# What exactly is delta-14C and why are there periodic plateaus in the delta-14C curve?

I downloaded the IntCal13, IntCal09, and IntCal04 datasets (parent site), and when I plotted the last 24k years was surprised to see a plateau or even reversal appear every ~5300 years. Apparently we are near the end of one of these periods now:

Two questions:

1. Is there any term for these periods during which the trend flattens or even reverses?
2. I may be missing something basic about what the delta-14C values represent, basically I thought it was the same equation used here. Is that correct?

Edit:

I came back to this as a reference and found that unfortunately radiocarbon.org seems to be down. There is a new site here, but the data is all still hosted at the radiocarbon.org domain...

• How is 14C made on Earth? What might influence that process? Commented Mar 20, 2019 at 13:02
• @JonCuster You can see I tagged this with sun, cosmic rays, and geophysics (closest to geomagnetism I saw). Besides an increase in 14C production, such net 14C increases could also be due to less "old" carbon being released from oceans/etc. However, my question is about whether someone has attached significance to this periodicity and what specifically they have proposed is happening every ~5k years. Commented Mar 20, 2019 at 13:37
• the half life of C14 is 5730 years. I wonder if what we see if this this break down?
– Rick
Commented Apr 2, 2019 at 23:26
• @Rick That is an interesting point. I am not sure what to make of it though. Commented Apr 3, 2019 at 2:25
• You could try multiplying by data by $2^{t/\lambda}$ to get rid of the exponential decay ($\lambda$ is the half-life) and then take the Fourier transform to see what those periodic components look like. Commented Apr 6, 2019 at 7:48

Delta 14C is just the difference in carbon-14 from modern carbon-14 levels one finds in plants that died at some date in the past. Delta 13C is the same for carbon-13. The difference is that carbon-13 is stable and carbon-14 is not.

The reason the Delta 13C has waves is because the amount of carbon 14 in the air depends on the amount of cosmic rays before that time, but the amount of cosmic rays goes up and down (mostly due to sun activity). Another isotope that depends on cosmic ray abundance in the atmosphere is beryllium-10.

Carbon 14 has a half life of about 6000 years. This accounts for the steep slope in the graph, compared to that for carbon 13, which is stable and so has a nearly flat graph. Be-10 has a half life of about a million years so its slope is much gentler. Here's an article about Be-10 from ice cores: https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/97JC01282

• I'm not sure if this answered the question. Are you saying you would attribute this to a ~5k year cycle in "sun activity"? Commented Apr 8, 2019 at 12:38
• There are two primary causes, sun activity and earth's magnetism. This is not attributed to me. See published literature, for example: agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2002GL014734 Commented Apr 8, 2019 at 23:37
• Interesting paper, they claim 3.5k and 7k cycles in IntCal98, which I did not find on radiocarbon.org. The 7k yr cycle is consistent with the most recent one, but when we look at the newer IntCal's that extend further back in time 5.3k fits much better. I'm not sure if +/- a few thousand years is meaningful for this type of data really. But anyway I will give you the bounty because Southon 2002 was exactly what I was looking for. Please add it to the answer to help future readers if you can, because the current content there does not really answer the question. Commented Apr 9, 2019 at 1:50
• I noticed my post may be confusing. By "the most recent one", I mean't the most recent cycle, that apparently began ~400 AD. This is ~7k years after the previous one began at ~6.7k BC. Commented Apr 9, 2019 at 2:20

That is a fascinating observation!

It appears to indicate some sort of 5300 year solar cycle?

That's news to me, but I did find reference to something called the "Bray Cycle," which is exactly half that length:

https://www.nature.com/articles/220672a0

BTW, the radiocarbon.org domain name has been taken over by a journal, but IntCal now has a website here:

https://intcal.org/

The general downward slope presumably reflects radioactive decay of 14C back into nitrogen, with a 5700 ±30 year half-life. It indicates that the rate of 14C creation in the atmosphere over the last 24,000 years has been insufficient to balance the loss of 14C through radioactive decay. The 50% higher 14C/C ratio 24,000 years ago suggests that sometime prior to 24,000 years ago a lot more nitrogen was being turned into 14C in the atmosphere. That presumably means that more cosmic rays were making it past the Sun's (presumably weaker) magnetosphere, to reach the Earth.

CORRECTION:
The 50% higher 14C/C ratio 24,000 years ago is mainly because that was (roughly) the last glacial maximum, when the amount of CO2 in the atmosphere was much lower (only about 190-200 ppmv). The processes which remove CO2 from the atmosphere during glaciations do not discriminate (much) between 14C and 12C, but on timescales which are long compared to 14C's 5700 ±30 year radioactive decay half-life, if the amount of CO2 in the atmosphere is lower, and the rate at which 14C is created from N stays the same, the percentage of 14C in the CO2 in the atmosphere must go up.