What causes large scale extincition

Mass Extinction

- Habit loss

- Over-fishing

- Invasive Species (Non-Native plants or animals)

- Super-volcano (Sulfur, ash, and lava all involved and may cover the sun for decades)

- Asteroid impact (May be what wiped out the Dinosaurs)

But because you've listed this in Chem. I am assuming that you need more Chemical related so;

-Radiation (If we humans create something that is likely to eliminate a very large population of life)

-Pollution can play a chemical role such as air borne vapors of many toxins; we are most likely to be the ones that cause that. (If you need some more explanation you may ask again ((you don't have to ask me)) but otherwise elements like heavy amounts of CO2 (Carbon Dioxide)))

That's all I've got but good luck