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Carbon isotope analysis of n-alkanes in recent lake sediments from the Tibetan Plateau

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Aichner, B. , Herzschuh, U. and Wilkes, H. (2007): Carbon isotope analysis of n-alkanes in recent lake sediments from the Tibetan Plateau , The 23rd International Meeting on Organic Geochemistry, 9th - 14th September, Torquay, England. .
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Abstract:

Since the Tibetan Plateau has an essential influence on atmospheric circulations, it is important for our understanding of the dynamics of global changes and therefore interesting for paleoecological research (Lehmkuhl and Owen, 2005). Sedimentary organic matter is an excellent recorder of environmental conditions. Different n-alkane patterns have been assigned to different sources of organic matter. δ13C of aquatic macrophytes is usually in the range of C3 plants, however limited CO2 availability due to high pH or alkalinity or due to high respiration at dense plant stands, can lead to HCO3- assimilation and hence to a shift towards more positive δ13C values (Allen and Spence, 1981). To test correlations between environmental and sedimentary organic matter parameters, recent sediments of 50 Tibetan lakes (1500 5300 m a.s.l) were analysed for amounts of n-alkanes, bulk organic δ13C and compound-specific δ13C values of n-alkanes. Furthermore, bulk δ13C of the dominant aquatic macrophyte species Potamogeton pectinatus (L) was measured. The sediments can be separated into three groups by hierarchical cluster analysis according to their n-alkane patterns. Some samples show a remarkable terrestrial influence of long-and mid-chain n-alkanes with a strong odd-over-even predominance (type I) while others are dominated primarily by mid-chain n-alkanes derived from aquatic macrophytes with no odd-over even predominance (type II). Type III sediments are similar to type II, but contain also a strong terrestrial component (high relative amounts of C29, C31 and C33 n-alkanes). Short-chain n-alkanes of algal origin are scarce in most of the lake sediments (Figure 1). δ13C values of Potamogeton bulk biomass and of bulk organic matter range from -6.0 to -21.4 and -18.4 to -28.1, respectively. δ13C values of long-chain n-alkanes (C29-C31) show relatively little variations between -29 and -34. However, in some samples a shift towards more positive values, up to -21, is visible in mid-chain n-alkanes. Generally, odd-numbered n-alkanes are more enriched in 13C than even-numbered ones (Figure 1). In our lake sample set, pHs of the lake waters are varying from 7.1 to 10.5. Comparing bulk δ13C values of sediments and plants with pHs, a trend to more positive values at high pHs is visible. The correlation between δ13C of Potamogeton biomass and of mid-chain n-alkanes (C20-C26) is weak in all samples, but rather good in type II sediments (R2: 0.35 - 0.72). There is also a good correlation between bulk δ13C values of sediments and plant organic matter (R2 = 0.71). We conclude that limited CO2 availability at high pHs can be one explanation for a positive shift of δ13C values of mid-chain n-alkanes in macrophyte dominated lakes, a fact which can be applied for the interpretation of δ13C values of n-alkanes in sediment cores.

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