Cosmogenic 10 Be dating of raised shorelines constrains the timing of lake levels in the eastern Lake Agassiz-Ojibway basin

Surface exposure dating was applied to erosional shorelines associated with the Angliers lake level that marks an important stage of Lake Ojibway. The distribution of 15 Be-10 ages from five sites shows a main group (10 samples) of coherent Be-10 ages yielding a mean age of 9.9 +/- 0.7 ka that assigns the development of this lake level to the early part of the Lake Ojibway history. A smaller group (3 samples) is part of a more scattered distribution of older Be-10 ages (with 2 outliers) that points to an inheritance of cosmogenic isotopes from a previous exposure, revealing an apparent mean age of 15.8 +/- 0.9 ka that is incompatible with the Ojibway inundation and the regional deglaciation. Our results provide the first direct Be-10 chronology on the sequence of lake levels in the Ojibway basin, which includes the lake stage presumably associated with the confluence and subsequent drainage of Lakes Agassiz and Ojibway. This study demonstrates the potential of this approach to date glacial lake shorelines and underlies the importance of obtaining additional chronological constraints on the Agassiz-Ojibway shoreline sequence to confidently assign a particular lake stage and/or lake-level drawdown to a specific time interval of the deglaciation.