Post-glacial variations in distributions, 13C and 14C contents of aliphatic hydrocarbons and bulk organic matter in three types of British acid upland soils

Abstract

The post-glacial variations in distributions and 13C contents for individual aliphatic hydrocarbons and bulk organic matter down the profiles of three stratified organic upland soils (peaty gley, podzol and acid brown earth) in the U.K. were studied by a combination of conventional 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) characterisation of bulk and individual compounds. The reduction with depth of the total organic carbon content (TOC) of the soils was accompanied by concentration decreases in aliphatic hydrocarbons, attributed primarily to diagenetic degradation and, secondly, to changes in primary production since the last deglaciation. In contrast, the 13C content of TOC generally increased with depth and age in the soils, which was again paralleled by the 13C content of individual aliphatic hydrocarbons. CO 2 contributed by fossil fuel burning can only explain changes of about 1‰, with the additional (2–4‰) 13C 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 plants) n-alkanes by soil micro-organisms. The large 13C enrichment (5–7‰) for TOC, aliphatic hydrocarbon fractions and individual n-alkanes from a peaty gley horizon older than 10,000 yr BP was also attributed to the effects of environmental conditions on isotopic fractionation during photosynthesis. Compared with n-alkanes from higher plants, the hopanoids were enriched on average by 4–5‰ in 13C and showed little variation down the soil cores, suggesting a source of heterotrophic bacteria using carbohydrates/proteins as their major carbon source, and effecting little isotopic fractionation during hopanoid biosynthesis, and/or from soil cyanobacteria using dissolved CO 2 in water. The radiocarbon ages of the soil TOCs showed a nearly linear increase with depth, suggesting little bio-disturbance and consistent accumulation of organic C at this site. The 14C ages 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 bulk soil organic matter. The presence of n-alkanes derived from higher-plant leaf waxes in the oldest horizons of peaty gley soil indicates a 3000 yr earlier development of vegetation since the last glaciation than that estimated simply by the age of TOC.