Assessment of the difference between mid- and long chain compound specific δDn-alkanes values in lacustrine sediments as a paleoclimatic indicator

Abstract

Previous studies have proposed that the difference between compound specific hydrogen isotopic composition of n-alkanes (δDn-alkanes) of possible terrestrial origin and aquatic origin in lacustrine sediments is an indicator of hydroclimatic conditions for a lake environment (e.g. terrestrial evapotranspiration of the lake drainage area or evaporation/inflow ratio for the lake: Sachse, D., Radke, J., Gleixner, G., 2004. Hydrogen isotope ratios of recent lacustrine sedimentary n-alkanes record modern climate variability. Geochimica et Cosmochimca Acta 68, 4877–4889; Sachse, D., Radke, J., Gleixner, G., 2006. δD values of individual n-alkanes from terrestrial plants along a climatic gradient – implications for the sedimentary biomarker record. Organic Geochemistry 37, 469–483; Mügler, I., Sachse, D., Werner, M., Xu, B.Q., Wu, G.J., Yao, T.D., Gleixner, G., 2008. Effect of lake evaporation on δD values of lacustrine n-alkanes: a comparison of Nam Co (Tibetan Plateau) and Holzmaar (Germany). Organic Geochemistry 39, 711–729) and this proposed indicator has been applied to a paleoclimatic study of Koucha Lake (KCL) on the northeastern Tibetan Plateau (NETP). However, the application was not very successful, due to an apparent discrepancy between the reconstructed possible hydroclimatic variation based on the proposed indicator and precipitation amount variation based on pollen in the same lake (Aichner, B., Herzschuh, U., Wilkes, H., Vieth, A., Böhner, J., 2010a. δD values of n-alkanes in Tibetan lake sediments and aquatic macrophytes – a surface sediment study and application to a 16ka record from Lake Koucha. Organic Geochemistry 41, 779–790). We report new compound specific δDn-alkanes data for sediments since the last glacial from Hurleg Lake (KLK) and Genggahai Lake (GGH), both also on the NETP (ca. 300km apart). Together, δDn-alkanes data from the three lakes were analyzed to assess the difference between mid-chain and long chain compound specific δDn-alkanes values in the sediments as a paleoclimatic indicator. Our results indicate that mid-chain (n-C23 and n-C25) and long chain (n-C27, n-C29 and n-C31) n-alkanes, as compounds representative of aquatic and terrestrial origins, respectively, were valid only for KLK; correspondingly, the use of differences between mid-chain and long chain compound specific δDn-alkanes as a possible paleoclimatic indicator was feasible only for this lake. Hence, our results emphasize the complex relationship between the chain length of lacustrine n-alkanes and their actual origins, and suggest that the correlation analysis of δDn-alkanes data for different chain length could be part of the possible evaluation method for the application of lacustrine compound specific δDn-alkanes data in paleoclimatic studies.