Traces of past life found in Earth’s deep mantle

Traces of the rapid development of fauna 540 million years ago have been found in rocks from the lower mantle of the Earth.

It is easy to see that processes inside the Earth influence what happens on the surface. For example, volcanoes unearth magmatic rocks and emit gases into the atmosphere, thereby influencing our planet’s biogeochemical cycles. What is less obvious, however, is that the reverse is also true: what happens on the Earth’s surface affects the Earth’s interior, even at great depths.

This is the conclusion reached by an international group of researchers led by Andrea Giuliani, from the Department of Earth Sciences at ETH Zurich, in a new study published in the journal Science Advances. According to this study, the development of life on our planet affects parts of the lower mantle of the Earth.

In their study, the researchers examined rare diamond-bearing volcanic rocks called kimberlites from different periods of the history of the Land. These special rocks are messengers from the lower regions of the Earth’s mantle. The scientists measured the isotopic composition of carbon in some 150 samples of these special rocks. They found that the composition of the kimberlites younger rocks, which are less than 250 million years old, varies considerably from that of the oldest rocks. In many of the younger samples, the carbon isotope composition is outside the range that would be expected for mantle rocks.

The researchers see a decisive trigger for this change in the composition of the younger kimberlites in the Cambrian Explosion. This relatively short phase, geologically speaking, took place over a period of a few tens of millions of years at the beginning of the Cambrian Epoch, about 540 million years ago. During this drastic transition, almost all of the animal tribes in existence today appeared on Earth for the first time.

“The enormous increase in life forms in the oceans decisively changed what was happening on the surface of the Earth,” Giuliani explains in a statement. “And this in turn affected the composition of the sediments on the ocean floor.”

For Earth’s lower mantle, this change is relevant because some of the seafloor sediments, in which material from dead living things is deposited, enter the mantle through plate tectonics. Along subduction zones, these sediments, along with the underlying oceanic crust, are transported to great depths. In this way, the carbon that was stored as organic matter in the sediments also reaches the Earth’s mantle. There, the sediments mix with other rocky material from the Earth’s mantle and, after a certain time, estimated to be at least 200-300 million years, rise again to the Earth’s surface in other places, for example, in the form of magmas of kimberlite.

It is remarkable that changes in marine sediments leave such deep footprints because typically only small amounts of sediment are transported deep into the mantle along a subduction zone. “This confirms that subducted rocky material in the Earth’s mantle is not distributed homogeneously, but rather moves along specific paths.”Giuliani explains.

In addition to carbon, the researchers also examined the isotopic composition of other chemical elements. For example, the two elements strontium and hafnium showed a pattern similar to carbon. “This means that the carbon signature cannot be explained by other processes such as degassing, because otherwise the isotopes of strontium and hafnium would not be correlated with those of carbon,” Giuliani points out.

The new findings open the door for more studies. For example, elements such as phosphorus or zinc, which were significantly affected by the emergence of life, could also provide clues about how processes on Earth’s surface influence Earth’s interior. “Earth is really a complex overall system,” says Giuliani. “And now we want to understand this system in more detail.”