Earth’s past includes episodes when environments in much of the ocean were very different from those of today. One of these episodes, that has enormous economic repercussions, occurred in the mid-Cretaceous between ∼80–125 Ma, and led to deposition of organic carbon (Corg)-rich sediments, informally known as “black shales,” over large regions of the ocean. Deposition of these sediments resulted from several possible factors, including warm oceanic deep waters with low dissolved-oxygen contents, sluggish circulation in tectonically restricted basins, and elevated levels of marine productivity (Ryan and Cita, 1977; Thiede and van Andel, 1977; Arthur et...

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