Laurentide Ice Sheet Disintegration, Freshwater Discharge and Sea Level Riseby Tommy on 5/10/2012
Ice-Sheet Sources of Sea-Level Rise and Freshwater Discharge During the Last Deglaciation, Anders E. Carlson and Peter U. Clark, Reviews of Geophysics (2012)
We review and synthesize the geologic record that constrains the sources of sea-level rise and freshwater discharge to the global oceans associated with retreat of ice sheets during the last deglaciation. The Last Glacial Maximum (~26-19 ka) was terminated by a rapid 5-10 m sea-level rise at 19.0-19.5 ka, sourced largely from Northern Hemisphere ice-sheet retreat in response to high-northern-latitude insolation forcing. Sea-level rise of 8-20 m from ~19 to 14.5 ka can be attributed to continued retreat of the Laurentide and Eurasian Ice Sheets, with an additional freshwater forcing of uncertain amount delivered by Heinrich Event 1. The source of the abrupt acceleration in sea-level rise ~14.6 ka (Meltwater Pulse 1A, ~14-15 m) includes contributions of 6.5-10 m from Northern Hemisphere ice sheets, of which 2-7 m represents an excess contribution above that derived from ongoing ice-sheet retreat. Widespread retreat of Antarctic ice sheets began at 14.0-15.0 ka, which, together with geophysical modeling of far-field sea-level records, suggest an Antarctic contribution to this meltwater pulse as well. The cause of the subsequent Younger Dryas cold event can be attributed to eastward freshwater runoff from the Lake Agassiz basin to the St. Lawrence estuary that agrees with existing Lake Agassiz outlet radiocarbon dates. Much of the early Holocene sea-level rise can be explained by Laurentide and Scandinavian Ice Sheet retreat, with collapse of Laurentide ice over Hudson Bay and drainage of Lake Agassiz basin runoff ~8.4-8.2 ka to the Labrador Sea causing the 8.2 ka event.
In summary, climate model simulations refute claims that because incised channels near the eastern outlet (presumably from floods) were not open until after the Younger Dryas, another mechanism is needed to explain the event [Lowell et al., 2005; 2009; Teller et al., 2005; Steig, 2006a; 2006b; Tarasov and Peltier, 2005; 2006; Peltier et al., 2008]. Instead, they show that a flood is inconsequential to the forcing of the Younger Dryas, rather requiring a sustained discharge of freshwater to the North Atlantic to suppress AMOC and cause a millennia-long cold event like the Younger Dryas [Liu et al., 2009; Singarayer and Valdes, 2010; Valdes, 2011]. The lack of an identifiable large incised channel from a flood therefore does not preclude eastward routing as the forcing of the Younger Dryas. Freshwater may have just drained east through the less incised yet still substantial outlets identified by Teller et al.  [Carlson and Clark, 2008]. Based on these climate model simulations, the routing of freshwater to the St. Lawrence remains the only known forcing mechanism that can explain the timing, magnitude and duration of the Younger Dryas cold event.
Other hypotheses or arguments against eastward routing have been based on ice-sheet models with poorly constrained climate forcings, misinterpretations of geologic data, or are inconsistent with sea-level constraints on meltwater discharge. Using cosmogenic nuclide ages to directly date when the ice margin retreated from the multiple eastern outlets south of the LIS would confirm hypotheses based on minimum-limiting radiocarbon dates and more distal runoff records. Similarly, directly dating when the northern LIS outlet opened is key to understanding late deglacial runoff routing and AMCO/climate evolution. In particular, the direction that LIS runoff was routed at the end of the Younger Dryas is poorly constrained. Its identification would provide a critical constraint for understanding the response of the AMOC and climate system to changes in the North Atlantic hydrologic cycle.