Should the detritus of modern human society, such as plastic bottles and tin cans, be used as geologic markers for a new, human-shaped geological era? CREDIT:

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Humans drive trillions of miles in cars, clear-cut forests for

agriculture and create vast landfills teeming with tin cans, soda

bottles and other detritus of industrialization. There's no doubt that

humans have radically reshaped the planet, and those changes leave

traces in the Earth's geological record.

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At the annual meeting of the American Geophysical Union this week, geologists are grappling with how to define the boundaries of that human-centered geologic era, referred to as the Anthropocene. Despite our dramatic impact on the planet, defining our era has proven a difficult task.

"If it's to be a geological period, it has to be visible in the

geological record," said Anthony Brown, a researcher at the University

of Southampton in the United Kingdom, who is trying to define the


Because geology looks at the deep past, such questions would normally

be examined in 100,000, even 1 million years' time, he said.

"In the absence of time travel, we have to work out whether we really do have enough around to define a new geological period."

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Shifting boundary

In one possible way of demarcating the boundary between natural

geologic eras and the human-shaped period, scientists would look at how agriculture changes sediments, Brown told LiveScience.

For instance, when farmers clear-cut forests and plant crops, they

change how sediments and runoff wash into the local rivers, often

creating a thick layer of silty, sandy clay on the flood plain, Brown

said. (Top 10 Ways to Destroy Earth)

But using such geologic clues to date the Anthropocene era runs into a

problem: agriculture began at different times around the globe. Some

areas, such as certain pockets in Africa, may not have had intensive

agriculture until recently.

Alternatively chemical deposits could date the boundary between human

and natural geologic eras. For instance, widespread use of leaded

gasoline and paint has left high levels of lead

in soils throughout the world, said Michael Kruge, a researcher at

Montclair State University in New Jersey. Polycyclic aromatic

hydrocarbons (PAH) could also serve as markers. These are formed from

combustion in natural wildfires, but also come largely from the burning

of fossil fuels.

"In the middle of the 20th century, you see a big spike in these compounds in sediment," Kruge said in a press conference.

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Using those measures, the Anthropocene era would begin around

industrialization, thousands of years after humans began reshaping the

planet with agriculture.

Yet another proposal would peg the time of the Anthropocene's birth to

the mass movement of soil, or the accumulation of minerals from coal

burning, cement production for construction, or the massive use of

nitrogen fertilizers. That would date the Anthropocene to the sharp

uptick in the production of these chemicals after World War II.

Other scientists hope to date the Anthropocene's onset using modern-day

fossils, for instance, layers and layers of plastic soda bottles and

tin cans piling up in landfills.

All of these approaches face a challenge, however: combining

human-caused changes with natural, global variations that normally

demarcate different geologic time periods. For instance, our current

geologic time period, the Holocene Epoch, governs our climate and the

extent of our glaciers, and is dictated by eccentricities in the Earth's

orbit (something humans' haven't yet managed to alter).  Since the

Earth's orbit isn't going to change any time soon, the Anthropocene

would somehow need to overlap with the Holocene.

"Nobody believes that the astronomical cycle, the 100,000-year cycle

that we're in, is suddenly coming to an end," Brown said. "We have to

combine the anthropogenic with the natural variability in the climate

system. It is a question that geologists never had to face before."

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