BP means “Before Present”, but in the radiocarbon world “present” is defined as the year 1950. This is why 4000 calBP is 2050 BC.

The horizontal axis is known as “cal BP” (or “cal BC” for the top horizontal axis), that is the calibrated or true calendar date which we know (particularly in this part of the graph) from tree-rings, since the calibration curve is simply the comparison of the radiocarbon age of a sample of precisely known age.

The vertical axis is the the radiocarbon age of any given sample.

The colour data points are the different data that makes up the curve. The black lines define the standard deviation of the data, and as such represent the uncertainty in the calibration curve measurements.

So if you want to know the true age and hence date of an artifact you would radiocarbon date it and get an 14C BP age. You would then find this value on the vertical axis, then move horizontally until you reach the black lines of the curve, then read of the calibrated age on the horizontal axis.

As an example, say I got an age of 4300 14C BP for a sample, I would see that this hits the black lines at around 2900 BC. However, you will note this example gives a nice accurate result since the curve is quite linear at this point, but if you were to find the radiocarbon age to be 4500 14C BP, the result would be less accurate since it falls in a part of the curve that is quite flat but with much variability, so the date could be between around 3100-3350 BC. In some cases you could even get two (or more) ranges of dates.

These are simplified examples, and the practice it a bit more complicated, since the actual radiocarbon age will have an uncertainty +/- with it, which means you have to consider the whole range of radiocarbon age and determine where it meats the curve.

The link that Steve provided is from the IntCal13 resource page of the journal Radiocarbon. It can be found here http://www.radiocarbon.org/IntCal13.htm. The first link provides the graphs that Steve linked to. The second link provides the data (the black line) for the IntCal13 curve, which you can download, and open in excel or any other graphing package and plot the curve yourself. The other links are different calibration curves for marine samples, and for the southern hemisphere.

]]>@ Steve G: You have a home-page for that link?

Thanks all.

]]>so OH darn it . ]]>

I am not aware of any other sudden radiocarbon excess events other than what has been highlighted in the literature, which at this moment in time consists of the confirmed events of AD 775 and AD 994. The goal of course is to try to find more since this would help to synchronise ice core and tree ring records, as well as better dating of history where in some cases historical documentation records possible celestial events.

]]>“And this “decade” thing – what is THAT all about? A C14 date comes back from the lab with a single number age and a +/-. WHat is this about decades?”

The decade thing comes from the radiocarbon calibration graphs. When they state in the article

“The problem, however, is that the tree-ring data is only available in blocks of decades rather than year by year. The paper proposes a cutting-edge mathematical method to filter out particular years within such a block when ‘change points’ in radiocarbon levels occurred.”

This is true. The radiocarbon calibration curve is constructed not by the radiocarbon dating of contiguous individual tree rings, but rather by the dating of blocks of 5 or 10 tree rings in succession. While it would be fantastic to have a calibration curve consisting of contiguous annual sampling from all tree ring records, the practice is prohibited by time and cost. So a single data point on the curve is equivalent to the mean age of the mid-point of the 10 year block of tree-rings.

You are mostly correct though that when you date a sample, the lab will return a radiocarbon age with a +/-. Using this age and uncertainty you then compare it to the radiocarbon calibration curve to find its real age range. For some points of the curve the real age range can be large and you can sometimes get occasions where the age range can extend over two or more non-overlapping time periods, by which you can then assign a statistical weighting, or confidence interval to each age range.

With regards the detection of the two events at AD 775 and AD 994. These were detected by looking at the delta 14C data. The period containing the AD 775 event was initially detected because of previous studies that highlighted 3 periods of radiocarbon enrichment (two of which had been investigated but the increase was not dramatic over a single year, the latter being the AD 775 period which had a sudden increase between 774 and 775). To put it into context, the AD 775 event is the largest rise in radiocarbon in the last 3000 years. I had seen the same step result in the data obtained on Irish oak in radiocarbon lab in Queen’s University, Belfast some time before the Japanese cedar results were published in Nature. The Japanese had rightfully beaten Queen’s to the punch.

The AD 994 event was detected solely on the basis of looking at the delta 14C data period AD 605-1015 at annual resolution. The AD 994 event is much weaker than the AD 775 event (about 60% weaker), which may explain why it is not as obvious in the coarse decadal IntCal data.

If we can probe the radiocarbon curve at annual resolution through the entirety of human history, what other events might we find? Hence the proposal in the recent paper for a Gaussian analysis of the radiocarbon curves to try to tease out some information in lieu of annual resolution data.

With regards the most recent paper, the proposal for anchoring dates is quite straight forward. The first step is the most difficult and requires the identification of sudden radiocarbon excess events in the tree ring record. This is essentially the same as identifying the events in the radiocarbon calibration curve. So ideally this would be achieved by radiocarbon dating contiguous annual rings. As mentioned before, while desirable it is not practical to do this, hence why the proposal of using statistical methods to tease out possible events from the radiocarbon calibration curves. Having identified (and presumably confirmed that such an event is real through annual dating at these points), we then require archaeological samples to test. Ideally these would be samples consisting of a series of annual horizons (i.e. wooden timbers), a one of sample of mixed aged materials would not cut it. So you would find the approximate age of the sample (through approximate radiocarbon dating). If it happened to bracket (or be near to a sudden radiocarbon event), you could then examine each annual layer and ascertain if it contains the same radiocarbon excess. If it does then one can then precisely date that layer. In fact the Gaussian approach seems to identify periods of periods of radiocarbon depletion (or inert carbon enrichment), so even this could be used as a dating signature in principal. As an example, the Gaussian model has identified a possible depletion in 676 BC, and an enrichment event in 656 BC, so if one were to find a piece of timber that has 50 rings and dated to around this time period, one might date the individual rings and find the depletion-enrichment signature and then determine when the last ring of the sample grew and hence get a precise (and hopefully more accurate date) than radiocarbon dating (even by wiggle match) could provide. Well that’s the dream.

Indeed, Mike and I are looking at some of the dates that are mentioned in the paper. If you email me your email address I can supply the papers for you (JonnyMcA@hotmail.com). ]]>