A study led by the researchers at Northwestern University shows a period of abrupt global warming and ocean acidification that occurred 56 million years ago. Microscopic fossilized shells are helping geologists reconstruct Earth's climate during the Paleocene-Eocene Thermal Maximum (PETM). The clues from these ancient cells are expected to help scientists predict future warming ocean acidification driven by human-caused carbon dioxide emissions.
The researchers analyzed shells from foraminifera, an ocean-dwelling unicellular organism with an external shell made of calcium carbonate. They were collected from two sites; the Southeast Atlantic Ocean and the Pacific ocean. After analyzing the calcium isotope composition of fossils, the researchers concluded that massive volcanic activity injected large amounts of carbon dioxide into the Earths' system, causing global warming and ocean acidification. They state that the organism also actively responded by reducing calcification rates when building their shells. As calcification slowed, the foraminifera consumed less alkalinity from seawater, which helped buffer increasing ocean acidity. The researchers also found that global warming and ocean acidification did not just affect foraminifera.
This is the first study to examine the calcium isotope composition of foraminifera to reconstruct conditions before and across the PETM and the third recent Northwestern study to find about ocean acidification occurring due to volcanic carbon dioxide emissions, preceding major prehistoric environmental catastrophes, such as mass extinctions, oceanic anoxic events and period of intense global warming.
Other records from the research indicate that the atmosphere-ocean system experienced a massive carbon dioxide release immediately before the PETM. When atmospheric carbon dioxide dissolves in seawater, it forms a weak acid that can inhibit calcium carbonate formation. Although still undermined, Earth scientists believe the carbon release most likely came from volcanic activity or cascading effects, such as the release of methane hydrates from the seafloor due to ocean warming.
The study was led by Jacobson, professor of Earth and planetary sciences at Northwestern's Weinberg College of Arts and Sciences. This is the third study led by Jacobson to find that ocean acidification precedes major environmental catastrophes that correlate with large igneous province eruptions. The other two studies include the findings that the volcanic activity triggered a biocalcification crisis before an ocean anoxic event that occurred 120 million years ago, and ocean acidification preceded the asteroid impact leading to the Cretaceous Paleogene mass extinction event 66 million years ago, which included the demise of dinosaurs.
Jacobson said, "In all of our studies, we consistently see an increase in calcium isotope ratios before the onset of major events or extinction horizons". He also added that many predictions for Earth's future climate rely on understanding what happened during the PETM.