Deglaciation of Boknafjorden, south‐western Norway

Abstract

We present 30 10Be ages from glacial erratic boulders to constrain the deglaciation of the Scandinavian Ice Sheet in the Boknafjorden region, south‐western Norway. The southern part of the island Karmøy, located at the mouth of this fjord system, became free of glacier ice before 16 ka, probably because of the sudden break up of the Norwegian Channel Ice Stream at 20–18 ka. The ice sheet margin then stabilized at the fjord mouth until a second retreat phase commenced at or slightly before 16 ka. A calving bay developed in Boknafjorden after 16 ka, and in the course of the next millennium the ice front retreated to the inner fjord branches. In contrast, a major outlet glacier that filled the Hardangerfjorden farther to the north did not start to retreat from the fjord mouth until after 15 ka, probably in response to the Bølling warming. Thus, not only did Boknafjorden experience major retreat before Bølling warmth, but there was a variable response of south‐western fjord glaciers in Norway consistent with prior observations of non‐climatic forcing of marine‐terminating outlet glaciers.

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

“…), might theoretically carry a significant (up to ~10%) inheritance due to deep production of 10 Be by muons (Briner et al , ). Our results seem to support this scenario, as a 10% reduction in age would bring the exposure dates from Utsira (20.3 ± 0.1 ka bp ) and the oldest ones from Karmøy (20.3 ± 0.2 ka bp ) (Gump et al , ) directly in line with our radiocarbon dates. However, if the pre‐Bølling Δ R values turn out to be large, an even higher inheritance is needed to explain the discrepancy.…”

“…() (see Supplementary Information for details on the isobase model). Published exposure dates are shown in blue frames as the mean age and standard deviation (in ka) calculated from a given number ( n ) of dates (Svendsen et al , ; Gump et al , ). Published core sites are shown in green frames, with the oldest radiocarbon date included (2σ k cal a bp ).…”

“…Seven macrofossil samples from Grødheimsvatnet and 11 from Kringlemyr were submitted for accelerator mass spectrometry (AMS) radiocarbon dating at the Poznan Radiocarbon Laboratory in Poland and Beta Analytics, Inc. in Miami, USA Published exposure dates are shown in blue frames as the mean age and standard deviation (in ka) calculated from a given number (n) of dates (Svendsen et al, 2015;Gump et al, 2017). Published core sites are shown in green frames, with the oldest radiocarbon date included (2σ k cal a BP).…”

“…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 , ). In the autumn of 2014, we discovered marine, foraminifera‐bearing sediments in the bog Kringlemyr, located on the island of Karmøy (Fig.…”

Atlantic Water advection vs. glacier dynamics in northern Spitsbergen since early deglaciation

“…Apart from oceanic heat advection, topographic features of the relatively wide Woodfjorden system may have promoted the glacier retreat. Gump et al (2017) reported a deglaciation of narrower fjords with higher surrounding mountains in southwestern Norway delayed more than 1000 years (see also Stokes et al, 2014).…”