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Detecting Near-shore Submarine Permafrost at Barrow, Alaska Using Electrical Resistivity

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Citation:
Overduin, P. P. , Westermann, S. , Haberlau, T. , Romanovsky, V. and Yoshikawa, K. (2010): Detecting Near-shore Submarine Permafrost at Barrow, Alaska Using Electrical Resistivity , Third European Conference on Permafrost, Longyearbyen, Svalbard, Norway, June 13-17 .
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Abstract:

Non-consolidated sections of the arctic coastline are generally rich in ground ice and highly dynamic due to the combined effects of thermal abrasion and ero-sion. Subsidence following thaw may contribute to shaping shore-face profile and speed erosion, analo-gous to the removal of material in the near-shore zone. Ice dynamics in the near shore are thus impor-tant to coastal erosion rates. We test electrical resis-tivity tomography as a means of measuring the depth of ice-bearing permafrost within the sediment and to gain insights into how sediment ice content may control coastal dynamics. Coastline position change rate studies at Elson Lagoon at Barrow, USA, pro-vide background information on long-term coastal evolution as far back as the end of the 19th century. We couple observations on change in coastal geo-morphology with geophysical observations along a submarine sub-bottom profile obtained using electri-cal resistivity soundings and drilling. Permafrost on the land-side of the coastline at Elson Lagoon (71° 18' N , 156° 32' W) is ice-rich near the ground surface. It is cold at Barrow (about -9 °C at 20 m, the depth of zero seasonal amplitude) and warmed by about 2°C during the last century. De-spite sheltering by barrier islands, coastal retreat rates on the western and southern coasts of the la-goon have been high over the past few decades. Permafrost evolution after inundation following coastal retreat was examined using a combination of electrical resistivity surveys, sediment sampling and temperature measurements. Sub-bottom apparent electrical resistivity was collected in the summer of 2008 using an IRIS Syscal Pro(TM) system and an electrode cable with 5 m spacing. A GPS and echo-sounder measured position and bathymetry. In spring 2009, 4 boreholes were drilled along 2 tran-sects at 1 and 2.25 m water depth. Borehole tem-peratures, sediment pore water salinity and stable isotope composition were measured. Over twenty offshore profiles were measured with a penetration depth of about 15 m. Resistivity profiles perpendicular to the coastline showed a sharp in-crease in resistivity at some depth. This depth in-creased with increasing distance from the shore. The underlying high-resistivity layer disappeared beyond approximately the 2 m isobath (at least in the upper 14 m of sediments). Drilling and sediment tempera-ture suggested that this layer coincided with the ice-bearing permafrost. Salinity of pore water within the sediment reached values over four times that of sea-water. We suggest that bottom-fast ice in water depths of less than 2 m works to preserve ice within the sediment. Beneath water columns deeper than the maximum annual ice thickness, penetration of highly saline bottom water into the sediment facilitates permafrost degradation.Coastline retreat rates have been reported by nu-merous authors for this site and currently generally increase towards the southeastern portion of the study site. The distribution of the interpreted ice-bearing permafrost along the coast changes too little to be related to differences in coastal retreat rates.

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