Climate change may be the poster child for the Anthropocene. But the scars we've left on Earth's subsurface might actually prove more likely to earn humanity a spot on the geologic timescale. Anthroturbation—the digging, drilling, mining, and blasting that mars our planet's subterranean environs—could end up being the key marker for the epoch of humans and machines, according to the scientists tasked with hashing out when a new geologic era begins.
"The extensive, large-scale disruption of underground rock fabrics, to depths of greater than 5 km, by a single biological species, represents a major geological innovation. It has no analogue in the Earth's 4.6 billion year history," writes Jan Zalasiewicz, the British geologist who chairs the International Commission on Stratigraphy's Anthropocene working group.
Zalasiewicz's recent paper on anthroturbation is the first to detail the myriad ways we've chewed up the ground, the unparalleled nature of the damage, and why subterranean scars—among all human traces— are uniquely geologic.
It's easy to bemoan the mess we've made of our planet's surface. Ocean garbage piles, decimated rain forests, and eroded coastlines pull fast at our heartstrings. By contrast, the wreckage we've wrought on Earth's underground strata has gone relatively unnoticed: Out of sight, out of mind. But rest assured, we've been busy there, as well.
Consider, if you will, all the hardware and infrastructure beneath your feet that quietly supports your existence. Gas pipes and electric cables, sewers and reservoirs, tunnels, subways and building foundations. No organism has ever before laid claim to such extensive subterranean transformation. And that's just scratching the surface.
Boreholes—used to extract oil, natural gas, coal, metals and minerals—drive hundreds to thousands of meters into the Earth. Together, the millions of oil boreholes we've drilled span some fifty million kilometers, roughly the distance from Earth to Mars. It's this "deep anthroturbation" that's truly unprecedented in our planet's history. And, most importantly to the geologists, this stuff's permanent.
"The only way these marks can go away is by coming to the surface and being eroded, or getting caught up in a continental collision, or some other tectonic activity," Zalasiewicz told me. "Any scenario for erasing them will take tens to hundreds of millions of years."
A big hang-up in the Anthropocene debate is the lack of a proverbial "golden spike": A marker for ourselves that will stick around throughout the ages. Does anthroturbation fit the bill? Better, it seems, than anything else, but some remain skeptical about introducing such a novel phenomenon to our planetary timescale.
"The difficulty in all this is the heterogeneous nature of how humans have scarred the planet," Zalasiewicz told me.
Typically, geologists use fossils, sedimentary rock layers, and geochemical evidence to decide where various epochs begin and end. There are rules for how this stuff works—younger rocks are deposited over older rocks, and layers of rock are laterally continuous over large areas. But if anthroturbation is to mark the age of humans, the rules are going to have to bend.
"We have a whole network of this stuff, extending underground and forming complex three dimensional patterns, cutting through layers of rock from Holocene to pre-Cambrian," Zalasiewicz told me. "Human activity simply doesn't behave like a stratigraphic layer."
And if anthroturbation doesn't play nice for geologists, consider the other lines of Anthropocene evidence being thrown around: So-called "technofossils" include everything from saran wrap and CDs to cars and spaceships. As Zalasiewicz wrote in a related paper published earlier this year, "The morphological range of technofossils is almost infinitely greater than the range of trace types produced by any other species." And that's not to mention all the animals we've scrambled about the planet, which, millions of years from now, are also going to make for some head-scratching fossil beds:
"Rats, pigs, cows and domestic cats will appear almost everywhere very suddenly, other species will disappear in a blink. It'll be a biological merry-go-round the likes of which paleontologists have never seen," Zalasiewicz said.
So, the Anthropocene is geologically weird, and hidebound geologists won't be hurried into a rash decision. (The first formal scientific meeting on the subject was convened this past October, and the working group has given itself until 2016 to hash out a proposal for the ICS.)
"I think the Anthropocene reflects a real phenomenon— geologically there's a real thing there," said Zalasiewicz. "For the formalization, it needs to be not only real, it has to be useful to the geological community."
There is one aspect of anthroturbation where conventional geologic wisdom still applies: After humans disappear, fresh, unpunctured rock layers will bury our traces. It'll be up to future alien geologists, digging into our planet's subsurface and discovering eons worm-eaten, trash-ridden rock, to try and sort the whole mess out.