ICE FRACTURING AND ENGLACIAL SEDIMENT DEPOSITION DURING JÖKULHLAUPS

Our current understanding of jökulhlaup hydrodynamics is based on empirical data from a small population of well-studied floods, where it is assumed that a single subglacial conduit links the jökulhlaup source directly to the glacier snout. However, such an assumption cannot hold true for all types of jökulhlaup, as field observations confirm sudden spatial and temporal changes in outlet positioning and morphology. In addition, previous studies of jökulhlaup sedimentology have focused predominantly on proglacial impact, rather than depositional processes within glaciers. This paper presents evidence for supraglacial outbursts and associated englacial debris entrainment during jökulhlaups. Field observations are presented from two Icelandic glaciers, Skeidarárjökull and Sólheimajökull. Video footage and aerial photographs, taken before, during and after the Skeidarárjökull jökulhlaup and immediately after the Sólheimajökull jökulhlaup, reveal changes in floodwater routing and the positioning and morphology of outlets. Post-flood field observations show that englacial jökulhlaup deposits were contained within interlinking networks of up-glacier dipping fractures. This type of supraglacial outlet is interpreted as the product of hydrofracturing, induced by negative effective pressure. Simultaneous englacial deposition of fines and boulder-sized sediment demonstrates that englacial fracture discharge had a high transport capacity. Fracture geometry was an important control on the architecture of englacial jökulhlaup deposits. The occurrence of pervasively frozen flood deposits within Skeidarárjökull is attributed to freeze-on by glaciohydraulic supercooling. Floodwater flowing through up-glacier dipping fractures would have supercooled as it was raised to the surface faster than its pressure melting point could increase as glaciostatic pressure decreased. Sedimentary evidence for fluidal supercooled accretion is apparent within stratified sands, deposited englacially at exceptionally high angles of rest in the absence of post-depositional disturbance. The position and morphology of supraglacial outlets provide important controls on the spatial and temporal impact of flooding. The development of supraglacial jökulhlaup outlets provides a new mechanism for rapid englacial debris entrainment.