Mobility of Pb in Sphagnum-derived peat

One important assumption in applying 210Pb-dating is that atmospherically deposited 210Pb is immobilized in the peat or sediment column. This assumption has been challenged widely, but has never been evaluated experimentally. We evaluated Pb mobility and the chemical forms in which Pb is stabilized in peat profiles by adding either soluble or particulate Pb to intact peat cores that were maintained under different water level regimes (permanently high, permanently low, fluctuating between high and low) and were subjected to simulated precipitation over a five month period. By analyzing the behavior of stable Pb we made inferences about the expected behavior of 210Pb. Results indicate that added soluble Pb2+ was retained in the peat through physiochemical binding to organic matter, and as such Pb2+ was largely immobile in peat even under conditions of a fluctuating water table. Added particulate Pb was largely (most likely by physical entrapment), but not completely, immobilized in peat. In none of the water table treatments was there evidence to support mobility of Pb by alternating formation and oxidation of sulfides, or by any other mechanism. The binding of Pb2+ with organic matter at the peat surface, and the absence of Pb mobility lend credence to 210Pbdating of Sphagnum-dominated peat deposits, which are over 90% organic matter throughout, and have high cation exchange capacities.