Antarctic glacial chronology : new constraints from surface exposure dating [book]

Robert P. Ackert
2000 unpublished
Surface exposure dating and mapping of moraines from key locations provide new constraints to West Antarctic Ice Sheet (WAIS) chronology. 3 He production rates are determined from a 125 ka lava flow at 46*S. Measurements of low initial 3 He/ 4 He in dolerite (pyroxene) and sandstone (quartz) allow measurement of ages as young as 6000 years in these common Antarctic lithologies. At Mt. Waesche, a moraine band records interior WAIS elevations 85 m higher than present. 3He and 36 C1 exposure ages
more » ... ndicate that the last ice highstand, 45 meters above the present ice level, occurred 10 + 1 ka. The data suggest that the WAIS was out of equilibrium during the last glacial maximum (LGM) and constrain past ice sheet volume. Exposure ages from higher elevations are scattered (10-282 ka), but consistently fall within glacial periods in the marine isotope record. The He and 2Ne exposure ages place the first direct age constraints on six drifts of the Beardmore outlet glacier. The Beardmore 1 and 2 drifts record damming of the glacier during the LGM by an expanded WAIS. Based on geometric arguments, Meyer and Dominion drift record earlier damming of the glacier by the WAIS. Exposure ages of Meyer 1 (-600 ka), Meyer 4 (~1 Ma) and Dominion drift (-2 Ma) imply earlier WAIS expansions similar to the LGM. On Dromedary Platform, moraines of Koettlitz Glacier also record damming by an expanded WAIS prior to the LGM. He exposure ages on moraine boulders up to 350 m above the LGM ice limit are scattered, but independent stratigraphic control allows evaluation of prior exposure and cover. The oldest moraines are older than 400 ka. Relatively high erosion rates (~65 cm/Myr) are determined on a 1.68 Ma volcanic cone. Uplift of Dromedary Platform is limited to less than 450 m/Myr. These results show that surface exposure dating is a powerful tool for constraining Antarctic glacial history and can be used to date moraines from 5 to 2000 ka. The results are consistent with sea level control of WAIS extent; the WAIS has fluctuated synchronously with the Northern Hemisphere ice sheets for at least the last 1 million years. Thesis Supervisor: Mark D. Kurz Title: Senior Scientist, Woods Hole Oceanographic Institution This thesis represents the culmination of my graduate career which, including my Master's Degree and a break, has spanned 17 years. Indeed, my background in Antarctic glacial geology, which I drew heavily on for this work, extends back 25 years. Consequently, I have a great many people to thank. First of all, I must acknowledge my wife, Ruby Ackert-Herzig, who has provided steadfast support during this long process and edited the final document. We began our family just before I entered the MIT/WHOI Joint Program and we have also renovated an old Cape Cod house. She has endured the rigors of single parenthood during my numerous long field seasons, my long work hours and erratic schedule, and living with unfinished projects at home. Our children, Freeland and Adele, have been a great source of comfort and provided needed distraction. They also put up gallantly with my absences. My parents have provided encouragement and generous financial support, and never asked when I was going to get a real job. The freedom to do work that I love has been a cherished gift from my family. My advisor, Mark Kurz has walked the sometimes fine line of being a true friend, colleague, field companion, and mentor. He has had the task of teaching a pick and shovel field geologist ("just give me the age") quantitative isotope geochemistry. He has also raised the funding necessary for most of my financial support and given me tremendous freedom to continue to follow my various (non-geochemical) interests. George Denton introduced me to the Antarctic while I was an undergraduate at the University of Maine and started me on the path which lead ultimately to this thesis. He showed me how one can address big scientific questions by using detailed geologic field work in key areas. His knowledge of Antarctic glacial geology, which he passed on to me, is the foundation of this work. My association with Harold Borns also spans 25 years. He first introduced me to the field of glacial geology as an undergraduate and supplied the inspiration and samples for the Mount Waesche chapter. Many people were involved in the sometimes tedious and demanding sampling necessary for this work. Dave Kammer, Gera Pantalev, Tim Kenna, Becky Weed, Paul Rappaport ,and Ken Sims assisted in the Antarctic work. The Mt. Waesche field party, in particular, Harold Borns and Parker Calkin and Dave Barclay supplied the samples, geologic map, and valuable feedback. Nelia Dunbar generously supplied a GPS map and samples of the tephra layers near Mt. Waesche and critical input concerning the formation of the moraine bands. Jim Fastook provided the output from his WAIS model, which provided the context for the Mt. Waesche results. Brad Singer provided the initial impetus for the Patagonian work and provided logistical support (primarily a Ford Falcon station wagon) and the Cerro Volcin " 0 Ar/ 39 Ar dates. Lynn Gualterri assisted in sample collection. The Ocean Ventures Fund provided additional financial support for the Patagonian field work and 36 C1 dates. Here at WHOI,
doi:10.1575/1912/4123 fatcat:6mrszlfgkrfidkfhqgzfmoidl4