The Dating of Rock Surfaces Using in Situ Produced 10Ae, 26Al and 36CI, with Examples from Antarctica and the Swiss Alps
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Diss. ETH No. 11763 The dating of rock surfaces using in situ produced 10ae, 26Al and 36CI, with examples from Antarctica and the Swiss Alps A dissertation submitted to the SWISS FEDERAL INSTITUTE OF TECHNOLOGY ZURICH for the degree of DOCTOR OF NATURAL SCIENCES presented by Susan Denise Ivy Ochs M.S. Geology M.S. Civil Engineering Oregon State University born May 8, 1958 citizen of U.S.A. accepted on the recommendation of Prof. Dr. Christian Schluchter, examiner Dr. Peter W. Kubik, co-examiner Dr. j(jrg Beer, co-examiner PD Dr. Rainer Wieler, co-examiner Zurich 1996 To Michael Cover illustration: 'Gletscherstein.' The dates 1660 and 1767 are carved into the surface of a boulder found at the foot of a moraine at Goschener Alp, Switzerland. They are thought to indicate the dates of pilgrimages undertaken to halt the advancing Damma Glacier (modified from Holzhauser 1982, original reference Schaller-Donauer 1925). Acknowledgments This project was funded by ETH research grant 0-20-624-92 to Christian Schluchter. The fieldwork was supported by Swi:;s National Science Foundation grant 21-28971.90 to Christian Schluchter and US National Science Foundation Polar Programs 9020975. The following people provided rne with much needed support and encouragement for which I can never thank them enough: Christian SchlOchter: for his boundless and contagious enthusiasm for all things Quaternary, being there when you needed him but otherwise giving me a long line, for the most careful sampling-really the most important step, cigars during sampling, and pleasant moments in a 'Beiz-thing' after sampling; Peter Kubik: for explaining many things about physics to me (luckily only what I really needed to know), those long inte·grations, and always putting me in the first cassette of the run; Jurg Beer: for letting me use the modern lab space at EAWAG, and making me feel like a member of UP; Rainer Wieler: for being a last minute coreferent, and careful reviews; Michael Ochs: for making me comfortable with the whole idea of wet chemistry, figuring out what the ICP-AES was actually doing, making it stop, and then getting consistent and reproducible numbers for Al, chiseling out many samples, carrying many kilos of rock, and taking beautiful pictures of the sampling sites; Robert Ivy: for smashing many rocks during his 'vacations' to Switzerland, making most of the maps for the Geology paper, and showing me how to make maps with a computer; lrka Hajdas (and Wojtek): for an infinite number of fruitful discussions, for cutting me slack in the 14C lab; Laura Bruno: for her persistence especially when we were writing the first proposal; 36 Hans Arno Synal: for CI measurements and enthusiastic support of investigating 36CI in rock surfaces; Martin Suter: for giving all of the members of the AMS group relatively free rein in their research interests; Peter Signer: for his relentless support of the geology-AMS-noble gas collaboration; Willi Wolfli: for inception of a varied and robust AMS research group; 14 14 Georges Bonani: for being a nice C boss, for measuring a few C when I needed them, and for keeping THE MACHINE happy; Alan Thompson: for supporting us in the ETH grant evaluation process; 14 Wally Broecker: for hiring me to do C and always encouraging the next step; 14 Sue Trumbore: for getting me the job in the C lab and many interesting discussions; Ed Brook: for answering any and all questions through email, and conscientious reviews; Hanns Kerschner: for his enthusiasm for glacial geomorphology, fixing up the introduction to the Julier Pass paper, and the tip about the boulders at the toe of Koefel's; George Denton and David Sugden: for the excitement they incite for glaciologic and geomorphologic problems, reading the first (very) rough version of the Sirius paper on the train to Jungfraujoch and patiently explaining the real meaning of my Antarctic exposure dates; George Denton: for collecting great samples from New Zealand, and of course again his enthusiasm; David Sugden and Mike Bentley: for fun days sampling in Iceland; Jeff Klein: for explaining the banana just one more time and encouraging me to write the Geology paper; Fred Phillips: for his helpfulnes~: in all matters 36CI and/or exposure dating; Marek Zreda: for showing me how to make 36CI samples; Markus Knauer: for showing me his extraction of 10Be and 26AI from meteorites. All the members of the Teilchenphysik group, every single person who did a shift to measure these samples (especially a night shift) deserves a million thank yous: Wolfgang Gruber: for taking care of many little things and right when I needed it done (ordering stuff--even worse returning stuff), making neat stands for the columns, covers for the samples etc. etc.; Peter Kagi: for babysitting THE MACHINE and keeping it happy; Nora Steiner: for pressing some of my targets and fun in the 14C lab. The whole Umweltphysik group at EAWAG for making me feel like I belong to them: 36 Silvia Boll ha Ider: for helping me through some of the CI sample preparation and many discussions in DS7; Alfred Luck: for some of the data reduction and his cheerfulness; Gerhard Wagner: for interesting discussions and for some of the data reduction; Yvo Weidmann: for the most beautiful SEM pictures and ordering lots of stuff; Caroline Stengel: for letting me into her labs without a fight; and many other friends at EAWAG who got me through lunches and other stuff: especially Meg (and David), Yael (and Tony), Therese, Bouziane and Raoul (for the Ottawa font). David Kistler: for keeping the ICP-AES running and letting me use it at any time. Thanks also go to the whole lngenieurgeologie group at ETH, especially: Conrad Schindler: for accepting me into the group; Rita Gysin: for help in innumerable matters (Fremdenpolizei etc. etc.). The whole Happy Ice Age group (now of Uni Bern) provided (and still do) a big friendly Quaternary family: especially Beni MOiier (and Catherine): for shedding light on the Swiss Quaternary paradigm; Sylvio Tshudi and Joerg Schafer: for some fresh ideas about exposure dating; Michael Helfer (and Otti Rohner) for sampling in Antarctica and insights into Antarctic geology. Eve SchlOchter deserves a big hug for not minding when Christian literally ran-off to sample whenever and wherever, and for yummy Happy Ice Age fests. And a few, more personal thanks: lrka: for always being my best friend, no matter what; My family: especially Mama and Baba, Robert and the mom for unflagging encouragement; Michael: he is clearly the shadow coauthor of this dissertation. From the sampling through the chemcial extraction on to the final writing of each paper, his input can be felt. He constantly patiently explained in great detail what was actually happening during the Be, Al, and Cl chemical extraction procedures. Without this insight, this whole thing might not have been possible. But all this is not meant to diminish the value of the affection, emotional support and belief in me which he gave endlessly. Abstract A primary concern today in ice age research is to elucidate the worldwide timing of glacial fluctuations. This information is needed to define the mechanisms by which climatic signals are initiated, and then transferred throughout the various global systems (i.e. oceans, atmosphere, and terrestrial environments). Because of the importance of integrating the terrestrial records of the former extent of both ice sheets and alpine glaciers, the direct dating of rock surfaces with in situ produced cosmogenic isotopes is 10 26 36 an essential tool. These isotopes (e.g. Be, AI, and CI) are produced in the surfaces of rocks due to interactions with cosmic rays. In order to be able to reliably determine exposure ages from rock surfaces with both long and short exposure times, ocr goal at the onset of this project was to set-up the extraction procedure for Be, Al, Cl from rock samples. 73 separate rock or mineral dissolutions, involving 52 different rock samples, were performed to 1) make sure that 0 meteoric ' Be was being removed, 2) verify the reproducibility of the procedure, and 3) eventually, address several questions on the timing of glacier fluctuations in two specific geographic areas: Antarctica and the Swiss Alps. Exposure dates we have determined from erratic boulders in the Sirius Group sediments at Mount Fleming proved conclusively that this outcrop of the Sirius Group is more than 5.8 million years old. Minimum ages for Sirius Group deposits at Table Mountain and Mount Feather are 2.9 Ma and 2.3 Ma, respectively. The respective concentrations of cosmogenic isotopes are erosion controlled and the ages are true minima. At Table Mountain, the plateau surface carved into the Beacon Sandstone 10 bedrock yielded minimum Be exposure ages of 5.5 Ma. No erosion (in situ weathering) rate higher than 70 cm/Ma could be calculated from our data. Indeed many surfaces have been affected by weathering rates of less than 5 cm/Ma (based on the accepted 10 26 10 production rates for Be and Al). The very high Be concentrations we have measured at all three sites completely rule out significant tectonic uplift of these surfaces during the last 3 Ma. Our resu Its have provided support for the idea that the East Antarctic Ice Sheet, a crucial variable in projections of greenhouse scenarios, is a stable feature of Antarctica. They also show that much of the landscape in Antarctica has not been significantly modified geomorphologically sino~ at least the early Pliocene and is indeed a relict landscape. We have investigated three key sites in Switzerland related to the timing of glaciations of the Alps.