Monday, 13 May, 2024 / Edition 7

Do you like drastic contrasts in geoscience? For example, the most recent geologic period contrasted with the earliest history of Earth? Or Earth’s surface compared with the core-mantle interface? Well, then you’'ll enjoy this edition of Core Elements. Ours will be a deep journey through space and time, but let’s start with here and now–our own human world and age–the Anthropocene …

Rasoul Sorkhabi

Editor, Core Elements

An Epoch Decision About the Anthropocene

Courtesy of AAPG Explorer

Since the late Nobel Prize-winning atmospheric chemist Paul Crutzen and Eugene Stoermer coined Anthropocene (“human age”) in their 2000 essay, the term has gained popularity and caught mass imagination. But is it really a geological epoch?

When does the Anthropocene start?

• Many geologists have suggested the 1950s as the Anthropocene’s beginning on the basis that human activities and impacts on Earth rapidly accelerated around that time and left relics such as plutonium and plastic.

• Some scientists would place this date in the Industrial Revolution.  

Is it officially a geological epoch?

• In 2009 the International Commission on Stratigraphy (part of the International Union of Geological Sciences, IUGS) established the Anthropocene Working Group to help decide whether to declare the Anthropocene a new geologic epoch.

• After a voting process in March this year (12 vs. 4), the IUGS announced that it would not consider the Anthropocene as an “epoch” (terminating the Holocene) but rather as a real ongoing “event.”  

The Response

Those wanting to make it official say: Some support the designation of the Anthropocene epoch because it stimulates much-needed environmental consciousness.

Opponents say: Others say that several decades cannot be a geologic epoch and that the geologic timescale is about the past not the future and that we should not push it into political provocation.

Common ground: Even those who do not want to officialize the term Anthropocene do acknowledge the huge impact of human activities on altering the environment.

The bottom line: Does it matter? The term Anthropocene will continue to be used informally because it has a meaning and serves a purpose.

What to watch: Perhaps, in the future, the geological community will officially call it an epoch. This has a precedence. For decades, the term Quaternary was informally used as a geological period but only in 2009 was it officially included in the geologic time scale.

Read more: Several magazines including Issues in Science and Technology, EOS, and AAPG Explorer have published articles about this new development and debate.

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When Theia Met Earth

Oliver Denker/Shutterstock.com

Speaking of epochs, the first half-billion eon of Earth is called the Hadean (from Hades, Greek god of the underworld). This eon witnessed giant impacts as well as the layered formation of the crust, the mantle, and the core.

Among the objects that impacted Earth was a hypothesized proto-planet called Theia, which hit proto-Earth at around 4.5 billion years ago as Earth was forming. It is believed that the Theia-Earth collision ejected materials into Earth’s orbit, and this debris later coalesced to form the moon.

What’s new: Recently, an article in Nature linked the Theia hypothesis to a totally different observation about the core-mantle boundary—the low-shear velocity provinces.

What are the low-shear velocity provinces?

• These are two continent-size blobs of denser materials in the lowermost mantle surrounding the core.

• We have no sample of them, but they do exist because they are characterized by their lower-than-average seismic wave velocities.

• They are denser than the adjacent rock materials in the lower mantle because seismic waves travel faster in these mysterious blobs.   

Their hypothesis: The new research uses giant-impact simulations to suggest that the low-velocity provinces are relics of Theia’s mantle material incorporated into Earth’s body.

Why it matters: If true, the new study suggests after Theia met and hit Earth, it did not simply vanish and vaporize, but some part of it has been within Earth for billions of years. This is an alternative to the “subducted slab graveyard” hypothesis, which suggests these deep blobs are subducted and sunken ocean crust materials.

But wait, there’s more: In a different study with Caltech, the same author and colleagues suggest that about 200 million years after the Theia-Earth collision, strong mantle plumes from the super-hot core-mantle boundary rose and induced the first subduction process in the Earth’s crust. This scenario is investigated by computer simulations of whole mantle convection in early Earth.

A potentially big impact:

• How and when subduction started in Earth’s early history is an enigma and various hypotheses have been offered.

• The new idea implies that plume tectonics predated and triggered plate tectonics.

Dive deeper:

• Read the full study by Qian Yuan and colleagues here.

• Here is a helpful animation clip for the first study.

• You can also read the second study, published in Geophysical Research Letters, here.

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