Résumés
Abstract
Peatlands cover extensive parts of northeastern Manitoba that have low relief and impermeable substrates, with peat thicknesses varying between 25 and 400 cm. Peat reserves average 1.5 x 106 m3/km2. Fens, forested plateau bogs, and polygonal plateau bogs are the prevalent peatland types. The thickest peat deposits consist of Sphagnum bogs that developed on glaciolacustrine or marine silt. Thinner deposits are composed of fen peat, or bog peat developed on sandy till. There is a positive relationship between peat thickness and time since postglacial emergence of the land. Also, recently emerged areas are dominated by fen peat, whereas bog peat is more prevalent on older surfaces. Pollen analysis of peat cores show that spruce trees have been abundant in the region south of Churchill for the past 6300 years. Local changes in peat type and accumulation rate occurred as bog and fen habitats changed, probably in response to changes in water table induced by aggradation and degradation of permafrost. Other taxa typical of boreal forest occupied suitable habitats similar to today, with bog and fen habitats increasing as paludification continued through time.
Résumé
Dans le nord-est du Manitoba, les tourbières occupent de vastes superficies peu accidentées et dont le substrat est imperméable. L’épaisseur de la tourbe varie entre 25 et 400 cm. À travers la région, le volume de tourbe moyen est de 1,5 x 106 m3/km2. Les principaux types de tourbières sont les tourbières minérotrophes, les plateaux palsiques arborés et les plateaux palsiques à polygones. Les dépôts les plus épais se trouvent dans les tourbières ombrotrophes à Sphagnum qui se sont développées sur des silts glaciolacustres ou marins. Des dépôts plus minces sont constitués de tourbe calcique ou de tourbe acide développées sur le till sableux. Il existe une relation positive entre l’épaisseur de la tourbe et le temps écoulé depuis l’émersion postglaciaire. On note également que les terrains récemment émergés sont dominés par la tourbe calcique, tandis que la tourbe acide est plus répandue sur les surfaces plus anciennes. L’analyse pollinique des carottes montre que les épinettes abondent depuis 6300 années. Des changements localisés quant au type de tourbe et à son taux d’accumulation résultent probablement de la variation du niveau de la nappe phréatique attribuable à la progression ou la dégradation du pergélisol. D’autres taxons caractéristiques des forêts boréales ont occupé des habitats semblables à ceux d'aujourd’hui, mais l'étendue des tourbières ombrotrophes et minérotrophes s’est accrue au fur et à mesure de l'entourbement .
Resumen
Las turberas cubren grandes zonas al noreste de Manitoba, éstas se caracterizan por un relieve bajo y substratos impermeables, con un grosor de musgos que varia entre 25 y 400 cm. La reserva de turba cubre alrededor de 1,5 x 106 m3/km2. Los principales tipos de turba son del tipo minerotrófica, de mesetas pantanosa forestal y de meseta pantanosa poligonal. Los depósitos más gruesos se encuentran en las turberas ombrotrófica de Sphagnum que se desarrollaron sobre limo glaciolacustre o marino. Los depósitos más delgados están constituidos de turba calcárea o de turba ácida desarrollada sobre limo arenoso. Existe una relación positiva entre el grosor de la turba y el tiempo transcurrido desde que sucedió la emersión postglaciar. Además, las zonas de emersión reciente están dominadas por turba calcárea mientras que la turbera ácida es más frecuente en áreas más antiguas. El análisis palinológico de muestras de turba pone en evidencia la presencia de abetos que fueron abundantes en la región sur de Churchill hace unos 6300 años. Los cambios locales en el tipo de turba y en la taza de acumulación ocurrieron a medida que las turberas calcáreas o pantanosas cambiaron, probablemente en respuesta a cambios del manto freático provocados por la elevación y degradación del gelisuelo. Otros taxa típicos del bosque boreal ocuparon hábitats similares al actual, con turberas ombrotróficas y minerotróficas que se fueron extendiendo a través del tiempo.
Parties annexes
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