Earth's earliest life forms could be responsible for the creation of much of the dry land we live on today, according to a new geophysical model of the evolution of Earth's crust and mantle.
On its face, the claim seems a bit outrageous, but this is how it works: Early life on land might have led to a lot more erosion, which dumped a lot more wet sediments into the deep trenches at subduction zones – those places where one tectonic plate is being shoved under another. When all that mud got mixed into the Earth's mantle, the trapped water dramatically livened up the geochemistry down there and caused a lot more volcanoes and granites to rise up through the crust, building more thick continental landmasses.
The bottom line is that when a team of German researchers modeled early Earth's mantle and crust with and without the potential added erosion caused by roots, bacteria, lichens and other biological agents of erosion, the planet evolved with less continental area – giving us a more watery world like that thought to have existed before the advent of life. Coming to this conclusion was not easy, however, because there aren't a lot of useful numbers they could plug into the model regarding how much living things are adding to the sediments pouring into subduction zones today.
“Altogether, it is very difficult to obtain a global value for the biological enhancement of weathering and erosion,” explained Dennis Höning of the Institute of Planetary Research in Berlin, Germany. “Also, the estimations which can be found in the literature vary from nearly no influence up to an enhancement of magnitudes. In our model, we decided to vary the erosion rate to obtain general tendencies.”
The role of water in the Earth's mantle is critical to the model, which is explained in the paper in the journal Planetary and Space Science by Höning and colleagues Hendrik Hansen-Goos, Alessandro Airo and Tilman Spohn. The model suggests that as Earth approached its one-billionth birthday, it was approaching a fixed point where it would have a small continental area and a dry mantle. Then life started messing with the geochemistry of the planet, revving up the mantle with more water and drove the planet to another fixed point with a lot more land area.
The work is part of a broader movement to better understand how life and water have fundamentally changed the evolution of the Earth.
“The list of elements that you can see affected by biology is increasing," said geologist Norman Sleep of Stanford University. Seismic reflection studies in recent years have backed up the idea that a lot of water is getting into the mantle via subduction zones and the geochemical signatures of that water and the life that helped it reach the mantle are in the rocks that rise from the wet mantle. "Biology has a way,” said Sleep. “It's not fully understood yet.”
The German team has also suggested that figuring out the details of life's effects on Earth's geological evolution could help in the search for signs of life on exoplanets.
"It might be possible to make remote measurements of the atmosphere or global surface topography in the near future," the team speculated. And as their study suggests, any signs of strong volcanic activity helped by a wet mantle, might be a hint of life.