Sources of sedimentary lipids deduced from stable carbon-isotope analyses of individual compounds

Abstract

COMPOUND-specific isotope analysis by gas chromatography combined with isotope-ratio mass spectrometry (GC-IRMS)1,2 provides a new tool with which to study the carbon cycle at the molecular scale3. Previous studies2,4 using this technique have been concerned with oceanic systems. Here we demonstrate that the potential for elucidating terrestrial sedimentary processes is equally important. By comparing the carbon isotope ratios (δ13C) of individual n-alkanes from the leaves of lakeside trees with those from the lake sediments, we are able to discriminate between the diverse sources of the sedimentary carbon. The leaf-wax n-alkanes show a large inter-species δ13C variation of -30.1 to -38.7‰, which may be the result of genetic differences in plant adaptation and physiology. Values of-30.1 to -35.9‰> were obtained for the corresponding n-alkanes extracted from the lake sediments, indicating that they derive from a mixed input of deciduous leaf waxes. Shorter-chain lipids in the sediments had δ13C values of -20 to -22‰o, implying that these originate from a different (probably algal) source. Information of this sort goes beyond that which can be deduced from bulk isotope or biomarker analyses alone.