Explaining ocean acidification patterns during ancient warming

About 55 million years ago, Earth underwent a period of global warming known as the Paleocene-Eocene Thermal Maximum (PETM). Atmospheric carbon dioxide levels rose, temperatures climbed, and myriad seafloor organisms went extinct. Today, scientists study the PETM to gain insights into the potential effects of ongoing climate change. Still, several aspects of this ancient warming event remain mysterious.

In a new study, Ilyina and Heinze address a key PETM mystery: why ocean acidification appears to have occurred to a greater degree in the Atlantic than in the Pacific Ocean. Ocean acidification happens when carbon dioxide builds up in Earth’s atmosphere, forcing the oceans to absorb more of the greenhouse gas than usual. The resulting chemical reactions make seawater more acidic while also reducing the amount of carbonate ions available for building shells, corals, and other organic structures.

The geological record shows that during the PETM, carbonate ions decreased to a greater degree in the Atlantic than in the Pacific. Researchers have previously proposed that this difference was due to chemical reactions resulting from the release of a massive source of subseafloor methane into the Atlantic. But the new work suggests an alternative explanation.

The researchers employed a model of Earth’s atmosphere, ocean, and land surface known as the Max Planck Institute for Meteorology Earth System Model. They used the model to simulate what happened in the ocean at the onset of the PETM, taking into account factors such as the speed of sinking organic matter and its breakdown rate, as well as climate conditions at the time.

Their analysis suggests that at the start of the PETM, global warming weakened the Meridional Overturning Circulation, in which warm seawater that has been carried to colder latitudes finally cools and sinks. This weakening disrupted ocean ventilation, the delivery of atmospheric gases in sinking water to deeper organisms. In the simulation, weakened circulation reduced ventilation to a greater degree in the Atlantic than in the Pacific.

After ocean circulation weakened, organisms at intermediate depths broke down sinking organic matter as they usually do, but because of the reduced ventilation, carbon dioxide built up, and oxygen levels dropped to unusual levels. Because of the greater degree of reduced ventilation in the Atlantic than in Pacific, these chemical conditions triggered a greater degree of acidification.

The researchers report that this mechanism can sufficiently account for the asymmetrical carbon and oxygen signatures seen in the Atlantic and Pacific geological records without the need for a large Atlantic methane source. As PETM research progresses, these findings could help clarify the potential long-term effects of global warming. (Geophysical Research Letters, https://doi.org/10.1029/2018GL080761, 2019)

Sarah Stanley, EOS Earth and Space Science News, 29 April 2019. Article.

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