Are you sure it isn't "just" ash, and not swamp ash? They both come from the same tree, but the swamp ash was the stuff under the water. Swamp ash is lighter since it isn't as dense due to being under water. The "northern" ash is just the stuff higher up on the tree, and it is more dense. Usually heavier and brighter than the swamp ash.
You also could just have a heavy piece of swamp ash or a really light piece of alder.