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.
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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- Kuypers MMM et al. (2004) N2-fixing cyanobacteria supplied nutrient N for Cretaceous oceanic anoxic events. Geology, 32: 853-856.