A compound-specific n-alkane δ13C and δD approach for assessing source and delivery processes of terrestrial organic matter within a forested watershed in northern Japan

We measured molecular distributions and compound-specific hydrogen (delta D) and stable carbon isotopic ratios (delta C-13) of mid- and long-chain n-alkanes in forest soils, wetland peats and lake sediments within the Dorokawa watershed, Hokkaido, Japan, to better understand sources and processes associate with delivery of terrestrial organic matter into the lake sediments. delta C-13 values of odd carbon numbered C-23-C-33 n-alkanes ranged from -37.2 parts per thousand to -31.5 parts per thousand, while delta D values of these alkanes showed a large degree of variability that ranged from -244 parts per thousand to -180 parts per thousand. Molecular distributions in combination with stable carbon isotopic compositions indicate a large contribution of C3 trees as the main source of n-alkanes in forested soils whereas n-alkanes in wetland soil are exclusively derived from marsh grass and/or moss. We found that the n-alkane delta D values are much higher in forest soils than wetland peat. The higher delta D values in forest samples could be explained by the enrichment of deuterium in leaf and soil waters due to increased evapotranspiration in the forest or differences in physiology of source plants between wetland and forest. A delta C-13 vs. delta D diagram of n-alkanes among forest, wetland and lake samples showed that C-25-C-31 n-alkanes deposited in lake sediments are mainly derived-from tree leaves due to the preferential transport of the forest soil organic matter over the wetland or an increased contribution of atmospheric input of tree leaf wax in the offshore sites. This study demonstrates that compound-specific delta D analysis provides a useful approach for better understanding source and transport of terrestrial biomarkers in a C3 plant-dominated catchment. (c) 2009 Elsevier Ltd. All rights reserved.