Glaciers that terminate in the ocean are referred to as tidewater glaciers. These glaciers are unique because they go through periods of slow advance and rapid retreat that are influenced by climate but driven by glacier dynamics. This cycle is referred to as the tidewater glacier cycle. A key feature of tidewater glaciers is that erosional processes tend to produce
overdeepened fjords, in which the depth of the glacier bed decreases towards the front of the glacier. Thus, as a tidewater glacier retreats from an extended position, its terminus retreats into deeper water. This results in a loss of flow resistance, leading to faster flow, thinning, higher iceberg calving rates, and rapid and sustained retreat that continues until the glacier terminus retreats into shallow water or a fjord constriction. The retreat phase may last several decades. Terminus readvance is enabled by the development and progradation of a moraine at the glacier terminus, which protects the terminus from deep water. Because the advance phase is limited by sediment supply, it tends to be much slower and can last centuries. Understanding the behavior of tidewater glaciers thus requires the combined understanding of climate and glacier and sediment dynamics. This understanding is important for interpretation of past glacier change, predicting the evolution of glaciers into
the future, and interpreting glacial sediment deposits.

There are about 50 tidewater glaciers in Alaska, of which a handful are currently advancing. Taku Glacier, located in Southeast Alaska near the town of Juneau, is one of the advancing glaciers. The glacier was in a retreated position in the early 1900?s, at which time the formerly deep proglacial fjord had filled with a combination of soft marine sediments and outwash from the Taku River. Fjord depths were on the order of 10-50 m. As the glacier began to advance it rapidly excavated the sediments, at rates in excess of 1 m/yr, and built a large protective moraine at the glacier terminus. The moraine has extended above sea level, so that the glacier is no longer calving into the ocean and is not being subjected to relatively warm ocean water.

Results from this work indicate that sediment excavation and moraine building occur through a combination of coupled processes. During winter the glacier accumulates mass and no melting occurs at the glacier terminus. This allows the terminus to push and prograde the moraine down fjord. Pushing of the moraine is enhanced by surface melting in the spring, which pressurizes the glacier bed and causes the glacier to speed up. Eventually, in early summer the subglacial hydrological system becomes more efficient, resulting in a drop in subglacial water pressure and a reduction in ice velocities. This, in combination with high melt rates at the glacier terminus, results in a seasonal terminus retreat. The seasonal advance only affects sediment within about 10 m of the glacier terminus, and alone cannnot explain the observations of rapid sediment excavation. Thus, fluvial erosion must also play an important role in sediment excavation and redistribution. During our study we observed a major rain event in winter that flushed out soft marine sediment from beneath the glacier and deposited upwards of 30 cm of sediment in the immediate proglacial region. This is corroborrated by comparing several highly accurate digital elevation models of the proglacial sediments. It suggests that episodic events may play an important role in tidewater glacier terminus advance, and need to be accounted for in tidewater glacier models.

The presence of subglacial sediments can influence the evolution of a subglacial sediment, and it determines the nature of the coupling of the glacier to its bed. Active seismic studies indicate the presence of a dilated till, but direct studies of englacial and basal motion in boreholes indicate a relative strong coupling of the glacier to its base. This indicates that sediment deformation is actively occurring and does contribute to the downfjord transport of sediments, either towards the front of the glacier or towards loci of active subglacial fluvial erosion. Repeat radar surveys of the glacier bed do show locations of active erosion, but also areas of subglacial sediment deposition. 

Understanding the evolution of a tidewater glacier thus presents an interesting coupled problem between glacier dynamics, sediment erosion and transport, and oceanography. As part of this work a coupled ice-water-sediment model was developed that was based on parameters from this work and that successfully reproduces the cyclicity observed in tidewater glaciers.

Last Modified: 03/28/2019
Modified by: Martin Truffer