Improved chronology of Scandinavian Ice Sheet deglaciation at Boknafjorden, Norway using 10Be dating.

摘要

The behavior of paleo-ice sheets during previous periods of global warming offer insight to the response current ice sheets are having to our historic record global temperature change. Recent large scale reconstructions of the Scandinavian Ice Sheet indicate there was an asynchronous ice marginal response to increasing temperatures and carbon dioxide levels following the Last Glacial Maximum (LGM; ∼27-20 ka). For these large reconstructions to be most precise and impactful to our current changing environment, a detailed chronology of ice sheet behavior is required.;The aim of this thesis is to add necessary high spatial and temporal resolution data to the deglaciation of Boknafjorden in southwest Norway following the LGM. Previous work from the outer fjord has identified ∼20.3 10Be ka as the time when the Norwegian Channel Ice Stream (NCIS) unzipped from continental Norway, updating previous assertions of ∼18.5 ka. Observations at the fjord head from outside the well-defined and continuous Younger Dryas moraine yielded ages of ∼14 10Be ka. The goal of this thesis was accomplished by obtaining 28 new samples for 10Be dating from glacial erratic boulders between the fjord mouth and fjord head filling the spatial and temporal gaps left by previous work. Our samples were also taken along a vertical transect up the flowline of Boknafjorden and applied to a rudimentary paleo-ice sheet profiler. This allowed me to estimate a basal shear stress of the Boknafjorden paleoglacier during the last deglaciation (∼20-10 ka) that can serve as a boundary condition for numerical ice sheet models to consider.;In southern Karmoy, I identified a 10Be age of ∼20 ka from an alternate sample site corroborating recent results that the NCIS had retreated some 400 km south of the trough mouth fan ∼2,000 years earlier than previously believed. I found that the 10Be ages of ∼15 ka from northern Karmoy and Ryvarden substantiate previously published ∼15 ka deglacial ages from the opposite shore of Bomlafjorden. Furthermore, these ∼15 ka 10Be ages coupled with the 10Be ages of ∼15 ka from the Cleng Peerson moraine likely make the 10Be ages of ∼16 ka from a previous study at Vag slightly too old. The large mode of 10Be ages between 16-15 ka indicate that a culmination of a re-advance of the Scandinavian Ice Sheet in southwest Norway occurred ∼16 ka, followed by a major pulse of fjord deglaciation between ∼16 and 14 ka. I quantified this retreat by doing a basic time-distance calculation between southern Bokn (16.3 ka) and the Leiken Moraine (14 ka), which yields a retreat rate of ∼24.7 m/yr. Finally, when my results were applied to a simplistic paleo-ice profiler model, I obtained an estimated basal shear stress of ∼0.7 bar for the Boknafjorden paleoglacier.