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Issue

Stanford researchers have found strong evidence to support the theory that a devastating extinction event was caused by a lack of oxygen in Earth’s oceans approximately 444 million years ago. 

 

Background

Today, human-caused global warming is the primary cause of marine oxygen loss, and a variety of species are at risk. 

 

Details

• The investigation was focused on the Late Ordovician Mass Extinction, which is one of the “Big Five” mass extinction events in Earth’s history.

• The most famous of the great die-offs is the Cretaceous-Paleogene event that wiped out three-quarters of all plant and animal species, including the dinosaurs, about 65 million years ago.

• During the Late Ordovician event nearly 450 million years ago, most life was confined to the oceans and only a few plants had moved onto land. The continents were still mostly connected as a single giant landmass called Gondwana.

• The initial wave of extinction was triggered by a global cooling that impacted much of Gondwana. Approximately 444 million years ago, a second wave of extinction set in that is theoretically tied to ocean anoxia. By the end of the Late Ordovician event, around 85 percent of marine species vanished from the fossil record.

• Previous studies have derived ocean oxygen concentrations through the analysis of ancient sediments containing isotopes of metals such as uranium and molybdenum. These elements have different chemical reactions in anoxic conditions compared to well-oxygenated conditions.

• The researchers developed a new model which combined the existing metal isotope data with new data from samples of black shale, which was recovered from the Murzuq Basin in Libya. The shale was deposited in the geological record during the mass extinction event. 

• Based on the model, the researchers concluded that severe and prolonged anoxia must have occurred across large volumes of Earth’s deep seabeds.

• The research indicates that the deoxygenation across modern oceans will push many species toward extinction. And by expanding our thinking of how oceans have behaved in the past, we could gain some insights into the oceans today.