I would assume that V, Zn, Mo, and Cr form some compound with carbon dioxide that isn't soluble - so it collects in sediment. Or, it could be the acidity levels

I would assume that V, Zn, Mo, and Cr form some compound with carbon dioxide that isn't soluble - so it collects in sediment. Or, it could be the acidity levels? I think it's often the case that metal concentrations come from the release of oxygen from primitive micro-organisms, as the oxygen reacts with elemental metals to form layers of insoluble metal oxides on the seabed.