RADIOMETRIC DATING (PB-210, CS-137, AM-241) OF RECENT OMBROTROPHIC PEAT ACCUMULATION AND EVIDENCE FOR CHANGES IN MASS-BALANCE

The paper summarizes and evaluates the results of Pb-210, Cs-137 and Am-241 analysis on 37 recent ombrotrophic peat profiles. From these and previously published data, it is concluded that radiometric measurements alone cannot be relied on to give an accurate chronology of peat accumulation. Where constrained by profiles of Am-241 activity in the upper part and one or more pollen-dated horizons below this, Pb-210 can provide good chronologies of peat accumulation provided the constraint does not displace the calculated Pb-210 dates by more than c. 10%. Profiles of Cs-137 activity, in most cases, fail to contribute significantly to dating recent peat. At somewhat degraded sites, the residual dry mass of peat representing the last c. 150 years is reduced. Further analysis of well-dated profiles suggests that this may reflect loss of catolelm peat newly exposed to aerobic decomposition processes after a fall in water table. Several mechanisms are proposed for the displacement and net loss of lead (including Pb-210) from peat profiles.