ABSTRACT: Glacial sea level variations and their effects on resonance of the ocean tide in the Labrador Sea: possible implications for ice sheet dynamics and millennial scale climate variation.

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Abstract

During the last 800,000 years, the ice age Earth was subject to a series of glacial cycles of period ca. 100,000 years. Over much of this time, large ice complexes covered most of Canada and Northern Europe. At the various glacial maxima, sea level was 100 m or more lower than during interglacial periods. As a result, the geometry of ocean basins was altered from that of the present day. High-resolution sea-level, sediment, and ice core data for the last glacial cycle have provided compelling evidence of a rich temporal evolution in ice age climate. Throughout the globe, a profound millennial scale variability has been found that correlates with Heinrich events, which are believed to represent massive discharges of ice from the Labrador Sea. The cause of this instability is a matter of ongoing debate. In this talk we report on work that suggests that Labrador Sea tides were anomalously large in glacial times. We therefore suggest that tides played a catalytic role in destabilizing the Hudson Strait ice stream and Labrador Sea ice shelf, and that tides thus provide an important, missing link in understanding the origin of millennial scale climate events. Present-day semi-diurnal and diurnal tides have been interpreted as resonant normal modes of the ocean basins, and thus relatively small changes in the shape of ocean basins may substantially alter the tidal dynamics. As a first step towards understanding this sensitivity, we have investigated tides over the last 65,000 years using a new global numerical model that captures 92 percent of the present-day open-ocean tidal height variance from ``first-principles'' (i.e. without assimilating observations). The tide model utilizes a new formalism for predicting gravitationally self-consistent sea level change on realistic, viscoelastic Earth models. The formulation yields maps of sea level change that exhibit complex spatial patterns reflecting the deformation and self-gravitation associated with the (ice plus ocean) load. From approximately 65,000 to 7,000 years before present, Hudson Bay was ice covered and the tides at the debouchement point of the Hudson Strait ice stream in the Labrador Sea are predicted to be exceptionally large. M$_{2}$ tides are 6 meters peak-to-peak and spring tides are 10 meters peak-to-peak. Since observations in present-day Antarctica demonstrate that tides significantly impact the dynamics of both continental ice streams and floating ice shelves, our results suggest that large paleotides in the Labrador Sea weakened the ice shelves and streams and served as a precondition for the Heinrich event instability.

Bibliography of Canadian Geomorphology