The Bonarelli Event (a.k.a. OAE2) is a well-studied episode of "worldwide" oceanic anoxic event at the Cenomanian-Turonian boundary (ca. 94 Ma). It is characterized by the presence of organic-rich marine sediments and a $\mathrm{\delta^{13}C}$ shift. It is thought to have provoked a minor mass extinction and an increased atmospheric and oceanic temperature.

What evidence do we have that the anoxia was indeed global?
Is there really no known ocean basin with sediments spanning this time interval that doesn't show those characteristic black shales?


1 Answer 1


Recent research and models, based on sulphur and carbon radioisotope data, reported in the article Sulfur isotopes track the global extent and dynamics of euxinia during Cretaceous Oceanic Anoxic Event 2 (Owens et al. 2013) suggest that the oxygen-free and hydrogen sulfide-rich waters extended across roughly 5% of the global ocean, compared to <<1% today.

Owens et al. model results place important controls on local conditions, rather than global conditions and point to the difficulty in sustaining whole-ocean euxinia.

The models based on the sulphur and carbon isotopes suggest that

the excursions were driven by increased burial of pyrite and OC. Although generally coupled, these C- and S-isotope excursions show a pronounced offset between peak magnitudes. The observed offset between these isotope signatures may not be unique in the geologic record but is certainly a pronounced characteristic of OAE 2.

and crucially, in terms of local over global controls,

The spatial variability in the excursion magnitudes could be due to local watermass differences and/or varying riverine fluxes to an ocean with substantially lower than modern sulfate concentrations, an effect noted also for the Toarcian OAE*

Further explanations are provided in the article Marine $\mathrm{^{187}Os/^{188}Os}$ isotope stratigraphy reveals the interaction of volcanism and ocean circulation during Oceanic Anoxic Event 2 (du Vivier et al. 2014), who found that, through analysis of marine $\mathrm{^{187}Os/^{188}Os}$ isotope stratigraphy

differential input of nutrients from continental and volcanogenic sources, coupled with efficient palaeocirculation of the global ocean and epeiric seas, enhanced productivity due to higher nutrient availability, which permitted penecontemporaneous transport of continental and LIP-derived nutrients to trans-equatorial basins.

LIP = Large Igneous Provinces

and that the tipping point was rising sea levels at the time, further, the authors suggest that from local sources and igneous activity, coupled with rising sea levels and palaeo-oceanic circulation at the time, concluding with the evidence suggesting a global reach of the anoxic event:

The close similarity of $\mathrm{Os_i}$ profiles from ∼50 kyr prior to the OAE 2 and through out the syn-OAE 2 interval indicates that transgression progressed to a point where a homogeneous global seawater signal was delivered to multiple proto-transatlantic basins by active ocean circulation.

This suggests that the anoxic event started locally with weathering from the land surface, coupled with large igneous province (LIP) activity, progressing due to rising sea levels and palaeo-oceanic circulation.

Additional references

*Newton et al. 2013, Low marine sulfate concentrations and the isolation of the European epicontinental sea during the Early Jurassic


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