Conditions de l'accumulation du carbone dans une tourbiere du Quebec meridional: L'influence des facteurs autogenes et des controles allogenes (French and English text)

Mirabel bog (St. Lawrence lowlands, southern Québec) was studied with the aim to specify the influences exerted by the different autogenic and allogenic parameters on postglacial carbon accumulation. (1) Regional vegetation dynamics reveal a general climatic control, locally modified by the palaeogeographical context, physiography and ecological processes. (2) The postglacial history of the regional climate was reconstructed by the modern analogues method, constrained by the water level fluctuations of Lac Hertel (Mont St-Hilaire). Obtained results provide evidence for an important climate improvement between 13 000 and 11 000 cal. BP, a thermal optimum around 8000 cal. BP and two dry periods (10 000–6500 and 5000–3000 cal. BP). (3) Postglacial internal dynamics were independently reconstructed in three dimensions by the multidisciplinary analysis of seven profiles. They show similar plant successions, conforming to the terrestrialisation model but were asynchronous. They moreover attest the occurrence of very humid conditions prior to 6700 cal. BP, followed by numerous asynchronous fluctuations throughout the site. By contrast, carbon accumulation presents similar trends in all profiles, characterised by high accumulation rates prior to 6200 cal. BP (averages from 7.1 to 19.1 gC.m−2. year−1) and low rates since this date (averages from 2.7 to 6.3 gC.m−2 .year−1). (4) The different postglacial conditions of peatland development are confronted. Carbon accumulation appears to have been essentially controlled by the interplay between physiography (initial minerotrophic supply) and autogenic processes, which progressively isolated the peat-forming vegetation from groundwater. Notably, neither climate nor fire seem to have played a significant role.