Ocean acidification and the end-Cretaceous mass extinction

Heterohelix globulosa foraminifera isolated from the K-Pg boundary clay at Geulhemmerberg in the Netherlands. Image credit: Michael J. Henehan/PNAS

The Cretaceous–Paleogene extinction that followed the Chicxulub impact was one of the five great Phanerozoic mass extinctions. Three-quarters of the plant and animal species on Earth disappeared, including non-avian dinosaurs, pterosaurs, marine reptiles, ammonites, and planktonic foraminifera. The impact released an estimated energy equivalent of 100 teratonnes of TNT, induced earthquakes, shelf collapse around the Yucatan platform, and widespread tsunamis that swept the coastal zones of the surrounding oceans. The event also produced high concentrations of dust, soot, and sulfate aerosols in the atmosphere. Global forest fires might have raged for months. Photosynthesis stopped and the food chain collapsed. The impacto also caused sudden ocean acidification, impacting marine ecosystems and the carbon cycle. Around the time of the impact, 23,000 to 230,000 cubic miles of magma erupted out of the mid-ocean ridges, all over the globe. One of the largest eruptive events in Earth’s history. This pulse of global marine volcanism played an important role in the environmental crisis at the end of the Cretaceous. Marine volcanism also provides a potential source of oceanic acidification, but a new study by Yale University indicates that the sudden ocean acidification was caused by the Chicxulub bolide impact (and not by the volcanic activity) that vaporised rocks containing sulphates and carbonates, causing sulphuric acid and carbonic acid to rain down. The evidence came from the shells of planktic and benthic foraminifera.

Foraminifera are crucial elements for our understanding of past and present oceans. Their skeletons take up chemical signals from the sea water, in particular isotopes of oxygen and carbon. Over millions of years, these skeletons accumulate in the deep ocean to become a major component of biogenic deep-sea sediments. Ocean acidification in the geological record is often inferred from a decrease in the accumulation and preservation of CaCO3 in marine sediments, potentially indicated by an increased degree of fragmentation of foraminiferal shells. In the early 1990’s it was recognised that the boron isotopic composition of marine carbonates was determined largely by ocean pH. Usingy the boron isotope-pH proxy to planktic and benthic foraminifera, the new study determinated the ocean pH drop following the Chicxulub impact.

The Cretaceous/Palaeogene extinction boundary clay at Geulhemmerberg Cave. Image credit: Michael J. Henehan

The boron isotope composition of carbonate samples obtained from a shallow-marine sample site (Geulhemmerberg Cave, The Netherlands) preserved sediments from the first 100 to 1000 years after the asteroid’s impact. The data from the Geulhemmerberg Cave indicate a marked ∼0.25 pH unit surface ocean acidification event within a thousand years. This change in pH corresponds to a rise in atmospheric partial pressure of CO2 (pCO2) from ∼900 ppm in the latest Maastrichtian to ∼1,600 ppm in the immediate aftermath of bolide impact.

Ocean acidification was the trigger for mass extinction in the marine realm. Acidification affects the biogeochemical dynamics of calcium carbonate, organic carbon, nitrogen, and phosphorus in the ocean and interferes with a range of processes including growth, calcification, development, reproduction and behaviour in a wide range of marine organisms like planktonic coccolithophores, foraminifera, echinoderms, corals, and coralline algae. Additionaly, ocean acidification can intensify the effects of global warming, in a dangerous feedback loop.

Anthropogenic climate change and ocean acidification resulting from the emission of vast quantities of CO2 and other greenhouse gases pose a considerable threat to ecosystems and modern society. Since the Industrial Revolution the pH within the ocean surface has decreased ~0.1 pH and is predicted to decrease an additional 0.2 – 0.3 units by the end of the century. This underlines the urgency for immediate action on global carbon emission reductions.

 

 

References:

Michael J. Henehan el al., “Rapid ocean acidification and protracted Earth system recovery followed the end-Cretaceous Chicxulub impact,” PNAS (2019). www.pnas.org/cgi/doi/10.1073/pnas.1905989116

Kump, L.R., T.J. Bralower, and A. Ridgwell. 2009. Ocean acidification in deep time. Oceanography 22(4):94–107, https://doi.org/10.5670/oceanog.2009.100.

 

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