Archaeologists use radiocarbon dating to determine the age of plants and objects containing organic material by using the properties of radiocarbon, a radioactive isotope of carbon (14C). But research by Cornell University archaeologists shows that commonly accepted radiocarbon dating standards can miss the mark, thus calling historical timelines into question.
Sturt Manning and colleagues have found variations in the radiocarbon cycle at certain periods of time, affecting frequently cited standards used in archaeological and historical research in the southern Levant region, which includes Israel, southern Jordan and Egypt. These offsets of up to 20 years in the calibration of precise radiocarbon dating could be related to climatic conditions. The work has been published in the Proceedings of the National Academy of Sciences (PNAS). Premodern radiocarbon chronologies rely on standardised Northern and Southern Hemisphere calibration curves to obtain calendar dates from organic material. These standard calibration curves assume that radiocarbon levels are similar and stable everywhere across each hemisphere. Cornell researchers have questioned these assumptions.
“We know from atmospheric measurements over the last 50 years,” said Manning, “that radiocarbon levels vary through the year, and we also know that plants typically grow at different times in different parts of the Northern Hemisphere. So we wondered whether the radiocarbon levels relevant to dating organic material might also vary for different areas and whether this might affect archaeological dating.” The researchers measured a series of carbon-14 ages in southern Jordan tree rings, with calendar dates between 1610 and 1940. They found that contemporary plants growing in the southern Levant show an average offset in radiocarbon age of about 19 years compared with the current Northern Hemisphere standard calibration curve.
“Scholars working on the early Iron Age and biblical chronology in Jordan and Israel are doing sophisticated projects with radiocarbon age analysis, which argue for very precise findings,” Manning said. “This becomes the timeline of history. But our work indicates that their fundamental basis is faulty—they are using a calibration curve that is not accurate for this region.”
Applying their results to previously published chronologies, the researchers show how even the relatively small offsets they observe can shift calendar dates by enough to alter ongoing archaeological, historical and paleoclimate debates. The work should prompt a re-examination of the timeline of the archaeology and early history, particularly of the southern Levant through the early biblical period.
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