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IGV | PhD defense by Henrik Swärd

PhD Thesis Defence in Marine Geology
"In the wake of deglaciation – sedimentary signatures of ice-sheet decay and sea-level change"

by Henrik Swärd
Department of Geological Sciences (IGV)

Time: 27 April 2018, 13h00
Place: William Olssonsalen, Geovetenskapens hus, Stockholm University, Svante Arrhenius väg 14

Main supervisor:
Matt O'Regan, Department of Geological Sciences, Stockholm University, Sweden
Assistant supervisor:
Martin Jakobsson, Department of Geological Sciences, Stockholm University, Sweden
Opponent:
Prof. Juha Pekka Lunkka, Dean, Geosciences, Oulu Mining School, Finland
Examiners:
Dr. Mark Johnson, (Head) Department of Earth Sciences, University of Gothenburg, Sweden
Prof. Johan Ingri, Department of Civil, Environmental and Natural Resources Engineering, Luleå University of Technology, Sweden
Dr. Monica Winsborrow, Center for Arctic Gas Hydrate, Environment and Climate, University of Tromsø, Norway
(Reserv) Prof. Arjen Stroeven, Department of Physical Geography, Stockholm University, Sweden

Abstract
Lacustrine and marine sedimentary archives help unravel details concerning the withdrawal of large ice sheets and resulting sea-level changes during the last deglaciation (22–11 kyr). In a series of four manuscripts, this PhD thesis investigates the sedimentological signatures from deglacial processes at three key locations in the northern hemisphere: (i) Lake Vättern (LV) in south-central Sweden, (ii) Herald Canyon (HC) in the western Chukchi Sea, and (iii) Mackenzie Trough (MT) on the westernmost edge of the Canadian Beaufort Shelf. One lacustrine (LV) and two marine (HC and MT) sediment cores were analyzed using a broad range of methods to describe the physical, chemical, mineralogical and biological characteristics, and used to construct paleoenvironmental interpretations.

Constituting the westernmost part of the Baltic Sea during parts of the last deglaciation, LV has long been envisaged as a key region for deglacial studies in southern Scandinavia. Sediments in LV highlight four major lake development stages following the withdrawal of the Fennoscandian Ice Sheet. These include the Baltic Ice Lake, the Yoldia Sea, the Ancylus Lake and the ultimate isolated lake stage. New radiocarbon dates indicate that the lake became isolated at 9530±50 cal. yr BP. A sharp transition from a varved clay unit to a partly sulfide laminated clay unit marks the final drainage of the Baltic Ice Lake, dated to 11 650±280 cal. yr BP. However, an earlier peak in pore water chlorinity identified in the sediment provides the most compelling evidence to date for an initial drainage of the Baltic Ice Lake (~12.8 cal. kyr BP) near the onset of the Younger Dryas cold event.

Located downstream from where Pacific water flows into the Arctic, HC is a key location for understanding the details of the early Holocene (~11 cal. yr BP) flooding of the Bering Strait, and investigating sedimentological proxies for Pacific water in Arctic Ocean sediment cores. The deglacial transgression of the shelf and opening of the Bering Strait is reflected in the grain size and biogenic silica content from the HC sediment core. However, a clear Pacific water signature is not seen in the clay mineralogy which exhibits increased variability after the opening of the Bering Strait. This is interpreted as a combination of Pacific and East Siberian sources for bottom waters in HC. The absence of a clear Pacific water signature in the clay mineralogy highlights potential limitations to using this proxy in other records from the western Arctic.

Far field studies from the Arctic Ocean have argued that Mackenzie River sediments in Younger Dryas age sediments can be recognized by a unique mineral and isotopic composition, but no detailed proximal study of Mackenzie River sediments exists to support this assertion. The mineral and isotopic (Sr and Nd) studies presented in this thesis from the third of the key regions, the MT, fills this gap. The results show that the mineral assemblage and ɛNd of fine fraction material remained relatively stable during the decay of the Laurentide Ice Sheet. An exception to this exists in a transitional sedimentary unit, deposited immediately after transgression at the site, and might be related to meltwater pulses associated with the drainage of the Lake Agassiz. The results suggest that the modern mineral and isotopic signature of Mackenzie River sediments may not be a suitable proxy for deglacial meltwater events in far field sedimentary records.


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