Occurrence characteristics and source appointment of polycyclic aromatic hydrocarbons and n-alkanes over the past 100 years in southwest China

The extensive anthropogenic activities and their potential impacts during the Anthropocene have led to a research focus on the sedimentary record. In the present study, the occurrence and temporal variations in the fluxes and compositions of n-alkanes and polycyclic aromatic hydrocarbon (PAHs) were investigated in ²¹⁰Pb-dated sediment cores from a small catchment near the outflow Tanglangchuan in the western Dianchi Lake, China. The continuing organic contamination (i.e. PAHs and n-alkanes) from inputs to outputs has been of concern. To trace the sources and driving forces, multi indicators were applied. Results showed that the total organic carbon (TOC) contents and C/N ratios varied in the range of 4.20–12.30 mg g⁻¹ dw and 8.64–15.65, respectively, indicating algae- and terrestrial plant-derived organic matter (OM). The flux of Σn-alkanes ranged from 0.67 to 38.86 μg cm⁻² a⁻¹ with a peak in 2013. The long-chain n-alkanes (Σn-alk₂₆–₃₅) and short-chain n-alkanes (Σn-alk₁₂–₂₀) accounted for 44.02%–49.38% and 35.32%–41.49% of the Σn-alkanes, respectively. A bimodal distribution of n-alkanes was displayed in the sediments implying the sedimentary OM may be derived from a mixed source of endogenous and exogenous origin. The posterior peak (≥n-C₂₆) compounds in the highest abundance were n-C₃₁ or n-C₃₃ with a significant odd-numbered C predominance, representing terrestrial plant-derived OM. Whereas n-C₁₆ was rich in all sediment profiles reflecting crude oil or incompletely combusted fossil fuel-derived source. The indicators analysis showed an increasing trend of the contribution from terrestrial plants and wet to drought climate during 1873–2019. The sedimentary flux of ΣPAHs ranged between 11.71 and 1231.54 ng cm⁻² a⁻¹ and the percent of high-ring PAHs rose annually indicating enhanced anthropogenic activities. In the past 147 years, the results of present study highlight the influence of the agricultural and industrial economy on the catchment outlets.