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36 Post Little Ice Age Glacial Rebound in Glacier Bay National Park and Surrounding Areas By Roman J. Motyka, Christopher F. 4) determining when this episode of uplift two to four occupations over three to six Larsen, Jeffrey T. Freymueller and began, and years. By assessing the change in position Keith A. Echelmeyer 5) assessing the total amount of uplift that over these intervals, we determined bed- has occurred. rock motion on an annual basis. The results Introduction of our GPS investigations for vertical The fastest measured rates of uplift in Uplift Measurements motion are shown in Figure 2. A double the world today are in Southeast Alaska: We used three complimentary methods bulls-eye of contours delineates two centers in Glacier Bay National Park and east of to measure uplift: of rapid uplift in Southeast Alaska: over Yakutat (Figure 1). The first measurements 1) precision GPS geodesy, Glacier Bay (1.18 in/yr, 30 mm/yr) and over of this rapid uplift were done the mid- 2) relative sea-level change from tide gauge the Yakutat Icefield (1.26 in/yr, 32 mm/yr) twentieth century through tide gauge measurements, and (Larsen et al. 2005). Uplift rates decrease studies, which suggested that land in the 3) raised shoreline studies. smoothly away from these peaks. The uplift Glacier Bay region was emerging at 1.18 Precision GPS geodesy has many dis- pattern documented here spans an area of in/yr (30 mm/yr) and faster in Glacier Bay tinct advantages—it can be done anywhere over 40,000 mi2 (100,000 km2). As we shall (Hicks and Shofnos 1965). However, the there is stable bedrock, thus allowing much see later, the centers of peak uplift coincide cause of the uplift was still being debated broader spatial coverage than the other two with places that have experienced some of (Left and Above) Researchers Chris Larsen and Ellie Boyce set up GPS units over when we initiated our investigations in methods; researchers can measure bench- the greatest losses of ice. benchmarks set into bedrock at various 1998. To determine whether this uplift was mark positions relative to a global reference The tide gauge data used in our second locations in Glacier Bay. driven by tectonics or glacial rebound we frame with high accuracy (± 0.08 in, ± 2 method come from permanent National Photographs by R.J. Motyka embarked on a field program that included: mm); and it measures both vertical changes Ocean Service (NOS) gauges, NOS tempo- 1) repeating and reviewing the original tide and horizontal motion, latter of which is rary gauges and our own temporary gauges gauge measurements, very important in this tectonically active (Larsen et al. 2003, 2004) for a total of 22 2) measuring the current rates of uplift region. We established over 70 measure- sites (Figure 3). Temporary tide gauges typi- with modern GPS geodetic techniques, ment stations distributed across the region cally record sea-level over the course of 3) defining the regional pattern of uplift, (Figure 2), with each site typically having one or more monthly tidal cycles, and the 37 Post Little Ice Age Glacial Rebound in Glacier Bay National Park and Surrounding Areas Figure 1. Location map, showing tectonic Figure 3. Negative setting and present day glacier wastage sea level rates (from Larsen et al. 2005). Ice thinning from tide gauge rates follows Arendt et al. (2002). The data (from Larsen fastest changes are occurring at lower et al. 2004). elevations, such as the termini of the Contour interval Bering (BER) and Malaspina (MAL) is 0.08 in/yr glaciers. The Yakutat Icefield (YI) is an (2 mm/yr). Red exception, where thinning rates are diamonds indicate about three times greater than the tide gauge sites. regional average and are driving the greatest ongoing unloading in Southeast Alaska. The Glacier Bay Little Ice Age Icefield is outlined (GB). Most of this icefield disappeared over the last ~250 years. Tectonic deformation along the North America Pacific Plate boundary occurs as strike-slip motion on the Fairweather Fault (FWF), and to a lesser degree, the Denali Fault (DF). elevation of the gauge is then surveyed agrees well with the pattern of uplift rates Glacier Bay National Park clearly show the benches (Motyka 2003). Raised shorelines relative to a local network of benchmarks. from GPS measurements within the Glacier effects of rapid uplift through recent land were identified by: When this procedure is repeated several Bay region (Figure 2). emergence, young shoreline forests, new 1) a wave cut step or riser in the slope, decades later, sea level change can then In the third method, we shoals, raised shorelines, and wave-cut 2) a change in thickness of organic-rich soil, be found relative to the benchmarks. The measured raised shorelines 3) termination of beach deposits, and/or average overall uncertainty in this method at 27 different coastal sites. 4) an abrupt change in age of trees (Figure 4). is 1 = ±0.2 in/yr (5 mm/yr). The pattern of Coastal regions in and around The difference in elevation between the sea level changes found at the tide gauge raised shoreline and the current maximum sites indicates that the fastest sea level high tide level provided us with the total changes in Southeast Alaska are found in amount of sea-level change. Dating tree Glacier Bay (Figure 3). This finding is in ages just below the raised shoreline provide Figure 2. GPS uplift general agreement with Hicks and Shofnos observations in Southeast a minimum estimate of the onset of land (1965), although we determined that the Alaska (from Larsen et al. emergence (Motyka 2003, Larsen et al. peak uplift rate found by Hicks and 2005). GPS uplift rates are 2005). The average overall uncertainty in in mm/yr; contour interval Shofnos at Bartlett Cove is almost certainly is 0.08 in/yr (2 mm/yr). estimating change in shoreline elevation is biased by unstable reference benchmarks GPS stations are shown 1 = ± 1 ft (0.3 m). The results shown in there. After we rejected the data from this with diamonds, colored Figure 5 show that the total sea level change according to the uplift point, we found that overall the pattern and found at the raised shoreline sites also rate error at each site as magnitude of regional sea level rates have indicated by the color describes a regional pattern surrounding remained essentially constant at the level scale bar. Peak uplift Glacier Bay. Quite notably, the greatest of measurement accuracy since the time of rates are found in Glacier amount of sea level change occurs at the Bay (southern peak) the earliest rate measurements (Larsen et al. and the Yakutat Icefield sites closest to where the peak rates of 2005). The pattern of sea level rates also (northern peak). uplift and sea level fall are found. Dates for 38 the initiation of emergence is estimated to when you add or remove weight in a float- Icy Straits (Figure 6). The period over warm period triggered a dramatic calving have begun 1770 ± 20 AD, the same period ing boat, it will sink or rise in the water. The which this ice accumulated and expanded retreat of glacier ice that lasted into the that Glacier Bay and other regional glaciers earth’s crust responds in a similar way but to fill the entire bay is known as the Little twentieth century. began retreating from their Little Ice Age with a substantial time lag: unlike water, the Ice Age (LIA), which began during the Glaciers leave behind tell-tale signs of maximums (Motyka and Beget 1996, Larsen material beneath the earth’s crust (know as thirteenth century in southeastern Alaska. their maximum expansion, and we utilized et al. 2005). mantle) is extremely viscous and it takes During the mid-eighteenth century a these markers to reconstruct what Glacier considerable time (centuries to millennia) period of climatic warming appears to Bay looked like at its LIA maximum. These Glacial Rebound for it to respond to a change in load. have affected the entire region as evidenced signs include forest trimlines, lateral The correlation of the onset of uplift Examples of long lasting viscoelastic by a number of glacial retreats starting moraines, terminal moraines, and glacier with the retreat of glaciers in and around response are well documented in regions about this time (Motyka and Beget 1996). outwash. We used light aircraft overflights Glacier Bay makes unloading of the earth’s that were covered by continental ice sheets Glacier Bay was no exception and this to identify these geomorphic markers as crust through loss of glacier ice a prime during the Last Glacial Maximum 16,000 Photographs by C suspect for driving the regional uplift. years ago, e.g., Hudson’s Bay and Scan- Raised Shoreline Measurements This is because one of the consequences of dinavia. In a similar way the rapid uplift changes in glacier ice mass is a readjust- . rates observed in Glacier Bay are related to F . Larsen ment of the earth’s crust and underlying recent ice loss. To test this hypothesis, our layers in response to the changing weight, next step was to determine how much a process known as “glacial isostatic glacier ice has been lost in the region and adjustment” (GIA). The effect is akin to whether GIA response to this changing load the well-known Archimedes Principle, e.g., could account for the entire strong uplift signal. If so, then the results would also tell us much about the properties of the earth’s crust and mantle in this region. Glacier Ice Loss Just over 250 years ago a vast ice field blanketed much of what is now Glacier Bay National Park.