1. Department of Physical Sciences; MacEwan University; Edmonton AB T5J 4S2 Canada
2. Canada and School of Ocean Sciences; College of Natural Sciences; Bangor University; Menai Bridge LL59 5AB UK
3. Geological Survey of Canada-Atlantic; Dartmouth NS B2Y 4A2 Canada
4. School of Geography, Geology and the Environment; Keele University; Keele ST5 5BG UK