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ScienceWeek
EARTH SCIENCE: ON RADIOCARBON DATING
The following points are made by T.P. Guilderson et al (Science 2005 307:362):
1) Radiocarbon (14C) dating [1,2] is widely used to determine the ages of samples that are less than approximately 50,000 years old. Natural radiocarbon is mainly formed in Earth's stratosphere through the interaction of neutrons produced by cosmic rays with 14-nitrogen. However, the rate of radiocarbon production is not constant [3], nor is its partitioning among the atmosphere, terrestrial biosphere, and oceans. After local corrections [4], radiocarbon ages must therefore be calibrated to obtain ages on an absolute time scale. For decades, the radiocarbon community has adopted international calibration standards, most recently IntCal98. There are inherent limitations faced when using radiocarbon dates to derive calendar ages.
2) From modern day to 11,800 years ago, IntCal98 is based on sets of tree-ring chronologies that each cover several thousand years and together provide an annually resolved, nearly absolute time frame. These data set a quality standard against which other proposed calibration datasets can be judged. Prior to 11,800 years ago, IntCal98 is based on marine data and contains additional assumptions and uncertainties associated with the translation of marine data into atmospheric radiocarbon values.
3) The authors examine how precisely calendar ages can be determined from individual radiocarbon dates. The authors focus on the tree-ring section of the IntCal98 calibration curve. Between 0 and 8000 years before the present (B.P.), the error in this curve is often less than 20 years, and -- except for a few brief intervals -- it is less than 30 years over the past 11,800 years. But the range of statistically possible calendar ages, or calibrated age ranges, corresponding to any particular radiocarbon date can be larger or smaller, depending on where it falls on the curve.
4) The authors have linearly interpolated the Int-Cal98 curve at intervals of 20 calendar years and determined the radiocarbon dates that correspond to the calendar ages. The authors then calibrated these resampled radiocarbon ages using CALIB v4.4 [4] assuming an uncertainty of +-40 radiocarbon years, which is currently typical of routine dating (calibration 1). The authors performed a second calibration with a constant uncertainty of +-15 radiocarbon years, which is typical of the IntCal98 tree-ring data (calibration 2).
5) The authors conclude: Scientists attempting to take advantage of the available IntCal98 calibration curve to establish subcentennial resolution chronologies must become more familiar with the calibration curve and its inherent limitations. In many circumstances, radiocarbon dates on a series of carefully chosen samples will allow considerable refinement of the derived calendar ages through constraints imposed by a priori information (such as stratigraphy) or by the pattern of the radiocarbon dates relative to calibration curve variations (an approach that is sometimes referred to as "wiggle-matching"). Even with the implementation of such methods, the establishment of reliable chronologies with centennial or better resolution will require substantial diligence and the devotion of appropriate resources to overcome the inherent limitations in the conversion of radiocarbon dates to calendar ages.
References (abridged):
1. W. F. Libby, E. C. Anderson, J. R. Arnold, Science 109, 227 (1949)
2. M. Stuiver, H. A. Polach, Radiocarbon 19, 355 (1977)
3. M. Stuiver, P. D. Quay, Earth Planet. Sci. Lett. 53, 349 (1981)
4. M. Stuiver, P. J. Reimer, Radiocarbon 35, 215 (1993)
5. E. S. Deevey et al., Proc. Natl. Acad. Sci. U.S.A. 40, 285 (1954)
Science http://www.sciencemag.org
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Related Material:
PALEOCLIMATE: ON RADIOCARBON CLOCKS
The following points are made by E. Bard et al (Science 2004 303:178):
1) An accurate chronometer that covers the past 50,000 years is a fundamental tool for geology and archaeology. For example, some prehistorians believe that Neanderthals overlapped chronologically with modern humans, whereas others claim that they predated modern humans by several millennia. Accurate dating is essential to understand this crucial time in human evolution. Accurate dating is also of paramount importance in fields such as geophysics, geochemistry, and paleoclimatology.
2) The main chronometer for this time range is radiocarbon (14C), but its use is not free of perils. This is because the atmospheric 14C/12C ratio -- the starting point of the radiocarbon clock -- varies over time. To obtain accurate dates, these fluctuations must be accounted for with a calibration curve. Fossil trees have been used to produce a high-resolution tree-ring calibration, which has been extended with data from layered sediments and tropical corals. The latest "official" calibration curve, INTCAL04, was recently ratified during the 18th International Radiocarbon Conference in Wellington, New Zealand (1). It refines the previous INTCAL98 curve (2), but stops at 26,000 calendar years before 1950 A.D. ("before present" or B.P.) (3), a period beyond which no consensus was reached.
3) Several groups have tried to extend this curve from 26,000 to 50,000 calendar years B.P. The records used for this purpose include annually laminated sediments (varves) from Lake Suigetsu in Japan (4,5), corals from the uplifted terraces of New Guinea, sediments of the former Lake Lisan in Israel, carbonate deposits from a submerged cave of the Bahamas, and deep-sea sediments whose stratigraphy can be tied to the Greenland Summit ice cores. The latter method is based on correlating a known paleoclimatic event dated by 14C -- such as transient warmings and massive glacial surges that occurred abruptly over periods of decades --with its equivalent in another record (such as a Greenland ice core) that has been dated with techniques other than radiocarbon.
4) Despite these efforts, it remains difficult to calibrate periods older than 22,000 14C years B.P., because residual concentrations of 14C in such samples are extremely low (a few percent of the concentration found in modern samples). In addition, old samples have often been altered by geochemical processes. In particular, most corals that grew before the sea-level minimum at 21,000 calendar years B.P. suffered intense meteoric alteration, precluding their use for 14C calibration. The only two published reconstructions with satisfactory analytical precision and low overall data scatter are the Lake Suigetsu record (4,5) and the Bahamian speleothem (speleothems are cave carbonates such as stalagmites and flowstones). However, these two records strongly disagree. Hence, at least one of them provides an inaccurate picture of the true calibration curve.
References (abridged):
1. 18th International Radiocarbon Conference, 1 to 5 September 2003, Wellington, New Zealand. A series of papers by the INTCAL Working Group led by Paula Reimer from the Lawrence Livermore National Laboratory will be published in Radiocarbon, providing full technical details on INTCAL04 and COMPARE04.
2. M. Stuiver et al., Radiocarbon 40, 1041 (1998)
3. P. J. Reimer et al., Radiocarbon 44, 653 (2002)
4. H. Kitagawa, J. van der Plicht, Science 279, 1187 (1998)
5. H. Kitagawa, J. van der Plicht, Radiocarbon 42, 369 (2000)
Science http://www.sciencemag.org
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RADIOCARBON DATING
The following points are made by Philip Ball (citation below):
1) The upper atmosphere of Earth is bombarded with cosmic rays: fast-moving subatomic particles produced by extremely energetic astrophysical processes such as nuclear fusion in the Sun. When cosmic rays hit molecules in the atmosphere, they induce nuclear reactions that spit out neutrons. Some of these neutrons react with nitrogen atoms in air, converting them into a radioactive isotope of carbon: carbon-14 or "radiocarbon", with eight neutrons in each nucleus. This carbon reacts with oxygen to form carbon dioxide. About one in every million million carbon atoms in atmospheric carbon dioxide is carbon-14.
2) Carbon-14 decays by emitting a beta particle, transforming it back into the most stable isotope of nitrogen. But it is in no hurry to do so: the half-life of C-14 is around 5730 years. This time scale makes radiocarbon the ideal archaeologist's tool.
3) Carbon is constantly taken in by living organisms. Plants pluck it from the air and fix it in their tissues by photosynthesis. Animals consume the carbon compounds of plants and other animals. The flux of carbon through living bodies means that they maintain a more or less constant, minuscule level of radiocarbon.
4) When an organism dies, it stops acquiring new carbon, and the amount of radiocarbon it contains begins to decline through radioactive decay. Wood from a tree that died (when felled for timber, say) 5730 years ago has only half as much radiocarbon as that from a similar tree felled recently. Wood that is 11,460 years old (assuming it is somehow preserved) has only a quarter as much. So by measuring the C-14 content of ancient wooden artefacts we can deduce how old they are. The same applies to bones, to cloth and paper and animal fat used to bind pigments in cave paintings. The measurement is done in a mass spectrometer, which separates the different isotopes of carbon.
5) The American chemist Willard Libby (1908-1980) realized in 1947 that C-14 could be used for archaeological dating. Libby studied radiochemistry at Berkeley in the 1930s and subsequently worked on the Manhattan Project. After the war he joined the Institute of Nuclear Studies in Chicago, where Fermi made the first nuclear reactor. Libby and his co-workers tested their dating technique on wood and charcoal found in Egyptian graves, whose age was already well known to archaeologists from historical analysis, and on very old redwood trees that could be independently dated by counting tree rings. Libby's technique was used to date the end of the last Ice Age and the creation of human settlements in regions ranging from North America to Iraq. The invention of radiocarbon dating earned Libby the Nobel Prize in Chemistry in 1960.
Adapted from: Philip Ball: The Ingredients: A Guided Tour of the Elements. Oxford University Press 2002.
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ON RADIOCARBON DATING
The following points are made by R.E. Taylor (American Scientist 2000 88:60):
1) Radiocarbon dating depends on a chain of natural events, some having taken place in deep space long ago. The sequence begins in various parts of the Galaxy, where charged particles are accelerated to immense velocities, forming what are known as cosmic rays. A fraction of these particles eventually rain down on the Earth and strike molecules of atmospheric gas, producing neutrons. Some of these neutrons in turn react with nitrogen, (sup14)N, to form (sup14)C, which quickly combines with oxygen to form molecules of radioactive carbon dioxide [(sup14)CO(sub2)]. By the time the radioactive carbon dioxide reaches the Earth's surface, it has mixed fully with normal carbon dioxide and accounts for about one molecule in 10^(12).
2) The vast majority of this (sup14)C eventually enters the oceans. But 1 or 2 percent goes into the terrestrial biosphere, because plants absorb carbon from carbon dioxide in the air during photosynthesis. Thus vegetation, and the animals that feed on it, are tagged with (sup14)C. Living things maintain a (sup14)C content that is about equal to the atmospheric concentration because the carbon atoms that undergo radioactive decay within their bodies are continually replaced [*Note #1]. But once an organism dies and its metabolic processes cease, the amount of (sup14)C begins to diminish. The rate of decline is measured by the (sup14)C half-life, about 5730 years." [*Note #2]
American Scientist http://www.americanscientist.org
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Notes:
Note #1: Carbon-14 decays by beta emission back to nitrogen-14. Beta emission (beta-decay) is a type of interaction in which an unstable atomic nucleus changes into a nucleus of the same mass number but different proton number. In general, the change involves the conversion of a neutron into a proton with the emission of an electron plus energy or a positron plus energy. The electrons or positrons emitted are called "beta particles". (Positrons are electron antiparticles; they have the same mass as the electron, but are of opposite charge.) Carbon-14 to nitrogen-14 decay involves the emission of an electron plus energy, another fundamental particle (electron antineutrino) accounting for the energy released.
Note #2: The accelerator mass spectrometry dating method makes use of smaller quantities of material than required for conventional radiocarbon dating and extends the radiocarbon dating range to beyond 50,000 years from the approximately 0 to 25,000 years for conventional radiocarbon dating. The technique of accelerator mass spectroscopy involves the combination of a mass spectrometer and an accelerator to measure the concentration of rare isotopes such as carbon-14 at levels lower than parts per trillion. An accelerator mass spectrometer can be used to count the carbon-14 atoms from only a milligram sample of carbon, whereas the conventional radiocarbon dating method would require as much as a gram of the same material to achieve the same resolution by counting current beta particle emission.
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