OCGC Seminar - New Insights on the Late-Stage History of Glacial Lake Ojibway




Dr. Martin Roy 
Département des sciences de la Terre et de l'atmosphère &GEOTOP, UQAM 



Thursday, December 10th, 2015
11:30 a.m.  

233 Advanced Research Complex (ARC) 
University of Ottawa 



The decay of the Laurentide ice sheet (LIS) is believed to be responsible for abrupt climate variations during the last deglaciation and early Holocene, notably through changes in ice volume and extent that allowed massive discharges of meltwater that had accumulated in large ice-dammed lakes such as Lake Agassiz and Lake Ojibway. One of the best examples of freshwater forcing occurred ~8,200 years ago, when the collapse of the LIS allowed the coalesced Lake Agassiz-Ojibway to drain abruptly into the North Atlantic, presumably causing a global-scale cooling. However, assessing the precise impact of this abrupt lake drainage on the ocean circulation is complicated by high-resolution marine records that indicate that the late deglacial interval was rather marked by several outbursts of meltwater. Yet, field evidence and geological data supporting a multi-step drawdown of Lake Agassiz-Ojibway are relatively limited. These issues also underlie important uncertainties in the late-stage history of these glacial lakes, notably regarding the drawdown mechanism and the configuration and timing of the pre-drainage lake phases. Recent investigations of the deglacial sediment and geomorphological records in NE Ontario and NW Quebec have revealed new insights on the evolution of Lake Ojibway. The microfossil content and associated stable isotope geochemistry of a James Bay sediment sequence comprising a drainage unit indicate that the final drawdown of the lake was preceded by episodic subglacial drainage events. Radiocarbon dating results have also refined the timing of this event, in addition to underlie the critical role played by the late-glacial ice dynamics in the final stages of the deglaciation and the demise of Lake Ojibway. Additionally, the study of low-elevation relict terraces has revealed the occurrence of late-stage phases that bring new evidence for significant changes in the areal extent and depth of Lake Ojibway near the end of the deglaciation. 10Be surface exposure dating was also applied to lakeshore erosional bedrock surfaces associated with a high and stable Ojibway lake level commonly used as the pre-drainage lake phase in paleogeographic reconstructions. The results represent the first direct chronology obtained on glacial lake shorelines and indicate that this lake level developed early in the deglaciation, thereby raising questions on the use of this lake level as a reference for a pre-drainage lake surface. Obtaining additional geochronological constraints on the Ojibway shoreline sequence being developed will ultimately improve paleogeographic reconstructions, as well as refine estimates of meltwater volumes, two aspects that are critically needed to evaluate the impact of freshwater discharges on ocean circulation.