Browsing University of Alaska Southeast by Author "Fahnestock, M."
Glacier, fjord, and seismic response to recent large calving events, Jakobshavn Isbræ, GreenlandAmundson, Jason M.; Truffer, M.; Luthi, M. P.; Fahnestock, M.; West, M.; Motyka, R. J. (American Geophysical Union, 2008-11-18)The recent loss of Jakobshavn Isbræ’s extensive floating ice tongue has been accompanied by a change in near terminus behavior. Calving currently occurs primarily in summer from a grounded terminus, involves the detachment and overturning of several icebergs within 30 – 60 min, and produces long-lasting and far-reaching ocean waves and seismic signals, including ‘‘glacial earthquakes’’. Calving also increases near-terminus glacier velocities by 3% but does not cause episodic rapid glacier slip, thereby contradicting the originally proposed glacial earthquake mechanism. We propose that the earthquakes are instead caused by icebergs scraping the fjord bottom during calving.
Ice me ́lange dynamics and implications for terminus stability, Jakobshavn Isbræ, GreenlandAmundson, Jason M.; Fahnestock, M.; Truffer, M.; Brown, J.; Luthi, M. P.; Motyka, R. J. (American Geophysical Union, 2010-01-21)We used time-lapse imagery, seismic and audio recordings, iceberg and glacier velocities, ocean wave measurements, and simple theoretical considerations to investigate the interactions between Jakobshavn Isbræ and its proglacial ice me ́lange. The me ́lange behaves as a weak, granular ice shelf whose rheology varies seasonally. Sea ice growth in winter stiffens the me ́lange matrix by binding iceberg clasts together, ultimately preventing the calving of full-glacier-thickness icebergs (the dominant style of calving) and enabling a several kilometer terminus advance. Each summer the me ́lange weakens and the terminus retreats. The me ́lange remains strong enough, however, to be largely unaffected by ocean currents (except during calving events) and to influence the timing and sequence of calving events. Furthermore, motion of the me ́lange is highly episodic: between calving events, including the entire winter, it is pushed down fjord by the advancing terminus (at 40 m d1), whereas during calving events it can move in excess of 50 103 m d1 for more than 10 min. By influencing the timing of calving events, the me ́lange contributes to the glacier’s several kilometer seasonal advance and retreat; the associated geometric changes of the terminus area affect glacier flow. Furthermore, a force balance analysis shows that large-scale calving is only possible from a terminus that is near floatation, especially in the presence of a resistive ice me ́lange. The net annual retreat of the glacier is therefore limited by its proximity to floatation, potentially providing a physical mechanism for a previously described near-floatation criterion for calving.