1. School of Earth Sciences, Ohio State University, 125 South Oval Mall, Columbus, Ohio 43210, U.S.A.
2. Byrd Polar Research Center, Ohio State University, 1090 Carmack Road, Columbus, Ohio 43210, U.S.A.
3. Institute for Geophysics, University of Texas at Austin, 10100 Burnet Road, Austin, Texas 78758-4445, U.S.A.
4. Department of Geology, Portland State University, P.O. Box 751, Portland, Oregon 97207-0751, U.S.A.