1. Department of Earth and Environmental Sciences, University of Minnesota, Minneapolis, Minnesota 55455, USA
2. Saint Anthony Falls Laboratory, University of Minnesota, Minneapolis, Minnesota 55455, USA
3. Department of Geology, Macalester College, Saint Paul, Minnesota 55105, USA
4. Centro de Investigación GAIA Antártica, Universidad de Magallanes, 6210427 Punta Arenas, Chile
5. Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, Colorado 80309, USA