DATING OF THE NEAR-SURFACE LAYER OF A PEATLAND IN NORTHWESTERN ONTARIO, CANADA

The near-surface layer of the dominant microhabitat types on a peatlan d in the Rainy River area, northwestern Ontario, Canada was dated usin g 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 sugg ested that the post-depositional mobility of Pb-210, and possible its atmospheric supply, are affected by microhabitat type. Pb-210 dates co nsistently 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 Pb-210 da tes, but were subject to bias where the moss remains were poorly prese rved. C-14 analyses on material from the base to the near-surface hori zon confirmed the modern (post-AD 1950) origin suggested by the Pb-210 and moss increment dates. Increased ash content at the base of the ne ar-surface layer was correlated with the dustbowl of the 1930s, but as signment of dates to other stratigraphic markers (charcoal, Ambrosia p ollen, and Larix needles) was less certain. The ash peak (55 yr before AD 1990) was used as datum for relative dating methods based on assum ptions of a constant rate of peat accumulation (constant bulk density method) and constant rate of pollen accumulation (pollen density metho d). The constant bulk density method provided quick, inexpensive estim ates of maximum age, but could not be extrapolated beyond datum, nor a pplied to cores showing marked stratigraphic changes. The pollen densi ty method was valuable but time-consuming alternative for dating peatl and depressions, which lack annual moss increments and are subject to Pb-210 leaching.