“Extraordinary” Radiocarbon Anomaly Found in Tree Rings

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Posted on January 19, 2017 in Amazing Facts, Dating Methods, Geology, Philosophy of Science, Physics, Solar System

A tree ring sample from a bristlecone pine reveals something weird happened to the sun around 5480 BC.

This paper’s title should set off alarms for radiocarbon dating: “Large 14C excursion in 5480 BC indicates an abnormal sun in the mid-Holocene” (PNAS). For one thing, Christians might celebrate the apparent trend away from designating dates BCE (“before Common Era”) and back to BC (“Before Christ”). But what about this “abnormal sun”? And what does the carbon-14 anomaly mean for radiocarbon dating?

Carbon-14 contents in tree rings tell us information of the past cosmic ray intensities because cosmic rays produce 14C in the atmosphere. We found a signature of a quite large increase of incoming cosmic ray intensity in the mid-Holocene (the 5480 BC event) from the measurement of 14C content in North American tree rings. The cause of this event is supposed to be an extremely weak sun, or a combination of successive strong solar bursts and variation of a solar magnetic activity. In any case, 14C variation of the 5480 BC event is extraordinary in the Holocene, and this event indicates the abnormal solar activity compared with other periods.

The paper does not quantify how “extraordinary” this “large” increase was, other than to say it “one of the largest increase rates (0.51‰/y) in the Holocene,” faster even than the Maunder Minimum and two other known excursions. Unfortunately, they do not state what it means for published radiocarbon dates. The authors mainly focus on the fact that tree ring dating from this slice of California bristlecone pine provided finer resolution than previously-known anomalies—and may be indicative of other rapid changes in radiocarbon production in the atmosphere arising from different mechanisms. Readers may not be aware of the unknowns in this dating method:

The 14C contents in tree rings are normally affected by the solar magnetic activities and the geomagnetic activities, which modulate the GCR flux [galactic cosmic rays] to Earth. There is an excellent tree ring record of 14C data in the international radiocarbon calibration curve IntCal. This record has a typically 10-y resolution extending to 13,900 y B.P. We can see solar and geomagnetic variations exhibited in the radiocarbon record as decadal to millennial time scale, i.e., 50- to 100-y variation such as grand solar minima, and ∼1,000-y variations of the geomagnetic dipole moment.

On the other hand, there is little understanding of annual 14C variations, due to the lack of annual 14C data for periods before AD 1510. Previously, it was considered that annual variations of 14C contents do not change rapidly because the original signal is diluted and attenuated by the carbon cycle. Although most of annual 14C data show a gradual variation, there are some periods that show significant and rapid annual changes. The AD 775 and AD 994 (or AD 993) events are two examples of large changes, which occur at annual resolution. The 14C variation of these two events have a characteristic increase over 1 y to 2 y followed by a decay that reflects a rapid input of cosmic rays to the atmosphere within 1 y and the decay by the global carbon cycle. The most likely explanation of these events is that they were the result of extreme SPEs [solar proton events], based on verifications of annual 14C measurements using worldwide tree samples and annual 10Be measurements in ice cores from Antarctica and Greenland. It is possible that there were more annual cosmic ray events like the 775 event and even other types of annual rapid 14C variation in the past.

What’s notable is that this “extraordinary” and “large” anomaly was just discovered after decades of trust in radiocarbon dating. Experts have their calibration curves, which are supposed to tell technicians how to account for anomalies. Now, here’s another “excursion” away from the norm. Some process, whether the sun or galactic cosmic rays,  may have produced more 14C than expected that year (however reliable the date 5480 BC can be regarded). If so, it seems likely that radiocarbon dates would be off by some unspecified amount. Perhaps it is not a big adjustment. But how many more excursions will surface in the future? What other unknown unknowns give false confidence in this highly-regarded dating method? Their last sentence sounds a warning: “In any case, the 14C variation of the 5480 BC event indicates an unprecedented anomaly in solar activity compared to other periods.

We alert interested geophysicists to this paper and solicit comments. Does it having any significant bearing on radiocarbon dates?

 

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