In the biochemistry book I'm reading (Box 10-1, Lehninger Principles of Biochemistry), an example is given in whales of how spermaceti (a wax located in the whale head), when at sea level is a liquid, and allows the whale to float due to its buoyancy.
As the whale dives deeper, the wax turns into a solid (due to the colder temperatures), and hence this more dense wax is less buoyant and prevents the whale from floating back up due to the greater density of the seawater at these depths.
My limited understanding is that density effects buoyancy in so far as that the weight of the object displaces an equivalent weight of water. If the quantity of the water displaced is small enough that the surface area of the object isn't completely submerged, it can float.
Now with the whale example, considering the wax is fully enclosed by the head, how would this change in local density effect the buoyancy of the whale as a whole? The weight of the wax still remains the same (in so far as the example used in the book), the only thing that changes is its density inside the whale.
So with a constant weight, and only a variable local density enclosed by an object (the head) and limited to these conditions only (as this is all the example in the book uses), how is this possible?