| Summary: | Sediment cores in the North Atlantic show evidence of periodic large-scale ice discharge events between 60 kyr and 10 kyr before present. These so-called Heinrich Events (HEs) occurred with a typical period between 5 kyr and 10 kyr. During each HE, a significant amount of ice was discharged from the Laurentide ice sheet into the North Atlantic. This input of freshwater through the melting of icebergs is thought to have strongly reduced the strength of the Atlantic thermohaline circulation. One theory for HEs is that they are due to an internal oscillation of the ice sheet under constant forcing. This implies that past multi-millennial scale climate variations in the North Atlantic region may have been driven by variations in the Laurentide ice sheet and were not externally forced by climate change or sea level variations. A second theory for HEs requires some variable external forcing on an unstable ice sheet to produce a discharge event. Using the 3-D ice sheet model of Marshall and Clarke (1997a,b), which includes ice sheet dynamics and thermodynamics, the possibility of internal oscillations within an ice sheet is verified and an analysis of the mechanisms associated with these oscillations is performed. The bed topography and strain heating are found to be critical for the formation and development of fast moving ice streams, which lead to large iceberg calving. In addition, the calving parameterization is found to be an important factor in the ability of the model to periodically discharge large amounts of ice.
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