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 studies^2,4 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.