Glacier Fluctuations Since the Last Glacial Maximum in Southwest Alaska

Glacier fluctuations since the last glacial maximum in southwest Alaska

Introduction

During the last glacial maximum (LGM), alpine glaciers in the western cordillera expanded, coalesced, and flowed onto newly-emergent continental shelves (Figure 1). During and after deglaciation, rising seas again submerged these shelves, obscuring the deposits left in the termination zones of these glaciers. Luckily for geologists, outlet glaciers also flowed into unglaciated lowlands on the landward side of the cordillera. The deposits associated with these land-terminating glaciers are relatively well-preserved, and can be used to reconstruct the extent of glaciation during the LGM, as well as the timing and style of deglaciation. Ongoing research in southwest Alaska provides an excellent learning example of the use of these geomorphic records to reconstruct landscape history.

Geomorphic evidence of the last glacial maximum in southwest Alaska

The Alaska Range is a rugged mountain range in south-central Alaska, home to Mt. McKinley (Denali), the highest peak in North America (6194 m). It is also the northernmost extent of the Cordilleran Ice Sheet, a continuous ice cap that formed along the Pacific coast of North America during the LGM. The western flank of the Alaska Range is an area of ongoing study of glacier dynamics during and following the LGM. Well-preserved moraines and associated glacial deposits record and permit reconstruction of the events that occurred there.

The most prominent depositional glacial landforms in the area are the terminal moraines emplaced by piedmont glaciers during their late Wisconsin maximum extent. Over many hundreds of years, glacial till melting out of the ablation zone of the glaciers accumulated at their snouts to form broad, 1- to 2-km-wide hummocky ridges. Melting of ice blocks buried within the moraine resulted in the formation of hundreds of small kettle lakes on this landform. These moraines are visible in elevation data and satellite imagery and can be traced for hundreds of miles along the western flank of the Alaska Range (Figures 2, 3).

Upstream from the terminal moraine, a series of similar landforms are present. These are recessional moraines, recording the long process of retreat following the LGM. These moraines were built by glaciers that either temporarily readvanced during overall retreat, or simply paused long enough to build a significant moraine. In many locations in southwest Alaska, at least four recessional moraine-building events, or stades, are recognized, often correlatable from valley to valley.

Determining the age of glacial landforms

How can we constrain the ages of the LGM and subsequent recessional stades? Because very little vegetation was present during the last glacial maximum, radiocarbon is rarely able to produce more than broadly-defined minimum and maximum ages. Optically-stimulated luminescence has the advantage of not requiring organic matter--it dates sand grains directly. However, OSL has not been thoroughly tested on glacial deposits in Alaska, and it only works on those grains that were sufficiently exposed to sunlight before deposition. Cosmogenic radionuclide (CRN) dating is a tool that is well-suited to determining the age of a glacial moraine. By measuring the levels of cosmogenic isotopes (isotopes produced by the interaction of high-energy particles from space with minerals on earth) in boulders on the crest of a moraine, geologists can determine how long the boulder has been exposed. However, workers must be careful to select only those boulders that appear to be relatively stable. Boulders that have shifted or rolled since deposition will have received less cosmogenic radiation, resulting in ages younger than the age of the landform. An additional source of uncertainty is shielding of cosmic rays by seasonal snowcover. This source of uncertainty can be reduced by sampling only those boulders that protrude well above the surrounding moraine surface.

CRN Samples taken from boulders on the LGM terminal moraine on the western flank of the Alaska Range suggest that the moraine stabilized between 19-23 ka. The actual age of the LGM is likely toward the older end of this range, because the moraine boulders were probably subject to shifting for several hundred years after deposition. The recessional moraines are as yet undated. However, other forms of evidence provide insight. Radiocarbon dates from the base of cores taken from several kettle lakes along the shores of Lake Clark indicates that the glacier front had retreated into mountain valleys by 15 ka. Deglaciation was probably complete in southwest Alaska by around 11 ka.

Neoglaciation

Temperatures were at or above modern levels during the earliest Holocene, a period dubbed the Holocene Thermal Maxmimum. Most glaciers were probably smaller during this period than they are today. By the mid-Holocene (~5 ka), global temperatures were once again dropping, causing glaciers to advance beyond their early Holocene minima. This renewed glacial activity is referred to neoglaciation.

On the western flank of the Alaska Range, 2-4 moraines of probable mid- to late-Holocene age are present. These relatively small moraines are overwhelmed and partially buried by a much larger, more voluminous moraine immediately downvalley of modern glacier termini (Figure 4) This younger moraine is often sharp-crested, unvegetated, and up to 100 m high. Field observations indicate that it is ice-cored, suggesting that its apparent volume may be transient.

Dating of the older neoglacial moraines could best be accomplished with cosmogenic radionuclides. However, CRNs are not as effective for the younger, ice-cored moraine. Instead, its age is best constrained with lichenometry. Lichenometry is a method wherein the age of a moraine is estimated by comparing the largest lichens growing on moraine boulders with a known lichen growth rate. Though this method is somewhat imprecise (uncertainty ~ +/- 20%), lichen sizes on the youngest, ice-cored moraines suggest that they date to the culmination of the Little Ice Age (LIA) toward the end of the 19^th century.

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