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 14 C‐dated by accelerator mass spectrometry (AMS) and compared with 14 C 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 14 C 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 (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.