Oceanic Anoxic Events

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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.

How might the lack of oxygen have impacted on the rest of the Earth System? You can explore one aspect of this using the nitrogen-phosphorus model.

Most phytoplankton in the oceans are reliant on 'fixed nitrogen' (e.g. nitrate, nitrite and ammonium). When this runs out (and it is scarce throughout most of the world's surface oceans) they are unable to satisfy their nitrogen needs from any other source. Two main processes are known to be responsible for depleting the ocean's supplies of fixed nitrogen, denitrification and anammox. These two processes destroy fixed nitrogen (convert it to N2), but only in low-oxygen environments. They do not take place in well-oxygenated waters or sediments. It is very likely, therefore, that fixed nitrogen was destroyed at a much greater rate during anoxic ocean events.

To simulate this, first of all launch the nitrogen-phosphorus model. On the main window, click on 'Model Parameters'. On the window that comes up, click on 'Nitrate

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This website is in its early stages of use. If you find it difficult to run a model in the way described, or find any other problems, your feedback will help us improve the site for future users.

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