Salt in Oceans

The problem with the methodology of counting sink and sources is that you can never be sure that you are not missing sources or sinks (or both). The only way to do that would be to measeare sodium concentration years after years. It would take time but that's the way science works after all. You can't expect instant results for every questions.
The way I see it, anyone wanting to use sodium concentration for any conclusion should make a model of it and see if it is confirmed (many years later) by data collected on sodium concentration.
The way it is used now doesn't allow for any conclusions.