Abstract

Ice sheets terminating in water may provide geologic evidence to reconstruct iceberg-calving rates and hence provide insight to ice-sheet mass balance at certain locations. Recent work reported an empirical relation for 12 modern tidewater glaciers in Alaska in which iceberg-calving rate is directly related to the average water depth. The former Laurentide ice sheet split into tidewater glaciers in the troughs of Mount Desert Island, Maine, and deposited moraine banks as sea level transgressed onto the island. Paleoshorelines were at least 65 m and probably as high as 90 m above present-day sea level. With 75 m shoreline elevation, the average water-depth values correspond to iceberg calving rates of 650 to 2100 m/yr. The total ice-volume flux minus the iceberg-calving flux is equal to the rate of change in terminus position times the terminus area. Because the terminus is stationary when moraines are deposited, the calving flux is equal to the discharge flux at that time. Calculated discharge fluxes for Mount Desert Island range from 0.65 to 2.9 x 105 m3/yr; the average is 1.7 x 105 m3/yr across a unit width of the terminus. Consideration of the ice-sheet geometry and estimated precipitation and temperature conditions indicates that most of this flux must come from ice-sheet thinning rather than mass accumulation. The geometry of this analysis dictates that all of these estimates are minimum values.

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