Paleolimnology of Lake Manitoba: the Lithostratigraphic Evidence

Abstract

Lake Manitoba, the largest lake in the prairie region of North America, is one of the most intensively studied lacustrine basins in western Canada. New AMS 14C dating, together with mineralogical, geochemical, and lithostratigraphic analyses of the 14-m-thick, offshore sediment sequence, document a complex Holocene history in which water levels and limnological conditions were controlled by the interplay of changing climate, variable river and groundwater inflow, and differential isostatic rebound. Varves, ice-rafted debris, and clast-rich laminated sediment record deposition in the lake when the basin was part of proglacial Lake Agassiz. As Agassiz retreated northward, Lake Manitoba became isolated by about 8500 14C yrs BP, and for the next 800 years was characterized by mainly shallow water to dry conditions. Deeper and more stable water conditions returned to the Lake Manitoba basin by 7700 14C yrs BP probably due to the damming effect of differential isostatic rebound and decreasing aridity. For the next 3000 years, relatively stable lake levels and water compositions were maintained, reflecting a delicate balance between differential isostatic rebound, climate, inflow of the Assiniboine River, and groundwater contribution. At ~4500 14C yrs BP the Assiniboine River was rerouted to its present easterly path, by-passing Lake Manitoba, and resulting in loss of a significant component of the lake’s hydrologic budget. Water levels dropped and the offshore sediments were once again subaerially exposed. By 3700 14C yrs BP, a cooler and wetter climate, together with continued southward transgression of water, compensated for the loss of fluvial input, resulting in reflooding of the basin. By about 2000 years ago the lake had evolved from a shallow, saline, and alkaline pool to its present depth and extent.Le lac Manitoba, le plus grand lac des Grandes Plaines de l’Amérique du Nord, est l’un des bassins lacustres le plus intensément étudiés de l’Ouest du Canada. De nouvelles dates 14C SMA, ainsi que des analyses minéralogiques, géochimiques et lithostratigraphiques de la séquence sédimentaire de 24 m d’épaisseur au large du rivage témoignent d’une évolution complexe du lac, à l’Holocène. Au cours de cette période, les niveaux lacustres et les conditions limnologiques ont été influencés par les effets combinés des changements climatiques, de la variation des apports d’eaux lacustre et souterraine et d’un relèvement isostatique différentiel. Les varves, les débris glaciels et les sédiments en lamines riches en éléments détritiques ont constitué l’apport sédimentaire au lac lorsque le bassin faisait partie du Lac glaciaire Agassiz. Après la vidange du lac glaciaire vers le nord, le lac Manitoba a été isolé vers 8500 BP ; pendant les 800 années suivantes, le lac était caractérisé par des eaux peu profondes et un climat sec. Des eaux plus profondes et plus stables ont caractérisé le lac vers 7700 BP, probablement en raison de l’endiguement qu’a entraîné le relèvement isostatique différentiel et une moins grande aridité. Pe...

Optically stimulated luminescence ages from the Lake Agassiz basin in Manitoba

“…Moisture content could not be measured in the samples because of logistical difficulties, and an estimate was used based on an estimate of the average of time-varying and seasonally fluctuating precipitation. Specifically, our estimates of average moisture content during the antiquity of the samples considered that (1) early postglacial climate would have resulted in higher moisture contents, (2) the mid-Holocene climate was drier and warmer as indicated by numerous paleoenvironmental studies across the region (e.g., Laird et al, 1996; Last and Teller, 2004; Grimm et al, 2011; Teller et al, 2018), and (3) modern precipitation. We also took cognizance of the topographic and stratigraphic setting and field estimates (Table 1) and were additionally guided by moisture contents of samples from a similar Lake Agassiz setting used by Lepper et al (2013).…”

A multi-proxy study of changing environmental conditions in a Younger Dryas sequence in southwestern Manitoba, Canada, and evidence for an extraterrestrial event

“…7) is incompatible with environmental conditions indicated by the pollen and macrofossil assemblages, so they are interpreted as secondary minerals related to diagenetic additions during later (Holocene) drying conditions in the region. A diagenetic origin of the gypsum is supported by (1) the more arid climatic conditions recorded in mid-Holocene Prairie lakes (e.g., Last and Teller, 2002; Grimm et al, 2011; Teller et al, 2018), (2) the more arid pollen types in sediments at this site that are younger than ~11,600 cal ka (Boyd, 2003), (3) the presence of secondary gypsum in Quaternary siliclastic sediments throughout the region (Young et al, 2000), and (4) the overlying marl in Unit C and eolian dunes indicative of drier conditions.…”

Subglacial deforming bed conditions recorded by late Quaternary sediments exposed in Vineland Quarry, Ontario, Canada

“…Slight coarsening and thickening of the laminae up-section suggest enhanced rates of sediment delivery to the site and increased proximity to the sediment source, likely caused by progressive ice advance into the region (Brazier et al, 1998;Last and Teller, 2004). Clasts composed of local bedrock (dolostone) observed in Unit 2b may have been rafted into the lake by icebergs or shore ice (Ovenshine 1970).…”