It’s indisputable that humanity has carved out a geological legacy for itself. The signs are clear that we are now the planetary kingmakers, and their scars are often grim reminders of unchecked societal progress: climate change, plastic pollution, the rate of species extinction, and radioactive fallout are markers of our march forwards.

Scientists generally agree that we’ve entered our own geological epoch, dubbed the Anthropocene, but working out where it begins depends on what global signals you're looking for, and how pervasive and persistent they are. A new study places its bet on a 1965 start date, all thanks to what the authors refer to as the “loneliest tree in the world”.

The signal that research groups are looking for is colloquially named the “golden spike” – a clear-cut signature that heralds the start of a new time. The golden spike that accompanied the end of the age of the dinosaurs, for example, was a huge jump in the concentration of iridium in a 66-million-year-old layer of rock found all over the world, a clear sign of a colossal asteroid impact.

Back in 2016, a panel of scientists at the Anthropocene Working Group (AWG) decided that the golden spike is likely to be the plutonium remnants left over from nuclear weapons testing in 1950, at the start of the Cold War. This isn’t binding, and other groups have suggested other dates – but this radioactive signature is the most popular option for the Anthropocene’s golden spike.

Now, using said radioactive compounds, a new paper published this month in the journal Scientific Reports has suggested a curiously later start date. The University of New South Wales and University College London-led team have based their timeline on an analysis of a Sitka spruce tree on Campbell Island, a lonely viridian speck within the frigid Southern Ocean.

As noted by BBC News, this single tree’s native habitat is actually in the North Pacific; its presence here can be traced back to 1905, when it was planted by visitors to the isle.

As is well known, tree rings are marvelous time capsules of chemistry long past. From their form to their composition, they provide excellent proxies for scientists to peer back in time.

Using this “alien Sitka spruce”, the team found that the ratio of more common carbon isotopes fell in response to a proportional rise in radioactive carbon-14. Carefully ruling out disturbances from other phenomena, they concluded that this major chemical change, which took place in October-December 1965, was a signal of the result of two decades of increasingly prolific nuclear weapons testing.

Their paper explains that as the signature can not only be found in the Northern Hemisphere in a variety of environments, but also on this “remote subantarctic island”, it provides “a truly global marker of human impact”, and thus serves as an excellent start date for the Anthropocene – one as clear as the 66-million-year-old iridium golden spike.

The researchers also note that this date coincides with the “Great Acceleration”, a post-World War II uptick in industrialization and consumption.

This date is one of the later suggestions for the start of the Anthropocene. Another separate paper, published in Earth-Science Reviews and led by the aforementioned AWG, argues for a 1952-1954 start date, primarily based on the emergence of a global signal of radioisotopes, particularly Plutonium-239.

Although the paper notes that an uptick in carbon-14 in trees can be detected starting in the early 1950s, which peaks in 1964, they describe them as “terrestrial markers only”, and add that “complex environmental processes can influence tree ring signals”.

"The work in this publication is very valuable and we could certainly consider using it in our assessment," Prof. Colin Waters of the University of Leicester, and the current Secretary of the AWG, told IFLScience.

However, he noted that "we would perhaps prefer using the onset of the radiocarbon signal" rather than the peak, which is what this new paper has used, breaking from current conventions.

Waters stresses that plutonium isotopes are considered by the AWG to be advantageous over carbon-14, as the former are "almost exclusively anthropogenic in origin." Their far longer half-life also ensures their highly abrupt signal will persist in the geological record far longer than the carbon-14 spike.

It's not a closed debate just yet. The search for the golden spike continues.