Earlier this year, the Great Barrier Reef was devastated by the largest mass bleaching event ever seen — as record-warm ocean temperatures turned large swaths of this vibrant 1,400-mile habitat into a ghastly white boneyard. Now scientists have finally tallied up the damage. Data released Monday by Australian researchers shows that an unprecedented fraction of the coral in the more pristine northern part of the reef has died, with average mortality rates of 67 percent.