Thomas Thuesen

Source-to-sink in a western Norwegian fjord: How does landslides, floods and grinding glaciers influence sediment supply in the Holocene?

Thomas Thuesen, William Helland-Hansen, Christian Haug Eide, Haflidi Haflidason, Atle Nesje

Western Norwegian fjord-valley systems represent archives of changes in sedimentary processes, and typically exhibit a pronounced change in depositional environment related to the transition from glacial to interglacial conditions. During a glacial situation, the fjord-valley system is emptied of its sediments and further deepened by sub-glacial erosion, both removing sediments and increasing accommodation space. This process is thought to excavate almost all sediments within a fjord-valley system, indicating that most of the sediments in the fjord today have been deposited during and after the retreat of the last glacial ice sheet. Western Norwegian fjord basins typically represent a depositional system where turbidity currents, avalanches and slides interplay with glacimarine and hemipelagic sedimentation.

Here, we present results from a cruise with R/V G.O. Sars to Fjærlandsfjorden in 2018, where several sediment cores, TOPAS high-resolution seismic profiles (<30 cm vertical resolution), and bathymetric data was collected. Fjærlandsfjorden is a 27 kilometers long tributary fjord of Sognefjorden in Western Norway. The fjord-valley system is located just southwest of Jostedalsbreen, and covers an area of about 300 km². The sediment cores are examined using several instruments (XRF, MSCL and CT-scanning) and sedimentological methods, including dating using radiometric methods (14C AMS).

The data reveal that the Fjærlandsfjorden basin infill consists of basal till, overlain by a thick, acoustically well-laminated glacimarine unit (up to a maximum of ~105 metres in thickness), occasionally disrupted by acoustically transparent lenses interpreted to be avalanche deposits. A 2-3 metre thick hemipelagic unit drapes the glacimarine unit. Seismic and bathymetry data also give evidence of massive avalanche deposits, the largest one measuring up to a minimum volume of 5 million cubic metres. Within the sediment cores several layers interpreted to be related to flooding events can be observed. Preliminary 14C dating indicate a sedimentation rate of 6.5 mm/yr for the prodelta basin.