Radiocarbon dating is a valuable tool for archaeologists. Crucial to obtaining accurate dates is the Northern Hemisphere Radiocarbon International Calibration Curve (IntCal20). The idea is that this calibration applies essentially universally across the Northern Hemisphere. Nevertheless, common sense would dictate that in areas where the environment has extraneous sources of carbon dioxide, it would affect the accuracy of those calibrations. Such extraneous sources include large bodies of water, tectonic plate boundaries, calderas with active volcanism, etc. Although IntCal20 provides reasonably accurate calibrations for carbon-14 measurements from samples originating in many areas, places with regional offsets are abundant enough to question the concept that IntCal20 is genuinely “international.”
Researchers who submit samples for radiocarbon dating analysis know three significant variables exist. First, the resulting laboratory measurements of 14C are not precise, with results expressed as a range of ages, usually as ± two standard deviations (or ± 2 σ), which supposedly means the laboratory-determined range of ages has a 95% chance of encompassing the actual age. This result is the uncalibrated age. Second, the archaeologist must determine how that range fits on the Radiocarbon International Calibration Curve, which is also typically expressed as a ± 2 σ range of calibrated ages. Third, the calibration curve has “waves” or “wiggles” in it. One uncalibrated radiocarbon age can cross the calibration curve at three or more places, resulting in three or more possible ranges of ages. Thus, the process of radiocarbon dating has some degree of inherent ambiguity.
Nevertheless, another source of inaccuracy often has a more significant impact than any of these problems: regional offsets. Due to these unaccounted variances, the actual ages of organic samples are frequently not within the laboratory-measured age ranges that are supposedly 95% accurate! Radiocarbon-dated samples from the Middle East originating in the second and first millennia BCE often have offsets of ± 50 to 100 uncalibrated years. Under some circumstances, such as an ancient archaeological site within the caldera of an active super volcano (Campi Flegrei), the outgassing CO2 devoid of 14C can cause measured radiocarbon dates to be hundreds to tens of thousands of years older than the actual ages! Although that is an extreme example, it illustrates how environmental circumstances can seriously affect the accuracy of radiocarbon dates.
Tectonic plate boundaries are constant sources of outgassing CO2, and the released gas has no 14C. Especially during tectonic plate instability (earthquakes), that outgassing increases. In rare instances when a meteorite impact pierced the earth’s oceanic crust, having a hydraulic effect on the planet’s liquid mantle, that instability could be worldwide and gradually diminish over decades or even centuries. Food plants grown in the soil near those plate boundaries would have deficient levels of 14C, and radiocarbon dates would appear much older than reality. If two sites seem to have artifacts of roughly the same age, but the radiocarbon dates of one are seemingly much older than the other, check to see how close the “older site” is to a tectonic plate boundary or a major fault radiating from that boundary.
Marine environments require a separate calibration curve. Organic materials from oceans and seas have much lower ratios of 14C:12C & 13C, typically making them appear more than four centuries older if compared to contemporaneous terrestrial samples and measured against the same calibration curve.
Common sense should tell us that the lower ratios of 14C in large bodies of water would have some influence on terrestrial 14CO2 levels in areas near and downwind from them. This effect would be especially influential in regions with strong upwelling currents or if seafloor tectonic plate boundaries are near those shores.
The list of causes of regional offsets is quite long. A few of them make apparent radiocarbon ages too young, but the majority have the opposite effect. When archaeologists receive their laboratory reports for their organic samples, they typically consider none of these potential causes of offsets. They think, ‘Radiocarbon dating is science. The results are unquestionably accurate.’
For more information on this topic, see this author’s forthcoming Scientific Explanations for Radiocarbon Offsets at Astronomically Dated Sites, Volume I.
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