The last deglaciation of the Norwegian Channel – geomorphology, stratigraphy and radiocarbon dating

Abstract

Based on high‐resolution TOPAS acoustic data, bathymetric data sets and sediment cores from the Norwegian Channel, the last retreat of the Norwegian Channel Ice Stream has been investigated. Mapping of ice‐marginal features such as grounding‐zone wedges and terminal moraines off western Norway suggest that the retreat of the grounding line in this part of the channel was interrupted by frequent stillstands, whereas the channel south of the threshold at Jæren does not have crossing ice‐marginal landforms. Three main seismic units have been identified, and, based on their seismic characteristics, in addition to study of sediment cores, these units are interpreted as till (U1), glacial marine sediment (U2) and Holocene hemipelagic sediment (U3). Based on new and published radiocarbon dates of the lower part of U2, combined with dates from the adjacent areas, it is concluded that the grounding line started to retreat from the shelf edge at about 19 ka and that the inner part of Skagerrak was ice free at 17.6 ka. This gives an average retreat rate of 450 m a−1, which is generally higher than mean retreat rates estimated for other palaeo‐ice streams (15–310 m a−1).

Meltwater sediment transport as the dominating process in mid-latitude trough mouth fan formation

“…The Norwegian Channel Ice Stream was located at or close to the shelf break allowing dense sediment flows to develop from meltwater. The ice-stream-fed channels have a minimum age of c. 17.5 ka, as by that point the Norwegian Channel was completely deglaciated 40 . Most channels were formed before 18.7 ± 0.2 kyr, marking the retreat of the Norwegian Channel Ice Stream from the shelf edge 37,39 .…”

“…2b). The grounding line of the Norwegian Channel Ice Stream started to retreat from the continental shelf edge by c. 19 ka with an average retreat rate of 450 m/a and the channel was completely deglaciated by c. 17.5 ka 40 .…”

Evidence of early deglaciation (18 000 cal a bp) and a postglacial relative sea‐level curve from southern Karmøy, south‐west Norway

“…Our basal radiocarbon dates can be compared directly with the marine deglacial chronology of the Norwegian Channel, as this is also based mainly on radiocarbon dates of benthic foraminifera (Sejrup et al , , ) that were recently calibrated using the Marine13 curve with a Δ R of zero (Morén et al , ). According to the marine chronology, the margin of the Norwegian Channel Ice Stream started to retreat from the shelf edge around 19k cal a bp , while most of the Norwegian Channel was ice‐free by 17.6k cal a bp (Morén et al , ). This would place the ice margin close to Karmøy and Utsira by 18.5 cal a bp , which fits well with our results.…”

“…However, as this sample was extracted 67 cm above the base of the core, it allows for an earlier deglaciation. Our basal radiocarbon dates can be compared directly with the marine deglacial chronology of the Norwegian Channel, as this is also based mainly on radiocarbon dates of benthic foraminifera (Sejrup et al, 1994(Sejrup et al, , 2009) that were recently calibrated using the Marine13 curve with a ΔR of zero (Morén et al, 2018). According to the marine chronology, the margin of the Norwegian Channel Ice Stream started to retreat from the shelf edge around 19k cal a BP, while most of the Norwegian Channel was ice-free by 17.6k cal a BP (Morén et al, 2018).…”

“…Recent studies using cosmogenic exposure dating indicate deglaciation ages of around 20 ka bp for the outer Boknafjorden area in south‐west Norway (Svendsen et al , ; Gump et al , ), making it an ideal location for obtaining long RSL records. However, the cosmogenic exposure age estimates are significantly older than those obtained from radiocarbon dating of adjacent marine records (~18.5k cal a bp ) (Sejrup et al , , ; Morén et al , ), and the timing of deglaciation is therefore poorly constrained in outer Boknafjorden. Most of the fjord was ice‐free well before the Younger Dryas (YD) period (Gump et al , ), but the marine limits in the inner and middle parts of the fjord are generally younger, coinciding with the culmination of a~10‐m transgression in the YD (Helle et al , ).…”