10Be ages of flood deposits west of Lake Nipigon, Ontario: evidence for eastward meltwater drainage during the early Holocene Epoch.

The Nipigon channels, located to the west and northwest of Lake Nipigon, Ontario, are thought to have enabled the eastward drainage of meltwater from glacial Lake Agassiz during the last deglaciation. Here we present the first direct ages of flood deposits in two of these channels using 10Be surface exposure dating. Five 10Be ages of a coarse-grained deposit near the Roaring River in the Kaiashk channel complex indicate deglaciation and cessation of water flow by ∼11 070 ± 430 years. To test for inherited nuclides in boulder samples, we also measured the 10Be concentrations of the undersides of two boulders at the Roaring River site. Five 10Be ages of boulders atop a large bedform near Mundell Lake in the Pillar channel complex indicate deglaciation and cessation of water flow by ∼10 770 ± 240 years. Two 10Be ages of nearby bedrock are slightly younger (10 340 ± 260 and 9860 ± 270 years). The 10Be ages from the two sites are statistically indistinguishable and indicate that Laurentide Ice Sheet recession occurred rapidly in the region. We used clast diameters and channel dimensions at the Mundell Lake site to estimate paleodischarge and evaluate the possibility that meltwater drainage influenced climate conditions. We estimate a large maximum discharge of 119 000-159 000 m3·s−1 at the site. However, the timing of meltwater discharge at both Roaring River and Mundell Lake is not contemporaneous with abrupt climate events. [ABSTRACT FROM AUTHOR]