A climate catastrophe in stone
In the Apennine Mountains of Italy's Umbria-Marche region, a geological formation known as the Gorgo a Cerbara section records a warning from the deep past. These layers of sedimentary rock were once mud at the bottom of the ancient Tethys Ocean. Today, they preserve a detailed record of a global environmental crisis that unfolded around 120.5 million years ago, during the Aptian stage of the Early Cretaceous. This event, named the Selli Event or Oceanic Anoxic Event 1a (OAE 1a), was a period when the world's oceans were starved of oxygen, leading to a mass extinction of marine life.
The record is in the rocks. The section shows a shift from the typical whitish pelagic limestones of the Maiolica Formation to a distinct, 1.9-meter-thick, black shale layer known as the Selli Level. This dark, organic-rich shale formed because the ocean floor became anoxic—it lacked the oxygen needed for microorganisms to decompose organic matter. As dead plankton and other organisms rained down from the surface, their carbon-rich remains were preserved instead of decaying, creating the black layers we see today.
The volcanic trigger
The cause of OAE 1a is a series of colossal volcanic eruptions on the other side of the planet. Scientists have linked the event to the formation of the Ontong Java Plateau, a massive oceanic plateau in the Pacific Ocean. This large igneous province erupted enormous volumes of basalt lava, releasing immense quantities of carbon dioxide into the atmosphere. Isotope analysis of the Italian sediments, particularly of the element osmium, shows a chemical signature consistent with a massive influx of mantle-derived material, directly tying the environmental changes in the Tethys Ocean to the Pacific volcanism.
This injection of CO2 caused several effects. Global average temperatures rose, and the warmer ocean water could hold less dissolved oxygen. The excess CO2 also dissolved in seawater, forming carbonic acid and causing ocean acidification. This "carbonate crisis" was devastating for marine organisms with calcium carbonate shells, particularly a group of calcareous nannoplankton called nannoconids, which suffered a major crisis just before the main anoxic event. The entire catastrophic event, from its volcanic trigger to the slow recovery of the Earth's systems, lasted for over a million years.