Modeling the Subglacial Sediment System of the Finnish Lake District Ice Lobe During Deglaciation

Abstract. The systematic connection of glacial conditions in ice models with subglacial geomorphological observations has been limited by an inability to model subglacial sediment processes. The Finnish Lake District Ice Lobe (FLDIL) presents an opportunity to apply new model approaches in a situation of relative simplicity, with a well-preserved sedimentary record of its past subglacial hydrology and ice flow. With a recent model of the FLDIL subglacial hydrology as driver, we derive a sediment system model ensemble using the Graphical Subglacial Sediment Transport model (GraphSSeT). Model scenarios analyse the impact of varying sedimentary conditions, and resolve spatial and temporal variations in basal sediment thickness, sediment flux rate, grain size and detrital provenance. Our results show the development of a supply-limited system within 10 years characterised by strong seasonal cycles of winter gains from bed erosion, spring and summer losses from the mobilisation of basal sediment and autumn gains from deposition. Modelled at-outlet grain size also varies seasonally and would yield clastic varves, if deposited in a proglacial lake. The results define a submarginal zone of basal sediment depletion extending 40–60 km back from the terminus, in line with the modern-day sediment thickness.The mobilisation of an extensive blanket of sediment from this submarginal zone is proposed to form the Salpausselkä II ice marginal complex. Our model approach provides a template for the validation of subglacial hydrology models against sedimentary observables, opening a path to employ such constraints to study hard-to-observe modern and past subglacial hydrology, and ice conditions.