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, to form the black shales.
How might the lack of oxygen have impacted on the rest of the Earth System? One impact is likely to have been on the oceanic nitrogen cycle, with potential knock-on consequences for global production of phytoplankton.
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. These processes do not operate in well-oxygenated waters or sediments. It is very likely, therefore, that, due to the much greater prevalence of anoxia, fixed nitrogen was destroyed at a much greater rate during anoxic ocean events. By dramatically increasing fixed nitrogen scarcity, this had the potential to strongly affect the total primary production of the oceans.

You can explore this change in nitrogen cycling, and potential consequences of it, using the nitrogen-phosphorus model. 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 Parameters'. Finally, in the "Nitrogen Model Parameters" window, edit the value of the DN parameter. Click in the text box and increase the denitrification rate ten-fold; in other words, alter the value from 0.015 to 0.15 and then click on 'Apply and Close'.
Return to the main window for the model and click on 'Run Model'.
You will see that the concentration of nitrate (fixed nitrogen) in the deep ocean decreases due to its increased rate of removal. When this relatively nitrate-poor water is mixed up into the surface box of the model, it make conditions more conducive to nitrogen-fixers, and you can see in the model output plots that their abundance goes up, as does nitrogen fixation flux.
What is the net impact on primary production? According to the model, because the nitrogen-fixers increase to compensate the increase in denitrification (nitrogen-fixation indirectly creates new fixed nitrogen), primary production is unaffected (see the second page of output plots).

There is some support for the model's interpretation of what happened, from measurements of cyanobacterial biomarkers in marine sediments (itrogen-fixation is carried out by cyanobacteria). Kuypers et al (2004) found extremely high levels of these biomarkers in sediments laid down during oceanic anoxic events in the Cretaceous. The levels during the anoxic events were much higher than at other times. This is consistent with the model results.

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