Hermann Pfefferkorn’s office, at the University of Pennsylvania, spills across a large, well-lit room that seems to have closed in on itself with shelves and cabinets and papers generated during his 43 years there. Pfefferkorn is a geologist and, naturally, has some rocks and fossils laying around. He picked up a small, black stone with a white fossil in it, straight like a straw, with subdivisions.
“If I look at some rock that has fossils for about an hour, I can tell you their ages,” Pfefferkorn said.
He examined the rock, declared the fossil a relative of the octopus, and said it formed in the Ordovician. The Ordovician, on the geologic time scale, lasted from about 490 to 445 million years ago. He picked up another rock and repeated his party trick.
Pfefferkorn was able to do this because the geologic time scale is strictly correlated to tangible rocks, which are, in turn, defined by their relative position to other rock sections and a marker, like a unique fossil.
The first time fossils were used to determine rock age was in 1815, by a geologist and surveyor named William Smith. “When he was building canals, he found again and again the same sequence of rocks,” Pfefferkorn said. Smith arranged the rocks in the order they were formed and looked at their fossils; he created a relative time scale, with ages expressed in relation to other rocks.
Although geologists eventually figured out how to attribute specific years to the units of the time scale, the physical evidence of rocks remains essential. It’s so important, in fact, that when geologists define a time unit and find a representative rock bed that shows where it started, they drive a golden spike into the rock face to mark the spot.
Pfefferkorn disappeared behind one of his bookcases and emerged holding an old railroad spike he had painted gold. “Here’s the golden spike!” he said. He held it up triumphantly.
He used to give these to geologists on field trips, doing the hard work of investigating new boundaries, the ultimate nerd souvenir. He said the geologic golden spike actually looks different, like an oversized pin, but the idea for it came from the railroad. When the eastern and western tracks first met in May of 1869, a golden railroad spike joined them.
“The image is actually useful,” he said. “Because it says here is the place, two things come together here.”
Right now, scientists around the world are seeking out a new golden spike site to mark the beginning of a fresh geologic epoch: the anthropocene.
The most recent epoch, the Holocene, has lasted for about 11,000 years, since glaciers covering much of the earth during the last great glaciation melted. The anthropocene’s defining trait, say those who support acknowledging the new epoch, is that humans have shaped it.
“In science, if something is real, if something is a distinct phenomenon, we tend to give it a name,” said Jan Zalasiewicz, a geologist in England.
The word has been increasingly used in popular culture to talk about our current time period, mostly in reference to human-caused climate change. In 2008, a group of scientists published a paper saying it’s time geologists took up the term and figured out if it merited official recognition on their time scale. Zalasiewicz was the lead author.
One thing led to another, and Zalasiewicz ended up head of a working group examining this question for the group that oversees the time scale, the International Union of Geological Sciences.
Geologists, when evaluating a potentially new time unit, look for signs of it in the rocks. “You need to lay out evidence that both the earth system function has changed substantially and effectively, permanently. And also that change is reflected in rocks and strata,” Zalasiewicz said.
Since Zalasiewicz started this work, he’s published more papers, saying, basically, yes, the evidence is there. “So, for instance, looking out of the window now, I can see roads and buildings and I know that under those roads and buildings I know there’s a mass of reworked ground which likely has bits of asphalt and concrete and brick and plastic, ceramic, all that kind of thing within it,” he said. Those human artifacts could function as markers in the rock strata, in the same way fossils do.
Then there are chemical signals from pollution; radioactive traces from atomic bombs; and biological signs in the form of extinctions and redistribution of animals around the globe.
“So there are a wide range of signals that reflect important kinds of global change, earth system change, which are being petrified already intro strata now forming,” he added.
Is it in the rocks?
Not everyone is on board with Zalasiewicz. Lucy Edwards, a U.S. Geological Survey geologist in Virginia, recently published a paper saying the evidence isn’t good enough.
“I’m kind of a wait and see person,” she said.
She acknowledges that humans are affecting the planet and signs of that are all around us. But she’s not convinced that the geologic record clearly reflects this, yet. And many geologists are with her.
“I guess if you had to pin me down, I’d say I think it’s premature, because I can’t go out and put my finger on a lot of rocks that I would call anthropocene,” Edwards said.
The issue is rocks take a long time to form, and the anthropocene is recent. We’ll probably see our handiwork in future rocks, but those rocks don’t exist, yet, say critics.
Knowing when the anthropocene started is another issue. One milepost that’s been thrown around is the start of the atomic age. Like a human-made meteor, the atomic bomb left its mark worldwide.
If the launch of the first atomic bomb is accepted as the start of the anthropocene, said Edwards, “that is a very short time period. And that means a lot of what humans have done and have left as deposits would no longer be anthropocene. A Roman road. An indigenous people’s campsite. And on and on and on.”
If scientists pick an arbitrary start date that doesn’t encompass all human traces in the rocks – anthropo, after all, stems form the Greek word for human – the anthropocene doesn’t live up to its definition.
Pfefferkorn, for one, doesn’t mind an arbitrary start date, as long as it’s clearly defined.
Zalasiewicz has taken these critiques seriously. He said he gets that a lot of people don’t want to separate the time scale from the rock record. That’s why the golden spike is so important, it reconnects the idea of the anthropocene to physical rocks.
But before anyone can put a new golden spike anywhere, the International Union of Geological Sciences will have to formally approve the addition of the anthropocene. That means evaluating evidence and potential golden spike sites, a process that could take years.