During the Surface Heat Budget of the Arctic Ocean (SHEBA) project in October 1997 the first measurements of 7Be ever made within the Arctic Ocean were used to reconstruct the evolution of the mixed layer over the previous season and confirmed that the reservoir of heat beneath the fall mixed layer was emplaced in the summer, rather than input cumulatively over several seasons or advected in from distant sources. As suggested by McPheeet al. <1998>, several times as much heat was emplaced at SHEBA than at Arctic Ice Dynamics Joint Experiment 20 years earlier. A likely mechanism for this would be substantially greater lead opening coupled with the positive feedback loop between increased open water, increased heat absorption, and further ice melting. In addition, a rate of net primary production ≥13 gC m-2 for the preceding spring-summer period was derived using this mixed layer history with an oxygen profile from the fall. ¿ 2000 American Geophysical Union