Volcanic mountains

Volcanic fertilization of the oceans led to a severe mass extinction

Scientists from the University of Southampton have found that two intense periods of volcanism triggered a period of global cooling and falling oxygen levels in the oceans, which caused one of the most severe mass extinctions in history. the history of the Earth.

The researchers, together with colleagues from the University of Oldenburg, the University of Leeds and the University of Plymouth, investigated the effects of volcanic ash and lava on ocean chemistry over a period of extreme environmental changes around 450 million years ago. Their findings are published in the journal nature geoscience.

This period resulted in intense planetary cooling, which culminated in glaciation and the major “Late Ordovician Mass Extinction”. This extinction led to the loss of around 85% of the species living in the oceans, reshaping the evolutionary course of life on Earth.

“It has been suggested that the global cooling was due to an increase in phosphorus input to the oceans,” says Dr. Jack Longman, lead author of the study based at the University of Oldenburg, and previously a postdoctoral researcher. in Southampton. “Phosphorus is one of the key elements of life, determining the rate at which tiny aquatic organisms like algae can use photosynthesis to convert carbon dioxide (CO2) into organic matter.” These organisms eventually settle to the seabed and are buried, which ultimately reduces carbon dioxide levels in the atmosphere, which then causes cooling.

“The unsolved riddle is why the glaciation and the extinction occurred in two distinct phases at this time, separated by about 10 million years,” says Dr. Tom Gernon, associate professor at the University of Southampton and study co-author. “It requires a mechanism to pulsate the phosphorus supply, which is difficult to explain.”

The team identified that two unusually large pulses of volcanic activity across the globe, occurring in parts of present-day North America and southern China, coincided very closely with the two peaks of glaciation and extinction. “But intense volcanic outbursts are more generally related to massive CO2 which is expected to lead to global warming, so another process must be responsible for the sudden cooling events,” says Dr Gernon.

This prompted the team to wonder if a secondary process – the natural breakdown or “weathering” of volcanic material – could have provided the increase in phosphorus needed to explain the glaciations.

“When volcanic materials are deposited in the oceans, they undergo rapid and profound chemical weathering, including the release of phosphorus, effectively fertilizing the oceans,” says co-author Professor Martin Palmer of the University of Southampton. “So that seemed like a viable hypothesis and certainly worth testing.”

“This led our team to study volcanic ash layers in much younger marine sediments to compare their phosphorus content before and after they were modified by interactions with seawater,” said Dr Hayley Manners, senior of lectures in organic chemistry at the University of Plymouth. Armed with this information, the team was in a better position to understand the potential geochemical impact of vast volcanic layers resulting from huge eruptions during the Ordovician.

“This prompted us to develop a global biogeochemical model to understand the ripple effects on the carbon cycle of the rapid addition of a leached push of phosphorus from volcanic deposits into the ocean,” says Dr Benjamin Mills, associate professor at the University of Leeds and co-author of the study.

The team found that extensive blankets of volcanic material deposited on the seabed during the Ordovician period would have released enough phosphorus into the ocean to drive a chain of events including global cooling, glaciation, widespread reduction of ocean oxygen levels and mass extinction.

While it might be tempting to think that seeding the oceans with phosphorus can help solve the current climate crisis, scientists warn it could have more damaging consequences. “Excessive nutrient runoff from sources such as agricultural fertilizers is a major cause of marine eutrophication – where algae grow rapidly and then break down, consuming oxygen and causing substantial damage to ecosystems today. “, warns Dr. Mills.

Scientists conclude that even if on short time scales, massive volcanic eruptions can warm the climate via CO2 emissions, they can also lead to global cooling on time scales of millions of years. “Our study may prompt a re-examination of other mass extinctions throughout Earth’s history,” Dr. Longman concludes.