Passive seismic methods form the core of this 3-year, multi-disciplinary proposal designed to investigate temporal and spatial relationships between ice motion, basal processes and iceberg calving at a rapidly changing tidewater glacier. The proposed measurements will link glacier-generated seismic signatures to physical glaciological processes. At present, glacier generated waveforms are poorly understood. Understanding their source mechanisms could advance knowledge of the dynamic mass balance of Earth?s cryosphere, as well as improve models and understanding of glacier erosion and landscape evolution. Preliminary results from a 1-year pilot study highlight the tidewater glaciers of Icy Bay, Alaska as the regionally dominant source of glacier-generated seismic energy. Icy Bay is the optimal study site because of its highly dynamic glaciers, logistical simplicity and similarities with disintegrating outlet glaciers in Greenland. Pilot study results show potential to make use of long duration, high-resolution records of motion, seismicity and changes in geometry in concert to develop quantitative proxies for glacier erosion.
The proposed work will record seismic events within a local network with dense station spacing, and simultaneously observe 3-D ice displacement and glacier geometry changes. Iceberg calving and basal processes are challenging to measure, and applying readily available and highly developed seismic methods to these problems holds significant promise. The proposal will support the collaboration of three early-career scientists with multi-disciplinary backgrounds. Data from the experiments will be archived at UNAVCO, IRIS, NSIDC and at the University of Alaska.
and iceberg calving flux
The Yahtse Glacier project, funded by the National Science Foundation under grant EAR-0810313, examined factors relating to mass loss from glaciers. Yahtse Glacier (60.2° N, 141.4° W) is a 1000 km2 (386 mi2) glacier that ends in Icy Bay, an inlet off of the Gulf of Alaska in the northeast Pacific Ocean. Yahtse Glacier is similar to many of the largest glaciers on Earth, because it loses ice at its terminus, or toe, directly to the ocean. This ice loss at the terminus is in addition to the ice loss that occurs from melting on its surface under a bright sun and warm air. However, unlike ice loss on glacier surfaces, the rate at which marine-terminating glaciers lose mass to the ocean can change very quickly and for reasons that are not yet very clear to the scientists that study these systems. Because these very large, marine-terminating glaciers can change their mass so quickly, it is hard to predict how they might contribute to increases in future sea level around the globe. In order to better understand ice mass loss at marine-terminating glaciers, our project team established a network of sensors to study the behavior of Yahtse Glacier. Measurements of ground shaking produced by the glacier, known as "icequakes," were central to our study. We found that when blocks of ice fall off the terminus of Yahtse Glacier and into the ocean, a process known as iceberg calving, powerful icequakes are produced that can be detected 100s of km away. These icequakes are produced largely through icebergs interacting with the sea surface, rather than during the fracturing process, as had been previously thought. Our team explored the relationship between icequakes and the icebergs that produced them and found that we could predict the size of an iceberg by the icequake it produces. Two years of icequake recordings reveal that calving reaches an annual minimum during mid-winter, in April, we observed an abrupt increase towards a late-summer maximum. We interpreted that the calving rate is significantly modulated by the rate at which warm ocean water melts and undercuts the glacier terminus. We also found that iceberg calving is sensitive to several-meter variations in tidal height. Measurements of seawater temperature and salinity from in front of Yahtse Glacier allowed us to explore the relationship between warm ocean water and submarine terminus melt in greater depth. We found that warm, 10 °C water flowing towards the glacier terminus can easily melt submarine ice at 17 m/d, the rate at which the glacier flows down towards the terminus. This suggests ice loss at the glacier terminus may be paced by the rate of submarine melt. Iceberg calving above sea level is likely controlled by the rate at which the foundations of would-be icebergs melt out from below them. Therefore, efforts to understand and predict rates of mass loss from ocean-terminating glaciers like Yahtse Glacier are unlikely to be successful unless submarine melt is explicitly accounted for. The above scientific results have been published in peer reviewed journals such as Earth and Planetary Science Letters and the Journal of Geophysical Research. We have also communicated these findings though 11 presentations at international scientific conferences and several additional invited lectures. We supported media activities that resulted in print and online exposure through Scientific American, the Anchorage Daily News, and a blog post by the American Geophysical Union. Our grant has also supported graduate and undergraduate education. Over the course of this grant, we have trained one Ph.D. student who will be graduating this December. Interdisciplinary research was a centerpiece of our funded proposal and he has made this a hallmark of his dissertation, by drawing on glaciology, seismology, oceanography and statistics. This student will be continuing with his focus on tidewater glaciers as a postdoctoral researcher at the University of Texas. We also hosted an undergraduate researcher at the UAF Geophysical Institute for a 3-month internship. The undergraduate, mentored by our Ph.D. student, presented her novel research on icequakes and glacier motion at the annual meeting of the American Geophysical Union.
