| Popis: |
The timing and extent of former ice sheet fluctuations can demonstrate leads and lags during periods of climatic change and the forcing factors responsible, but this requires robust glacial chronologies. Patagonia, in southern South America, offers a well preserved record of glacial geomorphology over a large latitudinal range that is affected by key climatic systems in the Southern Hemisphere, but establishing the timing of ice advances has proven problematic. This thesis targets five southernmost ice lobes that extended from the former Patagonian Ice Sheet during the Quaternary; from north to south: the Río Gallegos, Skyring, Otway, Magellan and Bahía Inútil – San Sebastián (BI-SSb) ice lobes. The region is chosen because there is ambiguity over the age of glacial limits, which have been hypothesised to relate to different glacial cycles over hundreds of thousands of years but yield cosmogenic nuclide exposure data dominantly < 50 ka. This contradiction is the focus of the thesis: was the sequence of glacial limits deposited over multiple glacial cycles, or during the last glacial cycle? A new geomorphological map is used to reconstruct glacial limits and to help target new dating. Cosmogenic nuclide depth-profiles through glacial outwash are used to date glacial limits whilst accounting for post-depositional processes. These reveal that limits of the BI-SSb lobe hypothesized to date from MIS 12 (ca. 450 ka) and 10 (ca. 350 ka) were actually deposited during the last glacial cycle, with the best-dated profile giving an MIS 3 age of ca. 30 ka, indicating an extensive advance prior to the global Last Glacial Maximum (gLGM). A glacial reconstruction indicates that this may not have been unique to the BI-SSb lobe, and a compilation of published dates reveals that similar advances during the last glacial cycle indicate related forcing factors operating across Patagonia and New Zealand. |
