Dating of the near‐surface layer of a peatland in northwestern Ontario, Canada

Abstract

The near‐surface layer of the dominant microhabitat types on a peatland in the Rainy River area, northwestern Ontario, Canada was dated using a number of previously published methods. No single method was found to be appropriate in all microhabitat types, and discrepancies of up to 15 yr over a time‐scale of 80 yr were encountered. The results suggested that the post‐depositional mobility of 210Pb, and possibly its atmospheric supply, are affected by microhabitat type. 210Pb dates consistently underestimated independently derived dates in the uppermost sections of the cores. Ages derived by measuring the length of annual increments on moss stems were in general agreement with the 210Pb dates, but were subject to bias where the moss remains were poorly preserved. 14C analyses on material from the base to the near‐surface horizon confirmed the modern (post‐AD 1950) origin suggested by the 210Pb and moss increment dates. Increased ash content at the base of the near‐surface layer was correlated with the dustbowl of the 1930s, but assignment of dates to other stratigraphic markers (charcoal, Ambrosia pollen, and Larix needles) was less certain. The ash peak (55 yr before AD 1990) was used as datum for relative dating methods based on assumptions of a constant rate of peat accumulation (constant bulk density method) and constant rate of pollen accumulation (pollen density method). The constant bulk density method provided quick, inexpensive estimates of maximum age, but could not be extrapolated beyond datum, nor applied to cores showing marked stratigraphic changes. The pollen density method was a valuable but time‐consuming alternative for dating peatland depressions, which lack annual moss increments and are subject to 210Pb leaching.