REORGANIZATION OF THE LAURENTIDE ICE SHEET - HOW LONG DOES IT TAKE?

The realization that ice sheets are dynamic has fueled discussion about ice-sheet stability. Direct observations of present ice sheets provide little insight about centennial or longer-scale changes. If one wants to know how an ice sheet died, look at the Laurentide Ice Sheet.

An early view is that ice sheets generally grew and decayed in a radial pattern. Yet modern ice sheets have domes, saddles, and regions of diverging and converging flow. One form of converging flow is represented by ice streams. The locations of most ice streams in the Laurentide Ice Sheet now have been mapped. One surprising finding is that footprints of some ice streams cross-cut other ice streams footprints, in places at right angles. On smaller spatial scales striations also show cross-cutting relationships, reflecting multiple generations of ice flow. These features indicate that the Laurentide Ice Sheet had to change flow direction on much larger spatial scales than local topography can impose. To help understand the dynamics of the Laurentide Ice Sheet, we ask how long did it take to change flow direction or reorganize itself?

In Antarctica, analysis of deformed flow bands on ice shelves and snow-filled crevasses are taken to reflect switching on and off of ice streams on century time-scales. It may take longer to change direction. We posit, based on our preliminary dating in North Dakota, that the Laurentide Ice Sheet reorganized itself on millennial times scales during the last deglaciation. If so, this implies that ice streams changing direction and flipping locations are diagnostic and essential features of ice sheets in decay. Dating such activity will provide a new target to construct the dynamics of stable and unstable ice sheets over their life cycle.