Scientists discover two intense periods of volcanism triggered a period of global cooling and falling oxygen levels in the oceans, causing one of the most severe mass extinctions in Earth history.
The study, published in the journal Nature Geoscience, examined the effects of volcanic ash and lava on ocean chemistry during a period of extreme environmental change roughly 450 million years ago.
This period brought about intense planetary cooling, which culminated in a glaciation and the major 'Late Ordovician Mass Extinction'. This extinction led to the loss of about 85% of species dwelling in the oceans, reshaping the course of evolution of life on Earth.
Scientists have been puzzled as to why glaciation and extinction occurred in two distinct phases at this period in Earth's history, separated by nearly 10 million years. The study examined what mechanism could have pulsed the supply of phosphorus which led to the glaciation and subsequent extinction.
The study was led by Dr Jack Longman, previously a postdoctoral researcher at Southampton, and included Dr Benjamin Mills, Associate Professor at the University of Leeds, as a co-author.
"It's been suggested that global cooling was driven by an increase in phosphorus input to the oceans" says Dr 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 organisms eventually settle to the seabed and are buried, ultimately reducing levels of carbon dioxide in the atmosphere, which then causes cooling.
The team identified that two exceptionally large pulses of volcanic activity across the globe, occurring in parts of present-day North America and South China, coincided very closely with the two peaks in glaciation and extinction.
Typically, intense bursts of volcanism are commonly linked to massive CO2 release, however this is a driver for global warming, rather than glaciation. Therefore, another process was likely to have been responsible for sudden cooling events.
This prompted the team to consider whether a secondary process—natural breakdown or 'weathering' of the volcanic material—may have provided the surge in phosphorus need to explain the glaciations.
Image caption: Volcanic deposits both on land and on the seafloor are rapidly weathered, releasing nutrients like phosphorus to the oceans (example shown here is Montserrat, West Indies). Credit: Dr Tom Gernon/University of Southampton
The team tested the hypothesis that when volcanic material is deposited in the oceans it undergoes rapid and profound chemical alteration, including release of phosphorus, effectively fertilising the oceans. They studied volcanic ash layers in much younger marine sediments to compare their phosphorus contents before and after they were modified by interactions with seawater.
Equipped with this information, the team were better placed to understand the potential geochemical impact of extensive volcanic layers from enormous eruptions during the Ordovician.
"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", says Dr Mills from the School of Earth and Environment at Leeds.
The team discovered that widespread blankets of volcanic material laid down on the seafloor during the Ordovician Period would have released sufficient phosphorus into the ocean to drive a chain of events, including climatic cooling, glaciation, widespread reduction in ocean oxygen levels, and mass extinction.
Whilst it might be tempting to think that seeding the oceans with phosphorus may help solve the current climate crisis, the scientists caution that this may have more damaging consequences.
"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", cautions Dr Mills.
The scientists conclude that whilst on short timescales massive volcanic eruptions can warm the climate via CO2 emissions, equally they can drive global cooling on multimillion-year timescales.
