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The most reliable method is stratigraphic matching, where dust, atmospheric composition, or water-stable isotopes of the horizontal core are compared with well-dated, regular ice core records to construct a chronology (10, 11). Determining the age of the ablating ice is the main difficulty in using BIAs for climate reconstructions (4). At present, ATTA Ice cores from the Greenland and Antarctic ice sheets provide highly resolved, well-dated climate records of past polar temperatures, atmospheric composition, and aerosol loading up to 800 ka before present (1–3). Our study paves the way for reliable radiometric dating of ancient ice in blue ice areas and margin sites where large samples are available, greatly enhancing their scientific value as archives of old ice and meteorites. Because the ice stratigraphy is exposed laterally along the BIA surface, such ice records are often referred to as horizontal ice cores.
At present, ATTA (81)Kr analysis requires a 40-80-kg ice sample; as sample requirements continue to decrease, (81)Kr dating of ice cores is a future possibility. Of particular interest is the middle Pleistocene transition (1200–800 ka B. Such old ice can potentially be found in Antarctic BIAs such as the Allan Hills site (23), providing a strong impetus to developing reliable (absolute) dating tools for glacial ice. Based on elemental analyses of rock samples from the sandstone aquifer as well as from the confining Bulldog shale the in situ flux of thermal neutrons and the corresponding 3He/4He and 36Cl/Cl ratios are calculated. These 81Kr ages are then used to calibrate the 36Cl and the 4He dating methods.AA(College of Earth, Ocean and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331), AB(Scripps Institution of Oceanography, University of California, San Diego, CA 92093), AC(Physics Division, Argonne National Laboratory, Argonne, IL 60439), AD(Climate and Environmental Physics, University of Bern, CH-3012 Bern, Switzerland), AE(Department of Earth and Environmental Sciences, University of Rochester, Rochester, NY 14627), AF(Physics Division, Argonne National Laboratory, Argonne, IL 60439; Department of Physics and Enrico Fermi Institute, University of Chicago, Chicago, IL 60637), AG(Physics Division, Argonne National Laboratory, Argonne, IL 60439), AH(Ice Drilling Design and Operations, University of Wisconsin, Madison, WI 53706), AI(College of Earth, Ocean and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331), AJ(Scripps Institution of Oceanography, University of California, San Diego, CA 92093), AK(College of Earth, Ocean and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331)Kr-Kr radiometric dating of ancient polar ice.Krypton was extracted from the air bubbles in four ∼350-kg polar ice samples from Taylor Glacier in the Mc Murdo Dry Valleys, Antarctica, and dated using Atom Trap Trace Analysis (ATTA).