Different styles of subglacial soft bed hydrology: Examples from Breiðamerkurjökull and Fjallsjökull, Iceland

Glacier behaviour and its response to climate change is partially controlled by the flow of water at the base of the glacier. The subglacial hydrology associated with soft -bedded glaciers is particularly poorly understood, even though numerous modern and palaeo-glaciers flow over unconsolidated sediments. We have investigated glacier dynamics at two adjacent soft-bedded Icelandic glaciers via a study of surface velocity. This included short-term changes in surface velocity using a custom-built low-cost GNSS system, as well as with Sentinel-1 velocity data. In addition, we have used PlanetScope scenes to investigate the occurrence of sediment plumes in a glacial lagoon as a proxy for meltwater discharge and modelled the hydrology associated with the lake. Our results show that at Fjallsjökull and Breiðamerkurjökull West, there is a pattern of winter speed-up events associated with warmer days (termed winter events) and similar events in spring. At Breiðamerkurjökull East and Central, there are fewer winter speed-up events and a distinct spring event. We suggest there are at least two different styles of soft-bedded glacier behaviour related to subglacial meltwater storage. At Fjallsjökull and Breiðamerkurjökull West, we propose there is a multichannel distributed subglacial hydrological system, whereby during summer, excess meltwater is stored within the subglacial hydrological system and the till, which is partly released throughout the year resulting in speed-up events. In contrast, at Breiðamerkurjökull East and Central, it is proposed there is a summer channelized drainage, caused by a combination of the presence of the deep proglacial lagoon and high meltwater flux and subglacial transmissivity. As a consequence of this, subglacial meltwater storage is limited, resulting in fewer winter speed-up events and a distinct spring event. Thus, we suggest that these different subglacial hydrological systems associated with soft-bedded glaciers can be distinguished via distinct variations in their seasonal velocity.