LAKE AGASSIZ DRAINAGE BRACKETED HOLOCENE HUDSON BAY ICE SADDLE COLLAPSE

The duration and intensity of the early Holocene freshening events has been explained by subglacial outburst(s) from Lake Agassiz, the collapse of a Laurentide Ice Sheet ‘saddle’ overlying Hudson Bay, and/or a combination of the two events. In particular, the 8.2 ka cold event has been linked to final drainage of Lake Agassiz. Lake Agassiz drainage is often cited as 8.47 ka; an age that was based on just 14 radiocarbon ages (Barber et al., 1999). Here, we provide new field evidence and geomorphic observations to assess the deglacial history of this important region, allowing for revision of the sequence of events. We show that the collapse of the Hudson Bay Ice Saddle in southwestern Hudson Bay occurred between 8.57 ±0.28 ka BP and 8.11 ±0.19 ka BP. This event was preceded by at least one subglacial-drainage event through numerous newly-mapped subglacial channels onshore of southwestern Hudson Bay. The Tyrrell Sea entered these subglacial drainage channels, starting at 8.57 ka BP. Lake Agassiz may have experienced multiple subglacial drainage events prior to the final HBIS collapse at 8.11 ka BP, accounting for the timing discrepancies in freshwater cooling observed in the North Atlantic. Importantly, this new work links the chronology of events on the southwest (land-based) side of the HBIS to the northeast (marine-based) side of the ice sheet. Additionally, this work provides a potential analog for the behaviour of other ice sheets whose beds lie well below sea level, such as the West Antarctic Ice Sheet, during periods of warming climate.