Anoxia causes massive extinction of oceanic fauna and the accumulation of organic matter. The absence of benthic and burrowing organisms allows the conservation of the original layers of the sediment. It forms finely laminated clays called schist-cardboard. This phenomenon appears to be limited to deep areas of the oceans. Marine life seems to subsist on the surface as evidenced by the presence in these clays:
leftover fish; and above all organic matter of algal origin. This may be due to algal blooms (algal bloom) on the surface, with rapid and enormous concentrations of one (or a few) species (s) of phytoplankton, including coccolithophorids.
The schist-cardboard deposit is the response to an important geological event called the Toarcian Ocean Anoxic Event (OAS). During the Lower Toarcian, about 182 million years ago, a phase of global warming of the Earth, probably due to strong volcanic activity, destabilized the methane hydrates trapped in the sediments of the deep seabed. These hydrates release large amounts of methane gas, which causes seabed anoxia over much of the globe.
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