Macrofossil record of a middle Holocene drop in relative sea level at the St. Lawrence estuary, Quebec

Macrofossil analysis of a peat layer overlying shallow-water estuarine sedi ments allows the reconstruction of vegetation changes between 7000 and 6000 C-14 yr B.P. near Montmagny, Quebec. About 7500-7000 C-14 yr B.P., the stu dy site was brackish and intertidal. Next, a cattail marsh dominated by Typ ha changed into a terrestrial fire-prone environment that contained xero-me sophilous tree species (Pinus/Tsuga) and lasted a few hundred years, until about 6800 C-14 yr B.P. This rapid change may have resulted from land tilti ng associated with the northward migration of the postglacial forebulge. An abrupt change from a Pinus/Tsuga-dominated cover to shrubby vegetation was due to a fire that affected the site 6800 C-14 yr B.P, Fossil mosses at th e site reflect local wetness likely produced by a rise in the water table s tarting about 6600 C-14 yr B.P. By 6500-6400 C-14 yr B.P., seawater returne d and freshwater plants were replaced by semihalophiious and halophilous pl ants. Thinning of annual rings in fossil larch reflects this change from a terrestrial peatland to a treeless coast. (C) 2000 University of Washington .