I am imagining a world where an astronomical event occurs, like Earth collides with a comet, and humanity is all but wiped out. In their desperate attempt to recover, humans enter a form of stone age. Most of the existing life forms are still there, though also decimated by the event. Modern humans apply their existing knowledge to set out hunting, gathering, farming, and ranching. But something is different; the bioavailability of iron in all the Earth's arable soil is diminished and consequently becomes a highly coveted resource. The entire planet is anemic. The drive for iron-rich foods drives humans crazy. They drink blood from their dead and hunt each other for it. New species with copper based blood like the horseshoe crab rise to prominence.
What kind of event might have led to these conditions?