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Rates of Ocean Acidification: linked to Oceanic Extinction Patterns?

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Deep-sea benthic organisms derive food from export of organic matter produced in the photic zone, so that pelagic and benthic productivity are coupled in the present oceans, and severe extinction of plankton and benthos in the geological past thus should have been coupled. Foraminiferal records, however, show decoupling between planktic and benthic extinctions. The asteroid impact at the end of the Cretaceous (K/Pg) caused mass extinction of calcifying plankton (foraminifera and nannoplankton) but not of the benthos, whereas the reverse occurred 10 myr later, during the carbon-cycle perturbation and global warming at the Paleocene-Eocene boundary. The K/Pg extinction has been interpreted as a darkness-caused collapse of productivity, but such a collapse is not supported by the lack of benthic extinction. Across the K/Pg boundary, the decrease in export productivity was moderate, regionally variable, and insufficient to explain the severe marine mass extinction at higher levels of the food chain. Across the P/E boundary, productivity increased at many locations close to the continents, whereas open ocean productivity may have declined, i.e., the trophic resource continuum increased. There is thus no solid evidence that planktic or benthic extinctions were linked to changes in (export) productivity. The unexpected difference between planktic and benthic extinction patterns may have been caused by the occurrence of ocean acidification at different rates. Very rapid surface ocean acidification at the K/Pg boundary may have been due to influx of impact-generated nitric acid, followed by rapid oceanic buffering, and have been a factor in the massive extinction of pelagic calcifyers, ammonites and top-level predators, while oceanic productivity in terms of biomass recovered rapidly. Acidification at the end of the Paleocene was triggered by the much slower injection of a large mass of carbon-compounds into the atmosphere and transfer into the oceans, leading to severe extinction of deep-sea calcifying benthos, much less severe turnover in the plankton. The study of the biogeography of biotic effects of events at the K/Pg and P/E boundaries thus may assist in the evaluation of the varying effects on oceanic biota with varying rates and sources of acidification.

This talk is part of the Sedgwick Club talks series.

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