SOURCE OF LONG CHAIN N-ALKANES AND THEIR STABLE ISOTOPE SIGNATURES IN LAKE SEDIMENTS: IMPLICATIONS TO PALEOENVIRONMENT RECONSTRUCTION

Stable isotope compositions of long chain n-alkanes in lake sediments have become a routine means of tracking the changes of terrestrial vegetation and climate. A key assumption of this application is that lacustrine long-chain leaf wax components used for isotope analysis are primarily derived from terrestrial higher plants. Recent studies indicated that a large portion of certain long chain n-alkanes in sediments deposited in lakes at the Qinghai-Tibetan Plateau was contributed by submerged aquatic plants, thus lacustrine sediments carry a mixed isotope signals from both terrestrial and aquatic environments. We analyzed molecular carbon and hydrogen isotope compositions from in situ plants with known terrestrial and aquatic sources as well as from lipids extracted from lake sediments at various depths of the Lake Qinghai. These data are compared with various vegetation, climate, and hydrological signals of both terrestrial and aquatic environments. Our combined carbon and hydrogen isotope data analysis (1) confirmed considerable contributions of various long-chain n-alkanes from aquatic plants that are controlled by lake water depth; (2) indicated that lipids from different sources can be distinguished by analyzing ratios of isotope values from individual n-alkanes; (3) helped explain variations and discrepancies between certain molecular isotope records and environmental factors previously observed in lacustrine sediments; (4) suggested necessary refinement of using lake sediment stable isotopic data to reconstruct ancient climate, hydrology, and terrestrial vegetation. While a better understanding of sources and stable isotope signals from lipids in lake deposits will improve molecular isotope analysis of lake sediments as an environmental proxy, the universal nature of what has been observed in the Lake Qinghai as well as the biochemical mechanism for lipid production and their isotope behaviors of aquatic plants remain to be further explored.