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DIVISION S-2-SOIL CHEMISTRY

Radiocarbon Dating of Aliphatic Hydrocarbons

A New Approach for Dating Passive-Fraction Carbon in Soil Horizons

Yongsong Huang^,a, Baocai Li^a, Charlotte Bryant^b, Roland Bol^c and Geoffrey Eglinton^a

^a Biogeochemistry Research Center, Dep. of Geology, Univ. of Bristol, Bristol, BS8 1RJ, UK
^b NERC Radiocarbon Lab., Scottish Enterprise Technology Park, East Kilbride, Glasgow G75 0QF, UK
^c Inst. of Grassland and Environmental Research, North Wyke, Okehampton, Devon EX20 2SB, UK

yongsong{at}essc.psu.edu

Aliphatic hydrocarbons isolated from three types of British upland soils at different depths were ¹⁴C-dated by accelerator mass spectrometry (AMS) and compared with ¹⁴C ages of total organic C (TOC) of bulk soils and acid-hydrolyzed residues. In all cases, aliphatic hydrocarbons were significantly older than TOC but comparable with (in some cases older than) hydrolyzed residues, indicating that the ¹⁴C content of aliphatic hydrocarbons reflects the age of a passive-fraction C. The age differences between the aliphatic hydrocarbons and TOC increase with the degree of mineralization: thus, up to a 10000 yr difference in age is observed for highly mineralized horizons in podzol and acid brown earth. The leaf-wax n-alkanes (C₂₅ to C₃₃) isolated from a peaty gley core show a virtually linear relationship between their ages and the depth. In contrast to bulk soil organic matter that contains younger C deposited by plant roots and by water leaching, leaf wax n-alkanes are contributed at the soil surface by the leaves of dead plants and are of low mobility due to their extremely low water-solubility. The low biodegradability of long-chain n-alkanes leads to their persistence in the soil horizons where they were originally deposited. Therefore, their ages are ideal as chronological indicators for soils and peats.

Abbreviations: AMS, accelerator mass spectrometry • BP, before present • GC, gas chromatography • GC–MS, gas chromatography–mass spectrometry • TOC, total organic C

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