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.