An Inside Look at Eroding Coastal Bluffs on Alaska’s North Slope
In September 2015, scientists from the U.S. Geological Survey (USGS) and the University of California, Santa Cruz (UCSC) surveyed rapidly eroding permafrost bluffs on Barter Island, a remnant of low-elevation tundra on Alaska’s Arctic coast. Warming air and sea temperatures in the Arctic are leading to longer periods of permafrost thaw and ice-free conditions during the summer months, which can weaken the coastal bluffs and increase their vulnerability to storm surge and wave impacts. The 2015 survey is part of a long-term effort to document seasonal to decadal coastal-bluff change on the island’s north coast.
In spite of bleak weather conditions that thwarted scheduled flights to Barter Island for many days, the survey team achieved its goals on the island. The researchers drilled into the permafrost to obtain samples of permafrost ice, pore water, and sediment. They are using geochemical techniques, such as measurement of radon and stable isotopes, to trace the movement of groundwater and examine its effects on sediment erosion. Several geophysical techniques were used to image the subsurface structure of permafrost features, such as ice-wedge polygons, and to measure the salt content and internal structure of materials that make up the frozen ground.
Repeat electrical resistivity tomograms (ERTs) had been collected on the coastal bluffs in Barter Island in early and late summer 2014 to evaluate the effects of one summer’s thaw cycle. ERTs provide a cross-sectional view of electrical resistivity within the bluffs. Because ice is a poor conductor of electricity and thus has high resistivity, ERTs reveal the distribution of subsurface permafrost. ERTs were collected from the same bluffs during the 2015 survey to examine annual change and to identify sites for drilling into the permafrost.
To complement these geo-electrical methods, the September 2015 survey team collected data with a phase-sensitive radio echo sounder (pRES). Whereas traditional echo sounders send sound waves through water to detect boundaries between materials with differing physical properties, the pRES sends radio waves through ice. pRES data can be used to image the base of an ice mass and also internal reflecting layers, such as layers of liquid water or variations in the size of air bubbles in the ice. The USGS researchers were particularly interested in another strong radio-wave reflector: the interface between freshwater and saltwater. They collected pRES data along select survey lines for comparison with the ERTs to determine where the subsurface gets salty and so document the influence of seawater in permafrost.
A primary focus of the 2015 effort was to ground truth the remote-sensing methods by collecting permafrost samples. A custom-designed drilling platform allowed the team to obtain samples from depths down to approximately 6 meters in permafrost. Preliminary results confirm that the permafrost pore-water salinities near the bottom of the cores exceeded seawater values, an observation supported by the 2014 and 2015 ERT images. The pore-water samples from these drill holes are being used for additional geochemical analyses to illuminate the oceanic and geologic evolution of this dynamic coastal environment.
The researchers aim to document seasonal to decadal coastal-bluff change and associated hydro-geologic processes along a 3-kilometer stretch of coast on Barter Island by using the techniques outlined above along with recently collected time-lapse photography; historical maps and imagery; GPS surveys of the beach and nearshore; sediment sampling and analysis; 3-D models of the terrain derived from aerial photography and airborne lidar (a laser-based surveying technique); photographs of the bluffs taken from an all-terrain vehicle (ATV); measurement of water levels, currents, and salinity in lagoons and nearshore waters; and numerical models of waves, storm surge, and inundation.
The Barter Island study is part of a larger investigation of climate-change impacts on Alaska’s Arctic coast. (See website, “Climate Change Impacts to the U.S. Pacific and Arctic Coasts: Research” and related Sound Waves article, “Northern Alaska Coastal Erosion Threatens Habitat and Infrastructure.”)
Scientists who contributed to the September 2015 survey included Peter Swarzenski, Bruce Richmond, Cordell Johnson, Tom Lorenson, Li Erikson, and contractor Amy West from the USGS Pacific Coastal and Marine Science Center, and Neil Foley and Slawek Tulaczyk from UCSC. The work falls under USGS projects on Coastal Aquifers and Climate Change Impacts to the U.S. Pacific and Arctic Coasts.
Essential support for this field effort was provided by aquatic biologist Greta Burkart and Arctic National Wildlife Refuge (ANWR) Manager Brian Glaspell, both of the U.S. Fish and Wildlife Service (USFWS). On the last day of fieldwork, Bruce Richmond was asked to give a briefing to USFWS Director Daniel Ashe, Senator Tim Kaine (D, Virginia), Senator Martin Heinrich (D, New Mexico), Deputy Regional Director of Alaska Region USFWS Karen Clark, and Brian Glaspell.
in this issue:
Eroding Coastal Bluffs on Alaska’s North Slope