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When the activity of the sun changes, it has direct effects on the earth. For example, when the sun is relatively inactive, the amount of a type of carbon called carbon-14 increases in the earth's atmosphere. Because carbon in the air is absorbed by trees, carbon-14 levels in tree rings actually reflect solar activity and unusual solar events in the past. The team took advantage of such a phenomenon by analyzing a specimen from a bristlecone pine tree, a species that can live for thousands of years, to look back deep into the history of the sun.
"We measured the 14C levels in the pine sample at three different laboratories in Japan, the US, and Switzerland, to ensure the reliability of our results," A. J. Timothy Jull of the University of Arizona says. "We found a change in 14C that was more abrupt than any found previously, except for cosmic ray events in AD 775 and AD 994, and our use of annual data rather than data for each decade allowed us to pinpoint exactly when this occurred."
[...] "Although this newly discovered event is more dramatic than others found to date, comparisons of the 14C data among them can help us to work out what happened to the sun at this time," Fusa Miyake of Nagoya University says. She adds, "We think that a change in the magnetic activity of the sun along with a series of strong solar bursts, or a very weak sun, may have caused the unusual tree ring data."
An abstract is available but the full article is paywalled; see: "Large 14C excursion in 5480 BC indicates an abnormal sun in the mid-Holocene" was published in PNAS at: www.pnas.org/cgi/doi/10.1073/pnas.1613144114
(Score: 3, Interesting) by Immerman on Wednesday February 08 2017, @04:12PM
There's not a whole lot of alternative explanations.
They're looking at the ratio of C14 to C13 in the wood. Trees don't care about the particular isotope of carbon they're using to grow new wood, so the ratio of C14 to C13 in the new wood will be the same as is present in the atmospheric CO2 it's building the wood out of.
Regional overcast, etc. may affect the speed of growth, but it won't affect the ratio of C14 to C13 in that ring. The *only* thing that will do that is changing the local isotope ratio of atmospheric carbon. Putting some sort of thermal neutron source nearby might change the ratios for carbon already in the wood, but neutrons pass through wood pretty easily, so the change wouldn't be restricted to just a few rings.
So, *something* happened to put a lot more C14 in the atmosphere that year, which pretty much means irradiating the atmosphere with thermal neutrons. Solar events are the most likely cause of that. The only other known sources are cosmic rays and open-air nuclear explosions.
So our options are:
Dramatic solar event
Massive cosmic ray burst
Natural fission reactor that somehow occurred on the surface instead of deep underground, and then exploded.
Aliens
Though I suppose a sufficiently radioactive asteroid strike might do something similar - vaporize a highly radioactive neutron emitter in the upper atmosphere and it could produce a bunch of C14 and then decay to something not immediately obvious. The only problem is that anything radioactive enough to do such a thing would have long since decayed while it was drifting through space. So we'd probably want to file that under aliens as well.
(Score: 2) by HiThere on Wednesday February 08 2017, @05:51PM
You seem to have left out a near-by nova. A slightly more distant supernova would leave more visible evidence, though it could also do the job, but nobody seems to have reported on such.
OTOH, 7000 years (plus a few to allow for lag) doesn't give the nova much time to get out of the area, and I'm not sure how close it would need to have been.
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(Score: 2) by Fishscene on Wednesday February 08 2017, @09:21PM
Your explanation was far more informative than the article. Thanks!
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(Score: 2) by dry on Thursday February 09 2017, @04:42AM
>Trees don't care about the particular isotope of carbon they're using to grow new wood
Actually they do, with a preference for lighter carbon isotopes. This preference is well known so I'd assume the study took it into account. Wiki has a condensed quick read on it and how the different types of photosynthesis result in different ratios of C13 and C12. https://en.wikipedia.org/wiki/Isotopic_signature#Carbon_isotopes [wikipedia.org]
BTW, another possibility for the production of more C14 is a weakening of the Earths magnetic field. Unlikely but possible.
(Score: 2) by Immerman on Friday February 10 2017, @03:35PM
Interesting. That link doesn't specifically mention a C14 bias, but if C12 is in fact preferred over C13 because it's lower mass facilitates transport through the plant's tissues, then it's reasonable to assume C14 would be even more strongly selected against.
Yeah, I considered mentioning the Earth's magnetic field, but so far as I'm aware there's no known mechanism for the magnetic field to weaken dramatically in a single year. But I suppose it's still a poorly understood area of science, so it might be possible.
(Score: 2) by dry on Saturday February 11 2017, @04:46AM
I used to assume that different isotopes didn't matter in chemistry until I started reading up on it and was somewhat surprised that it did matter though it actually makes sense. Another variable is the total spin of a molecule.
As for the magnetic field, there is evidence that the magnetic field (pole) reversing happens fairly quickly, but it leaves evidence and with the lack of evidence, it's unlikely but not impossible.
(Score: 2) by Immerman on Sunday February 12 2017, @02:38PM
If you happen to know of decent article on the mechanisms of those chemical biases, I'd love to read it.
As for the field changes, quickly on a geological scale, certainly. But fast enough to let them pinpoint the particular year of a sudden crash in tree rings? If so, that's...kinda scary to even consider.
Then again, I suppose we already have some relatively "dead"zones in the field, even a few extra poles. Those can change quite a bit on a yearly basis, and I imagine the area immediately below the "Northern lights" of a pole probably sees a far more dramatic change in ambient C14 levels than most of the planet's atmosphere. We are dealing with only one sample here, so all sort sorts of localized anomalies
(Score: 2) by dry on Monday February 13 2017, @07:08AM
I've just read bits here and there. Started when I was curious about deuterium, and discovered that trying to live on D2O will kill you, mostly IIRC, as it doesn't cross membranes the same as H2O. It is a subject I find interesting especially as I started with the default understanding that chemical reactions didn't care about isotopes.
As for the magnetic reversals, as far as I understand, the geological record is too smeared to really say how fast they happen. The good news is that there doesn't seem to be any extinction events associated with them. As usual, the more we learn the more we find we don't know.