Deglaciation and early Holocene fire record from Scotstown Bog, Eastern Townships, Québec, Canada

Abstract:

During the end of the last glaciation, the final retreat of the Laurentide Icesheet affected the landscape and vegetation across vast regions of Canada and the eastern United States. Sedimentary deposits at Scotstown Bog, Eastern Townships, Québec, are among the few sedimentary sequences studied at high resolution from this period. A sediment core of six meters was taken from the site. The bottom layer consists of a silty clay deposited during the years the site was a pro-glacial lake; this is surmounted by layers of gyttja (organic lake mud), then peat from riparian marshes, and finally bog sediments deposited as lake was filled in.  The bottommost Scotstown Bog sediments date back ~12,800 years BP. The bottom 105 cm were analysed for the presence of micro-charcoal at high resolution (using contiguous 1 cm subsamples) to gain information about the occurrence and frequency of forest fires in the region. Surprisingly, the proglacial lake sediments contained little micro-charcoal, even though the northern border of the boreal forest was only ~100 km to the south. The transition from the silty clay of the proglacial lake to gyttja was very rapid, occurring over 2 cm. This shift is accompanied by a transition from a near-complete lack of micro-charcoal to substantial amounts. Hence, we conclude that the lack of micro-charcoal at the bottom of the core is likely due to the nearby Laurentide Ice sheet suppressing fires in the nearby boreal forests, until it recedes far enough northward that it no longer affects the local pyro-climate.

Keywords: fire regimes, deglaciation, Québec, Holocene, micro-charcoal, post-glacial vegetation

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