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NEWS
Study Finds Volcanic Eruptions Directly Triggered Ocean Acidification
December 28, 2020

When thinking about the term ‘mass extinction’, the first mental picture to emerge is that of the dinosaurs and other land animals. However, approximately 120 million years ago, the earth experienced an extreme environmental disruption that choked oxygen from its oceans. During this age in the Early Cretaceous Period, known as oceanic anoxic event (OAE) 1a, an entire family of sea-dwelling nannoplankton virtually disappeared.

Now, according to information provided by Northwestern University (Northwestern), by measuring calcium and strontium isotope abundances in nannoplankton fossils, earth scientists have concluded the eruption of the Ontong Java Plateau large igneous province (LIP) directly triggered OAE1a.

Published online in the journal “Geologyhe”, funding for the study, “Stable Ca and Sr isotopes support volcanically-triggered biocalcification crisis during Oceanic Anoxic Event 1a,” was provided by the David and Lucile Packard Foundation, the National Science Foundation and the Ubben Program for Climate and Carbon Science at Northwestern. It is the first study to apply stable strontium isotope measurements to the study of ancient ocean anoxic events.

Roughly the size of Alaska, the Ontong Java LIP erupted for seven million years, making it one of the largest known LIP events ever. During this time, it spewed tons of carbon dioxide (CO2) into the atmosphere, pushing Earth into a greenhouse period that acidified seawater and suffocated the oceans.

“We go back in time to study greenhouse periods because Earth is headed toward another greenhouse period now,” stated Jiuyuan Wang, a Northwestern Ph.D. student and first author of the study. “The only way to look into the future is to understand the past.”

Nannoplankton and many other marine organisms build their shells out of calcium carbonate, which is the same mineral found in chalk, limestone and some antacid tablets. When atmospheric CO2 dissolves in seawater, it forms a weak acid that can inhibit calcium carbonate formation and may even dissolve preexisting carbonate.

To study the earth’s climate during the Early Cretaceous, the Northwestern researchers examined a 1,600-meter-long sediment core taken from the mid-Pacific Mountains. The carbonates in the core formed in a shallow-water, tropical environment approximately 127 to 100 million years ago and are presently found in the deep ocean.

“When you consider the Earth’s carbon cycle, carbonate is one of the biggest reservoirs for carbon,” noted Bradley Sageman, a professor of earth and planetary sciences at Northwestern. “When the ocean acidifies, it basically melts the carbonate. We can see this process impacting the biomineralization process of organisms that use carbonate to build their shells and skeletons right now, and it is a consequence of the observed increase in atmospheric CO2 due to human activities.”

Several previous studies have analyzed the calcium isotope composition of marine carbonate from the geologic past. The data can be interpreted in a variety of ways, however, and calcium carbonate can change throughout time, obscuring signals acquired during its formation. In this study, the Northwestern researchers also analyzed stable isotopes of strontium — a trace element found in carbonate fossils — to gain a fuller picture.

According to Andrew Jacobson, a professor of earth and planetary sciences at Northwestern, “Our study exploits observations that calcium and strontium isotopes behave similarly during calcium carbonate formation, but not during alteration that occurs upon burial. In this study, the calcium-strontium isotope ‘multi-proxy’ provides strong evidence that the signals are ‘primary’ and relate to the chemistry of seawater during OAE1a.”

“Stable strontium isotopes are less likely to undergo physical or chemical alteration over time,” Wang added. “Calcium isotopes, on the other hand, can be easily altered under certain conditions.”

The team analyzed calcium and strontium isotopes using high-precision techniques. The methods involve dissolving carbonate samples and separating the elements, followed by analysis with a thermal ionization mass spectrometer.

Researchers have long suspected that LIP eruptions cause ocean acidification. “There is a direct link between ocean acidification and atmospheric CO2 levels,” Prof. Jacobson said. “Our study provides key evidence linking eruption of the Ontong Java Plateau LIP to ocean acidification. This is something people expected should be the case based on clues from the fossil record, but geochemical data were lacking.”


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