A string of volcanic eruptions spanning a million years spelled doom for the dinosaurs even before the devastating asteroid impact 66 million years ago – and finished off any that survived it – a new study claims.
The Chicxulub crater in modern-day Mexico is evidence of a huge space rock crashing into the Earth, which is believed to have kick-started a chain of events leading to a mass extinction.
But at the same time, a volcanic province known as the Deccan Traps in modern-day Asia was extremely active, belching out huge amounts of carbon dioxide, lava and mercury.
These mercury levels were detected in ancient fossilised shells, indicating the eruptions had started to poison the world’s oceans and the animals living within them even before the Chicxulub asteroid hit.
It is now thought that a ‘double-whammy’ of the devastating six-mile wide asteroid and huge amounts of toxic lava triggered the demise of the dinosaurs – not a single event.
Scroll down for video
A fossil specimen of the uniquely ornamented extinct oyster Agerostrea ungulata, retrieved from the Fezzan region of Libya and found across portions of Africa, Asia, Europe, and the Middle East. This specimen is 66 million to 72 million years old and documents mercury levels
Four bivalve specimens from Antarctica’s Seymour Island analysed in the University of Michigan study, showing the range of sizes of the different mollusks. Shells form a valuable resource to see what the world was like at any given time
The study by the University of Michigan found fossilised oysters and clams with high concentrations of the toxic metal.
The Deccan Traps are one of the largest volcanic provinces in the world – covering an area of nearly 200,000 square miles and eruptions are the largest natural source of mercury entering the atmosphere.
Volcanic eruptions emit traces of mercury in the plumes of gas that rise into the sky and spread throughout the atmosphere.
It is then deposited on land or in the world’s oceans and retained for tens of millions of years.
Lead author Dr Kyle Meyer used a new technique to study the fossilised shells which assessed how the environment affected the soft calcium they are made of.
He said of the new method: ‘For the first time we can provide insights into the distinct climatic and environmental impacts of Deccan Traps volcanism by analysing a single material.’
Dr Meyer, now at Portland State University in Oregon, then compared the level of mercury in the ancient samples with a modern site, known to be polluted with the toxic element.
At Virginia’s South River industrially contaminated site, where the researchers collected freshwater clam shells, signs warn residents not to eat fish from the river.
Co-author Dr Sierra Petersen, of Michigan University who advised Dr Meyer while he was a graduate student there, said: ‘The modern site has a fishing ban for humans because of high mercury levels.
‘So, imagine the environmental impact of having this level of mercury contamination globally for tens to hundreds of thousands of years.’
A well-preserved shell of the extinct oyster Exogyra costata, common to the southeastern United States and retrieved from along the Tombigbee River near Moscow Landing, Alabama. Isotope analysis of the shells revealed the devastating impact mercury from Deccan Traps eruptions had on the world’s ecosystems
The preservation of Cretaceous mollusk fossils from Seymour Island is excellent, with shells preserving original mother-of-pearl material as in these two specimens of Eselaevitrigonia regina which were used in the study
The remarkably well preserved shells were collected in Antarctica, the US, Argentina, India, Egypt, Libya and Sweden.
Fossilised shells are a good resource for tracing changes this far back in time as the soft calcium carbonate shells trap the current water conditions, including mercury levels.
Dr Petersen said: ‘Mercury anomalies had been documented in sediments but never before in shells.
‘Having the ability to reconstruct both climate and a volcanism indicator in the exact same materials helps us circumvent lots of problems related to relative dating.
‘So one of the big firsts in this study is the technical proof of concept.’
A recently developed technique was used in both studies which measures the calcium isotope composition of fossilised clam and snail shells.
Any changes to the shell due to environmental conditions are trapped.
Researchers from Northwestern University say the technique provides a ‘short, preserved snapshot of the ocean’s chemistry’.
A recent study measured calcium in prehistoric shell fossils and found carbon dioxide levels from the Deccan Traps put Earth under severe stress before the asteroid hit.
This latest, and unrelated research, adds fuel to the theory that the asteroid impact was not the sole cause of mass extinction.
The Deccan traps, with both carbon dioxide induced acidified oceans and mercury-laden water, put Earth’s ecosystems under immense strain.
The research was published in the journal Nature Communications.
Two small drill holes below a larger drilled area show where sample material was extracted from the umbo (hinge) region of this Cucullaea antarctica shell for analysis. Experts say calcium in shells acts as a snapshot of the prehistoric world’s environment
