Carbon isotope composition of long chain leaf wax n-alkanes in lake sediments: A dual indicator of paleoenvironment in the Qinghai-Tibet Plateau

The carbon isotope composition (delta C-13 values) of long chain n-alkanes in lake sediments has been considered a reliable means of tracking changes in the terrigenous contribution of plants with C-3 and C-4 photosynthetic pathways. A key premise is that long chain leaf wax components used for isotope analysis are derived primarily from terrigenous higher plants. The role of aquatic plants in affecting delta C-13 values of long chain n-alkanes in lacustrine sediments may, however, have long been underestimated. In this study, we found that a large portion of long chain n-alkanes (C-27 and C-29) in nearshore sediments of the Lake Qinghai catchment was contributed by submerged aquatic plants, which displayed a relatively positive carbon isotope composition (e.g. -26.7% to -15.7% for C-29) similar to that of terrestrial C-4 plants. Thus, the use of delta C-13 values of sedimentary C-27 and C-29 n-alkanes for tracing terrigenous vegetation composition may create a bias toward significant overestimation/underestimation of the proportion of terrestrial C-4 plants. For sedimentary C-31, however, the contribution from submerged plants was minor, so that the delta C-13 values for C-31 n-alkane in surface sediments were in accord with those of the modern terrestrial vegetation in the Lake Qinghai region. Moreover, we found that changes in the delta C-13 values of sedimentary C-27 and C-29 n-alkanes were closely related to water depth variation. Downcore analysis further demonstrated the significant influence of endogenous lipids in lake sediments for the interpretation of terrestrial C-4 vegetation and associated environment/climate reconstruction. In conclusion, our results suggest that the delta C-13 values of sedimentary long chain n-alkanes (C-27, C-29 and C-31) may carry different environmental signals. While the delta C-13 values of C-31 were a reliable proxy for C-4/C-3 terrestrial vegetation composition, the delta C-13 values of C-27 and C-29 n-alkanes may have recorded lake ecological conditions and sources of organic carbon, which might be affected by lake water depth. (C) 2015 Elsevier Ltd. All rights reserved.