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

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Citation:
Aichner, B. , Herzschuh, U. and Wilkes, H. (2007): Carbon isotope analysis of n-alkanes in recent lake sediments from the Tibetan Plateau , 4th International Limnogeology Congress, 11th -14th July, Barcelona, Spain. .
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Abstract:

d13C values of aquatic macrophytes are 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 d13C values. Epicuticular leaf waxes of plants contain n-alkanes and different n-alkane patterns have been assigned to different sources of organic matter. Long- and mid-chain n-alkanes (C23 -C31) with a strong odd-over-even predominance are usually interpreted to be of terrestrial origin while mid-chain n-alkanes (C20 -C26) with no or weak odd-over-even predominance were assigned to aquatic macrophytes.Recent sediments of 50 Tibetan lakes were analysed for amounts of n-alkanes, bulk organic d13C and compound-specific d13C values of n-alkanes. Furthermore, bulk d13C of the dominant aquatic macrophyte species Potamogeton pectinatus (L) was measured. Sediments could be separated into three groups by hierarchical cluster analysis according to their n-alkane patterns. Some samples show a remarkable terrestrial influence (type I) while others were obviously dominated primarily by aquatic macrophytes (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.d13C values of Potamogeton bulk biomass and of bulk organic matter range from -6.0 to -21.4 and -18.4 to -28.1, respectively. d13C values of long-chain n-alkanes 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. In our sample set, pHs of the lake waters are varying from 7.1 to 10.5. Comparing bulk d13C values of sediments and plants with pHs, a trend to more positive values at high pHs is visible. The correlation between d13C of Potamogeton biomass and of mid-chain n-alkanes 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 d13C values of sediments and plant organic matter (R2 = 0.71). We conclude, that limited carbon availability at high pHs can be one explanation for a positive shift of d13C values of mid-chain n-alkanes in macrophyte dominated lakes.

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