This is a discovery that could transform the way we believe life evolved on our planet .
The first appearance and persistence of the continental crust on Earth during the archaic eon (4 billion to 2.5 billion years ago) has important implications for plate tectonics, ocean chemistry and biological evolution and , according to a new study, it occurred approximately 500 million years earlier than previously thought .
When the earth emerged on our planet, we were able to understand the circumstances in which primitive life began. Today, the new oceanic crust grows on the ridges of the mid-ocean where tectonic plates flow separately. The continental crust is very old, formed by volcanoes where plates collide with each other and where nutrients and minerals are patent in the oldest rock record . The weathering of the continental crust adds nutrients to the ocean, a process that could help us understand the conditions in which early life began. But when did this process begin? When was the continental crust formed?
To answer this question, the scientists used a new approach to track the first appearance of ancient rocks using a different mineral than usual: “barite” (instead of strontium isotopes in marine carbonates).
Barite or barite, the main ore of barium (Ba), is formed when sulfate in ocean water reacts with barium from hydrothermal events and has been found thanks to an outcrop of this mineral from 3,230-3,260 million years old greenstone bed, in the Mapepe formation in the Barberton greenstone belt in South Africa. The rock has barite domes within the formation ; barite that was examined by the researchers.
“The composition of the piece of barite that we collect in the field now that it has been on Earth for 3.5 billion years is exactly the same as it was when it was precipitated,” says Desiree Roerdink, a geochemist at the University of Bergen, Norway, and team leader of the new investigation. “So in essence, it’s really a great recorder for looking at processes on early Earth.”
They tested six different barite deposits, located on three different continents . For each deposit, the team calculated the proportion of different strontium isotopes retained within the rock, from which they could infer when the degraded continental rock reached the ocean and ended up incorporated into the barite. Based on the results, they concluded that the weathering of the continents began about 3.7 billion years ago, 500 million years earlier than previously thought.
” That is a huge period of time. It essentially has implications for the way we think about how life evolved, ” Roerdink said. “We don’t really know if it is possible that life developed at the same time on earth, but then that earth has to be there.”
When Earth formed 4.5 billion years ago, it was nothing but a hellish landscape of molten rock. When the outer layer of the planet cooled enough, it began a new geological eon about 4 billion years ago, the archaic eon, which is when scientists believe life first emerged. However, it remains unclear when and how life began.
The full results of the study will be presented at the EGU General Assembly in 2021, which is held virtually from April 19 to 30.
Referencia: “The emergence of subaerial crust and onset of weathering 3.7 billion years ago” by Desiree Roerdink, Yuval Ronen, Harald Strauss and Paul Mason, 25 April 2021, EGU General Assembly 2021. DOI: 10.5194/egusphere-egu21-4701
