Slashdot Mirror
Carbon Dating Gets an Update
ananyo writes "Climate records from a Japanese lake are set to improve the accuracy of carbon dating, which could help to shed light on archaeological mysteries such as why Neanderthals became extinct. Carbon dating is used to work out the age of organic material. But the technique assumes that the amount of carbon-14 in the atmosphere was constant — any variation would speed up or slow down the clock. Since the 1960s, scientists have started accounting for the variations by calibrating the clock against the known ages of tree rings. The problem is that tree rings provide a direct record that only goes as far back as about 14,000 years. Now, using sediment from bed of Lake Suigetsu, west of Tokyo, researchers have pushed the calibration limit back much further. Two distinct sediment layers have formed in the lake every summer and winter over tens of thousands of years. The researchers collected roughly 70-meter core samples from the lake and painstakingly counted the layers to come up with a direct record stretching back 52,000 years. The re-calibrated clock could help to narrow the window of key events in human history. Take the extinction of Neanderthals, which occurred in western Europe less than 30,000 years ago. Archaeologists disagree over the effects changing climate and competition from recently arriving humans had on the Neanderthals' demise. The more accurate carbon clock should yield better dates for any overlap of humans and Neanderthals, as well as for determining how climate changes influenced the extinction of Neanderthals."
Maybe if you knew what a half life was you wouldn't find it so confusing?
Usually it's not just "count". Every layer must be checked to see if there are no bioturbations or other disturbances, that could hint a missing layer. Also layer thickness is measured in order to create deposition speed time model, to allow to correlate particular log with other borehole (varve) logs.
http://en.wikipedia.org/wiki/Varve
http://en.wikipedia.org/wiki/Rhythmite
Of course, I haven't RTFA, as I'm located in a "leading science university" (rectors quote) and I don't have an access to Science.
Maybe if you knew what a half life was you wouldn't find it so confusing?
His comment doesn't show that he doesn't understand the concept of a half life.
I'ts a superficialy reasonable question - if we know that C14 is decaying then it must of come from somewhere.
But his error is to assume the only place it could have come from is the decay of something else.
In fact it's generated by cosmic rays hitting Nitrogen, a beta particle (electron) is captured by N14 giving C14.
Watch this Heartland Institute video
No, they could go back further, up to 55,000 years, I think, by assuming that the C-14/C-12 ratio has always been the same. This is not quite the case, as the levels of cosmic radiation changes, so the production rate of C-14 changes. If we have an object that is known to have a certain age, we don't need to guess, we can compare directly. However, really old objects with known dates are hard to come by. You more or less need an annual cycle going up to today, so that you have an anchored chronology. Previously, tree ring data was the best anchored chronology that could be carbon dated (I assume there is too little C in glacial ice, I don't know why corals can't be used. Perhaps sea water might not always be in C equilibrium with the atmosphere?), so we could only go back 14,000 years. Now, we have an carbon-dated anchored chronology going back 52,000 years, so carbon dating gets much more accurate.
The last magnetic reversal of the poles was 780,000 years ago, 720,000 years before carbon 14 dating is useful. I doubt it has any effect.
That carbon dating has always been as accurate as you can afford. You decide the date that you need in order to confirm your thesis, send your sample to as many labs or as many times as your budget allows, then pick the closest answer from the essentially random set of results.
Anyone on the inside of the inside care to confirm or refute that?
I'm not on the inside, but I've read some of the papers.
Every few years there is an International Radiocarbon Intercomparison, where a batch of different types of samples are sent to most of the world's labs (~100) to date. The results are then compared. Overall stats are published anonymously, and individual labs can publish their results if they want.
The most accurate method (AMS) shows error rates of ~1%, while older methods give error rates of up to 10%.
Of course there are some classes of samples which present special problems; the study samples are ones which don't present major contamination issues.
The full study from 2003 is open access: here
Usually when I see a post moderated as informative, it leads me to believe it may contain information of some kind. I think this would be better characterized as insightful.
If anyone does't understand what the parent is talking about, the half-life of Carbon 14 is 5,730±40 years. That means that 52,000 years is a little more than 9 half-lives. By taking 1/2 and raising it to the power of 9, we can conclude that about 0.2% of the original carbon 14 will remain in the oldest layers of sediment.
As for the question of where the Carbon 14 is coming from, we know that it's formed by cosmic radiation striking the atmosphere, and that the amount in the atmosphere varies slightly from year to year. As this article has explained, the purpose of this research is to get a better idea of how much Carbon 14 was in the atmosphere every year so that we can get a better idea of how old a piece organic matter might be based on it's isotopic ratio (the fraction of the carbon that is Carbon 14).