POSTGLACIAL VARIATIONS IN DISTRIBUTIONS, C-13 AND C-14 CONTENTS OF ALIPHATIC-HYDROCARBONS AND BULK ORGANIC-MATTER IN 3 TYPES OF BRITISH ACID UPLAND SOILS

The post-glacial variations in distributions and C-13 contents for ind ividual aliphatic hydrocarbons and bulk organic matter down the profil es of three stratified organic upland soils (peaty gley, podzol and ac id brown earth) in the U.K. were studied by a combination of conventio nal radiocarbon dating and accelerator mass spectrometry (AMS) and gas chromatography (GC), gas chromatography-mass spectrometry (GC-MS) and gas chromatography-isotope ratio mass spectrometry (GC-IRMS) characte risation of bulk and individual compounds. The reduction with depth of the total organic carbon content (TOC) of the soils was accompanied b y concentration decreases in aliphatic hydrocarbons, attributed primar ily to diagenetic degradation and, secondly, to changes in primary pro duction since the last deglaciation. In contrast, the C-13 content of TOC generally increased with depth and age in the soils, which was aga in paralleled by the C-13 content of individual aliphatic hydrocarbons . CO2 contributed by fossil fuel burning can only explain changes of a bout 1 parts per thousand, with the additional (2-4 parts per thousand ) C-13 enrichment for individual n-alkanes from horizons in the podzol ( < 2000 yr BP) and acid brown earth ( < 3000 yr BP) being attributed primarily to the contributions of isotopically heavier (relative to pl ants) n-alkanes by soil micro-organisms. The large C-13 enrichment (5- 7 parts per thousand) for TOC, aliphatic hydrocarbon fractions and ind ividual II-alkanes from a peaty gley horizon older than 10,000 yr BP w as also attributed to the effects of environmental conditions on isoto pic fractionation during photosynthesis. Compared with n-alkanes from higher plants, the hopanoids were enriched on average by 4-5 parts per thousand in C-13 and showed little variation down the soil cores, sug gesting a source of heterotrophic bacteria using carbohydrates/protein s as their major carbon source, and effecting little isotopic fraction ation during hopanoid biosynthesis, and/or from soil cyanobacteria usi ng dissolved CO2 in water. The radiocarbon ages of the soil TOCs showe d a nearly linear increase with depth, suggesting little bio-disturban ce and consistent accumulation of organic C at this site. The C-14 age s of the aliphatic hydrocarbon fractions isolated from peaty gley soil horizons (measured by AMS) increased linearly with depth and the age of the lower gleyed horizon was ca. 3000 yr older than that of the bul k soil organic matter. The presence of n-alkanes derived from higher-p lant leaf waxes in the oldest horizons of peaty gley soil indicates a 3000 yr earlier development of vegetation since the last glaciation th an that estimated simply by the age of TOC. Copyright (C) 1996 Elsevie r Science Ltd