Anomalous cosmogenic 3He production and elevation scaling in the high Himalaya |
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| Authors: | William H Amidon Kenneth A Farley Douglas W Burbank Beth Pratt-Sitaula |
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| Institution: | 1. California Institute of Technology, GPS Division, 1200 E. California Blvd., Pasadena, CA 91125, United States;2. University of California Santa Barbara, Department of Earth Science, Webb Hall, Santa Barbara, CA 93106, United States;3. Central Washington University, Department of Geological Sciences, 400 E. University Way, Ellensburg, WA 98926, United States;1. Institut für Geologie und Paläontologie, Westfälische Wilhelms-Universität Münster, Corrensstr. 24, D-48149 Münster, Germany;2. Helmholtz-Zentrum Potsdam – Deutsches GeoForschungsZentrum (GFZ), Telegrafenberg, D-14473 Potsdam, Germany;3. Dokuz Eylül University, Engineering Faculty, Department of Geological Engineering, T?naztepe Campus, Buca, TR-35160 Izmir, Turkey;1. Department of Earth and Planetary Science, University of California, Berkeley, 307 McCone Hall #4767, Berkeley, CA 94720-4767, USA;2. Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709, USA;1. Geology Department, Middlebury College, Middlebury, VT, 05753, USA;2. Department of Geosciences, Penn State University, University Park, PA 16802, USA;1. CRPG, Université de Lorraine, UMR 7358, CNRS, Vandoeuvre-lès-Nancy, France;2. Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA, USA;3. Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA;4. Deutsches GeoForschungsZentrum GFZ, Potsdam, Germany;5. Lamont-Doherty Earth Observatory, Geochemistry, Palisades, NY, USA;6. Department of Earth and Environmental Sciences, Columbia University, New York, NY, USA;7. Department of Geosciences, NSF Arizona AMS Laboratory, Tucson, AZ, USA;8. School of Mathematics and Statistics, University of Glasgow, Glasgow, Scotland, UK;9. Department Earth and Planetary Science, University of California, Berkeley, CA, USA;10. Scottish Universities Environmental Research Centre, Glasgow, UK;1. Antarctic Research Centre, Victoria University of Wellington, P.O. Box 600, Wellington 6140, New Zealand;2. School of Geography Environment and Earth Sciences, Victoria University of Wellington, P.O. Box 600, Wellington 6140, New Zealand;3. Lamont-Doherty Earth Observatory of Columbia University P.O. Box 1000, Palisades, NY 10964, USA;4. GNS Science, P.O. Box 30-368, Lower Hutt 5040, New Zealand;1. Geosciences Research Division, Scripps Institution of Oceanography, La Jolla, CA 92093-0244, USA;2. Department of Earth Sciences, University of Oxford, Oxford OX1 3AN, UK;3. School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester M13 9PL, UK;4. Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada;5. Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, TN 37996, USA |
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| Abstract: | The production rate of cosmogenic 3He in apatite, zircon, kyanite and garnet was obtained by cross-calibration against 10Be in co-existing quartz in glacial moraine boulders from the Nepalese Himalaya. The boulders have 10Be ages between 6 and 16 kyr and span elevations from 3200 to 4800 m. In all of these minerals 3He correlates with 10Be and is dominantly cosmogenic in origin. After modest correction for non-cosmogenic components, 3He/10Be systematics imply apparent sea-level high-latitude (SLHL) apparent production rates for 3He of 226 atoms g? 1 yr? 1 in zircon, 254 atoms g? 1 yr? 1 in apatite, 177 atoms g? 1 yr? 1 in kyanite, and 153 atoms g? 1 yr? 1 in garnet. These production rates are unexpectedly high compared with rates measured elsewhere in the world, and also compared with proposed element-specific production rates. For apatite and zircon, the data are sufficient to conclude that the 3He/10Be ratio increases with elevation. If this reflects different altitudinal scaling between production rates for the two isotopes then the SLHL production rates estimated by our approach are overestimates. We consider several hypotheses to explain these observations, including production of 3He via thermal neutron capture on 6Li, altitudinal variations in the energy spectrum of cosmic-ray neutrons, and the effects of snow cover. Because all of these effects are small, we conclude that the altitudinal variations in production rates of cosmogenic 3He and 10Be are distinct from each other at least at this location over the last ～ 10 kyr. This conclusion calls into question commonly adopted geographic scaling laws for at least some cosmogenic nuclides. If confirmed, this distinction may provide a mechanism by which to obtain paleoelevation estimates. |
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