When the world-girdling ice came at the end of the Ordovician period roughly 440 million years ago, only a few species of graptolite survived the mass extinction. Graptolites, whose name means written in rock, were tiny animals that lived in colonies of little cuplike structures known as theca. The graptolites built many theca together to form a branching structure that then drifted in ancient seas and therefore can be found in sedimentary rocks of a certain vintage all around the world. And their geologically abrupt disappearance makes them the perfect fossil to mark the end of the Ordovician, or so think geologists like Jan Zalasiewicz of the University of Leicester who has spent much of his professional life studying the beautiful shapes left behind by these long-gone animals.

There is a golden spike hiding in a rock face outside the village of Moffat in Scotland that marks the end of the Ordovician, denoted by the appearance of these graptolite survivors—Akidograptus ascensus and Parakidograptus acuminatus, to give them their scientific names. It's not a real golden spike but a line of darker shale and a marker of these graptolites' significance as what's called an index fossil: Wherever such a fossil is found a geologist can be sure those rocks are of a certain age.

Zalasiewicz now thinks he's found the perfect marker for the Anthropocene, or the new epoch of humans, so dubbed for Homo sapiens’s world-changing impacts. It's a rather precise start date, thanks to some unusual isotopes: July 16, 1945, at 5:29 in the predawn New Mexico desert. That's when U.S. scientists exploded the world's first atomic bomb and when the human-induced radioactive isotope clock started ticking.

The three isotopes in question are cesium 137 and plutonium 239 and 240, which will take millennia or more to decay. There are no known natural sources of cesium 137. As a result of the subsequent detonations of hundreds of such weapons around the globe, there will be plenty of these isotopes still around far into the future. Like the meteorite that helped end the Cretaceous period about 65 million years ago, and possibly the reign of the dinosaurs as well, the nuclear detonation may mark for future geologists a turning point in Earth's history.

Biology, magnetism and the chemistry of isotopes are the markers geologists typically use to tell the tale of changeovers in deep time, whether the Ordovician period or the Anthropocene epoch (the latter just a small slice of the Quaternary period that began more than 2.5 million years ago and has its own golden spike in Sicily). Lingering isotopes will also tell the tale of skies filled with carbon dioxide from burning fossil fuels. This human habit has already shifted atmospheric CO2 concentrations by more than 100 parts per million, which is the usual shift in concentration between a planet swathed in ice and the more temperate climes in which human civilization developed.

That is hardly the only change. Nitrogen has been wrested from the sky, turned into plant food and, ultimately, more people—a doubling of the amount of nitrogen cycling through planetary systems. The sediments in Sky Pond in the Rocky Mountains show that around 1950 nitrogen began to flood the lake, an event unprecedented in at least 14,000 years and this isotopic record too could have a pretty precise start date: July 2, 1909 when Fritz Haber first demonstrated how to make ammonia from the air in Germany.

In Greenland lead isotopes in ice cores reveal a record of lead pollution from Roman smelting in Spain some 2,000 years ago. Lead really takes off in the ice record in the 20th century, however, with the addition of this element to gasoline. More recently, China's coal-fired power plants have taken the lead in lead pollution.

As traces of the Romans show, humans have been leaving their mark for quite some time now, if not necessarily everywhere, before the middle of the 20th century, a time some have dubbed the “Great Acceleration.” So “the Bomb” is not the only proposal for the start this new epoch.

: The cesium and plutonium from weapons testing will last for millions of years, providing a record in rock of new globe-spanning impacts. That's why Zalasiewicz, among others, thinks the Anthropocene should date from this "golden spike"—a clear record in the rock that can be found everywhere in the world.

An end to atmospheric testing may have produced the declining bomb curve in these radioactive isotopes but it didn't stop geologic scale impacts, like the plutons produced by atomic blasts taking place underground until the 1990s. These masses of shattered rock atop the radioactive melt core of a bomb test can be hundreds of meters across, and melt as much rock as a medium-size volcano.

OVERVIEW: Another alternative is that the Anthropocene begins when people first become aware of global human impacts, an event often tied to the first comprehensive look at the planet, provided first by satellites after Sputnik. Astronauts joined the cavalcade of whole-Earth imaging not much later.

CARBON CONUNDRUM: The future rock record will reveal a void of fossil fuels after about 1800. That's when a new steam engine fueled by coal allowed for greater production of the dirty black rock that then fired ever more new steam engines and inaugurated the industrial revolution—as well as the problem of climate change. And climate changes, like the shift from the icy world of the Pleistocene to the summery clime of the Holocene epochs, have marked some past shifts in the geologic record. Uniquely spherical magnetic minerals wafted over the world by coal burning can be found from peat bogs to lake sediments and may furnish a record of this carbon combustion for future geologists.

DISTURBED SOIL: Another proposal pegs the start of the Anthropocene to the widespread creation of human-affected soils around the globe, dubbed the "archaeosphere," a unique mix of waste, ancient infrastructure, plowed soils and other oddities that can be tens of meters deep. This would not create a geologic record synchronized across the globe, but the archaeosphere may reflect the reality that the Anthropocene is not evenly distributed even today and, from 100 million years in the future, this may all look like a geologic instant anyway. This idea began percolating nearly a century ago when geochemist Vladimir Vernadsky of Russia suggested that human changes to the soil constituted "a new phenomenon in geological history."

ANCIENT METHANE: Some scientists argue for an Anthropocene, stretching far back to the start of wide-scale farming at least 10,000 years ago. Methane from the swampy rice farming in Asia may have really kicked this off 6,000 years ago or so and, in part, be responsible for maintaining the summery clime of the Holocene. Future geologists will also find a steady shift from tree pollen in sediment cores and other records to the pollen of grassier plants, particularly corn, as farming came to predominate human activity. This idea would potentially supplant the Holocene epoch and replace it with the Anthropocene.

BIG DEATH: Some argue that widespread extinctions of large mammals, stretching as far back as 40,000 years ago, deserves recognition as the starting point of any human-induced epoch. Paired with the mastery of fire, this would make the Anthropocene at least 13,000 years old and possibly more than 100,000 years old.

Zalasiewicz is one geologist who has given some thought to what will remain in 10,000 years. Certainly some record of cities, plastics and the millions of fossil-fuel wells and mines will persist as what he calls technofossils. The concentrations of carbon dioxide in the atmosphere could still be out of whack from the emissions resulting from the burning of all those fossil fuels in just the last few decades. In a million years, barring profound shifts, the climate should have returned to its natural rhythms but any cities buried in sediment by rising seas should still be preserved, along with those signs of anthroturbation, human-induced disturbances underground, like the plutons from underground explosions of nuclear bombs. Those are good for 10 million even 100 million years, or until plate tectonics lifts them back to the surface and exposes those strata to the rain that will, ever so slowly, erode these signs away. For certain, nothing made by contemporary humanity will be left at the surface that far in the future, even stone edifices like the pyramids or Mount Rushmore will be wiped away, though the fine imprints of plastic objects, like say a vinyl record, may be legible—and even perhaps listenable—in the rock like the fronds of a fern.

Deep time
The geologic time scale—deep time in general, like the age of the universe itself—is perhaps one of the most difficult scientific concepts for the human mind to grasp. A generation of our species encompasses 25 years or so and is ill-suited to match up with a planet that measures age in billions of years.

Ten thousand years is all that separates today's people from those who lived at Catalhöyük in Turkey, a city whose mud-brick homes had doors in the roofs. The inhabitants were seemingly obsessed with leopards, slept on their own ancestors’ graves and occasionally kept the skulls as mementoes. In the far future will anyone even understand the binary code and English scrawlings in the Roman alphabet in which the very idea of the Anthropocene is recorded? The Rosetta stone was required to unlock the mysteries of hieroglyphics from a mere 5,000 years ago and the world is no closer to understanding the hash marks left by ancient hominins hundreds of thousands of years ago. One million years ago, Homo sapiens did not exist and our hominin ancestors stalked the savannas of Africa and perhaps not much else, the human population explosion still far in the future.

In the long run of geologic time, the Holocene, or "entirely new," is already special, the only summery span accorded its own epoch between periods of an icier planet. It is also the longest period of globally stable climate and sea level in at least the last 400,000 most recent years of seesaw between glaciation and warmer times. And it is the only epoch in the last 542 million years worth of the geologic record to be defined from the "present." It is 11,703 ice-layer years before A.D. 2000. That translates to a depth of roughly 1,492 meters below the surface of Greenland and is marked by its own isotopic changes, thanks to all that melting ice.

That may prove the Holocene's undoing as an epoch, if the Anthropocene's golden spike turns out to be widespread burning or agriculture, both of which occupy large swathes of Holocene time. Or perhaps just the most recent 2,000 years will get lopped off if lead residue becomes a marker.

Will the Anthropocene be a blip in time or merit an epoch or even more? If this new epoch lasts for only a few centuries or even decades how will anyone find it in 10,000 years time? If climate change gets catastrophic—and the world sees more than 6 degrees Celsius warming of average temperatures—the planet will have left the current geologic period, known as the Quaternary and a distant successor to the Ordovician, and have returned to temperatures last seen in the Paleogene period more than 30 million years ago. Perhaps then the Anthropocene deserves recognition as the beginning of a new geologic period, perhaps known as the Quinary or even the Anthropogene. For the time being, Zalasiewicz and other geologists pondering this new epoch prefer to be conservative, given the uncertain future trajectories of climate change, sea level rise and mass extinction.

Still, people's impact keeps growing larger, as the population swells beyond seven billion. Civilization now moves some 57 billion metric tons of rock, dirt, sand and coal, among other things, or three times as much geologic stuff as is moved by all the world's rivers. That's a volume of 30 cubic kilometers shifted about by people every year. Or, as Zalasiewicz wrote in his book The Earth after Us: "it is hard, as humans, to get a proper perspective on the human race." The Anthropocene, as a geologic epoch, puts people back in the center of the action, the collective protagonist of a planetary drama. As an idea, the Anthropocene represents the makers of geology inscribing themselves into their own rock record.