BEDROCK SIGNATURES OF THE JAKOBSHAVNS ISBRAE ICE STREAM, WEST GREENLAND: IMPLICATIONS FOR ICE STREAM AND ICE SHEET DYNAMICS

Geomorphological evidence for palaeo-ice streams includes streamlined bedforms, lateral shear moraines, dispersal trains, drift lineations , pervasively deformed till or general landform assemblages. However, such drift features are absent in formerly glaciated areas where bedrock landforms and landscapes of areal scour dominate. In these areas other control mechanisms, particularly topography, influence ice stream initiation, location, and bedform signature. However, the identification and understanding of ‘rigid bed’ ice stream bedforms remains a poorly investigated area of glacial geomorphology.

One of the main reasons bedrock bedforms have not been used to map the flow-paths of palaeo-ice streams is because they can represent ice flow dynamics through a number of glacial cycles. In addition, local bedrock geology can exert strong controls over bedrock form, with joint plane orientation, joint frequency and joint dip all affecting the resultant landform. However, the presence of cross-cutting striations on a bedform, or the identification of double-plucked bedforms, can be used as evidence for changing flow direction. In addition, differences in bedform morphology (roche moutonnee or whaleback) can provide evidence of bed separation, which can be used to infer slow versus fast ice flow. Furthermore, bedform configuration (elongation, bed form density) can also be used to reconstruct ice flow velocity.

This study investigates the bedform ice stream signal of Jakobshavns Isbrae, the fastest flowing ice stream in West Greenland. Based on a combination of air photo interpretation and detailed field mapping, we i) characterise the bedform signal of the current ice stream margin, ii) compare and contrast the ice stream signature with bedforms formed beneath adjacent ice sheet areas and hence, iii) reconstruct the interplay between ice stream and ice sheet dynamics. Evidence of overtopping events and significant switches in ice flow direction and bed morphology in areas adjacent to the current ice stream are evident. These show that Jakobshavns Isbrae may have become partially unconfined during maximum growth phases, causing significant regional changes in ice sheet dynamics and ice discharge along the west coast of Greenland.