Radiocarbon Record in Anatolia

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Posted by Paula Reimer ( on January 01, 2002 at 13:06:59:

Science, Vol. 294, Issue 5551, 2494-2495, December 21, 2001

A New Twist in the Radiocarbon Tale

Paula J. Reimer*

Radiocarbon (14C) is generated in the atmosphere by cosmic rays. Its decay is widely used to date organic materials in archaeological and paleoclimate records. But radiocarbon ages can give a warped perspective of time. Without calibration to data sets with known ages, they may deviate from calendar time by up to a few thousand years at the time of the Last Glacial Maximum (about 21,000 years ago). Much effort has been invested in generating long, precise records with known ages to calibrate radiocarbon ages.

It has been assumed that for any given time period the radiocarbon concentration of the atmosphere is the same throughout each hemisphere within the error of measurement. Combined Northern Hemisphere data sets have therefore been used to calibrate radiocarbon ages of samples throughout the hemisphere (1). A few studies have identified 14C differences between various Northern Hemisphere locations (1-3). However, the uncertainty in measurement differences between laboratories can mask small regional offsets. Intralaboratory comparisons between regions have previously only covered relatively short time periods. A general circulation model found only small offsets within hemispheres at mid-latitudes (4).

On page 2532 of this issue, Manning et al. (5) provide convincing evidence that a regional, time-varying 14C offset can occur within a hemisphere. The authors attempted to match the radiocarbon ages of a "floating" tree-ring sequence (with unknown calendar age) from archaeological monuments in Anatolia (see the first figure) to the combined Northern Hemisphere data set (1). Only by excluding the 14C measurements from the floating tree-ring sequence from ~750 to 800 B.C. could they find a good match. During this interval, the sequence shows a rapid increase in atmospheric 14C that is not mirrored in the combined data set.


Figure 1. Dating archaeological monuments. The biggest of the many Gordion tombs (top) is some 300 m in diameter and about 67 m high. It is entirely manmade. (Right) Remains of the wooden burial chamber inside the mound. This structure is the oldest known wooden building that is still standing, albeit with the help of some modern steel posts and wooden supports. The logs were up to 918 years old at the time they were cut. Manning et al. show that they were felled in about 740 B.C.


Now that the tree-ring chronology is "anchored" with an uncertainty of only a few years, the ring width pattern may be used to date wood with well-preserved ring sequences from archaeological sites in the region. This independent scientific evidence may resolve several ongoing, and often intense, debates in Near Eastern and eastern Mediterranean archaeology. For example, using the new tree-ring chronology, Manning et al. reevaluated the felling dates of timbers used in the construction of palaces at Acemhöyük and Kültepe IB. Together with seal imprints and documents of King Sami-Adad I, these dates constrain the problematic Assyrian-Mesopotamian chronology to the more reliable Middle or low-Middle Chronology (5). Manning et al. are also able to add support to the standard chronology of ancient Egypt from the dating of pieces of cargo wood from the Uluburun shipwreck, although this requires confirmation from timbers from the wreck itself. Furthermore, the 3 to 5 year growth anomaly found in the tree rings of the Anatolian sequence (~1650 B.C.), which may be a result of the volcanic eruption of Thera on Santorini, reinforces ice-core evidence for an older eruption date than previously indicated (6), with important implications for Aegean Bronze Age chronology.

On page 2529, Kromer et al. (7) further explore the magnitude and likely mechanisms of 14C offsets. They report evidence that during a more recent interval of high 14C production, radiocarbon ages for trees growing in Turkey are 17 years older than those of German trees with the same calendar age. The difference in the individual measurements is not statistically significant, but the trend in the data set is convincing. The authors conclude that the different growing season of the trees in the Mediterranean compared with those in Germany is responsible for the offset, which is seen only during a solar minimum when 14C production is high.

To understand their reasoning, we must look at the mechanism of 14C production. 14C is primarily produced at high latitudes in the lower stratosphere by the collision of cosmic ray-produced neutrons with nitrogen. During periods of high solar activity, distortion of Earth's geomagnetic field by the solar wind prevents charged particles from entering the atmosphere and little 14C is produced, whereas 14C production peaks during periods of low solar activity (solar minima). The atomic 14C is quickly oxidized to 14CO2 and enters the troposphere during the late spring, a period of high stratospheric-tropospheric exchange. By the next spring, the higher 14C concentration in the atmosphere has been well mixed and diluted by exchange with other carbon reservoirs, particularly the surface ocean. The German trees, which grow mostly in the mid to late summer, take up more 14CO2 during photosynthesis than do the Mediterranean trees, which grow in the spring and early summer.

Kromer et al. suggest that the colder and wetter climate associated with the solar minima may further accentuate the different growth patterns. The regional offset between the Turkish and German trees occurs during the early stages of the cooling associated with "Little Ice Age" glacial advances in Europe (8), and the Anatolian offset occurs during a well-documented cold period in the Northern Hemisphere (9, 10).

Regional radiocarbon offsets such as that between Germany and the Mediterranean will not have a noticeable effect on most radiocarbon calibrations, but they do make a difference to high-precision chronologies (see the second figure). It will be important to establish the maximum regional offsets for other regions and for other intervals of high 14C production. Whether regional calibration data sets will become necessary or whether a correction can be made in the calibration process will depend on how well we can understand and predict these offsets.


Figure 2. The importance of regional radiocarbon offsets. Calibration of a hypothetical radiocarbon age of 2650 ± 20 14C years before the present (B.P.) (black) with the Northern Hemisphere calibration data set (blue data points) and the Anatolian tree-ring sequence (red data points). The 2 s calibrated age ranges (shaded areas) for the resulting probability distributions are 829 to 798 B.C. and 825 to 765 B.C., respectively. Calibration was done with CALIB 4.3 (11).


The confirmation of regional 14C offsets has important implications, not only for high-precision chronologies in archaeological, geophysical, and paleoclimatic studies, but also for our understanding of variations in the exchange between Earth's carbon reservoirs. These offsets will be a challenge for climate modelers to explain.

References and Notes:

1. M. Stuiver et al., Radiocarbon 40, 1041 (1998) [GEOREF].
2. P. E. Damon, Radiocarbon 37, 829 (1995) [GEOREF].
3. F. G. McCormac, M. G. L. Baillie, J. R. Pilcher, R. 4. M. Kalin, Radiocarbon 37, 395 (1995) [GEOREF].
5. T. F. Braziunas, I. Y. Fung, M. Stuiver, Global Biogeochem. Cycles 9, 565 (1995).
6. S. W. Manning, B. Kromer, P. I. Kuniholm, M. W. Newton, Science 294, 2532 (2001); published online 6 December 2001 (10.1126/science.1066112).
7. S. W. Manning, A Test of Time: The Volcano of Thera and the Chronology and History of the Aegean and East Mediterranean in the Mid Second Millennium BC (Oxbow Books, Oxford, 1999) [publisher's information].
8. B. Kromer et al., Science 294, 2529 (2001); published online 6 December 2001 (10.1126/science.1066114).
9. J. M. Grove, Clim. Change 48, 53 (2001) [GEOREF].
10. D. A. Meese et al., Science 266, 1680 (1994) [GEOREF].
11. B. van Geel et al., Radiocarbon 40, 1163 (1998).
12. M. Stuiver, P. J. Reimer, Radiocarbon 35, 215 (1993) [GEOREF].
This work was performed in part under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory, under Contract No. W-7405-Eng-48.

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