younger C ages.
Regardless of their origin, these fluctuations necessitate calibration of the 14C ages. By calibration, the chronologically irrelevant conventional age is converted into a calendar age. This is achieved by calibration curves in which conventional 14C ages are plotted versus the true calendar ages. For the Holocene, this is done with high accuracy by dendro-calibration. For the Late Pleistocene, other archives than tree-rings are needed due to the scarcity of trees during glacial climate. Annually varved sediments and shallow-water corals, which can be independently dated with uranium series techniques, were used for this purpose. The results of such efforts were presented as INTCAL98 curve (Stuiver et al. 1998), which reaches back to 24,000 calendar years, and the subsequent IntCal04
(Reimer et al. 2002). In order to extend the calibration back to 50 ka, several attempts were undertaken, using lake sediments, corals from uplifted marine terraces, submerged speleothems, and deep-sea sediments (cf. Bard et al. 2004). In this early period, the conventional 14C ages are consistently younger than the true ages by 4-5 ka. A particular complication is posed by the strong 14C fluctuations that are associated with the magnetic minima during the Laschamp and Mono Lake geomagnetic excursions around 40 and 33 ka, respectively, which actually prevent reliable 14C dating for that period (Conard and Bolus 2003). In order to distinguish calibrated 14C ages from conventional ones, they are characterized by the notations "cal bc'' (calendar years bc), "cal ad'' (calendar years ad) or "cal bp'' (calendar years before 1950 ad).
Apart from the temporal fluctuations, there are spatial 14C nonhomogene-ities in the materials and reservoirs participating in the carbon cycle. One distinguishes isotope fractionation and reservoir effects. Photosynthesis, for instance, enriches the light 12C over the heavy 13C, and in turn the latter over the even heavier 14C, so that the carbon in plants is isotopically lighter than in the atmosphere. For most materials, the age corrections that result from isotope fractionation are less than 80 a but may amount up to several hundred years, as in the case ofmarine limestones and organisms. The reservoir effect deals with the isotopic variation of carbon within the reservoir from which the organisms extract their carbon. Such spatial changes may have various causes. If the carbon stays long—with respect to the 14C half-life—within the same reservoir, the 14C concentration declines ("aging" of carbon). A prominent example is the "marine reservoir effect'' in the oceans where upwelling regions have apparent 14C ages around 400 a. Also the admixture of "aged" carbon lowers the 14C concentration, such as "hard-water effect'' in carbonaceous ground and surface waters. Reservoir effects result in an apparent increase of the 14C age. They are difficult to assess.
As for the other radiometric dating systems, the 14C system also has to remain closed, i.e., carbon must neither enter nor leave the sample. The 14C age is lowered by uptake of recent carbon. Common sources of contamination with recent carbon are the presence of rootlets, humic acid infiltration, and bioturba-tion. The danger of contamination by modern carbon is the greater, the smaller is the authigenic 14C amount and the older the sample. For this reason, the applicability of 14C dating at high ages is limited by unavoidable contamination rather than by the instrumental capabilities of 14C detection. The upper dating limit (maximum age) is reached when the 14C of an old sample cannot be discriminated with sufficient statistical confidence from the background. The AMS technique has the great potential to lower the instrumental background and thus to extend the maximum age but effectively is limited by the 14C background due to contamination.
The amount of sample required for 14C dating depends on the carbon content, the conditions of preservation, the degree of contamination, and the technique of 14C detection. For b-counting, either in gas or liquid scintillation counters, 5-10 g of extracted carbon usually is needed. The AMS technique requires carbon in the milligram range. Note that the quoted amounts refer to carbon and not sample. The required amount of the latter one might be larger by a factor of 10 or so, depending on the carbon content.
Bones and antler are the most frequently used paleoanthropologic sample materials for 14C dating. As long as their inorganic fraction was used, bones were considered as a problematic material for 14C dating due to open system behavior. However, their organic substance consisting predominantly of various proteins, generally classed as collagen, is more resistant to exchange. The collagen is chemically extracted as acid-insoluble residue and is then usually subjected to AMS analysis. In an extensive program, Conard and Bolus (2003) dated numerous animal bones, the majority of them with anthropogenic modifications, from several Upper Paleolithic sites in the Swabian Jura, Germany. Among these sites were the famous Vogelherd and Geissenklosterle caves with rich finds of small figurines and flutes made of bone. The conventional 14C ages indicate that in this area, the Aurignacian spans the period between 40 and 30 ka BP and the Gravettian was well-established not later than 29 ka bp. This chronology—which actually is supported by luminescence dating of burnt flint (Richter et al. 2000)— seems to be significantly higher than those at other European sites with comparable Upper Paleolithic assemblages. However, the true chronostratigraphic position and duration of the Aurignacian cultural group cannot be directly deduced from these data due to the strong global fluctuation of the atmospheric 14C level around this period. Unfortunately, this "Middle Paleolithic Dating Anomaly'' coincides roughly with the period of the Upper/Middle Paleolithic transition and the arrival of modern humans as well as the extinction of the Neanderthals, and thus seriously hampers the chronological solution of the basic questions connected with these culture and population changes (Conard and Bolus 2003). Commonly it is believed that the beginning of the Aurignacian is associated with the arrival of modern humans and among the best evidence appeared to be the presence of modern human skeletal remains within the Aurignacian layers at the Vogelherd cave. 14C dating of these bones, however, revealed that they are Holocene intrusive burials within the Paleolithic levels, leaving open whether modern humans indeed produced the Aurignacian artifacts (Conard et al. 2004).
Another important dating material is charcoal, as for instance at El Castillo cave, Spain. Charcoal samples were taken from different parts of the lowermost beds containing Aurignacian artifacts. The mean 14C-AMS age of the three samples amounts to 38.7 ± 1.9 ka bp (Valdes and Bischoff 1989).
Limnic sediments form an important archive for the climatic fluctuations of the past and if they contain organic matter 14C dating can be directly applied, in particular to peat and sapropel. An excellent material for AMS 14C dating are macrofossils, such as nutlets, fruit scales, and leaves, from the sediments. Such fossils are short-lived and free of the hard-water effect. When dating secondary calcareous sinter, the uptake of a certain fraction of "dead" carbon from geologically old limestone must be taken into account, which lowers the reliability of such dates. When using mollusc shells, the marine or hard-water reservoir effect, depending on the habitat of the molluscs, need to be considered. Paleolithic rock paintings commonly contain organic material, such as charcoal, carbonized plant matter, pigments, plant fibers, blood, fatty acids, and beeswax, which enables 14C dating. The recently discovered rock paintings at Chauvet-Pont d'Arc, France, gave 14C ages ~31,000 a bp, using microsamples of charcoal from the paintings (Clottes et al. 1995), and thus belong the earliest examples of prehistoric rock art so far discovered.
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