Comment on Half-million-year-old wooden structure unearthed in Zambia

northendtrooper@lemmy.ca ⁨1⁩ ⁨year⁩ ago

• wjrii@kbin.social ⁨1⁩ ⁨year⁩ ago

  The article says they used luminescence dating to measure the amount of radiation the surrounding soil had soaked up over time:

  Grains of rock absorb natural radioactivity from the environment over time - essentially charging up like tiny batteries, as Prof Duller put it.

  And that radioactivity can be released and measured by heating up the grains and analysing the light emitted.

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• fossilesque@mander.xyz ⁨1⁩ ⁨year⁩ ago

  Wood is decent for C14 dating. They likely took a direct sample. There’s not really a lot of comparable material this old, mostly just due to time.

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  • northendtrooper@lemmy.ca ⁨1⁩ ⁨year⁩ ago

    Interesting, I didn’t think of the tree rings for fine tuning.

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    • fossilesque@mander.xyz ⁨1⁩ ⁨year⁩ ago

      www.youtube.com/watch?v=xcib9Fnv7-Y

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  • TauZero@mander.xyz ⁨1⁩ ⁨year⁩ ago

    How could carbon-14 possibly work for anything half a million years old? That’s 87 half-lives!

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    • fossilesque@mander.xyz ⁨1⁩ ⁨year⁩ ago

      Ha, you’re right. Totally forgot there’s that upper limit (To be fair I don’t really do anything with stuff that old and I’m having waiting for dinner scatterbrains. :) ) . Going to have to dive into the paper I guess!

      www.nature.com/articles/s41586-023-06557-9

      Looks like they did luminescence. More about that here: projects.arch.ox.ac.uk/luminescence.html

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      • TauZero@mander.xyz ⁨1⁩ ⁨year⁩ ago

        Aha! The supplemental materials contain all the interesting stuff, like some cool photos of the excavation site:

        Image

        And section 2 goes into details about how the soil samples were collected and prepared for luminescence measurement, including keeping them in the dark/under red light lab room conditions while they were washed in acid for 3 days, before shining a laser on them. They even stuck gamma radiation probes into the holes they dug the samples from to measure the current background radiation levels there.

        Interestingly in their calculation, only 5% of radiation comes from cosmic rays, and 95% from decay of nearby radioisotopes of uranium, thorium, and potassium. They were worried whether cyclical sedimentation/erosion activity of the adjacent river would change the thickness of the soil overlying the archeological site, thus affecting amount of cosmic radiation reaching it. Turns out for the final age calculation it didn’t make much of a difference whether the soil was usually 5 meters thick or 10.

        I would still need to read the reference papers to figure out how accumulated radiation is related to luminescence under a laser in the first place.

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  • Shdwdrgn@mander.xyz ⁨1⁩ ⁨year⁩ ago

    Ohhh is this the technique where they line up the tree rings with known climate events? It’s certainly cool how many different methods they come up with to date things, although judging by the small size of those logs it doesn’t seem like they would have very many years to work with.

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    • fossilesque@mander.xyz ⁨1⁩ ⁨year⁩ ago

      Rangefinding helps narrow the window.

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