Radiocarbon dating of aliphatic hydrocarbons: A new approach for dating passive-fraction carbon in soil horizons

Aliphatic hydrocarbons isolated from three types of British upland soils at different depths were C-14-dated by accelerator mass spectrometry (AMS) and compared with C-14 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 C-14 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 10 000 Sr difference in age is observed for highly mineralized horizons in podzol and acid brown earth. The leaf-wax n-alkanes (C-25 to C-33) 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.