JÖKULHLAUPS AS AGENTS OF GLACIAL SEDIMENT TRANSFER

Glaciers and ice sheets erode, entrain, and deposit massive quantities of debris. Subglacial meltwater fluxes exert a fundamental control on ice dynamics and sediment transport budgets. Glacier outburst floods (jökulhlaups) are high meltwater fluxes, constituting high frequency, high magnitude events within many glacial systems. Using field evidence from Iceland, we examine the primary controls on sub-, en-, and proglacial sediment transfer by the action of jökulhlaups.

Jökulhlaups produce significant glaciological and sedimentological impacts because they can simultaneously erode, deposit, and re-work sediment. Jökulhlaups that impose flood waves at the glacier bed can induce increased glacier sliding on a localised scale. Jökulhlaups can re-work and entrain huge volumes of sediment in sub- and englacial zones, thereby extending the recognisable impact of jökulhlaups from the proglacial to intraglacial environment. Most jökulhlaups transport sediment to proglacial sandar, and often directly to oceans. Proglacial jökulhlaup deposits form distinctive sedimentary assemblages, coupled with suites of high-energy erosional landforms.

Skeiðarárjökull and Skeiðarársandur, Iceland are the world’s largest glacier and outwash plain subject to frequent and regular jökulhlaups, collectively providing an analogue for Quaternary ice sheet margins subject to high magnitude jökulhlaups. Sediment budget studies based on Skeiðarársandur suggest that high-magnitude jökulhlaups are the dominant sediment transport events. Comparatively little is however known about sub- or englacial jökulhlaup sedimentary regimes. In this presentation, we (i) present evidence for jökulhlaup sediment entrainment within sub- and englacial locations, and jökulhlaup erosional and depositional impacts within subglacial sediments; (ii) identify glaciofluvial sedimentary characteristics diagnostic of fluvial accretion from hydraulically supercooled floodwater; and (iii) produce a model of the controls and characteristics of jökulhlaup erosion, transport, and deposition. Our synthesis of catchment-scale jökulhlaup processes, sedimentary deposits, and landforms provides a physical basis for inferring jökulhlaup dynamics from the stratigraphic record of formerly glaciated regions.