ABSTRACT: A varved paleoecological record From West Hawk Lake meteorite impact crater, Southeastern Manitoba, Canada: Initial results.

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Abstract

West Hawk Lake is an unusually deep (111 m) basin in the Canadian Shield, located in southeastern Manitoba, Canada. Drilled by the Dominion Observatory in the 1960s, the West Hawk Lake (WHL) sedimentary sequence consists of ~ 70 m of unconsolidated sediment above Mesozoic siltstone, impact breccia, and Precambrian (Archean) bedrock. In this study we present initial paleoecological interpretations of the upper 11 m of this sedimentary sequence. Generally, sediment in the upper 11 m is a slightly silty clay (median grain size <0.4 m) with subtle light-dark couplets and mostly uniform mineralogy. Differences were observed, however, in organic content and style and thickness of laminae. A total of about 860 light-dark couplets were counted. The couplets are interpreted as varves. In the lower unit (D), most couplets are ~ 1 cm thick. This is overlain by a half meter of thinly-laminated sediment (unit C) and 2 m of sapropelic clay (units A & B). Although radiocarbon dates are problematic due to low concentrations of organic carbon in the varved portion of the core, we suggest that the lower 8 m of this sequence represents the last ~ 900 years of deposition by glacial Lake Agassiz in this region. Unlike varved Agassiz deposits elsewhere, the WHL sequence is associated with abundant pollen, charcoal, and algae. Notable palynological trends include: (1) synchronous peaks in Ambrosia-Chenopodiineae throughout the lower 9 m of the core, interpreted as evidence of short term, cyclical (< 200 yr.), drought during the last ~ 900 years when Agassiz covered the region. This component of the WHL paleoecological record offers a rare opportunity to investigate the temporal complexity of early Holocene drought with high (annual) resolution. (2) A rise (from <5 to 20%) in Pinus strobus at 2.2 m is also apparent. Based on correlation with pollen assemblages from N. Minnesota and NW Ontario, this trend probably records (at least initially) long-distance transport of white pine pollen from the Great Lakes region beginning ~ 8000-7000 BP. Algal microfossil analysis indicates four trophic periods. Above a barren zone (11-8.6 m), algal remains are dominated between 8.6-4 m by a mix of planktonic and benthic taxa. A change in the drainage basin and probably lake size is indicated in the interval from 4-2 m, with higher abundance of planktonic microfossils such as Fragilaria crotenensis, and Aulacoseira islandica as well as several species of Stephanodiscus indicating an increase in productivity. Sediment chemistry above 2 m shows increased nitrogen and phosphorous, which matches an increase in N-fixing bluegreen akinetes. Planktonic taxa such as Cyclotella bodanica, C. stelligera as well as Aulacoseira subarctica and chrysophyte cysts also become dominant at this time, indicating conditions more typical of a boreal lake. Thus, it appears that the lake evolved from a barren cold water/ low nutrient/ turbid basin before 8.6 m to a more productive basin before modern conditions were established. This may reflect a change in nutrient supply, which may have resulted from a change in extent of the drainage basin as glacial Lake Agassiz regressed.

Bibliography of Canadian Geomorphology