Oceanic Anoxic Events
Cretaceous Ocean Anoxic Events
Most seafloor sediments today are well-oxygenated and as a result are host to communities of organisms that make a living by decomposing the detritus that falls out of the waters above. It has not always been so. During certain periods in the Earth's history, organic-rich black shale sediments were laid down. The prevalence of shales from these times is attributed to low levels of oxygen in the oceans. Low levels of oxygen excluded most organisms, with the result that organic remains accumulated untouched by scavengers or bacteria.
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