Seasonal changes in stable carbon isotopic composition of n-alkanes in the marine aerosols from the western North Pacific: Implications for the source and atmospheric transport

Abstract

To constrain seasonal changes in the long-range atmospheric transport of land-derived lipid biomarker compounds, we investigated the compound-specific stable isotopic composition of marine aerosol n-alkanes collected from 1990 to 1993 at a remote island, Chichi-Jima (27°04′N, 142°13′E), in the western North Pacific. Compound-specific isotope analysis revealed, in particular, strong seasonal changes in the δ 13C values of the C 29 and C 31 n-alkanes (biomarkers for higher plants). Lighter δ 13C values were observed in winter (typically −32 to −34‰), with a transition to heavier values in summer (typically −28 to −31‰). Using a mixing equation and typical end members for C 3 and C 4 plants, we found that this is due to relative increases in the contributions from C 4 plants in the summer season. Using backward air-mass trajectory analyses, it was shown that the Asian continent was the major source region for C 3 plant material during winter/spring, whereas Indonesia/Australia and possibly the Americas were the major source regions for C 4 material during the summer/autumn. Also observed was an enhanced atmospheric transport of n-alkanes from C 4 plants in 1991 summer/autumn during a strong El Nino event, which was associated with forest and bushfires in Indonesia and Australia. In addition to providing information on contemporary processes, this study also provides a base for future paleoclimatological work in ocean sediments.