On Campbell Island, some 400 miles south of New Zealand, sits a single Sitka spruce. Planted in the early 20th century, the tree is considered the world's loneliest tree.

It may also be the "golden spike" that could serve as the marker for the hotly debated Anthropocene epoch. A study published Feb. 19 in Scientific Reports outlines how this isolated tree in this isolated part of the Southern Ocean shows a spike in radioactive elements from the tail end of 1965. To researchers, it's a clear sign of humans' far-reaching influence on the environment.

"The impact that humanity’s nuclear weapons testing has had on the Earth's atmosphere provides a global signal that unambiguously demonstrates that humans have become the major agent of change on the planet," Christopher Fogwill, a professor at Keele University, said in a statement about the study. "This is an important, yet worrying finding."

A human epoch

Geologists divide the Earth's geologic history into epochs, and we're currently in the Holocene epoch. It started about 11,700 years ago, after the last major ice age. As such, it encompasses all of written history and a transition from rural to urban living.

In recent years, however, there has been a push to declare a new epoch — the Anthropocene — to reflect the ways in which humans have significantly altered the environment, from climate change to billions of pieces of plastic in the ocean to driving species and wildlife and plants to extinction. These reasons, however, are grounded in environmental impact and not, as some geologists might prefer, a geological one.

"When you start naming geologic-time terms, you need to define what exactly the boundary is, where it appears in the rock strata," Whitney Autin, a stratigrapher (someone who studies rock layers) at the SUNY College of Brockport told Smithsonian.com in 2013.

The groups that decide when we change epochs, the International Commission on Stratigraphy and the International Union of Geological Science, have not adopted the Anthropocene as an epoch. Despite that, the term is used among scholars from across a number of fields (including various disciplines in the humanities).

A tree as a 'spike'

A color photo of the Trinity explosion The Trinity test was the first nuclear test conducted by the U.S. on July 16, 1945, and it could be considered the start of the Anthropocene epoch. (Photo: Jack W. Aeby/Wikimedia Commons)

Suggestions for when the Anthropocene started differ. The start of the Industrial Revolution in the late 1700s is a contender, and so is the Trinity test, the first nuclear detonation in July 1945. Other suggestions go back much further in time. A 2015 study points to 1610 as a potential start date for the Anthropocene to mark New World pollen arriving in Europe and a massive dip in carbon dioxide levels.

All of these suggestions are attempts to identify a Global Boundary Stratotype Section and Point (GSSP), or a "golden spike." The presence of this spike must be seen globally in rock layers. For example, the meteorite that struck Earth and scattered iridium into the soil (and killed off the dinosaurs, of course) is one such spike. For the Holocene epoch, the GSSP was an ice sample from the Greenland ice sheet that showed an uptick in warming as we exited the last ice age.

Finding such a global spike is what has made the push for the Anthropocene epoch so difficult. Radioactive elements from nuclear weapons, like the above-mentioned Trinity test, are seen as possibilities, according to the Guardian in 2016. The problem with this particular potential spike, however, is that the evidence of the radioactivity, whether it be in lake sediment or tree rings, is largely located in the Northern Hemisphere and in the weapon's surrounding test areas. In short, they're not seen as global spikes.

Enter Campbell Island's lonely tree.

Based on work conducted during the Australasian Antarctic Expedition in 2013-2014 which was co-led by Fogwill, the researchers behind the Scientific Reports study say we can start the Anthropocene epoch between October and December 1965 thanks to an actual spike in radioactive carbon-14 detected in the tree ring from around that time.

The scientists took various core samples from the lone Sitka spruce and scrub vegetation (Dracophyllum longifolium) from various locations around the island. They also took samples from Homestead Scarp, an escarpment on the edge of a raised bog. The scrubs and the Homestead Scarp also demonstrated increased carbon-14 levels, though not with the same clarity as the spruce's rings.

The date corresponds with the Great Acceleration, a post-1950s period that saw a drastic increase in a number of socio-economic and Earth science trends, including urban living, water use, tropical forest loss and carbon dioxide emissions.

The researchers argue that the Sitka spruce, given its location in the Southern Hemisphere and generally far from nuclear testing, constitutes the global signal that Anthropocene proponents have been seeking.

"We were incredibly excited to find this signal in the Southern Hemisphere on a remote island, because for the first time it gave us a well defined global signature for a new geological epoch that could be preserved in the geological record," lead author and professor from University of New South Wales Chris Turney said in the Keele University statement. "Thousands of years from now, this golden spike should still stand as a detectable marker for the transformation of the Earth by humankind."

This study is not the end of the debate, of course. As Colin Waters of the University of Leicester and chairman of the Anthropocene Working Group told IFLScience, "The work in this publication is very valuable and we could certainly consider using it in our assessment" but that its use of carbon-14, given that it can be produced by other, non-human ways and has a shorter half-life than plutonium isotopes, limits the potential value of the spike as a geological signifier."

Still, this is a bright spotlight on a very lonely tree.