杏吧原创

A long-running effort to declare that the global impact of聽humans is enough to establish a聽new geological epoch will come聽to a head this year聽鈥� when a聽decision is made on the best site to聽officially mark the beginning of聽the Anthropocene.

The past 11,650 or so years form a geological unit of time known as聽the Holocene, considered a climatically benign epoch in the planet鈥檚 history that allowed civilisation to flourish.

But some scientists think there聽is now sufficient evidence to聽suggest that we have left the Holocene. They argue that the environmental presence of radionuclides from nuclear weapons, ash from coal-burning, plastics in sediments and other phenomena is enough to clear the聽bar for designating that we are聽in a new human-dominated epoch, which started聽in the 1950s.

In December, a group will announce a specific site somewhere on Earth that it thinks offers the clearest evidence of the dawn of the Anthropocene. This site will then be put forward for official consideration as the 鈥済olden spike鈥� marking the epoch鈥檚 start, formally known as聽a聽Global Boundary Stratotype Section and Point (GSSP).

GSSPs are being established for聽every boundary between named geological time periods. Each provides one or more clear signals of a significant and lasting change to Earth鈥檚 biosphere: the聽extinction or appearance of聽a聽key species, for instance, or a聽significant chemical signature. As聽an example, a cliff face near the聽town of El Kef in Tunisia is the GSSP for the end of the Cretaceous, 66 million years ago, because it聽preserves a particularly clear iridium signature from an asteroid that triggered a major extinction.

鈥淚 suspect that there will be several sites that will be very strong candidates,鈥� says 聽at the University of聽Leicester, UK.

Which one wins will depend聽partly on what a team of聽34聽researchers on the (AWG)聽鈥� established by a subcomission of the 聽鈥撀� decides should be the primary marker, or the main signal of humanity鈥檚 fingerprint worldwide. The choice is due in the聽second half of this year.

There is a strong move towards making this indicator plutonium from nuclear weapons testing. This is because it has such a clear absence and then appearance, says geologist , secretary of the AWG. But choosing a marker will involve weighing the clarity of聽the signal versus how much it聽has affected the world.

鈥淭he bomb spike from radionuclides is very pristine, a聽very clear signal,鈥� says Waters, but it doesn鈥檛 change the planet, whereas burning fossil fuels does because of climate change. Ice cores are another candidate because they contain a methane record that shows the advent of widespread fossil fuel use and large changes to the biosphere.

There are 12 sites in the race to be declared the golden spike (see map, below). They include the mud in a Japanese bay,聽which records a signature of atomic bomb testing and also contains fish scales showing the intensification of human fishing practices. Mud in a聽tiny Canadian lake, coral on the Great Barrier Reef and mineral deposits in an Italian cave all preserve evidence of atomic bomb testing. The Canadian lake also contains microplastics.

Teams of researchers, funded by聽German cultural institute (House of World Cultures), are racing to assemble their stratigraphic data聽for an exhibition at the institute in Berlin this May.

The findings will then be published in journals and a database later this year so the AWG聽can pore over them. A smaller circle of the group, comprised of 22聽members, will then vote in November on which site to put forward as the best candidate for the dawn of the Anthropocene. The point when the primary marker first appears at that site will be聽the聽GSSP.

So far, the 1950s have been proposed as the rough start of the聽Anthropocene. But the site and聽marker together should give a specific year, a remarkable level of precision in geology, where error bars for units of time can be in the thousands or even millions of years. It may be even more precise. 鈥淚t鈥檚 feasible with things like the corals we could tell you a specific season, and in some cases it may even be possible to link back to a specific [nuclear] detonation event and date,鈥� says Waters.

Deciding on a candidate site and primary marker this year would be聽a major milestone for proponents of the idea of the Anthropocene. But it doesn鈥檛 mean a new epoch is聽guaranteed to be officially declared. The choice聽of GSSP will first be voted on next year by a subcommission of the ICS, the arbiter of the geological timescale.聽The ICS itself would then need to approve the existence of聽the Anthropocene, which could聽happen in 2024.

But it isn鈥檛 guaranteed to do so. Some people disagree with the idea of adding the Anthropocene to the geological timescale, and establishing a specific time and place it began, . 鈥淎lmost nobody questions that we are in聽the Anthropocene. Is there a聽utility to defining a precise time聽when this begins? I think no,鈥澛爏ays at the University聽of Maryland, Baltimore County.

at University College London says that while a聽date around 1950 fits with the idea of a great acceleration in human activity around then, 鈥渋t聽misses out major processes and聽impacts that occurred before聽1950鈥�.

That is unlikely to deter the researchers who hope the ICS will designate the Anthropocene on the basis of the GSSP they pick this year. But will it happen? 鈥淚 think the odds are low,鈥� says Ellis, although even he concedes it is plausible the ICS could approve it.

Maslin says it would be significant: 鈥淚t will be a formal scientific statement that humans have altered the world so much that we have entered a new human- dominated period.鈥�

at University College London says that while it聽is聽easy to be pessimistic about the Anthropocene, its official declaration could be a catalyst for聽people making positive environmental changes.

鈥淧eople understand we are a聽geological agent,鈥� he says. 鈥淵ou聽always hope people will suddenly realise this is the only聽planet we have.鈥�