Thats the only signature of supernova blasts? How did the iron get under the ocean....shouldnt there be some on land? Or in recent uplifted mountain stratigraphic layers?
There is some I’m sure, but it would be hard to find. Erosion would carry it into the sea.
Land material is “usually” one of two types: Exposed (upper surface few meters) that are very recent material (mountain building and upthrust eroded material), very-very old material (Canadian shield and Australian rocks 3.5 to 4 billion years old), or edges of faults and upthrusts exposing many layers from recent to medium (20,000 year to 65 million year old) ages. The Grand Canyon goes back hundreds of millions of years from surface to the depths, the dinosaur fossils in the Badlands of ND are exposing now rocks and fossils from 200 MYA, along with debris exposed tens of years ago. Finding that small layer of the right rock in the middle of the erosion before, during and after the assumed supernova is hard. Access to the rocks is easier. But two million years of deposits could be almost any thickness of material depending on your specific 100 foot location.
Ocean floor debris tends to be more slowly moving as sits still in the “mud” as the ocean floors separate. So mud and debris slowly accumulate on top, but the ocean abyssal plain is “nice and flat” over much of the earth. It makes studying this kind of effect a little easier.