Holocene period 14C dates can be calibrated by means of dendrochronology (Stuiver et al. 1993). At present, the tree-ring curve based on absolutely dated trees and a matched floating German pine chronology (Kromer & Spurk 1998), and a new recommended curve INTCAL98, has been constructed to c. 10,200 BP or 9900 cal BC (Stuiver & van der Plicht 1998). This new curve is extended through the Late Glacial using measurements of corals, dated by both U-series isotopes and 14C (Bard et al. 1993).
There is no recommended curve for the Glacial Period; nevertheless, datasets are available which contain calibration information [ILLUSTRATION FOR FIGURE 1 OMITTED]. Firstly, the U-series/14C-dated coral series has been extended by a few isolated measurements (Bard et al. 1993). Secondly, another U-series/14C dataset for speleothems is available, albeit measured with large error bars (Vogel & Kronfeld 1997), and low in time resolution. Thirdly, a 100,000-year-old varved sediment from Lake Suigetsu, Japan has produced more than 250 14C samples, which were measured by AMS, yielding a high time resolution dataset to about 45,000 BP (Kitagawa & van der Plicht 1998). These latter data were also presented at the 16th International Radiocarbon Conference in Groningen, June 1997, where it was decided which datasets would be included in INTCAL98.
The Japanese varved record is not included in this new curve because
a it is floating and has to be matched to the tree ring curve for the Holocene part; and
b there are possible problems with the chronology of the older parts (in particular before 30,000 BP, as will be discussed below) probably caused by missing varves and/or varve-counting errors.
The speleothem data has also not been included because of potential problems such as the unknown initial 14C content, and the possibility of detrital Th contamination. Indeed, the 'speleothem curve' and 'varve curve' deviate markedly [ILLUSTRATION FOR FIGURE 1 OMITTED]. Additionally, the coral data are obviously a marine dataset; a marine reservoir age must be subtracted in order to construct a calibration curve. This reservoir age is taken as c. 500 years for INTCAL98.
Recently, van Andel (1998) presented both a calibration curve back to 45,000 cal BC, based on values of the geomagnetic field strength as measured by Laj et al. (1996), and made comparisons with the GRIP ice-core chronology, in particular the Denekamp (IS-8) and Hengelo (IS-12) interstadials, in terms of the frequency of 14C-date occurrence. Two problems arise with this treatment of calibration. First, the magnetic data used for deduction of a calibration curve are not the most suitable. Second, the chronologies carry uncertainties that do not yet justify an exact calibration of the interstadials used; a precise and accurate calibration is necessary for the deduction of calibrated frequency distributions. These two points will be discussed in detail below.
The geomagnetic record used exhibits large fluctuations of magnetic grains (Laj et al. 1996). It is based on only three cores from the same Azores area, which are not dated by 14C. It is preferable to use the palaeointensity curve reconstructed from compiling records from 18 different locations (Guyodo & Valet 1996). This record averages both local mineralogical and sedimentological problems and also regional heterogeneous effects of the geomagnetic field. Together with the global 10Be flux reconstructed by Frank et al. (1997), it was also used as input for a carbon cycle model to calculate time-dependent 14C variations (Bard 1997). This geomagnetic record explains the general trend observed in the varved chronology from Lake Suigetsu. These data are plotted in FIGURE 2 as [Delta]14C (see footnote). Note that 1 per mil [[Delta].sup.14]C corresponds to 8 BP; thus 500 per mil [[Delta].sup.14]C means that 4000 BP is the difference between radiocarbon years and calendar years (deviation from the Libby line; compare [ILLUSTRATION FOR FIGURE 1 OMITTED]).
On the general trend, two pronounced peaks can be recognized at 23,000 and 31,000 cal BP, These apparent [[Delta].sup.14]C increases correspond to an increased concentration of another cosmogenic isotope, 10Be, also observed at 23,000 (Beer et al. 1992) and times ranging from 32,000 to 40,000 cal. BP, depending on which chronologies are used (see discussion below). The latter peak is probably caused by a magnetic excursion with a sharp change in the inclination of the geomagnetic field, such as those observed as the Mono Lake and Laschamp excursions dated around 28,000 and 33,000 BP, respectively (Liddicoat, 1992; Vlag et al. 1996).
This large peak was first observed in 10Be in Antarctic ice (Raisbeck et al. 1987); the magnitude is a factor of 2. Enhanced cosmic ray flux of this magnitude over a period of 2000 years would correspond to a 14C peak of about 300 per mil (Beer et al. 1992) which is exactly what is observed in the Japanese varves.
The GRIP ice core used by van Andel shows the interstadials IS-8 (Denekamp) and IS-12 (Hengelo) at 34,000 and 42,000 cal BP, respectively (Dansgaard et al. 1993). There is, however, a major problem with the ice-core chronology: the GISP2 ice core shows these interstadials at 37,000 and 45,000 cal BP (Grootes et al. 1993). Both Greenland ice cores were taken at the same time and are only a few kilometres apart (see the Journal of Geophysical Research (AGU 1997) for information on all GRIP and GISP2 science).
All problems with the chronologies have yet to be resolved, so it is dangerous to use the ice-core data for either calibration or comparison with histograms presenting a calibrated time-scale. For instance, the Denekamp interstadial is 14C dated at 30,000 BP at la Grande Pile, France (Woillard & Mook 1982). The sedimentation curve shows an anomaly near Denekamp: for a depth range of more than 1 m, all 14C dates are C. 30,000 BP. This anomaly may correspond to one of the 'plateaux' in the Lake Suigetsu dataset. A plateau is defined as a horizontal stretch in the calibration curve - such as, for example, the Hallstart and Younger Dryas plateaux. Such plateaux follow peaks (increases) in the atmospheric 14C level. In frequency diagrams, plateaux correspond to clusters of (non-calibrated) 14C dates. It is not clear why there seems to he a clustering effect in calibrated dates such as shown by van Andel (1998: [ILLUSTRATION FOR FIGURE 4 OMITTED]). Considering the information available, it is premature to assign frequency maxima to climatic optima, especially since the calibration curve used by van Andel (1998) is smoothed, whereas we know that significant peaks or 'wiggles' exist for the period considered.
The calibration curve from Lake Suigetsu is the best terrestrial record available at present, but it is not perfect, especially for 30,000 years ago and beyond. In FIGURE 2, an error estimate is shown of 2000 years at 40,000 cal BP. This is due to either miscounting of varves, or varves missing from the sediment. We consider all data beyond 20,000 cal BP as minimum ages (Kitagawa & van der Plicht 1998).
Many 'missing' varves could bring the Lake Suigetsu data into agreement with the U/Th data. The 14C peak at 31,000 cal BP is likely to have been caused by the Mono Lake magnetic excursion, which temporarily increased the production of cosmogenic isotopes such as 14C in the upper atmosphere. An increase in the production of another cosmogenic isotope, 10Be, has recently been observed at several sites during the last decade: at 35,000 cal BP in the Antarctic Vostok ice core (Raisbeck et al. 1987), at 32,000 cal BP in marine sediments in the Gulf of California (McHargue et al. 1995), at 35,000 cal BP in the Mediterranean (Castagnoli et al. 1995) and at 40,000 cal BP in the GRIP Arctic ice core (Yiou et al. 1997). Marine sediments from the North Atlantic dated by 14C have also shown the Mono Lake event (Voelker et al. 1998); but the chronology of this dataset is derived from GISP2. Finally, another independent cosmogenic isotope, 38Cl, peaks at 38,000 cal BP in the GRIP ice core (Baumgartner et al. 1998). In addition, new 10Be data from a marine core from the Caribbean (Aldahan & Possnert 1998) show peaks at 23-24,000, 37-39,000 and 60-65,000 cal BP, in accordance again with the Antartic ice core (Raisbeck et al. 1987).
All this information shows that the chronologies for the various ice cores, marine records and terrestrial varved sediments deviate from each other by several millennia before 30,000 years ago. A remarkable amount of progress has been made in the last few years, but before we can deduce detailed environmental/prehistoric conclusions in calendar time, the correlations must be improved significantly. Thus, a warning to prehistorians: it is still too early for detailed calibration of the Middle/Upper Palaeolithic. The calibration curve as shown by van Andel, in any case, is too simple and not necessarily correct.
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