The Cambrian explosion (about 541 million years ago) - was when life and living things really started to survive on Earth. Now, new research reveals how the explosion of life left its mark deep in the mantle.
For scientists, it shows the interconnections and interactions between the surface and below the surface as sediments carrying organic material are pushed underground through subduction on huge geological time scales.
Top: Carbonate-rich kimberlite flake.
This new study looks at the rare, diamond-filled volcanic rock kimberlites. As they are pushed to the surface, they tell us what is happening deep in the mantle, and the researchers measured the carbon composition of 144 samples from 60 sites around the world.
Geologists generally agree that the carbon within diamonds has changed little over time scales of hundreds of millions of years.
However, the researchers found here that the ratio of specific carbon isotopes changed about 250 million years ago, around the time that sediments from the Cambrian explosion were folded into the mantle. This is a potential shift, a dramatic change in the carbon cycle at a time when the mass and diversity of the biosphere was increasing.
The researchers said, "These observations suggest that biogeochemical processes at the Earth's surface have profound effects on the deep mantle, revealing an overall link between the deep and shallow carbon cycles."
This connection between the carbon cycle near the surface and deep underground is not easy to measure, and in fact it has changed significantly rather than remained constant over the billions of years that the Earth has existed.
However, it seems clear that the dead organisms trapped in the sediments entered the mantle through plate tectonics. Their carbon remains mixed with other material before eventually reaching the surface again through events such as volcanic eruptions.
This connection was confirmed by further observations of strontium and tellurium in the samples. They match the carbon pattern, narrowing the possibility that the composition of these rocks has been altered.
Andrea Giuliani, a geochemist at ETH Zurich, Switzerland, said, "This means that the characteristics of carbon cannot be explained by other processes such as extinction, otherwise strontium and hafnium isotopes would not be associated with the isotopes of carbon."
Technically, we're dealing with sedimentary subduction fluxes here, and these carbon cycle details are important for understanding what's happening on Earth - especially as the effects of the climate crisis persist.
New research continues to reveal more about how carbon is absorbed from and released back into the atmosphere, particularly through the constant circulation of the tectonic plates that make up the Earth's surface.
Scientists know that relatively little sediment was pushed deep into the mantle through the subduction zone, which means that traces of the Cambrian explosion must have reached directly deep into the mantle.
Geochemist Andrea Giuliani said, "This confirms that the rocky material subducted in the mantle is not uniformly distributed, but moves along specific trajectories. The Earth is a very complex overall system. Now we want to understand this system in more detail."
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