| Abstrakt: |
Alpine glacier deposits in the McMurdo Dry Valleys of Antarctica have been interpreted to indicate that early Pliocene climate in that region was not warmer than it is today. Correlation of these alpine-glacier till sheets to marine deposits that contain evidence consistent with warmer-than-present climate has been used to constrain the age of both deposits, preclude the warm interpretation of the marine evidence, and constrain mountain uplift as determined from the marine deposit. We tested the interpretation that, in the early Pliocene, the alpine glaciers in Wright Valley terminated in a fjord and, thereby, constrain the age, temperature, and depth of the fjord. We did this by mapping the surficial geology in this region using the newly available microtopography based on the light detection and ranging (LIDAR) technique. Stratigraphic issues like these need to be resolved in order to quantify early Pliocene climate in Antarctica and contribute to understanding warm global-climate dynamics. We found that the Pliocene Alpine-IIIA (A-IIIA), A-IIIB and A-IV drift sheets were more likely deposited from terrestrial alpine-glacier lobes than glaciers terminating either at tidewater or with a floating appendage. The principal evidence is the occurrence of moraine fragments weir below the minimum elevation of the early Pliocene fjord surface, and moraine arrangement in arcs indicative of arcuate glacier fronts without flairing near the proposed shoreline. Our A-IIIA till is more widespread in the proglacial areas of the five alpine glaciers examined than previously proposed. We propose that the existing distribution of A-IIIB till reflects glaciers even less extensive than today rather than truncation at a hypothetical fjord surface. Additionally, the A-IV moraine remnants outline glaciers that were significantly larger than those associated with A-III moraines. If we are correct, the age of the A-III till, 3.4 ± 0.1 Ma at maximum, does not constrain the age of the Prospect fjord episode which can be closer to 5.5 ± 0.4 Ma as previously inferred. Moreover, if the alpine tills are not as old as the Prospect fjord episode, the polar paleoclimatic interpretation from those tills does not preclude the high temperature (0–3°C) and reduced salinity previously inferred for the Prospect fjord. However, if alpine glacier extent was not limited by Prospect fjord surface elevation, then paleoclimate during the A-IIIA, A-IIIB, and A-IV glacial episodes can be quantitatively reconstructed. The Prospect fjord might have been deep, not shallow, and, hence, mountain uplift might be greater than currently thought which would explain minimal alpine-glacier erosion into the valley sides. [ABSTRACT FROM AUTHOR] |
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