The size and frequency of icebergs and bergy bits derived from tidewater glaciers in Kongsfjorden, northwest Spitsbergen

Abstract

Tidewater glaciers constitute over 1000 or 20% of the coast of Svalbard. The dimensions and frequency of the occurrence of icebergs and bergy bits produced from these tidewater glaciers in Kongsfjorden, northwest Spitsbergen, were measured during the summers of 1991 and 1992. In 1991, 35-40% of 275 observed icebergs and bergy bits were <0.5 m wide, and <10% exceeded 5 m in width. 70-80% of the freeboard measurements were <0.5 m and only 10–15% were >1 m in height. The largest observed freeboard was 6 m. In 1992, small icebergs were significantly less common. 50% of the observed icebergs were > 10 m in width and >2 m in freeboard. This is interpreted to be the result of a major calving event prior to the 1992 observations. Side-scan sonar data on sea floor morphology showed frequent scouring by iceberg keels to a depth of 35 m, but no scouring below 40 m, thus defining the maximum iceberg keel depth and the depth to which sediment reworking by these keels occurs. Calculations of the melt rate of icebergs allows an estimation of the life expectancy of icebergs calved into Kongfjorden. Melting by forced convection lies between approximately 0.1 and a maximum of 1.0 m d?1, depending on iceberg relative velocity, size and water temperature. Melting linked to wave action is also approximately 0.5-1.0 m d?1. These calculations imply that icebergs of the dimensions commonly observed in Kongsfjorden will seldom survive travelling beyond the fjord mouth. Radar observations of iceberg occurrence from FS POLARSTERN during its summer 1991 circumnavigation of Svalbard also showed that no larger icebergs were escaping beyond the mouths of the major fjords of western and northern Spitsbergen. Iceberg derivation from Spitsbergen fjords is therefore not likely to be an important mechanism for sediment rafting and deposition on the continental shelf and in the deep ocean, but it is of significance to local fjord sedimentation. Comparison with evidence on iceberg dimensions from the Barents Sea and an East Greenland fjord shows that the larger icebergs there are derived from parent ice masses with quite different characteristics than those calving into the Spitsbergen fjords.