Researchers unravel mass extinction history, how Earth became habitable
Researchers have uncovered how ancient volcanic eruptions caused global mass extinction and how meteoroid bombardment may have made Earth more habitable.
SCIENTISTS at the University of Leeds, United Kingdom (UK), have uncovered a previously unknown giant volcanic eruption that could have led to global mass extinction 260 million years ago.
The eruption in the Emeishan province of south-west China unleashed around half a million cubic kilometres of lava, covering an area five times the size of Wales, and wiping out marine life around the world.
Unusually, scientists were able to pinpoint the exact timing of the eruption and directly link it to a mass extinction event in the study published in Science. This is because the eruptions occurred in a shallow sea - meaning that the lava appears today as a distinctive layer of igneous rock sandwiched between layers of sedimentary rock containing easily datable fossilised marine life.
The layer of fossilised rock directly after the eruption shows mass extinction of different life forms, clearly linking the onset of the eruptions with a major environmental catastrophe.
Also, results of a new study suggest large bombardments of meteoroids approximately four billion years ago could have helped to make the early Earth and Mars more habitable for life by modifying their atmospheres.
When a meteoroid from space enters a planet's atmosphere, extreme heat causes some of the minerals and organic matter on its outer crust to be released as water and carbon dioxide (as a meteor burning up in the atmosphere) before it breaks up and hits the ground (and becomes a meteorite).
Researchers suggest the delivery of this water could have made Earth's and Mars' atmospheres wetter. The release of the greenhouse gas carbon dioxide could have trapped more energy from sunlight to make Earth and Mars warm enough to sustain liquid oceans.
Meanwhile, the global effect of the eruption is also due to the proximity of the volcano to a shallow sea. The collision of fast flowing lava with shallow seawater caused a violent explosion at the start of the eruptions - throwing huge quantities of sulphur dioxide into the stratosphere.
"When fast flowing, low viscosity magma meets shallow sea it's like throwing water into a chip pan - there's spectacular explosion producing gigantic clouds of steam," explains Prof. Paul Wignall, a palaeontologist at the University of Leeds, and the lead author of the paper.
The injection of sulphur dioxide into the atmosphere would have lead to massive cloud formation spreading around the world - cooling the planet and ultimately resulting in a torrent of acid rain. Scientists estimate from the fossil record that the environmental disaster happened at the start of the eruption.
"The abrupt extinction of marine life we can clearly see in the fossil record firmly links giant volcanic eruptions with global environmental catastrophe, a correlation that has often been controversial," adds Wignall.
Previous studies have linked increased carbon dioxide produced by volcanic eruptions with mass extinction. However, because of the very long term warming effect that occurs with increased atmospheric carbon dioxide (as we see with current climate change) the causal link between global environmental changes and volcanic eruptions has been hard to confirm.
This work was done in collaboration with the Chinese University of Geosciences in Wuhan and funded by a grant from the Natural Environment Research Council, UK.
In the new study, researchers from Imperial College London analysed the remaining mineral and organic content of 15 fragments of ancient meteorites that had crashed around the world to see how much water vapour and carbon dioxide they would release when subjected to very high temperatures like those that they would experience upon entering the Earth's atmosphere.
The researchers used a new technique called pyrolysis-FTIR, which uses electricity to rapidly heat the fragments at a rate of 20,000 degrees Celsius per second, and they then measured the gases released.
They found that on average, each meteorite was capable of releasing up to 12 per cent of the object's mass as water vapour and six per cent of its mass as carbon dioxide when entering an atmosphere. They concluded that contributions from individual meteorites were small and were unlikely to have a significant impact on the atmospheres of planets on their own.
The researchers then analysed data from an ancient meteor shower called the Late Heavy Bombardment (LHB), which occurred four billion years ago, where millions of rocks crashed to Earth and Mars over a period of 20 million years.
Using published models of meteoritic impact rates during the LHB, the researchers calculated that 10 billion tonnes of carbon dioxide and 10 billion tonnes of water vapour could have been delivered to the atmospheres of Earth and Mars each year.
This suggests that the LHB could have delivered enough carbon dioxide and water vapour to turn the atmospheres of the two planets into warmer and wetter environments that were more habitable for life, say the researchers.
Prof. Mark Sephton, from Imperial's Department of Earth Science and Engineering believes the study provides important clues about Earth's ancient past: "For a long time, scientists have been trying to understand why Earth is so water rich compared to other planets in our solar system. The LHB may provide a clue. This may have been a pivotal moment in our early history where Earth's gaseous envelope finally had enough of the right ingredients to nurture life on our planet."
Lead author of the study, Dr. Richard Court from Imperial's Department of Earth Science and Engineering, adds: "Because of their chemistry, ancient meteorites have been suggested as a way of furnishing the early Earth with its liquid water. Now we have data that reveals just how much water and carbon dioxide was directly injected into the atmosphere by meteorites. These gases could have got to work immediately, boosting the water cycle and warming the planet."
However, researchers say Mars' good fortune did not last. Unlike Earth, Mars doesn't have a magnetic field to act as a protective shield from the Sun's solar wind. As a consequence, Mars was stripped of most of its atmosphere. A reduction in volcanic activity also cooled the planet. This caused its liquid oceans to retreat to the poles where they became ice.