ABSTRACT: Holocene environmental history of Lake Winnipeg: Thecamoebians and stable lead isotopes

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This study is a paleolimnological and detailed taxonomic account of the thecamoebian fauna and stable lead isotopic composition of Lake Winnipeg sediments. Holocene sediments in Lake Winnipeg consist of the lower Lake Agassiz sequence unconformably overlain by the Lake Winnipeg sequence. Nine piston/gravity core sites and three box core sites, covering the North and South basins and the connecting Narrows, were selected for analysis. The identification of thecamoebians is complicated by their wide range of intra- and interspecific variability. The term strain is introduced for designating intraspecific variations in thecamoebians. Strains were assigned previously published names according to the original designation and/or commonly accepted synonymy in order to facilitate comparison between studies. The paleolimnological study indicates that biologic productivity and consequently the type of organic material in the sediments is the main control on thecamoebian distribution. Other factors are water chemistry and turbidity whereas inorganic sediment geochemistry and water temperature do not appear to significantly influence the thecamoebian fauna. Variations in abundance of key thecamoebian species along a north-south transect divide Lake Winnipeg into three distinct areas. Environmental changes are more significant in the restricted South Basin, resulting in distinct thecamoebian assemblages. In contrast, the North Basin provided a more stable environment throughout the late Holocene. Stable lead isotope composition can yield information on sources of lead in lake sediments. In this reconnaissance study, lead isotopic compositions were determined for acid-soluble and residual sediment fractions of samples from a north-south transect of Lake Winnipeg. Results show that lead in the sediment is derived from natural and anthropogenic sources and both interbasinal and vertical variations in source material can be distinguished. The natural-source lead in Lake Winnipeg sediments was predominantly derived from the Superior Province (model age 2.5 Ga). The North Basin sediment contains lead which was derived directly from Archean granitic rocks. In the South Basin common lead was derived from the Superior Province but recycled through Paleozoic and Mesozoic sedimentary rock before entering Lake Winnipeg. Reduced isotopic ratios of acid-soluble lead are indicative of anthropogenic lead sources in the most recent sediments. Specific sources of anthropogenic lead for Lake Winnipeg cannot be determined with the present data since the anthropogenic isotopic composition is unknown.

Bibliography of Canadian Geomorphology