Abstract
Sea-floor landforms and acoustic-stratigraphic records allow interpretation of the past form and flow of a westward-draining ice stream of the Greenland Ice Sheet, Rink Isbrae. The Late Pliocene–Pleistocene glacial package is several hundred metres thick and down-laps onto an upper Miocene horizon. Several acoustic facies are mapped from sub-bottom profiler records of the 400 km-long Uummannaq fjord-shelf-slope system. An acoustically stratified facies covers much of the fjord and trough floor, interpreted as glacimarine sediment from rain-out of fine-grained debris in turbid meltwater. Beneath this facies is a semi-transparent deformation-till unit, which includes buried streamlined landforms. Landform distribution in the Uummannaq system is used to reconstruct past ice extent and flow directions. The presence of streamlined landforms (mega-scale glacial lineations, drumlins, crag-and-tails) shows that an ice stream advanced through the fjord system to fill Uummannaq Trough, reaching the shelf edge at the Last Glacial Maximum. Beyond the trough there is a major fan built mainly of glacigenic debris flows. Turbidity-current channels were not observed on Uummannaq Fan, contrasting with well-developed channels on Disko Fan, 300 km to the south. Ice retreat had begun by 14.8 cal. ka ago. Grounding-zone wedges (GZW) in Uummannaq Trough imply that retreat was episodic, punctuated by several still-stands. Ice retreat between GZWs may have been relatively rapid. There is little sedimentary evidence for still-stands in the inner fjords, except for a major moraine ridge marking a Little Ice Age maximum position. On the shallow banks either side of Uummannaq Trough, iceberg ploughing has reworked any morphological evidence of earlier ice-sheet activity.
Highlights
Today, the outlet glaciers draining huge interior basins of the Greenland Ice Sheet are among the fastest-flowing on Earth (Rignot and Kanagaratnam, 2006)
The Greenland Ice Sheet is known to have expanded during the Last Glacial Maximum (LGM), about 20,000 years ago, providing an important increment of global sea-level fall
We present the full details of marine-geophysical observations of sea-floor landforms and sediments from the 250 km-wide continental shelf offshore of Uummannaq Fjord in West Greenland (70300 to 71N), and in the 150 km-long fjord system that links the present ice sheet with the Uummannaq crossshelf trough beyond (Fig. 1)
Summary
The outlet glaciers draining huge interior basins of the Greenland Ice Sheet are among the fastest-flowing on Earth (Rignot and Kanagaratnam, 2006). Funder and Hansen, 1996; Funder et al, 2011; Roberts et al, 2013) Swath-bathymetric data revealing sea-floor morphology, and accompanying acousticstratigraphic records, allow us to interpret the form and flow of a major outlet glacier of the ice sheet at, and following the Last Glacial Maximum (LGM), including both its past extent across the West Greenland shelf and its flow regime and style of deglaciation Swath-bathymetric data revealing sea-floor morphology, and accompanying acousticstratigraphic records, allow us to interpret the form and flow of a major outlet glacier of the ice sheet at, and following the Last Glacial Maximum (LGM), including both its past extent across the West Greenland shelf and its flow regime and style of deglaciation (e.g. Dowdeswell et al, 2008a)
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