Seismic Geomorphology of Glaciated Margins

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Abstract

One of the biggest concerns regarding contemporary climate change is how the Earth̢۪s major ice sheets will respond to rising temperatures. The late Cenozoic history of high-latitude margins has been dominated by glacial-interglacial cycles as marine and terrestrially-based ice sheets waxed and waned. Most studies have tended to focus on the most recent glacial cycle, which is relatively well known. However, reconstructing the dynamics of older glaciations has become increasingly important because it appears that the global climate may be on the cusp of entering conditions not observed during the last glacial cycle. Using ~240,000 km2 of 3D seismic reflection data, ~260,000 line kilometres of 2D seismic reflection data, borehole data, and ~60,000 km2 of multibeam bathymetry information across both hemispheres, this thesis investigates what climate change insights can be gathered by looking into the longer-term, late Pliocene and Pleistocene record of palaeo-environmental changes. Seafloor records from the Barents Sea have shown how the Barents Sea Ice Sheet evolved through the last glacial cycle. Ice streams in the central Barents Sea led to the eventual uncoupling of the Fennoscandian and Barents Sea Ice Sheets between 16-14 ka. After separation, the ice stream margin retreated episodically, rather than rapidly. Offshore Northwest Greenland, similar high-resolution seafloor records from the last glacial cycle show that the Melville Bugt Ice Stream retreated from the shelf edge to the mid-shelf at the Younger Dryas (~12.9-11.7 ka). Unlike the ice cover in the central Barents Sea, once the Melville Bugt Ice Stream was destabilised, it retreated rapidly onto the reverse slope and toward the coastline. This rapid retreat has implications for our understanding of marine ice sheet instability because the Melville Bugt Ice Stream is comparable in size to many contemporary ice streams whose stability is a cause for concern. Seismic geomorphological analysis on the mid-Norwegian and Northwest Greenland continental shelves shows that ice sheets provided the dominant control on margin evolution in the latest Cenozoic. Offshore Norway, observations of landforms indicative of grounded ice sheets suggest ice frequently traversed the Norwegian coastline and delivered material directly to the shelf edge. Offshore Northwest Greenland, similar landform records have provided the first understanding of how glacial processes, antecedent and the evolving shelf morphology controlled the margin evolution since ~3.4 Ma. Observations of iceberg scours on the mid-Norwegian margin have shown that throughout many of the glacial cycles since 2.8 Ma the North Atlantic Current has persistently flowed northward along the Norwegian margin. A similarly-long record of iceberg scouring from the North Sea has been combined with ocean-climate modelling to invoke the presence of large ice sheets through many of the Early Pleistocene glacials, moving the onset of extensive marine-terminating ice sheets back by ~1.4 Myr to the start of the Pleistocene. A method for inferring ocean current speeds from spiral-shaped iceberg scours has been developed. This has shown that during the stage 12 deglacial, northward currents along the mid-Norwegian margin were 50% slower than the present. This is thought to relate to ice sheet collapse and provides evidence of the impact of freshwater flux on North Atlantic ocean currents. This is important because modern observations offshore Southeast Greenland have shown a freshening of surface waters in response to increased ablation of the Greenland Ice Sheet. Iceberg scours observed in the South Atlantic offshore the Falkland Islands show that the Falkland/Malvinas Current, a key current in the South Atlantic, was operational during the last deglacial cycle. This thesis interprets regional, basin-wide, glaciological and oceanographic records of glaciogenic deposition with embedded morphological evidence of ice streaming and iceberg scouring. By providing evidence for how marine, marginal-marine, and grounded glacial signatures vary across different margins with varied geological histories, major insights can be gathered on past climate change. Such findings are crucial for testing the ability of predictive climate models to reproduce observations of the past, particularly for time periods with temperatures that are comparable to those that are predicted for the future.

Keyword(s)

Baffin Bay; Barents Sea; Falkland Islands; Glacial Geomorphology; Greenland; Greenland Ice Sheet; Grounding-zone Wedges; Ice Sheets; Ice Streams; Mega-scale Glacial Lineations; Mid-Norway; North Atlantic Current; North Sea; Palaeo-glaciology; Palaeo-oceanography; Pleistocene; Seismic Geomorphology; Seismic Reflection Data; Seismic Stratigraphy

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