ALBERT

Description: We utilized dendrochronology and precise elevation-constrained mapping to date glacially overridden and drowned trees at the margin of Brady Glacier in southeast Alaska. This technique allowed determination of the timing of the former tidewater glacier’s last advance and consequent formation and filling of two marginal lakes. The subfossil tree-ring chronology spans the interval from AD 1370 to 1861. Brady Glacier impounded Spur Lake to an elevation of 83 m a.s.l. around 1830 and 121 m a.s.l. around 1839. Soon after, Spur Lake reached 125 m a.s.l. and began to overflow a stable bedrock sill. The glacier continued to advance, thickening by at least 77 m between c . 1844 and 1859 at a site down-glacier of Spur Lake on the opposite glacier margin. Farther down-glacier, North Trick Lake began to form by 1861 and reached its highest elevation at approximately 130 m a.s.l. when Brady Glacier reached its maximum extent around 1880. Our findings add precision to the chronology of the last advance of Brady Glacier and provide insight into the evolution of glacier-dammed lakes and calving glaciers.

Description: Volcanic ash layers preserved within the geologic record represent precise time markers that correlate disparate depositional environments and enable the investigation of synchronous and/or asynchronous behaviors in Earth system and archaeological sciences. However, it is generally assumed that only exceptionally powerful events, such as supereruptions (≥450 km 3 of ejecta as dense-rock equivalent; recurrence interval of ~10 5 yr), distribute ash broadly enough to have an impact on human society, or allow us to address geologic, climatic, and cultural questions on an intercontinental scale. Here we use geochemical, age, and morphological evidence to show that the Alaskan White River Ash (eastern lobe; A.D. 833–850) correlates to the "AD860B" ash (A.D. 846–848) found in Greenland and northern Europe. These occurrences represent the distribution of an ash over 7000 km, linking marine, terrestrial, and ice-core records. Our results indicate that tephra from more moderate-size eruptions, with recurrence intervals of ~100 yr, can have substantially greater distributions than previously thought, with direct implications for volcanic dispersal studies, correlation of widely distributed proxy records, and volcanic hazard assessment.

Description: We studied 30 large debris fans along the Alaska Highway between the Alaska-Yukon boundary near Beaver Creek and the south end of Kluane Lake to document late Holocene and historic debris flow activity and to evaluate the hazard that debris flows pose to the highway and other infrastructure. We used dendrochronology and tephrochronology to date surfaces on the fans and to estimate debris flow recurrence. All of the fans are paraglacial landforms of largely latest Pleistocene and early Holocene age. Debris flows continued to occur, probably at a diminishing rate, during the middle and late Holocene, but have only left an irregular carapace of deposits on the early Holocene fans. The White River tephra, which is about 1,200 years old, occurs across the surface of most of the fans, indicating that few debris flows and floods have escaped existing channels of streams on the fans. We conclude that future debris flows, like those that have occurred on nine fans in the past few decades, will mostly be restricted to present stream crossings.

Description: Glaciers in the Canadian Rocky Mountains constitute an important freshwater resource. To enhance our understanding of the influence climate and local topography have on glacier area, large numbers of glaciers of different sizes and attributes need to be monitored over periods of many decades. We used Interprovincial Boundary Commission Survey (IBCS) maps of the Alberta–British Columbia (BC) border (1903–1924), BC Terrain Resource Information Management (TRIM) data (1982–1987), and Landsat Thematic Mapper (TM) and Enhanced Thematic Mapper (ETM+) imagery (2000–2002 and 2006) to document planimetric changes in glacier cover in the central and southern Canadian Rocky Mountains between 1919 and 2006. Over this period, glacier cover in the study area decreased by 590 ± 70 km2 (40 ± 5%), 17 of 523 glaciers disappeared and 124 glaciers fragmented into multiple ice masses. Glaciers smaller than 1.0 km2 experienced the greatest relative area loss (64 ± 8%), and relative area loss is more variable with small glaciers, suggesting that the local topographic setting controls the response of these glaciers to climate change. Small glaciers with low slopes, low mean/median elevations, south to west aspects, and high insolation experienced the largest reduction in area. Similar rates of area change characterize the periods 1919–1985 and 1985–2001; −6.3 ± 0.6 km2 yr−1 (−0.4 ± 0.1% yr−1) and −5.0 ± 0.5 km2 yr−1 (−0.5 ± 0.1% yr−1), respectively. The rate of area loss, however, increased over the period 2001–2006; −19.3 ± 2.4 km2 yr−1 (−2.0 ± 0.2% yr−1). Applying size class-specific scaling factors, we estimate a total reduction in glacier cover in the central and southern Canadian Rocky Mountains for the period 1919–2006 of 750 km2 (30%).

Description: A large rock avalanche occurred at 03:27:30 PDT, 6 August 2010, in the Mount Meager Volcanic Complex southwest British Columbia. The landslide initiated as a rock slide in Pleistocene rhyodacitic volcanic rock with the collapse of the secondary peak of Mount Meager. The detached rock mass impacted the volcano's weathered and saturated flanks, creating a visible seismic signature on nearby seismographs. Undrained loading of the sloping flank caused the immediate and extremely rapid evacuation of the entire flank with a strong horizontal force, as the rock slide transformed into a debris flow. The disintegrating mass travelled down Capricorn Creek at an average velocity of 64 m s−1, exhibiting dramatic super-elevation in bends to the intersection of Meager Creek, 7.8 km from the source. At Meager Creek the debris impacted the south side of Meager valley, causing a runup of 270 m above the valley floor and the deflection of the landslide debris both upstream (for 3.7 km) and downstream into the Lillooet River valley (for 4.9 km), where it blocked the Lillooet River river for a couple of hours, approximately 10 km from the landslide source. Deposition at the Capricorn–Meager confluence also dammed Meager Creek for about 19 h creating a lake 1.5 km long. The overtopping of the dam and the predicted outburst flood was the basis for a night time evacuation of 1500 residents in the town of Pemberton, 65 km downstream. High-resolution GeoEye satellite imagery obtained on 16 October 2010 was used to create a post-event digital elevation model. Comparing pre- and post-event topography we estimate the volume of the initial displaced mass from the flank of Mount Meager to be 48.5 × 106 m3, the height of the path (H) to be 2183 m and the total length of the path (L) to be 12.7 km. This yields H/L = 0.172 and a fahrböschung (travel angle) of 9.75°. The movement was recorded on seismographs in British Columbia and Washington State with the initial impact, the debris flow travelling through bends in Capricorn Creek, and the impact with Meager Creek are all evident on a number of seismograms. The landslide had a seismic trace equivalent to a M = 2.6 earthquake. Velocities and dynamics of the movement were simulated using DAN-W. The 2010 event is the third major landslide in the Capricorn Creek watershed since 1998 and the fifth large-scale mass flow in the Meager Creek watershed since 1930. No lives were lost in the event, but despite its relatively remote location direct costs of the 2010 landslide are estimated to be in the order of $10 M CAD.

Description: Brady Glacier is a large Alaskan tidewater glacier that is beginning a period of substantial retreat. Examination of 27 Landsat and MODIS images from the period 2003 to 2011 indicates that Brady Glacier has a mean equilibrium line altitude (ELA) of 745m and accumulation area ratio (AAR) of 0.40. The zero balance ELA is 600m and equilibrium AAR 0.65. The negative mass balance associated with the increased ELA has triggered thinning of 20-100m over most of the glacier below the ELA from 1948 to 2010. The thinning has caused substantial retreat of seven calving distributary termini of the glacier. Thinning and retreat have led to an increase in the width of and water depth at the calving fronts. In contrast, the main terminus has undergone only minor retreat since 1948. In 2010, several small proglacial lakes were evident at the terminus. By 2000, a permanent outlet river issuing from Trick Lake had developed along the western glacier margin. Initial lake development at the terminus combined with continued mass losses will lead to expansion of the lakes at the main terminus and retreat by calving. The glacier bed is likely below sea level along the main axis of Brady Glacier to the glacier divide. Retreat of the main terminus in the lake will likely lead to a rapid calving retreat similar to Bear, Excelsior, Norris, Portage and Yakutat glaciers. © 2013 John Wiley & Sons, Ltd.

Description: A reanalysis of the varve chronology from hydraulic piston sediment cores was carried out to establish better uncertainty estimates on ages of prehistoric debris-flow deposits (DFDs) in the last 4000 yr. Saanich Inlet is an anoxic fiord located in southeast Vancouver Island near the city of Victoria, British Columbia. It contains annually laminated (varved) marine mud deposited in anoxic conditions. Interlayered with these Holocene varves are massive layers of coarser sediments deposited by submarine debris flows. It has been previously interpreted that these flows were induced by earthquake shaking. Two of the DFDs correspond to known earthquakes: A.D. 1946 Vancouver Island (M 7.3) and the A.D. 1700 Cascadia plate-boundary subduction earthquake (M 9). Based on varve counts, 18 DFDs (310, 410-435, 493-582, 767-887, 874-950, 1001-1133, 1163-1292, 1238-1348, 1546-1741, 1694-1811, 1859-2104, 2197-2509, 2296-2483, 2525-2844, 2987-3298, 3164-3392, 3654-4569, 3989-4284 yr ago from A.D. 2010 datum) were correlated among two or more cores during this time period, suggesting an average return period of strong shaking from earthquakes of about 220 yr. Nine of the DFDs overlap with the age ranges for great plate-boundary earthquakes that have been determined by other paleoseismic studies: coastal subsidence and offshore turbidity deposits. The remaining nine events give an average return period of about 470 yr for strong shaking from local earthquakes. The peak ground acceleration calculated from a recurrence relation based on statistics from local earthquakes for a 470-yr period is 0.30 g, which corresponds to the upper range of Modified Mercalli Intensity (MMI) VII (seven). Historical data from Vancouver Island and other areas show that this level of shaking (MMI VII) is sufficient to trigger submarine landslides.

Description: Volcanic ash layers preserved within the geologic record represent precise time markers that correlate disparate depositional environments and enable the investigation of synchronous and/or asynchronous behaviors in Earth system and archaeological sciences. However, it is generally assumed that only exceptionally powerful events, such as supereruptions (≥450 km3 of ejecta as dense-rock equivalent; recurrence interval of ∼105 yr), distribute ash broadly enough to have an impact on human society, or allow us to address geologic, climatic, and cultural questions on an intercontinental scale. Here we use geochemical, age, and morphological evidence to show that the Alaskan White River Ash (eastern lobe; A.D. 833–850) correlates to the “AD860B” ash (A.D. 846–848) found in Greenland and northern Europe. These occurrences represent the distribution of an ash over 7000 km, linking marine, terrestrial, and ice-core records. Our results indicate that tephra from more moderate-size eruptions, with recurrence intervals of ∼100 yr, can have substantially greater distributions than previously thought, with direct implications for volcanic dispersal studies, correlation of widely distributed proxy records, and volcanic hazard assessment.