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Meltwater drainage from glacial Lake Agassiz has been implicated for nearly 15 years as a trigger for thermohaline circulation changes producing the abrupt cold period known as the Younger Dryas. On the basis of initial field reconnaissance to the lake's proposed outlets, regional geomorphic mapping, and preliminary chronological data, an alternative hypothesis may be warranted. Should ongoing data collection continue to support preliminary results, it could be concluded that Lake Agassiz did not flood catastrophically into the Lake Superior basin preceding the Younger Dryas (Figure 1). All preliminary findings imply a retreating ice sheet margin approximately 1000 years younger than previously thought, which would have blocked key meltwater corridors at the start of the Younger Dryas.

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SUMMARYThe present paper introduces a new limiter formulation for the definition of bounded higher-order convection schemes within a finite-volume context. The new formulation emphasizes the symmetry property and brings into clearer view certain types of solution behaviour, in particular strong positive solution curvature. The new formulation can be shown graphically, in a diagram similar to the well-known Sweby diagram, and the various boundedness criteria in current use, in particular the total-variation diminishing (TVD) and positivity conditions, can be shown as regions in the new diagram. The formulation allows the definition of smooth limiters with simple and flexible functional forms, of which some example classes are given (along with the transformed versions of some existing limiters). The smooth classes can be extended to maintain positive solution behaviour on non-uniform grids in a simple and natural manner.