Ocean anoxia cause for past mass extinction (Image 2)

Geologists collect rock samples of Ordovician limestones along the coast of Anticosti Island, located in the Saint Lawrence Seaway of Quebec, Canada. [Image 2 of 2 related images. Back to Image 1.] More about this image For decades now, ...This is an NSF Multimedia Gallery item.
Source: NSF Multimedia Gallery - Category: Science Source Type: video