Two powerful phases of volcanism sparked a period of global cooling and dropping oxygen levels in the oceans, resulting in one of the most devastating mass extinctions in Earth history, according to scientists from the University of Southampton.
The authors assessed the effects of volcanic ash and lava on ocean chemistry during a period of significant environmental change around 450 million years ago, in collaboration with colleagues from the University of Oldenburg, the University of Leeds, and the University of Plymouth.
During this time, the world experienced extreme cooling, culminating in a glacier and the huge ‘Late Ordovician Mass Extinction.’ This extinction resulted in the extinction of around 85 percent of ocean-dwelling organisms, changing the evolution of life on Earth.
“It’s been suggested that global cooling was driven by an increase in phosphorus input to the oceans” said study lead author Dr Jack Longman.
“Phosphorus is one of the key elements of life, determining the pace at which tiny aquatic organisms like algae can use photosynthesis to convert carbon dioxide (CO2) into organic matter”.
These animals eventually sink to the seafloor and are buried, lowering carbon dioxide levels in the atmosphere and causing cooling.
“The unresolved puzzle is why glaciation and extinction occurred in two distinct phases at this time, separated by about 10 million years,” added Dr Tom Gernon, co-author of the study.
“That requires some mechanism to pulse the supply of phosphorus, which is hard to explain”.
Two extremely significant bursts of volcanic activity across the globe, happening in regions of modern-day North America and South China, coincided very closely with the two maxima in glaciation and extinction, according to the researchers.
“But intense bursts of volcanism are more typically linked to massive CO2 release, which should drive global warming, so another process must be responsible for sudden cooling events”, explained Dr Gernon.
This led the researchers to wonder if a secondary process—natural breakdown or ‘weathering’ of the volcanic material—could have provided the phosphorus boost required to explain the glaciations.
“When volcanic material is deposited in the oceans it undergoes rapid and profound chemical alteration, including release of phosphorus, effectively fertilizing the oceans,” said co-author Professor Martin Palmer.
“So, it is seemed viable hypothesis and certainly one worth testing”.
“This led our team to study volcanic ash layers in much younger marine sediments to compare their phosphorus contents before and after they were modified by interactions with seawater” said Dr Hayley Manners.
The team was better able to evaluate the probable geochemical impact of broad volcanic strata from massive eruptions throughout the Ordovician with this knowledge.
“This prompted us to develop a global biogeochemical model to understand the knock-on effects on the carbon cycle of rapidly adding a surge of phosphorus leached from volcanic deposits into the ocean”, said Dr Benjamin Mills, another co-author on the study.
The results revealed that blankets of volcanic material put down on the seafloor during the Ordovician Period released enough phosphorus into the ocean to trigger a cascade of events that included climatic cooling, glaciation, broad reductions in ocean oxygen levels, and mass extinction.
While it may be tempting to believe that adding phosphorus to the oceans could help solve the present climate catastrophe, scientists warn that this could have far more negative implications.
“Excess nutrient runoff from sources like agricultural fertilisers is a major cause of marine eutrophication – where algae grow rapidly and then decay, consuming oxygen and causing substantial damage to ecosystems at the present day,” cautioned Dr Mills.
Massive volcanic eruptions can warm the climate via CO2 emissions on short timescales, but they can also cause global cooling over multimillion-year timescales, according to the scientists.
“Our study may prompt reinvestigations of other mass extinctions during Earth history,” concluded Dr Longman.
Image Credit: Dr Tom Gernon/University of Southampton