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

Since the turn of the 21st century the appearance and expansion of the most recent proglacial lakes fronting Skeiarrjkull 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 jkulhlaup 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, Grimsvtn subglacial lake drained 0.9 km3 of water as a jkulhlaup from Skeiarrjkull. 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 jkulhlaup within a subaqueous setting and the downstream fluvial system. We present a model of the controls on jkulhlaup impact on landform and sedimentary assemblages within the proglacial lakes and connected glacifluvial system of Skeiarrsandur. This provides a modern analogue for Quaternary glacier and ice sheet margins.