MOLECULAR, C-13, AND C-14 EVIDENCE FOR THE ALLOCHTHONOUS AND ANCIENT ORIGIN OF C-16-C-18 N-ALKANES IN MODERN SOILS

The heterogeneous isotopic composition of C-3 and C-4 plants can be us ed to to follow the fate of plant carbon into soil organic molecules. Thus, after 23 years of maize cropping (C-4) on a soil which was previ ously under C-3 vegetation, C-25-C-33 soil n-alkanes are C-13-enriched up to 9 parts per thousand relatively to the initial C) soil, reflect ing the input of C-13-enriched n-alkanes from maize waxes. In sharp co ntrast, C-16-C-18 soil n-alkanes do not show any significant C-13/C-12 variation over the same time interval. This absence of isotopic varia tion, along with consideration of their relative concentration, absolu te concentration, and biodegradability, demonstrate that these substan ces must represent a regular input from an external source. Evidence o f a large contribution of an ancient source, amounting to more than 65 % of the alkane fraction, is given by a C-14-age of 8510 yrs sp. Moreo ver, short-chain n-alkanes from soils, diesel fuel, diesel automobile exhaust, and petroleum products exhibit similar distributions and delt a(13)C values. These findings suggests that C-16-C-18 soil n-alkanes r epresent a non-point source pollution of ancient hydrocarbons either c arried by aerosols or entering the soil via continuous hydrocarbon see page from the deep sedimentary rocks of the Paris basin. Copyright (C) 1997 Elsevier Science Ltd.