December 2021 Jökulhlaup impact on landform and sedimentary assemblages on the decoupled Skeiðarársandur system, SE Iceland: implications for the Quaternary record.

Since the turn of the 21^st century the appearance and expansion of the most recent proglacial lakes fronting Skeiðarárjökull in SE Iceland, has led to the sandur being disconnected or decoupled from the glacier. Consequently, the sediment that would otherwise be deposited on the sandur is instead trapped within these lakes, leading to sediment deprivation of the distal sandar which in-turn impacts the fluvial and coastal systems. The recent formation of proglacial lakes also provides new challenges for jökulhlaup hazard assessment. Despite their importance, there have been no detailed studies of this large-scale proglacial sedimentary systems undergoing active decoupling, and the role of this process for sediment flux and landscape development remains unclear. In December 2021, Grimsvötn subglacial lake drained 0.9 km³ of water as a jökulhlaup from Skeiðarárjökull. A comprehensive survey of the proglacial lakes (sub-bottom profiling and single beam echosounder) and proximal sandur system (ground penetrating radar (GPR) and UAS survey), along with the collection of sediment cores, was conducted after the event. This provides a rare opportunity to capture the geomorphological and sedimentary signature of a jökulhlaup within a subaqueous setting and the downstream fluvial system. We present a model of the controls on jökulhlaup impact on landform and sedimentary assemblages within the proglacial lakes and connected glacifluvial system of Skeiðarársandur. This provides a modern analogue for Quaternary glacier and ice sheet margins.