Scientists develop a statistical fix for archaeology’s dating problem

Archeologists have long had a dating problem. The radiocarbon analysis typically used to reconstruct past human demographic changes relies on a method easily skewed by radiocarbon calibration curves and measurement uncertainty. And there’s never been a statistical fix that works—until now.

Nobody has systematically explored the problem, or shown how you can statistically deal with it,” says Santa Fe Insitute archeologist Michael Price, lead author on a paper in the Journal of Archeological Science about a new method he developed for summarizing sets of radiocarbon dates. “It’s really exciting how this work came together. We identified a fundamental problem and fixed it.

In recent decades, archeologists have increasingly relied on sets of radiocarbon dates to reconstruct past population size through an approach called “dates as data.” The core assumption is that the number of radiocarbon samples from a given period is proportional to the region’s population size at that time.

Archeologists have traditionally used “summed probability densities,” or SPDs, to summarize these sets of radiocarbon dates. “But there are a lot of inherent issues with SPDs,” says Julie Hoggarth, Baylor University archeologist and a co-author on the paper.

Radiocarbon dating measures the decay of carbon-14 in organic matter. But the amount of carbon-14 in the atmosphere fluctuates through time; it’s not a constant baseline. So researchers create radiocarbon calibration curves that map the carbon-14 values to dates.

Yet a single carbon-14 value can correspond to different dates—a problem known as “equifinality,” which can naturally bias the SPD curves. “That’s been a major issue,” and a hurdle for demographic analyses, says Hoggarth. “How do you know that the change you’re looking at is an actual change in population size, and it isn’t a change in the shape of the calibration curve?

When she discussed the problem with Price several years ago, he told her he wasn’t a fan of SPDs, either. She asked what archeologists should do instead. Essentially, he said, “Well, there is no alternative.

That realization led to a years-long quest. Price has developed an approach to estimating prehistoric populations that uses Bayesian reasoning and a flexible probability model that allows researchers to overcome the problem of equifinality. The approach also allows them to combine additional archeological information with radiocarbon analyses to get a more accurate population estimate.

He and his team applied the approach to existing radiocarbon dates from the Maya city of Tikal, which has extensive prior archeological research. “It serves as a really good test case,” says Hoggarth, a Maya scholar. For a long time, archeologists debated two demographic reconstructions: Tikal’s population spiked in the early Classic period and then plateaued, or it spiked in the late Classic period.

When the team applied the new Bayesian algorithm, “it showed a really steep population increase associated with the late Classic,” she says, “so that was really wonderful confirmation for us.

The authors produced an open-source package that implements the new approach, and website links and code are included in their paper. “The reason I’m excited for this,” Price says, “is that it’s pointing out a mistake that matters, fixing it, and laying the groundwork for future work.

This paper is just the first step. Next, through “data fusion,” the team will add ancient DNA and other data to radiocarbon dates for even more reliable demographic reconstructions. “That’s the long-term plan,” Price says.

And it could help resolve a second issue with the dates as data approach: A “bias problem” if and when radiocarbon dates are skewed toward a particular time period, leading to inaccurate analyses.

But that’s a topic for another paper.

See more here: phys.org

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Comments (5)

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    Joseph Olson

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    Carbon-14 comes from the solar radiation of atmospheric Nitrogen-14, the C-14 atoms combine with Oxygen O2 molecules, forming CO2 that is absorbed by plants and converted to sugars, starches and cellulose. Volcanic CO2 does not have any C-14, so lifeforms fed off other nutrient streams do not show correct Carbon dating.

    “New! Amazing! Wrongco Proxy Crock” and “Proxy Crock Memos” at CanadaFreePress

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    • Avatar

      Jerry Krause

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      Hi Joseph and PSI Readers,

      This article is another consequence that I term my GOD SIGHTING.

      This morning the hour before I came to my computer I had been pondering what I had observed about my property which had been burned by the Bootleg Wildfire. Which result I have only observed for a few hours and know that I miss observing a possible critical observation of a long dead juniper tree which stood less than hundred feet from a big ponderosa tree which needles were all burned off and maybe some limbs also.

      So, I was considering, if the dead juniper was now gone, I could date the approximate time of the last wildfire by falling this big ponderosa and counting its yearly tree rings.

      And a fact is that the Bootleg Wildfire started in an area of lodgepole pine which regrow very densely after the previous wildfire. So, the tree rings of these now standing dead trees should with good precision date the last wildfire of this area.

      Joseph, you have supplied information which should allow any PSI reader to SIMPLY understand how various natural factors have influenced the weather, hence climate, of this natural laboratory of this part of arid Oregon. And maybe allow PSI Readers better understand this article.

      For I suspect my big, dead, ponderosa tree will be found at least 100 years old. But it must be felled, the rings counted and the widths of the rings evaluated. Only by doing this, can one begin to understand NATURAL WEATHER AND CLIMATE.

      Have a good day, Jerry

      Reply

      • Avatar

        Joseph Olson

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        ARCHAEA, the single cell lifeform, so different from any other on Earth, it was given it’s own family in 1978. It is the only lifeform that can exist without photosynthesis, or photosynthesis food source. Archaea live in the intestines of termites and rudimentary animals where they produce Methane waste. Archaea live at seafloor vents, under extreme temperatures and pressure, where they eat Volcanic Methane. Archaea are the base of Antarctica food chain, with no C-14, the krill they feed are eaten by penguins and seals, when fresh killed Carbon date to 3,500 years old.

        Reply

      • Avatar

        Jerry Krause

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        Hi Joseph and other PSI Readers,

        Hope you other PSI Readers appreciate this information las I do because I am only a physical scientist instead while Joseph is the complete package (both a physical and life scientist)!!!

        Thank you Joseph!!!

        Have a good day, Jerry
        Have a good day,

        Reply

  • Avatar

    MattH

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    Hi readers. I thought some may find this interesting from spaceweather.com

    EARTH CAN MAKE ITS OWN AURORAS: No solar storms? No problem. Earth has learned to make its own auroras. New results from NASA’s THEMIS-ARTEMIS spacecraft show that a type of Northern Lights called “diffuse auroras” comes from our own planet–no solar storms required.

    Diffuse auroras look a bit like pea soup. They spread across the sky in a dim green haze, sometimes rippling as if stirred by a spoon. They’re not as flamboyant as auroras caused by solar storms. Nevertheless, they are important because they represent a whopping 75% of the energy input into Earth’s upper atmosphere at night. Researchers have been struggling to understand them for decades.
    “We believe we have found the source of these auroras,” says UCLA space physicist Xu Zhang, lead author of two papers reporting the results in the Journal of Geophysical Research and Physics of Plasmas.

    It is Earth itself. Earth performs the trick using electron beams. High above our planet’s poles, beams of negatively-charged particles shoot upward into space, accelerated by electric fields in Earth’s magnetosphere. Sounding rockets and satellites discovered the beams decades ago. It turns out, they can power the diffuse auroras.

    The video, below, shows how it works. The beams travel in great arcs through the space near Earth. As they go, they excite ripples in the magnetosphere called Electron Cyclotron Harmonic (ECH) waves. Turn up the volume and listen to the waves recorded by THEMIS-ARTEMIS:

    ECH waves, in turn, knock other electrons out of their orbits, forcing them to fall back down onto the atmosphere. This rain of secondary electrons powers the diffuse auroras.

    “This is exciting,” says UCLA professor Vassilis Angelopoulos, a co-author of the papers and lead of the THEMIS-ARTEMIS mission. “We have found a totally new way that particle energy can be transferred from Earth’s own atmosphere out to the magnetosphere and back again, creating a giant feedback loop in space.”

    According to Angelopoulos, Earth’s polar electron beams sometimes weaken but they never completely go away, not even during periods of low solar activity. This means Earth can make auroras without solar storms.

    The sun is currently experiencing periods of quiet as young Solar Cycle 25 sputters to life. Pea soup, anyone?
    An expanded and sharable version of this story may be found here.

    Reply

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