Subglacial Drainage Networks of the Fennoscandian Ice Sheet

Ice sheet dynamics are modulated by subglacial meltwater, which can promote basal sliding and affect ice flow velocity. However, logistical challenges of measuring subglacial processes beneath contemporary ice sheets hinder our understanding of their spatio-temporal evolution. This thesis uses the extensive landform record of subglacial meltwater landforms in Fennoscandia to forward our understanding of the evolution of subglacial drainage networks and their relation to ice sheet retreat. Using high-resolution (2-5 m) digital elevation models, integrated networks of subglacial meltwater landforms – herein called subglacial meltwater routes – are mapped at an ice sheet- scale (~1.4 million km^2). Subglacial meltwater routes comprise eskers, tunnel valleys, subglacial meltwater channels and subglacial meltwater corridors. The analyses of >34,000 features show that subglacial meltwater routes preferentially occur in thick drift and form integrated networks. Elongated network geometries are interpreted to reflect the control of ice-surface gradients on subglacial drainage. Abrupt and laterally traceable line density changes are interpreted to record periods of greater cumulative geomorphic work during pauses in retreat. Esker enlargements – significant ridge- widenings along esker ridges – are interpreted to form due to collapse of subglacial conduits which is a potentially underestimated process during deglaciation.

The orientation of subglacial meltwater routes is used to build a geometrical reconstruction of the retreat of the Fennoscandian Ice Sheet using formlines – subglacial hydrologic equipotential lines constructed perpendicular to the prevalent subglacial meltwater route direction. Cross-cutting relationships between formlines are used to disentangle the relative drainage chronology in Fennoscandia. Finally, these observations are combined with theoretical considerations to derive a conceptual model of the time-transgressive development of subglacial drainage networks during deglaciation. The model invokes progressive headward growth of subglacial meltwater routes, such that already established routes grow at the expense of other routes that become abandoned in order to sustain an efficient thread network.