1. National Institute of Water and Atmospheric Research, Private Bag 14901, Wellington 6021, New Zealand
2. Geological Survey of Canada, Natural Resources Canada, PO Box 1006, Dartmouth, Nova Scotia B2Y 4A2, Canada
3. Present address: 4169 Highway 2, Wellington, Nova Scotia B2T 1A3, Canada
4. University of Otago, PO Box 56, Dunedin 9054, New Zealand