May 31, 2018 | David F. Coppedge

Radiocarbon Dating Is Not Globally Uniform

Adjustment required for dates in the Levant could undermine published results by decades and impact debates about Biblical chronology.

Radiocarbon dating has become a staple method for corroborating dates of archaeological sites. Often the results appear to confirm dates arrived at by other methods. A new paper in PNAS by  Manning et al.,1 however, warns of anomalies that could undermine published dates:

We observe a substantive and fluctuating offset in measured radiocarbon ages between plant material growing in the southern Levant versus the standard Northern Hemisphere radiocarbon calibration dataset derived from trees growing in central and northern Europe and North America. This likely relates to differences in growing seasons with a climate imprint. This finding is significant for, and affects, any radiocarbon application in the southern Levant region and especially for high-resolution archaeological dating—the focus of much recent work and scholarly debate, especially surrounding the timeframe of the earlier Iron Age (earlier Biblical period). Our findings change the basis of this debate; our data point to lower (more recent) ages by variously a few years to several decades.

Readers will recall that radiocarbon dating only concerns biological artifacts within thousands of years, not millions. Because of the C-14 half-life of 5,730 years, no radiocarbon should be left after 100,000 years or more. Even so, radiocarbon dating is often looked to as one of the most reliable radioactive dating methods.

These scientists measured material from native Junipers in the Levant (region including Lebanon and Israel), calibrating it against tree rings. To their surprise, the results did not match the standard dataset used globally for radiocarbon calibration. Like they say, climate conditions apparently cause an “imprint” of local fluctuations, and “more interestingly, this offset seems to vary in importance through time.

Hezekiah praying in the Temple about the Assyrian threat (II Kings 19). Does the Bible need science to “prove” its reliability?

Their findings become important for Biblical interpretation. “Scholarly debate” has raged for years about the “high chronology” vs the “low chronology” of the Iron Age, which overlaps with the times of Kings David and Solomon in Jerusalem.

While relatively small, such an offset has substantial relevance to high-resolution 14C chronologies for the southern Levant, both archaeological and paleoenvironmental. For example, reconsidering two published studies, we find differences, on average, of 60% between the 95.4% probability ranges determined from IntCal13 [the current standard Northern Hemisphere 14C calibration curve] versus those approximately allowing for the observed offset pattern. Such differences affect, and even potentially undermine, several current archaeological and historical positions and controversies.

Complicating the picture is that the offset fluctuates. It cannot be easily solved by adding an adjustment factor.

Although, overall, the 14C offset identified here produces what may seem to be relatively small dating changes, these are revealed to be of a scale that is important for high-resolution chronological work. They are especially important for the contested and detailed chronology debates in archaeological scholarship on the southern Levant region, particularly for those focused on differences of only a few decades to ∼50 y to 100 y in recent “high” (or conventional) versus “low” chronology debates. Thus, we recommend that users must proceed with caution when dating plant material from the southern Levant with a winter to spring growing season. It also seems likely that the offset we observe fluctuates, and thus is not best compensated for via a static, systematic, adjustment.

It remains to be seen which side of the controversy will “win” with revised radiocarbon dates that include the new offset. More significant, perhaps, is the recognition of a new factor that had not been considered in previous uses of radiocarbon dating. Here we see an “unknown unknown” coming to light that changes things. What other unknowns are out there?

If radiocarbon has this previously-unknown factor that is significant enough to change interpretation of recent material, how much more could other dating methods (the ones that yield millions or billions of years) be off by significant factors that could undermine evolutionary beliefs? Scientists cannot go back millions of years to check their dates with a stopwatch. They rely on radioactive decay as a kind of stopwatch, but what if the stopwatch fluctuates due to “unknown unknown” factors? They calibrate their dates with evolutionary assumptions, but what if those assumptions are invalid? (See cartoon below.)

This new paper should remind all scientists that reliability on methods is only tentative at best, and at worst, is like a reed that can poke the side of the person leaning on it.


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