Understanding ocean chemistry in the Western Interior Seaway during the Cenomanian–Turonian Extinction Event.
[sciencythoughts.blogspot.com]
The boundary between the Cenomanian and Turonian stages of the Cretaceous Period is marked by a mass extinction event that saw the demise of a quarter of the marine invertebrates present at the onset of the crisis, combined with carbon, oxygen, and sulphur isotope levels, the deposition of a thick (up to 3 m in places), organic-rich, black shale in many ocean basins, and the onset of a greenhouse climate, known as the Cretaceous Climatic Maximum, which peaked in the early Turonian, then gradually cooled off over the remaining 24 million years of the Cretaceous. Numerous causes have been proposed for the Cenomanian–Turonian Extinction, but the most likely is thought to be massive volcanic emplacements, possibly in the Caribbean Large Igneous Province, which injected large amounts of carbon dioxide, hydrogen sulphide, and sulphur dioxide, as well as a variety of metal compounds, into the ocean-atmosphere system. Increasing atmospheric carbon dioxide would have led to higher global temperatures and higher precipitation on land, which in turn would have led to higher erosion on land, more nutrients being washed into the oceans, and vast Algal Blooms, which would be recorded as a higher burial rate for organic carbon, causing the global carbon isotope excursion, which can be observed from both black shales and carbonate rocks spanning the Cenomanian–Turonian boundary. At the time much of the world's ocean system was dominated by shallow, epicontinental seas (i.e. seaways covering continents in the already warm Cretaceous world), which would have quickly become stagnant when these Algal Blooms were combined with a combination of an injection of oxygen consuming metals and a break-down in ocean circulation caused by the rising temperatures, resulting in large portions of the global ocean becoming anoxic and hostile to multicellular life. The widespread occurrence of black shales at the Cenomanian–Turonian boundary is thought to be a reflection of this. Curiously, however, these phenomena are not recorded in all sequences spanning the Cenomanian–Turonian boundary, with many shallow marine environments (which would be predicted to be the most severely impacted by such events) seemingly unaffected. This variability, with the event leaving a strong signal in some sequences, a light one in others, and being totally absent in some places, leads to the conclusion that the 'global event' may in fact have been a series of overlapping local occurrences, driven by multiple factors rather than a single change in global atmospheric composition.