T208 Lakes, Playas, and Soils

Monday, 6 October 2008
1:30 PM - 5:30 PM

An 8900-Year-Old Spruce Forest Buried by Lake Superior: Paleoecological and Lake-Level Implications

Paper number: T208-8
Program: Technical Sessions
Time: 3:15 PM – 3:30 PM

Authors

Abstract

A series of exposures along the lower Kaministiquia River (near Thunder Bay, Ontario, Canada) provide exceptional insight into middle Holocene lake level fluctuations, and paleoenvironments, in the western Lake Superior basin. In general, this sedimentary sequence is composed of >2 m of massive to cross-bedded sand which is overlain by ~9 m of rhythmically laminated silt. At the contact between these two units is a well-preserved organic deposit with abundant plant macrofossils and several large trees trunks (up to 17 cm in diameter), including one in upright position. Three AMS radiocarbon dates were obtained on wood and cones from this deposit: 8135 +/- 25, 8010 +/- 25, and 7970 +/- 30 14C yrs BP. Macroremains from a variety of terrestrial (Picea, Alnus), emergent (Carex spp.), and bryophyte (Pleurozium schreberi, Dicranum polysetum) species were recovered; all of these plant taxa are common constituents of the modern boreal forest.

From this evidence we reconstruct a lowstand phase of Lake Superior characterized by fluvial deposition and local landscape stabilization by ~8000 14C BP (8900 cal BP). At this time, vegetation at the sample site was a mature spruce forest with a Pleurozium-Dicranum understory, interspersed with shallow wetland communities. Low amounts of charcoal particles, and bryophyte ecology, suggest that fire frequencies were locally very low, perhaps due to climate and/or landscape (e.g., presence of natural fire breaks). Sometime shortly after 8000 14C BP, the level of Lake Superior rose well above the modern water plane, depositing a thick sequence of varves in a deep-water depositional environment with high sediment flux. This transgression likely records the onset of the Nipissing Phase, when the level of Lake Superior rose from an earlier lowstand (Houghton) due to isostatic uplift of its outlet at Sault Ste. Marie.