Geomorphic evidence of ice stream shut down within the Iceland Ice Sheet, northeast Iceland

Abstract

Both modern and palaeo ice streams experience shut down which has critical implications for their mass balance and influence on relative sea level rise. Reconstructions of palaeo-ice streams have mainly focused on their phase of active flow but less is understood of their shutdown and style of deglaciation. Mapping of streamlined subglacial bedforms (SSBs), including drumlins and mega scale glacial lineations (MSGLs), in NE-Iceland reveals cross-cutting flow-sets of palaeo-ice streams within the Iceland Ice Sheet (IIS) during and following the Last Glacial Maximum (LGM). Here we map geometrical ridges (linear and reticular) in the Bakkaflói and Þistilfjörður areas and combine the morphological data with sedimentological analyses to increase our understanding of the dynamics of the IIS in NE-Iceland. We interpret the ridges as crevasse-squeeze ridges (CSRs), based on their interconnected network, primary orientation transverse and/or oblique to former ice flow, and internal composition of homogenous subglacial till. In both areas, the CSRs are superimposed on the SSBs, indicating that they post-date the formation of the SSBs and signify the waning stage of ice streaming associated with the readvance of the IIS during the Younger Dryas period. The preservation of the CSRs suggests ice stagnation following the readvance and ice stream shutdown. The morphological variance of the CSRs between the flow-sets may indicate different spatial-setting within the ice streams; the linear CSRs in Bakkaflói formed further upstream (dominated by extensional forces parallel to ice flow). Comparatively, the reticular CSRs in Þistilfjörður are more characteristic of the down-ice region (effected by mixed mode of transverse and longitudinal forces), proximal to the ice margin or piedmont. Future research reconstructing past glacial behaviour and ice dynamics would benefit from high-resolution bathymetric data from the adjoining shelf as well as enhanced geochronological constraints.