We acknowledge the laboratory assistance of Shelley Thibaudeau, Mary Jo Mittelholtz, Ted Little, and Bill Parkins, and the assistance of Mike Lozon in producing figures. This work was supported by an NSERC operating grant to F. McCarthy and AGC Project 820044 funds to P. Mudie. Discussions with Martin Head, Graham Williams, Rob Fensome, and Anne de Vernal were valuable, as were the insights and data of Eric Collins and other members of the Shipboard Scientific Party, L. Liu, E. de Kaenel, D. Milkert, E. Gervais. This paper greatly benefited from helpful critical reviews by Martin Head and Rex Harland.