Macrofossil Record of a Middle Holocene Drop in Relative Sea Level at the St. Lawrence Estuary, Québec

Macrofossil analysis of a peat layer overlying shallow-water estuarine sediments allows the reconstruction of vegetation changes between 7000 and 6000 14C yr B.P. near Montmagny, Québec. About 7500–7000 14C yr B.P., the study site was brackish and intertidal. Next, a cattail marsh dominated by Typha changed into a terrestrial fire-prone environment that contained xero-mesophilous tree species (Pinus/Tsuga) and lasted a few hundred years, until about 6800 14C yr B.P. This rapid change may have resulted from land tilting 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 14C yr B.P. Fossil mosses at the site reflect local wetness likely produced by a rise in the water table starting about 6600 14C yr B.P. By 6500–6400 14C yr B.P., seawater returned and freshwater plants were replaced by semihalophilous and halophilous plants. Thinning of annual rings in fossil larch reflects this change from a terrestrial peatland to a treeless coast.