What most likely happened in sediment layer 5?

A massive volcano erupted, causing global warming and lava flow, which in turn caused mass extinction.
More iridium was produced by space objects and dumped by local factories.
A massive dust cloud formed, causing global cooling and a lack of sunshine. This in turn caused a lack of plants and a mass extinction.
Fewer impacts lead to a drop in iridium levels and a mass extinction caused by a lack of sunshine.