Fluctuating radiocarbon offsets observed in the southern Levant and implications for archaeological chronology debates

Abstract

SignificanceWe 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.

Mediterranean radiocarbon offsets and calendar dates for prehistory

“…Radiocarbon measurements on wood from these trees (19,27) thus usually show no apparent offset in the periods of reversals and plateaus in the 14 C calibration record that were identified in the Jordanian juniper time series (~1685 to 1762 CE and 1818 to 1912 CE) ( Fig. 1A) (18). This negative case observation indicates the relevance of specific growing season differences-where present, as in the Jordan example-above, and beyond, the small latitudinal gradient recognized in 14 C values (5)(6)(7)(8).…”

“…Two episodes of substantial 14 C offsets have been observed through comparisons of measurements on known-age wood from southern Jordan versus the record from central and northern Europe. These offsets appear episodic and, hence, not amenable to the application of a simple constant offset or error enlargement (18). However, the limiting factor in a number of the cases where offsets or differences are reported is the problem that, partly because they are so small, we cannot always discern the sources of variability, for example, interlaboratory variations in methods and instruments (1-4, 10, 12), versus real differences in 14 C levels from contemporary samples.…”

“…The Jordanian and Egyptian cases (16,18,23) suggest that a recurring Mediterranean region offset, versus just a few special cases exhibiting impacts of major solar minima or large-scale climate change (15,17), applies typically only for plants with growing seasons substantially offset from those standard in central and northern Europe (e.g., winter to early/earlier summer versus spring through the entire summer). This is typical of plants in many areas of the lower-to moderate-elevation Mediterranean growing under either Mediterranean or continental conditions, with rainfall/snow in autumn, winter, and spring, and growth limited by hot, dry summers (18,28). We might therefore assume that the instances of substantive offsets, of relevance, occur especially under circumstances that stretch the normal differences in the growing seasons of plants.…”

“…The potential importance of a diverging (i.e., intra-annual temporally offset) growing season for lower-elevation regions with a typical Mediterranean climate for archaeology is self-evident: Most of the major historical and archaeological centers in the Mediterranean lie in lower-elevation settings governed by such a typical Mediterranean (table S1) compared with the IntCal13 calibration curve (4) and Oxford (OxA) and Arizona (AA) data on Jordan juniper (JJ) (18). Calendar dates B.P.…”

“…In each of these cases, this offset includes the Mediterranean offset. If we maintain the same LLGPC IntCal04/09 reference value [the recent tree ring part of the two calibration curves was the same (2, 3)] and an OxCal R 0,10 test, then the OxA and AA Jordan juniper AMS 14 C offsets (18) are ( ± ) 18.2 ± 2.8 and 18.1 ± 4.1 14 C years, respectively. All these values are noticeably in a very similar range.…”