1. Department of Marine, Earth, and Atmospheric Sciences, North Carolina State University, Raleigh, North Carolina 27695-8208
2. Cary Institute of Ecosystem Studies, Millbrook, New York 12545
3. Energy Research Centre of The Netherlands, P.O. Box 1, 1755 ZG Petten, The Netherlands, and Department of Earth Sciences, VU University of Amsterdam, 1081 HV Amsterdam, The Netherlands