| Résumé | Ten multi-year ice floes sampled in the high Arctic in late summer 2009 and spring 2010 are described. Ice thickness obtained from detailed dirl hole profiles indicated average floe ticknesses from 3.4 to 14.7 m (±1.3 to 4.3 m). A maximum tichness of 21.1 m was measured, which was limit of the two-person drill team, however the sail height of some other features on the floes suggested much ticker ice. Voids, pockets and loose blocks on the underside of the ice were sometimes noted while drilling through multi-year ice in August 2009 and in May 2010 albeit less frequently. Thicknesses obtained from drill-hole measurements on four of the very thick multi-year ice floes were compared to thicknesses obtained over the same profile areas by a helicopter-based electromagnetic introduction (HEM) system. Compared to drill-hole measurements, the HEM underestimated the average thicknesses of the four floes by 15 to 24 %. Result showed that the HEM did not reproduce thickness larger than about 12 m and it overestimated the percentage of ice from 3 to 7 m thick. It is expected that the HEM provided no data about ice thicker than 12 m because the thickness of deformed multi-year ice within the sensor's footprint was so variable and also because of the attenuating efect that large, sea-water filled voids had upon the EM soundings. Two deformed, multi-year ice floes instrumented with 11 m long temperature chains in August 2009. The instrumentation extended through 12.4 m thick ice on the first floe (Floe L03, 74°N) and 13.5 m thick ice on the second floe (Floe L08, 77°N). Both floes had a ‘C-shaped’ temperature profile to a depth of about 7.5 m in August 2009, below which the ice was isothermal at near melting temperatures. The coldest temperature (-5°C) occured towards the interior of the floe (4 to 5 m depth). The temperature vs. time series suggests that about 4 m of ice was lost from the bottom of the Floe L03 (12.4 m thick) as it drifsted from Kane Basin to the northern part of Baffin Bay, prior to the last data transmission in September 2009. Floe L08 continued to transmit temperature data for one year. In August 2010, Floe L08 was near-isothermal throughout its full tickness, with the coldest temperature being -3°C at an ice depth of 4 to 5 m - compared to the “C-shaped” temperature profile that the floe had in August 2009. About 1 mof ice ablated from the top surface of Floe L08 during the summer of 2010 and undetermined amoint of thinning occured on the underside of the floe. Flow L08 was re-visited in May 2010, when cores were taken to a depth of 5 m and borehole strength tests were conducted at 30 cm depth intervals to an ice depth of 5.40 m. The uppermost 5 m of ice had an average temperature of -11.3°C and an average salinity of 1.4%. Borehole strength tests were conducted in two boreholeson Floe L08 to a depth of 5.40 m. The maximum ice pressure attained due to the limited capacity of the borehole system. As anticipated, the strength of the ice was very dependent upon the ice temperature, which is consitent with past four years of borehole strength tests multi-year ice. |
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