210Pb radiometric dating of peat accumulation and environmental changes during the last centuries in taiga and boreal forests, Quebec, Canada

Adam Ali, Julie Loisel, Michelle Garneau, H. Asnong
GEOTOP-UQAM-McGill and Department of Geography,
Université du Québec à Montréal

B. Ghaleb
GEOTOP-UQAM-McGill,
Université du Québec à Montréal

The use of  210Pb to reconstruct recent peat accumulation represents an unusual and controversial issue. The principal limitations for using this radiometric method securely are irregular sedimentation rates from both biotic and abiotic perturbations that take place in the upper peatlands horizons. In order to reconstruct environmental changes that occurred during the last centuries in the James Bay region (La Grande and Eastmain drainage basins), several peat cores were dated using 210Pb method. Unexpected results testified that this technique can provide appropriate chronologies.

In fact, in regard to radiocarbon dating and paleoenvionmental reconstructions carried out in the same sequences, accurate chronologies were established. An increase of peat accumulation rate at the top of all sequences related with less compaction and variable decomposition at the acrotelm/catotelm contact was recorded with a synchronous pattern.  Downwards into these sequences, potential impacts of the Neo-glacial and the Little Ice Age cooling events on the water-table fluctuations were also recorded, although these fluctuations varied spatially. Failure in 210Pb results is related with the assumption that physical perturbations through peat accumulation might have been induced from variations of the water-table depth up to peat surfaces. In order to validate and secure 210Pb carried out on peat sediments, it is necessary to take into account possible compaction and density variations along cores. The combination of several dating methods (14C, 210Pb, 137CS and 241Am) may also allow to obtain more accurate chronologies and might therefore be useful to better understand paleoenvironmental changes deduced from recent peat sediments.


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