Canadian lake chosen to mark start of proposed Anthropocene epoch

Scientists have picked the bottom of Crawford Lake in Ontario as the "golden spike" to mark the start of a new proposed geological epoch — the Anthropocene. The announcement was made at a media conference in Berlin Tuesday by a group of scientists called the Anthropocene Working Group — more on them later. 

Here's why the lake was chosen and what evidence it provides that humans have made such big changes to the Earth that we may be in a new geologic time period.

Geologists measure the history of the Earth using the geologic time scale (its official name is the International Chronostratigraphic Chart) — kind of like a calendar, except that it's divided into much bigger divisions than days, weeks or months. 

For example, "periods" like the Jurassic and Cretaceous are tens of millions of years long and divided into epochs that are typically millions of years long.

Until now, our current epoch has been the Holocene, which started at the end of the last ice age 11,700 years ago.

But in many fields, including science, researchers and thinkers had already been discussing the huge impact humans have had on the Earth — including mass extinction and climate change, the kind of changes that typically mark the start and end of epochs. 

About two decades ago, Nobel prizing winning chemist Paul Crutzen popularized the idea that science should recognize that impact with a new geological epoch, the Anthropocene, Prof. Jürgen Renn, director at the Max Planck Institute for the History of Science in Berlin, said at a news briefing about the new announcement.

"He said, 'You know, we cannot say with all these changes that we are living still in the Holocene,'" Renn said. "It's not just about climate change. It's not just biodiversity loss. It's not just the sediments that humans are moving. It's all of this together."

He added that much of the ongoing change is effectively irreversible, such as the melting of glaciers around the world: "These changes that we have induced already will only unfold in the next decades and centuries."

That said, the proposal that we're in a new epoch has been controversial.

The International Commission on Stratigraphy, a group of geologists within the International Union of Geological Sciences, is the scientific body that officially decides when epochs begin and end. In 2009, it asked a group of geologists, paleontologists and other scientists, the Anthropocene Working Group, to look into whether there was enough scientific evidence to back up Crutzen's proposal of a new epoch.

One of the group's key jobs has been to identify and describe a "golden spike" marking the start of the new proposed epoch.

Crawford Lake was chosen among 12 "golden spike" candidates around the world after a series of votes by the Anthropocene Working Group.

A "golden spike" is the ideal marker showing where one epoch ends and another begins — typically at a major global event such as a mass extinction or climatic shift. Such events are generally visible in the form of layered evidence like changing fossils in rock deposited over time.