directly due to fire events in this region. Although TOC contents of the sediments were constant at about 0.5% in Lake Shinji and 1.0% in Lake Nakaumi before ca. 1900 CE, and increased up to about 4% due to recent nutrient discharge, the profiles of PAHs contents were completely different. The sediment cores contained the following PAHs: phenanthrene (P), fluoranthene (Fla), pyrene (Py), benz[a]anthracene (BaAn), benzofluoranthenes (Bflas), benzo[e]pyrene (BePy), indeno[1,2,3-cd]pyrene (InPy) and benzo[ghi]perylene (BghiP). The contents of 6-ring InPy and BghiP, high temperature markers, are lower than those of the other low ring Py, BaAn, Bflas and BePy, showing low to medium intensity/temperature fire events. Relatively higher BaAn contents suggest that the PAHs were carried and deposited in the sediments for a short period after the fire. The PAHs peak groups B (1830–1870 CE) and C (1920 CE) may be attributed to the burning of wood buildings in Matsue City, and the PAHs peak group period D (1970–1980 and 2000–2007 CE) could be mainly due to forest fires. The general increase in PAHs after 1850 CE could be caused by climate change after the Little Ice Age. The PAHs with maximum total peak concentrations at 30–34 cm depth (1850 CE: 4890 ng g−1) in Lake Shinji and at 8–12 cm depth (1970–1980 CE: 2030 ng g−1) in Lake Nakaumi are not at a toxic level.