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Abstract

Some glacial sediment samples recovered from beneath the West Antarctic ice sheet at ice stream B contain Quaternary diatoms and up to 108 atoms of beryllium-10 per gram. Other samples contain no Quaternary diatoms and only background levels of beryllium-10 (less than 106 atoms per gram). The occurrence of young diatoms and high concentrations of beryllium-10 beneath grounded ice indicates that the Ross Embayment was an open marine environment after a late Pleistocene collapse of the marine ice sheet.
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REFERENCES AND NOTES

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We estimated the absolute abundance of diatoms using a modification of a recognized method [R. P. Scherer, J. Paleolimnol. 12, 171 (1994)]. Abundance is normally estimated on the basis of whole diatoms, with the use of standardized counting criteria, but whole diatoms are too rare in these sediments to allow statistically significant estimates. Instead, we counted diatom fragments in the size class from 2 to 5 μm (Table 1). The abundance of whole diatom valves is at least three orders of magnitude lower. Extracting identifiable diatoms from these diamictons for biostratigraphic application requires specialized laboratory methods and extreme care to avoid sample contamination. All glassware and reusable supplies used in sample preparation are soaked in warm 15% NaOH for 4 hours and repeatedly rinsed with distilled and filtered water to remove any diatom residues that may be present. To test the reproducibility of abundance estimates and to test for potential laboratory contamination, we routinely prepare and analyze both sample duplicates and blanks. Duplicate preparations demonstrated consistent and reproducible results in diatom abundance (<10% error) and assemblage analyses. Thorough analysis of blanks revealed no evidence of sample contamination by identifiable diatom fragments.
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AMS analysis of 10Be was performed at the Uppsala Tandem Laboratory with a machine and statistical error of <5% (1 SD); the error may become >5% because of boron correction and correction for the NIST SRM 4325 standard. 10Be was extracted from 0.5 to 1 g of a dried silt-clay fraction, using 200 μg of Be carrier, total dissolution (with hydrochloric acid and hydrofluoric acid), and cation column separation. 10Be (measured in atoms per gram) was normalized for the total sample dried at 110oC for 6 hours. The silt-clay fraction used is dominated by quartz, with lesser quantities of feldspar and clay minerals and negligible carbonate (8).
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Ice samples [M. Jackson and B. Kamb, J. Glaciol. 43, 415 (1998)] were washed under a stream of distilled deionized water to remove approximately 0.5 cm of the outer layers, then melted in new nylon beakers and filtered onto 25-mm microfilters. Additionally, two water blanks were filtered and analyzed (9). Filters were mounted on cover glass with Norland optical adhesive and analyzed in their entirety at ×1200 magnification. The age of the ice samples is unknown but is likely to be early Holocene.
13
This sample was taken during field season 1988–1989 (borehole 2, sample 1, Winnower sample). Much of the clay and other fine-grained material was lost in recovery of this sample (3).
14
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16
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Support for this study came from Uppsala University, the Swedish Natural Sciences Research Council, and the Knut and Alice Wallenberg Foundation to R.P.S., A.A., and G.P., and from a U.S. NSF grant OPP9319018 to B.K. and H.E.

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Science
Volume 281 | Issue 5373
3 July 1998

Submission history

Received: 14 April 1998
Accepted: 14 May 1998
Published in print: 3 July 1998

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Reed P. Scherer*
R. P. Scherer and A. Aldahan, Department of Earth Sciences, Uppsala University, Villavägen 16, S-752 36 Uppsala, Sweden. S. Tulaczyk, H. Engelhardt, B. Kamb, Department of Geological and Planetary Sciences, California Institute of Technology, Mail Code 170-25, Pasadena, CA 91125, USA. G. Possnert, Tandem Laboratory, Ångström Laboratory, Box , Uppsala University, Uppsala, Sweden.
Ala Aldahan
R. P. Scherer and A. Aldahan, Department of Earth Sciences, Uppsala University, Villavägen 16, S-752 36 Uppsala, Sweden. S. Tulaczyk, H. Engelhardt, B. Kamb, Department of Geological and Planetary Sciences, California Institute of Technology, Mail Code 170-25, Pasadena, CA 91125, USA. G. Possnert, Tandem Laboratory, Ångström Laboratory, Box , Uppsala University, Uppsala, Sweden.
Slawek Tulaczyk
R. P. Scherer and A. Aldahan, Department of Earth Sciences, Uppsala University, Villavägen 16, S-752 36 Uppsala, Sweden. S. Tulaczyk, H. Engelhardt, B. Kamb, Department of Geological and Planetary Sciences, California Institute of Technology, Mail Code 170-25, Pasadena, CA 91125, USA. G. Possnert, Tandem Laboratory, Ångström Laboratory, Box , Uppsala University, Uppsala, Sweden.
Göran Possnert
R. P. Scherer and A. Aldahan, Department of Earth Sciences, Uppsala University, Villavägen 16, S-752 36 Uppsala, Sweden. S. Tulaczyk, H. Engelhardt, B. Kamb, Department of Geological and Planetary Sciences, California Institute of Technology, Mail Code 170-25, Pasadena, CA 91125, USA. G. Possnert, Tandem Laboratory, Ångström Laboratory, Box , Uppsala University, Uppsala, Sweden.
Hermann Engelhardt
R. P. Scherer and A. Aldahan, Department of Earth Sciences, Uppsala University, Villavägen 16, S-752 36 Uppsala, Sweden. S. Tulaczyk, H. Engelhardt, B. Kamb, Department of Geological and Planetary Sciences, California Institute of Technology, Mail Code 170-25, Pasadena, CA 91125, USA. G. Possnert, Tandem Laboratory, Ångström Laboratory, Box , Uppsala University, Uppsala, Sweden.
Barclay Kamb
R. P. Scherer and A. Aldahan, Department of Earth Sciences, Uppsala University, Villavägen 16, S-752 36 Uppsala, Sweden. S. Tulaczyk, H. Engelhardt, B. Kamb, Department of Geological and Planetary Sciences, California Institute of Technology, Mail Code 170-25, Pasadena, CA 91125, USA. G. Possnert, Tandem Laboratory, Ångström Laboratory, Box , Uppsala University, Uppsala, Sweden.

Notes

*
To whom correspondence should be addressed. E-mail: [email protected]

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