Submarine landform assemblages and sedimentary processes in front of Spitsbergen tidewater glaciers

Abstract

New swath-bathymetric data from the inner parts of the three Svalbard fjords Ymerbukta, Trygghamna, and Magdalenefjorden reveal the landform assemblages deposited in front of tidewater glaciers in west and northwest Spitsbergen. Overridden moraines in Ymerbukta, a tributary of Isfjorden in central west Spitsbergen, record several re-advances of the Esmarkbreen glacier at the head of the fjord after deglaciation, and glacial lineations formed in seafloor sediments are indicative of fast ice advance during one of these events. A terminal moraine and associated debris lobe mark the maximum ice extent during the Holocene, which, implied by the presence of crevasse-squeeze ridges, is likely related to a surge of Esmarkbreen. Several De Geer moraines provide evidence for subsequent slow and step-wise retreat. In the adjacent Trygghamna and in Magdalenefjorden in northwest Spitsbergen the landforms are similar but the absence of overridden moraines and glacial lineations shows that the glaciers probably only re-advanced once during the Holocene and that ice flow was relatively slow. Terminal moraines and associated debris lobes mark the maximum extent of these advances and formed during the Little Ice Age (LIA). In Magdalenefjorden the relatively small size of the debris lobe suggests that the ice margin was at its maximum position for only a short period of time, or that sediment availability was restricted during the LIA advance. Similar to Esmarkbreen the retreat phase of the glaciers in Trygghamna and Magdalenefjorden was also characterised by periods of still-stand or small re-advances, although the comparatively small number of De Geer moraines in all three fjords shows that these landforms probably formed much less frequently than previously thought. Sub-bottom profiler data, four sediment cores and six radiocarbon dates from Magdalenefjorden further provide information about the Holocene sedimentary environments in a northwest Spitsbergen fjord. The main source of sediment is glacial meltwater entering the fjord from the surrounding coastline, which has led to the accumulation of thick sequences of fine-grained mud. Stratified and laminated muds record glacier-proximal conditions, probably related to a LIA re-advance of Waggonwaybreen around 300 cal a BP, where the interplay of a range of glacimarine processes led to the formation of partially rhythmic couplets of one coarser and one finer layer, accumulated at a rate of around 3 cm a −1. Multiple sandy layers intercalated with the glacimarine mud provide evidence for the occurrence of gravitational mass-flow events like turbidity currents. In ice-distal settings, massive to weakly stratified, occasionally bioturbated mud accumulated at a lower rate of 0.04–0.49 cm a −1. Occasional clasts and diamictic layers show that the depositional environment in Magdalenefjorden is also influenced by sedimentation from icebergs and sea ice, but the ubiquitous glacimarine mud underscores the predominance of meltwater-related sedimentation in the fjord.