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Surface-Exposure Dating of Antarctic Glacial Deposits

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Surface-Exposure Dating of Antarctic Glacial Deposits Denton, G.H., J.G. Bockheim, S.C. Wilson, and M. Stuiver. 1989. Late Kurz, M.D., D. Colodner, T.W. Trull, R.B. Moore, and K. OBrien. 1990. Wisconsin and early Holocene glacial history, inner Ross Embay- Cosmic ray exposure dating with in-situ produced cosmogenic He: ment, Antarctica. Quaternary Research, 31, 151-182. Results from young lava flows. Earth and Planetary Science Letters, 97, Hendy, CH., T.R. Healy, E.M. Rayner, J. Shaw, and A.T. Wilson. 1979. 177-189. Late Pleistocene glacial chronology of the Taylor Valley, Antarctica, Marchant, D. 1990. Surficial geology and stratigraphy in Arena Valley, and the global climate. Quaternary Research, 11, 172-184. Antarctica: Implications for Antarctic Tertiary glacial history. (Un- Kurz, M.D., J.J. Gurney, W.J. Jenkins, and D.E. Lott. 1987 Helium published doctoral dissertation, University of Maine, Orono, Maine.) isotope variability within single diamonds from the Orapa kimberlite pipe. Earth and Planetary Science Letters, 86, 57-68. there are significant uncertainties both in the determinations Surface-exposure dating of and in extrapolating the results to high latitude relevant to antarctic glacial deposits antarctic samples. The most detailed field work was performed on quartz sand- stone moraine boulders from Arena Valley (Brook and Kurz in MARK D. KURZ, EDWARD J . BROOK, and ROBERT P. ACKERT, JR press; Brook et al. in press; Brook et al., Antarctic Journal, this issue); helium data on quartz suggest that the moraines range Chemistry Department in age from approximately 113,000 years (Taylor II) to >1.10 Woods Hole Oceanographic Institution million years (Taylor IVB). Based on the helium-3 data, a subset Woods Hole, Massachusetts 02543 of the samples was selected for beryllium-10 and aluminum-26 measurements. The subset yielded reasonable agreement with Cosmic rays produce many different nuclides at the surface the helium ages (Brown et al. 1991), although there is evidence of the Earth, dominantly by spallation reactions with the major that the older samples have lost some helium-3. These data elements of the rocks (Lal 1988). If the production rate of a provide the first direct dates on the glacial deposits of Arena cosmogenic nuclide is known, measurement of the amount Valley. present in the rock can then be used to obtain an exposure age. Preliminary exposure ages from volcanic rocks in Taylor Val- This technique has particular importance for antarctic glacial ley are as old as 1.2 million years, but the ages are in all cases geology because direct dating of glacial deposits is critical to younger than the eruption ages (based on Wilch et al., 1989), understanding past fluctuations in the antarctic ice sheets. Dur- suggesting that the volcanic outcrops have been glaciated. The ing the 1989-1990 and 1990-1991 field seasons, we collected exposure ages increase with altitude, a phenomenon that sug- samples from glacial deposits and lava flows in the McMurdo gests that the older fluctuations in the Taylor glacier involved Dry Valleys to test and apply this method. the largest ice volumes. The emphasis of this research has been the use of cosmic- Several aspects of the field work were designed to help con- ray-produced helium-3, because it is stable, has the highest strain the production rates. Samples of well-preserved lava- production rate of any cosmogenic nuclide, and can be meas- flow surfaces were collected from Mount Erebus, Mount Morn- ured with a conventional mass spectrometer (e.g., Kurz 1986). ing, and the foothills of the Royal Society Range; measurements Collection efforts have focused on rocks containing quartz and of helium-3 in these samples coupled with independant potas- olivine, which are minerals with slow helium-diffusion rates, sium-argon dates will be used to calibrate the production rates. thus minimizing loss problems (Trull, Kurz, and Jenkins 1991). We are also attempting to measure production rates in experi- Preliminary studies indicate that some cosmogenic helium is mental targets during a known exposure period. In this exper- lost from some quartz samples (Brook et al. in press); however, iment, tritium-free water is placed in a high-vacuum, stainless- this mineral should be useful for younger samples (i.e., steel vessel, atmospheric gases are removed by degassing, and <100,000 years), and there is no evidence of helium loss from the vessels are placed at various locations. The helium-3 accu- olivine. Samples are collected from top surfaces, or from known mulation during the exposure period of at least 1 year is then depths within the rock, because production rates are depth attributed to cosmic ray spallation reactions with oxygen. Pre- dependent and are significant only within the top several liminary results from a single vessel (left at 2,700 meters ele- meters. vation on Mount Feather in the Quartermain Mountains for 1 Several important geological assumptions are inherent to the year) yielded a production rate of 611 atoms per gram per year. technique. The samples to be dated cannot have experienced Assuming the tritium/helium-3 production ratio is 1.17 (Lal and significant erosion or soil cover in the past or, in the case of Peters 1967), this corresponds to a sea-level production rate of glacial moraine boulders, cannot have changed orientation dur- approximately 190 atoms of helium-3 per gram per year, a rate ing the exposure period (i.e., through boulder rolling). Erosion, that agrees well with the production rate extrapolation from soil cover, or boulder rolling all will decrease the accumulation low latitudes (Brook and Kurz in press). Although there are of the nuclide to be measured, yielding an exposure age that is significant uncertainties in this estimate caused by solar-cycle lower than moraine age. Exposure to cosmic rays prior to dep- variations and corrections for helium-3 production during air osition or exposure to any other source of the nuclide (such as transport of the vessels, the result demonstrates that the ap- nucleogenic processes) can yield erroneously old exposure proach will be extremely useful in calibrating helium-3 produc- ages. In addition to these assumptions, it is necessary to know tion rates. Deployment of additional water vessels is planned the production rates, which are a function of latitude and alti- for the future. tude. Although there have been a number of attempts to mea- The preliminary results demonstrate that surface-exposure sure production rates at other locations (e.g., Kurz et al. 1990), dating can yield extremely useful geochronological data. The 1991 REVIEW 85 data coupled with field observations, however, indicate the im- Brook, E.J., M.D. Kurz, R.P Ackert, Jr. and G.H. Denton. 1991. Chro- portance of well-constrained field geology. Due to the assump- nology of Taylor glacier advances in Arena Valley using in situ pro- tions inherent to the technique, it is criticial to collect from well- duced cosmogenic He. Antarctic Journal of the U.S., 26(5). defined moraine ridges, and to evaluate the exposure history Brown, E.T., J.M. Edmond, G.M. Raisbeck, F. Yiou, M.D. Kurz, and of each sample. Additional studies will be required to evaluate E.J. Brook. 1991. Examination of surface exposure ages of Antarctic moraines using the assumptions of exposure-age dating and to refine the pro- in-situ, produced beryllium-10 and aluminium-26. Geochimica Cosmochimica Acta, 55, 2269-2283. duction rate estimates. Kurz, M.D. 1986. In-situ production of terrestrial cosmogenic helium This work was supported by National Science Foundation and some applications to geochronology. Geochimica Cosmochimica grant DPP 88-17406. We gratefully acknowledge the collabora- Acta, 50, 28551ed2862. tion of C. Denton, D. Marchant, WJ. Jenkins, E. Brown and T. Kurz, M.D., D. Colodner, T.W. Trull, R. Moore, and K. OBrien. 1990. Wilch. We are also grateful for excellent logistical support from Cosmic ray exposure dating with in-situ produced cosmogenic He: the personnel of VXE-6, ITT/ANS, ASA, and NSF Results from young Hawaiian lava flows. Earth and Planetary Science Letters, 97, 177-189. Lal, D., and B. Peters. 1967 Cosmic-ray produced radioactivity on the earth. In S. Flugge (ed.), Handbuch der Physik XLVII2. Berlin: Springer Verlag. References Lal, D. 1988. In-situ produced cosmogenic isotopes in terrestrial rocks. Annual Review of Earth and Planetary Science, 16, 355-388. Brook, E.J., and M.D. Kurz. In press. Surface exposure chronology Trull, T.W, M.D. Kurz, and W.J. Jenkins. 1991. Diffusion of cosmogenic using in situ cosmogenic He in Antarctic quartz sandstone boulders. helium in olivine and quartz: Implications for surface exposure dat- Quaternary Research ing. Earth and Planetary Science Letters, 103, 241-256. Brook, E.J., M.D. Kurz, G.H. Denton, and RE Ackert, Jr. In press. Wilch, TI., D.R. Lux, W.C. McIntosh, and G.H. Denton. 1989. Plio- Chronology of Taylor glacier advances in Arena Valley, Antarctica, Pleistocene uplift of the McMurdo Dry Valley sector of the Transant- using in situ produced He and 10Be. Quaternary Research. arctic Mountains. Antartic Journal of the US, 24(5), 30-33. Annual ablation rates with calipers that have a reading error of only 0.00254 centi- meters. of the Lewis Cliff ice tongue The annual ablation rates (365 days) across the upper Lewis Cliff ice tongue based on six of the seven stations (61 to 68, figure) range from 3.4 to 5.6 centimeters per year with an av- C. FAURE and D. BUCHANAN erage of 4.4 ± 0.3 centimeters per year (the error here and else- where is one standard deviation of the mean). Station 64 was Byrd Polar Research Center excluded because a snowdrift had covered the ice surface in Ohio State University which the bamboo pole was originally implanted.
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