A peace of Earth offering

In recent weeks this interesting science story has caught my attention. A group of scientists has been tasked with deciding whether we are in the midst of a new geologic epoch, and if so, what historical event can be used to mark its beginning. The potential new epoch is known as the anthropocene—the 'human epoch'—and is defined as the period in which human activities have had a significant impact on the Earth and its ecosystems.

Several sensible options have been put forward as possible start points for the anthropocene. These include the invention and rise of agriculture about 10,000 years ago, or the start of the Industrial Revolution in the 18th Century. It seems, however, that these choices have lost out to the might and power of an atomic option.

A group of researchers, led by Jan Zalasiewicz at the University of Leicester, have proposed that the new epoch began with the dawn of the atomic age. They suggest that the first nuclear bomb test, on 16th July 1945 in the Jornada del Muerto desert of New Mexico, marks the moment in time when humankind left its first permanent and global imprint on Earth.

Rising fireball and forming mushroom cloud, nine seconds after the world's first atomic bomb was detonated on 16th July 1945 in New Mexico. Credit: U.S. Department of Defense

Earth's geologic timescale is divided into units of various lengths that stretch back to the planet's formation about 4.54 billion years ago. Any given eon, era, period, or epoch consists of characteristic rock strata that can be distinguished from those that are directly above (younger) and below (older) it. Often, these stratigraphic boundaries are marked by major compositional or paleontological changes, such as mass extinctions. If the anthropocene is to be added to the top of Earth's stratigraphic column, it therefore makes sense to identify its beginning with a similarly global signature.

The geologic timescale. Credit: Science Education Resource Center at Carleton College

Luckily, as Zalasiewicz and colleagues point out, the nuclear tests that took place between 1945 and 1963 caused an unambiguous and detectable change in the Earth's atmosphere. The nuclear explosions were the primary source of man-made radionuclides (i.e., radioactive isotopes) found in the atmosphere. The most abundant of these nuclides—caesium-137—has no natural sources and is the product of nuclear fission processes. The first pronounced increase in the atmospheric concentration of Cs-137 occurred in 1954, and there was an additional peak in 1963. Man-made radionuclides therefore represent the best chronological markers for the atomically-heralded anthropocene.

Fallout of anthropogenic radionuclides (including caesium-137). Credit: Hancock et al., 2014, Geol. Soc. London

I think it is pretty obvious that a piece of the anthropocene Earth should be sent into the cosmos as our next geological postcard. This period, in which humans have begun to leave an indelible mark on our planet, is uniquely representative of Earth's capacity to support intelligent life. Caesium-137, however, with a half-life of only 30 years, is not a great choice. It will not be able to journey very far into the vast expanse of the universe before it decays away to almost nothing. So instead, I'd like to send an actual rock that is a direct product of the first nuclear test.

After the bomb detonation on 16 July 1945, a glassy residue was left on the desert floor at the 'Trinity' test site. The glassy deposit is composed mainly of feldspar and quartz-rich sand grains that were melted during the blast. This rock—known as trinitite, after its type locality—is usually light green in colour, and is mildly radioactive.

A trinitite hand specimen, produced by the first nuclear bomb detonation. Credit: Paul M. Schumacher

 

I like to think that our hypothetical alien planetary geologists are a friendly race and would extend a hand of peace if we ever encounter them. As evidenced by the anthropocene, we are now able to alter the workings of our own planet. So let those aliens beware the destruction we would unleash—Independence Day style—if they turn out to be not quite that welcoming. A piece of trinitite should therefore serve as a suitable example of the awesome and destructive power humankind can wield when we put our mind to it.