New studies confirm that volcanoes in the mid-Cretaceous period caused ocean acidification


Almost 100 million years ago, the Earth experienced severe environmental disturbance that suffocated oxygen from the oceans and led to high levels of marine extinctions that affected the entire world.

Now, in a pair of new complementary studies, two Northwestern University-led teams of geoscientists report new findings about the chronology and character of the events that led to this co-discovered occurrence, known as Ocean Anoxic Event 2 (OAE2). More than 40 years ago by the late Northwestern University professor Seymour Schlanger.

By studying preserved microplankton fossils and bulk sediments extracted from three locations around the world, the team gathered direct evidence indicating that ocean acidification occurred during the early stages of the event, due to carbon dioxide (CO).2) Emissions from the eruption of massive volcanic complexes on the sea floor.

In one of the new studies, the researchers also proposed a new hypothesis to explain why ocean acidification caused a strange wave of cooler temperatures (dubbed a “chill-even event”), which briefly interrupted the extremely hot period of global warming.

By analyzing how carbon dioxide flows2 From volcanoes affecting ocean chemistry, biominerals and climate, researchers hope to better understand how Earth today responds to an increase in carbon dioxide.2 Due to human activities, which are likely to lead to solutions to adapt to and mitigate the expected consequences.

On Thursday (January 19) a paper with findings from the deep-sea core, including a newly drilled site near southwestern Australia, will be published in the journal Nature Geoscience. A supplementary paper detailing the findings from ancient deformed microfossils was published Dec. 13 in the Nature Journal Communications Earth & Environment.

Ocean acidification and anoxia are caused by large amounts of carbon dioxide2 said Brad Sagman of Northwestern University, a senior co-author of both studies.2 Resurgence events in Earth’s history provide the best examples we have of how the Earth system responded to very large inputs of carbon dioxide2. This work has fundamental applicability to our understanding of the climate system, and our ability to predict what will happen in the future. “

“Based on isotopic analyzes of calcium, we suggest a possible explanation for the Plenus Cold Event is that a slowdown in biogenesis rates due to ocean acidification allowed alkalinity to build up in seawater,” said Andrew Jacobson of Northwestern, a senior study author. Author of both studies. Increased alkalinity led to lower carbon dioxide2 From the air. It could be the case that such cooling is a predictable—but transient—consequence of warming. Our results for OAE2 provide a geological counterpart to improving ocean alkalinity, a leading strategy for mitigating the human climate crisis.”

Experts on Cretaceous climate and isotope geochemistry, Sageman and Jacobson are professors of Earth and planetary sciences at Northwestern’s Weinberg College of Arts and Sciences. The two studies were led by a former Ph.D. Students, Gabriella Ketch and Matthew M. Jones, who began this research while at Northwestern.

Reconstructing Cretaceous Conditions

Based on more than 40 years of study, OAE2 is one of the most significant perturbations to the global carbon cycle that has occurred on planet Earth. The researchers hypothesized that oxygen levels in the oceans dropped sharply during OAE2 which greatly increased marine extinction rates. To better understand this event and the conditions that led to it, the researchers studied ancient, organic carbon-rich, fossil-bearing layers of sedimentary rocks at widely distributed outcropping sites, as well as deep-sea samples obtained by the International Ocean Discovery Program (IODP) (funded by the National Science Foundation). and its international partners).

Sites have included Gubbio, Italy (a famous area in mainland Italy that was once a deep ocean basin), the Western Interior Seaway (the ancient sea floor stretching from the Gulf of Mexico to the Arctic Ocean in North America) and several deep-sea sites, including the Fresh from the eastern Indian Ocean, off the coast of southwestern Australia.

Deep sea cores provide an invaluable record of conditions in parts of the ancient ocean that were completely unknown before ocean drilling programs were developed. Across all three cores, the researchers focused on sections from the mid-Cretaceous period, before the boundary of the Turonian and Cenomanian periods, in order to reconstruct the conditions that led to OAE2.

“The difficult part of studying ocean acidification in the geological past is that we don’t have ancient seawater,” said Jones, now a Peter Buck postdoctoral fellow at the Smithsonian Institution. “It’s very rare to find anything that looks like ancient seawater trapped in a rock or mineral. So we have to look for indirect evidence, particularly changes in the chemistry of fossil shells and rock sediments.”

mutilated fossils

For the study published in Communications Earth & Environment, Ketch and her co-authors focused on fossilized foraminifera, single-celled ocean-dwelling organisms with an outer shell made of calcium carbonate, which were collected at the Gubbio site by an Italian collaborator, Professor Rodolfo Coccini at the University of Urbino.

Kitsch and her collaborators were drawn to Gubbio’s samples because Coccioni’s visual observations and measurements of their shells showed abnormalities, including a consistent pattern of “dwarfism,” or a decrease in overall size, coinciding with the appearance of OAE2.

“These are visual signs of stress,” said Ketch, now a Knauss Fellow at the National Oceanic and Atmospheric Administration. “We hypothesized that the stress might have been caused by ocean acidification, which then affected the way the organisms build their shells.”

To test this hypothesis, Ketch analyzed the calcium isotope composition of the fossils. After dissolving the fossilized shells and analyzing their composition with a thermal ionization mass spectrometer, the Northwestern team observed that the calcium isotope ratios changed in the deformed samples in a way consistent with stress from acidification.

“This is the first paper to marry calcium isotope evidence for acidification with observations of biological indicators of stress,” Sageman said. “It is these independent biological and geochemical observations that confirm an effect on biomineralization during the onset of OAE2.”

Cause and effect relationship

For the second study, published in Nature Geoscience, Jones and colleagues focused on the deep-sea cores of lithic sediments from southwestern Australia, which he and his colleagues had collected during the IODP expedition in 2017. For this piece of the puzzle, they were less interested in what was in the sediments. And more interested in what sediment is noticeably lacking.

The core contains heaps of limestone, rich in calcium carbonate minerals, but punctuated by a surprising absence of carbonates just before OAE2.

“For this time period, we found calcite to be absent,” Jones said. “There are no carbonate minerals. This part of the core looks visibly darker; it just jumped right at us. The carbonates were either dissolved on the sea floor or fewer organisms making calcium carbonate shells in the surface waters. It’s direct observation of an ocean acidification event.”

In his geochemical analyses, carried out in collaboration with Professor Dave Selby at Durham University, Jones noted that carbonates were not the only component showing significant change. Coinciding with the onset of OAE2, there is also a marked shift in osmium isotope ratios that indicates a massive introduction of mantle-derived osmium, a fingerprint of a major submarine volcanic event. This observation is consistent with the work of several other investigators, who have found evidence of a large igneous interrupt (LIP) eruption prior to OAE2.

These massive volcanic activity events occur throughout Earth’s history and are increasingly recognized as major drivers of global change. Many of the LIPs were submarines, injecting tons of carbon dioxide2 directly into the oceans. When CO2 Dissolved in seawater, it forms a weak acid that can prevent the formation of calcium carbonate and may dissolve pre-existing carbonate scales and sediments.

“At the onset of OAE2, the osmium isotope ratios turn to really low values,” Jones said. “The only way that could have happened is through a large volcanic eruption. That helps us establish a cause-and-effect relationship. We can see the evidence that the volcanoes were really active because the osmium values ​​crashed. Then suddenly, there was no carbonate.”

biological feedback

While ocean acidification after LIP isn’t necessarily surprising, the Northwestern team discovered something unusual. The acidic conditions during OAE2 lasted much longer than other acidification events widely recognized in the Old World. Jones hypothesizes that a lack of oxygen in ocean waters may have extended the acidification state.

Organisms that consumed sinking plankton and organic matter in the water column during OAE2 were also breathing carbon dioxide.2which contributed to ocean acidification that was initially caused by carbon dioxide2 Jones said the emissions are from LIP volcanic activity. Therefore, marine anoxia could be a ‘positive feedback’ on ocean acidification. This is important because today’s global ocean, in addition to declining pH levels, is also losing oxygen content. This suggests that hypoxia may prolong the acidification period and highlights that the two phenomena are closely linked.”

In Kitsch’s study, I found that biology played another role during the event. Global warming and ocean acidification have not only negatively affected foraminifera. The organisms also actively responded by reducing calcification rates when building their shells. As calcification slowed, the foraminifera took up less alkalinity from the seawater, which helped buffer the ocean’s increasing acidity. This also increased the ocean’s ability to absorb carbon dioxide2potentially triggering a Plenus Cold Event.

“We call this phase the ‘greenhouse period’ because the temperatures were really warm,” Ketch said. “However, there is evidence of relative cooling during the OAE2 period. No one has been able to explain why this cooling occurred. Our study shows that by reducing carbonate production in the ocean, you actually bump up the alkalinity, which gives the ocean room to absorb carbon dioxide.”2. The ocean suddenly had the ability to take in carbon dioxide2 and balancing carbon flows.”

Stability ‘has a cost’

But just because a brief cooling halted this period of global warming, so did the oceans’ natural ability to sequester carbon dioxide, the researchers warn.2 It is not the solution to the current man-made climate change. Sageman explains the scenario by comparing climate change to cancer.

“It’s like if a patient had cancer, and the cancer was gone for a month,” Sageman said. “But then he came back and killed the patient. Don’t be fooled into thinking the ocean will cool us down and everything will be fine. It was great for a little sliver of time.”

“Although the Earth rebounded and healed itself, the extinctions in the sea world helped make this happen,” Jacobson added. “The Earth has some stabilizing feedback, but it has a cost.”

Kitsch’s study, “Calcium isotope ratios in deformed foraminifera reveal biocalcification stress preceding the Ocean Anoxic Event 2”, was supported by the National Science Foundation (award numbers DGE-1842165 and EAR 0723151) and the David and Lucile Packard Foundation (award number 2007-31757) .

Jones’ study, “Mid-Cretaceous Acidification Associated with Supervolcanoes,” was supported by the National Science Foundation (Award No. EAR 1338312) and the US Science Support Program/IODP.



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