Caroline C. Clason

Integrated use of LiDAR and multibeam bathymetry reveals onset of ice streaming in the northern Bothnian Sea

Abstract Geomorphological mapping from the new LiDAR (Light Detection and Ranging)-derived digital elevation model for Sweden and a high-resolution multibeam bathymetry data-set for the Gulf of Bothnia reveals a continuous system of glacial landforms crossing the transition between the modern terrestrial and marine environments. A palaeo-ice stream in the northern Bothnian Sea is reconstructed, with an onset tributary over the present-day Ångermanland–Västerbotten coastline. Systematic contrasts in landform morphology and lineation length indicate that this ice stream comprised a relatively narrow (∼40 km) corridor of fast flow, flowing first SW then S, and likely fed by converging flow around the upper Bothnian Sea. The geometry and landform associations of this system imply that ice, at the time period represented here, did not flow across the Gulf of Bothnia: SSE-ward ice flow indicators on the northern Swedish coast do not correspond directly with landform assemblages of the large SE-oriented Finnish deglacial lobes. Instead, we suggest they may contribute to a late-stage fast-flow event to the S and SW. Multibeam bathymetry data offer entirely new access into the rich, landform-scale geomorphological record on the seafloor of the Gulf of Bothnia. The combination of offshore multibeam with the new terrestrial LiDAR data provides unprecedented insight into and renewed understanding of the glacial dynamics of the Bothnian Sea sector of the Fennoscandian Ice Sheet, hitherto interpreted over large areas of unmapped ice sheet bed.

Annual down-glacier drainage of lakes and water-filled crevasses at Helheim Glacier, southeast Greenland

Supraglacial lake drainage events are common on the Greenland Ice Sheet. Observations on the west coast typically show an up-glacier progression of drainage as the annual melt extent spreads inland. We use a suite of remote sensing and modelling techniques in order to study a series of lakes and water-filled crevasses within 20km of the terminus of Helheim Glacier, south east Greenland. Automatic classification of surface water areas shows a down-glacier progression of drainage, which occurs in the majority of years between 2007 and 2014. We demonstrate that a linear elastic fracture mechanics model can reliably predict the drainage of the uppermost supraglacial lake in the system, but cannot explain the pattern of filling and draining observed in areas of surface water downstream. We propose that the water levels in crevasses downstream of the supraglacial lake can be explained by a transient high-pressure wave passing through the subglacial system following the lake drainage. We support this hypothesis with analysis of the subglacial hydrological conditions, which can explain both the position and interannual variation in filling order of these crevasses. Similar behaviour has been observed in association with jokulhaups, surging glaciers, and Antarctic subglacial lakes, but has not previously been observed on major outlets of the Greenland Ice Sheet. Our results suggest that the behaviour of near-terminus surface water may differ considerably from that of inland supraglacial lakes, with the potential for basal water pressures to influence the presence of surface water in crevasses close to the terminus of tidewater glaciers.

Controls on the early Holocene collapse of the Bothnian Sea Ice Stream

New high resolution multibeam data in the Gulf of Bothnia reveal for the first time the subglacial environment of a Bothnian Sea Ice Stream. The geomorphological record suggests that increased meltwater production may have been important in driving rapid retreat of Bothnian Sea ice during deglaciation. Here we apply a well-established one-dimensional flowline model to simulate ice flow through the Gulf of Bothnia and investigate controls on retreat of the ice stream during the post-Younger Dryas deglaciation of the Fennoscandian Ice Sheet. The relative influence of atmospheric and marine forcings are investigated, with the modelled ice stream exhibiting much greater sensitivity to surface melting, implemented through surface mass balance and hydrofracture-induced calving, than to submarine melting or relative sea level change. Such sensitivity is supported by the presence of extensive meltwater features in the geomorphological record. The modelled ice stream does not demonstrate significant sensitivity to changes in prescribed ice stream width or overall bed slope, but local variations in basal topography and ice stream width result in non-linear retreat of the grounding line, notably demonstrating points of short-lived retreat slowdown on reverse bed slopes. Retreat of the ice stream was most likely governed by increased ice surface meltwater production, with the modelled retreat rate less sensitive to marine forcings despite the marine setting.

Ice-flow and meltwater landform assemblages in the Gulf of Bothnia

The Gulf of Bothnia is a shallow, elongate basin dividing Sweden and Finland, and is located in the centre of the terrain formerly occupied by the Fennoscandian Ice Sheet (FIS). It is considered to have hosted a variety of different glaci-dynamic environments during the evolution of the FIS, but direct glacial geological or geomorphological evidence from the Gulf itself is almost entirely lacking. Recent acquisition of high-resolution multibeam data enables direct investigation of the basin for the first time. These data reveal a wealth of glacial landforms in the Gulf of Bothnia including glacial lineations, ribbed moraine, eskers, meltwater channels, moraines, crevasse-squeeze ridges and iceberg ploughmarks with a variety of different morphologies. Distinct landform assemblages record multiple ice-flow events or phases of differing glaci-dynamic character. Two contrasting glacial landform assemblages are presented from the western and northern Bothnian Sea. Assemblage 1, located in the western Bothnian Sea (Fig. 1), comprises an 80 km long field of approximately 950 pronounced, elongate and aligned mounds (Jakobsson et al. 2016). These mounds have a typical length of about 900 m, width of c. 300 m and amplitude of c. 6 m, and are formed in glacial till, often clustered locally with multiple individuals formed in/of a single till patch. Many mounds appear to be bedrock-cored, however, with preferential till accumulation over bedrock obstacles (Fig. 1e). Within this population, the mounds show progressive downstream elongation. Overall, they possess a SSE orientation, with some slight southward shift in the downstream and western parts of the assemblage. The SSE-orientated features are cross-cut and terminated at their downstream end by a SW-orientated group of much narrower (<50 m), lower-amplitude ( c. 1–4 m) and elongate lineations (Fig. 1h). A well-developed channelized landform system weaves southward through the streamlined mounds (Fig. 1f), comprising both incised channel …