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Freiburger Geographische Hefte, Heft 50

Arne Friedmann (1996): Das Taku-Gletschersystem (Jeneau Icefield, Alaska): Seine Dynamik und Fluktuationen


The Taku Glacier System (710 km2, 59.5 km in length), a Tidewater Glacier of the southern Juneau Icefield, advances since 1899 into the north end of Taku Inlet. The Main Terminus advanced 7.3 km and the Hole-in-the-Wall-Glacier-Terminus 3.3 km, whereby the advance rates continuously diminished and the ice front stagnates since 1989. The advance produced a massive push moraine which fringes the glacier front and prevents it from calving into the inlet waters. This protective push moraine makes it difficult to detect the Tidewater Glacier-nature of the Taku Glacier. As a Tidewater Glacier the Taku has a periodic advance and retreat cycle and at the moment the glacier is in the advance phase of this cycle.

The Matthes Glacier is the largest tributary glacier and the only true Tidewater Glacier of the Taku Glacier System, because it is the only tributary to reach the main ice front. The Demorest Glacier and the NW-Branch tributaries also transport substantial ice volumes into the Main Taku Icestream, but all the ice of the Demorest Glacier flows into the Hole-in-the-Water-Glacier outlet. The SW-Branch tributary is small and doesn't influence the dynamics of the Main Icestream.

The presnt dynamics of the Taku Glacier System are the result of a strongly positive mass balance and the Tidewater Glacier cycle (compensation for the catastrophic retreat phase). Hereby the Tidewater cycle controls the periodicity, but not the individual advance rates of a glacier, which are dominately influenced by the glacier's mass balance and external factors. The strongly positive mass balance of the Taku Glacier leads to ice surface elevation increases in the accumulation and ablation areas and generates high iceflow velocities.

Two different velocity phases occured on the Taku Glacier System. In the first phase calving dynamics at the ice front controlled ice velocities in the glacier system. This phase endet 1953, when the glacier ceased calving. In the second phase (until today), glacier flow velocities are mainly influenced by mass balance fluctuations. The flow pattern on the Taku Glacier System is mostly parabolic, but below the ELA (Profiles 1 and 2) a transition to partly Blockschollen - Flow with velocities of nearly 1 m/d occur.

Seismic measurements on the Taku Glacier System revealed ice thicknesses of over 1400 m in the Main Icestream, which makes the glacier the thickest so far measured alpine glacier in the world. The total ice volume of the Taku Glacier exceeds 300 km3.

The balance flux near the ELA exceeds the actual flux of 1994 (6.70 x 108 m3/a). This demonstrates, that the glacier yearly transports less ice into the ablation area as snow falls on its accumulation area, which confirms the observed ice surface elevation increases in the accumulation area.

Because of its length and area, ice from the accumulation area is transported with a flow lag (delay factor) of 60 to 380 years into the ablation area. This shows that the present terminus is nourished by snow, which fell hundreds of years ago.

The very warm summers of the last years led to high ablation rates which exceed the mean of the last 50 years, especially in the ablation area. This was followed by stagnation and slow downwasting at the glacier front. At the moment the ice front is still stable, therefore the ablation rates of the next years ind the iceflux into the terminus area determine the future behaviour of the ice front. The iceflux from the accumulation area to the ablation area over the last years was relatively constant, which is underlined by the regular iceflow velocities at the ELA. The crucial parameter for the actual mass balance and terminus behaviour is therefore the ablation. However, variations in the iceflux at the ELA can alter this quickly. A small retreat from the push moraine will inevitably lead to the catastrophic retreat-phase of the Tidewater Glacier cycle. For a substantial further advance of the Taku Glacier, a significant increase in ice surface elevation of the terminal area is essential.

On the Norris Ridge and the Brassiere Hills moraine ridges were identified, which date from the "Little Ice Age" - advance in the 18th century. This advance culminated between 1720 and 1770. The configuration of the "Little Ace Age" - Taku Glacier can be reconstructed.

A detailed investigation of Taku Point showed, that the Taku Glacier ice front did not reach the eastern shores of Taku Inlet and the Taku River was not ice blocked. No large ice dammed lake reaching into Canada was formed in the 18th century.

In addition, several moraine ridges in the central part of the Norris Ridge and on the south side of the Brassiere Hills were discovered for the first time. Through diggings and 14C-dating of imbedded wood it was found, that these moraines were formed by the fluctuations of the retreating lateglacial and early holocene Taku ice margin. The first neoglacial advance followed ca. 1700 years BP, which was indicated by glaciofluvial outwash in a peat pit. Until the "Little Ice Age" - advance in the 18th century, which was dated dendrochonologically, no other traces of periods of glacier expansion could be found so far for the Taku Glacier System.

The Norris Glacier /180 km2, 27 km long) reached its outermost position since the lateglacial in 1916. No traces were found for an adavance during the "Little Ice Age", which might be due to subsequent further advance until 1916, that overran older moraines. The Norris Glacier seemed to have a high ice level for several centuries until the culmination in 1916. Since then the ice front, now ending in a meltwater lake, is retreating with increasing recession rate.

The Grizzly Bar, a glaciofluvial outwash area infront of the Norris Glacier, is in the process of forestation, which can be documented by several stades of successions. Its northern end is being actively eroded by the still advancing west side of the Taku Terminus Lobe and the icemarginal Norris River.

This is the first time, that the late- und postglacial fluctuations of the Taku and Norris glaciers have been reconstructed in detail.