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Grounding-line systems as second-order controls on fluctuations of tidewater termini of temperate glaciers

By
Ross D. Powell
Ross D. Powell
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Published:
January 01, 1991

Climate is the first-order control on fluctuations of glacial tidewater termini. However, factors affecting relative water depth such as eustasy, isostasy, and sediment yield are important second-order controls on termini fluctuations. Deposition of grounding-line systems, which include ice-contact deltas, morainal banks, and grounding-line fans, can change relative water depth and consequently alter the stability of tidewater termini independent of climate. Therefore, the rate at which sediment accumulates in these grounding-line systems can control the rate of terminus movement.

Modern temperate glaciers at Glacier Bay, Alaska, on average produce about 106 m3/yr of sediment, most of which accumulates at grounding lines. Sediment volume decreases logarithmically downfjord from a grounding line. In Glacier Bay, readvance rates that average about 10 m/yr are one to two orders of magnitude slower than retreat rates. Rates of glacial advance may be a function of sediment yield (determined by drainage area and rates of glacial erosion, transportation and debris release), total sediment volume of grounding-line systems (determined by fjord width, fjord depth less the maximum water depth for terminus stability, and angle of repose of sediment making the system), and sediment dispersal patterns (determined by process of debris release and dispersion, type of sediment, and marine currents).

Consequently, average rates of advance are slower than retreat rates because of the time required for sediment to accumulate as a bathymetric high at a grounding line, which effectively decreases relative water depth and increases terminus stability. Retreat rates are faster because they occur during terminus instability in deep water and cannot be altered by sediment accumulation rates until quasi-stability of the terminus is caused by other means, such as bedrock pinning-points or a decrease in ablation area.

As marine-ending glaciers expand, total sediment delivered to a grounding line increases; as they retreat, sediment volume decreases because of the change in size of the drainage basin. Grounding lines are the major depocenters receiving this sediment, and resulting lithofacies packages and geometries are primarily controlled by grounding-line movement over a glacial advance-retreat cycle.

During glacial minima, fjords trap sediment while the continental shelf and slope are starved. During glacial advances and retreats, fjords are eroded while the continental shelf receives sediment and the slope may remain starved. At glacial maxima, fjords and commonly the shelf are eroded, and the continental slope receives sediment. Because grounding-line advance and retreat may or may not coincide with eustatic sea-level changes, the sedimentary record may vary depending on environmental settings and the magnitude of glacial cycles.

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GSA Special Papers

Glacial marine sedimentation; Paleoclimatic significance

John B. Anderson
John B. Anderson
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Gail M. Ashley
Gail M. Ashley
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Geological Society of America
Volume
261
ISBN print:
9780813722610
Publication date:
January 01, 1991

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