Abstract
When icebergs touch and scour through unconsolidated seafloor sediment the keel ice may fracture and crush, creating individual ice blocks that may rotate independently of each other under confining pressure. Blocks of ice also may be forced into and embedded in the seafloor beneath the keel. Evidence for these dynamic processes is presented based on seafloor features observed during submersible dives on the Labrador shelf in 1985 and later. Additional evidence for crushing and recrystallization of ice from the keel of a grounded iceberg is provided from thin sections of ice collected from growlers that floated to the surface after breaking free from the margin of the keel. The constant width of upslope scouring events, some traversing more than 20 m of bathymetric change on Makkovik Bank, clearly indicate that scouring keels remain relatively stable after initial crushing and modification of the keel as it first contacts the seafloor. Such stability is very likely the result of keel armouring, the action of ice protection by adhesion and freezing of coarse seafloor material into the keel. The mechanical removal of keel ice by crushing early in a scour event reduces iceberg draft so that the potential amount of drop-down of a scouring iceberg into an Excavated Drill Centre is reduced.
