Cpdb

A change in 814C of only l%o (i.e., a change in 813C of 0.5%o) corresponds to an age difference of -8 yr. Consequently, if two contemporaneous samples differed in 813C by 25%o, they would appear to have an age difference of -400 years (Olsson and

Osadebe, 1974). To avoid such confusion, it has been recommended that the 13C value of all samples be normalized to -25%o, the average value for wood. By adopting this reference value, comparability of dates is possible. This is particularly important in the case of marine shell samples, which characteristically have 813C values in the range +3 to -2%o. Standardization to 813C = -25%o thus involves an age adjustment of as much as 450 yr (to be added to the uncorrected date).

This is further complicated because of the low 14C content of the oceans, which gives modern seawater an "apparent age" of 400-2500 yr (see Section 3.2.1.4b) and results in a correction in the opposite direction from the correction for fractionation effects (a value to be subtracted from the uncorrected age). For the North Atlantic region, the apparent age of seawater is ~400 yr (Stuiver et al., 1986) so the fractionation and oceanic effect more or less cancel out. In other areas, particularly at high latitudes, the oceanic adjustment is >450 yr, so the adjusted date will be lower (younger) than the original estimate, before correction for fractionation effects. Of course, for comparison of dates on similar materials within one area, these adjustments are irrelevant. However, if one wishes to compare, for example, dates on terrestrial peat with dates on marine shells, or to compare a shell date from high latitudes with one from another area, care must be taken to ascertain what corrections, if any, have been applied. Details of reservoir corrections for different locations can be found in Stuiver et al. (1986).

3.2.1.5 Long-term Changes in Atmospheric ,4C Content

Fundamental to the principles of radiocarbon dating is the assumption that atmospheric 14C levels have remained constant during the period useful for 14C dating. However, even in the early days of radiocarbon dating, comparisons between archeologically established Egyptian chronologies and 14C dates suggested that the assumption of temporal constancy in 14C levels might not be correct. It is now abundantly clear that 14C levels have varied over time, though fortunately the magnitude of these variations can be assessed, at least since the Late Glacial. Variations in atmospheric 14C concentration may result from a wide variety of factors, as indicated in Table 3.2, and it is worth noting that many of these factors may themselves be important influences on climate. It is a sobering thought that fluctuations in the concentration of radiocarbon may help to explain the very paleoclimatic events to which radiocarbon dating has been applied for so many years; there could be no better illustration of the essential unity of science (Damon, 1970).

Early work on carefully dated tree rings indicated that 14C estimates showed systematic, time-dependent variations (de Vries, 1958). Both European and North American tree-ring samples spanning the last 400 yr showed departures from the average of up to 2%, with 14C maxima around A.D. 1500 and A.D. 1700 (Fig. 3.8). These secular 14C variations appear to be closely related to variations in solar activity, as discussed further in Section 3.2.1.6 (Suess, 1980). Also seen in Fig. 3.8 is the marked decline in 14C activity during the last 100 yr. This resulted primarily from the combustion of fossil fuel during that period (Suess, 1965), causing a rapid increase in the abundance of "old" (essentially 14C-free) carbon in the atmosphere (the so-called "Suess effect").

H TABLE 3.2 Possible Causes of Radiocarbon Fluctuations

I. Variations in the rate of radiocarbon production in the atmosphere

(1) Variations in the cosmic-ray flux throughout the solar system

(a) Cosmic-ray bursts from supernovae and other stellar phenomena

(b) Interstellar modulation of the cosmic-ray flux

(2) Modulation of the cosmic-ray flux by solar activity

(3) Modulation of the cosmic-ray flux by changes in the geomagnetic field

(4) Production by antimatter meteorite collisions with the Earth

(5) Production by nuclear weapons testing and nuclear technology

II. Variations in the rate of exchange of radiocarbon between various geochemical reservoirs and changes in the relative carbon dioxide content of the reservoirs

(1) Control of C02 solubility and dissolution as well as residence times by temperature variations

(2) Effect of sea-level variations on ocean circulation and capacity

(3) Assimilation of C02 by the terrestrial biosphere in proportion to biomass and C02 concentration, and dependence of C02 on temperature, humidity, and human activity

(4) Dependence of CO, assimilation by the marine biosphere upon ocean temperature and salinity, availability of nutrients, upwelling of C02-rich deep water, and turbidity of the mixed layer of the ocean

III. Variations in the total amount of carbon dioxide in the atmosphere, biosphere, and hydrosphere

(1) Changes in the rate of introduction of COz into the atmosphere by volcanism and other processes that result in C02 degassing of the lithosphere

(2) The various sedimentary reservoirs serving as a sink of C02 and 14C. Tendency for changes in the rate of sedimentation to cause changes in the total C02 content of the atmosphere

(3) Combustion of fossil fuels by human industrial and domestic activity

These studies generated further interest in testing the assumptions of radiocarbon dating and led to hundreds of checks being made between 14C dates and corresponding wood samples, each carefully dated according to dendrochronological principles (see Chapter 10). The longest tree-ring calibration set is the German oak and pine chronology of >11,000 yr, made up of more than 5000 different overlapping tree-ring sections from living trees, medieval housing timbers, and subfossil wood excavated from river gravels (Becker, 1993). Similar chronologies have been constructed from Irish oaks (to 5289 B.C.) and from Douglas fir and Bristlecone pine in the U.S. Pacific Northwest and California (to >6000 B.C.) Other "floating chronologies," derived from radiocarbon-dated subfossil wood, have now been accurately fixed in time by matching de Vries-type 14C variations in the chronology with those observed in the well-dated continuous tree-ring records (Kuniholm et al., 1996). By being able to precisely match such "wiggles," recorded in wood from

14c Suess Effect

FIGURE 3.8 Radiocarbon variations over the last 2000 yr expressed as departures from the long-term av-erage.Trend due to geomagnetic field variation shown by curved line. Positive KC anomalies around A.D. 1500 and A.D. 1700 correspond to periods of reduced solar activity (the Sporer and Maunder Minima, S and M, respectively). At these times the increased cosmic ray flux produces more l4C in the upper atmosphere. Combustion of fossil fuel has contaminated the atmosphere with l4C-free C02, hence the large negative departures shown since 1850 (from Eddy, 1977). Note that the ordinate is plotted with positive departures lowermost, corresponding to periods of reduced solar activity.

FIGURE 3.8 Radiocarbon variations over the last 2000 yr expressed as departures from the long-term av-erage.Trend due to geomagnetic field variation shown by curved line. Positive KC anomalies around A.D. 1500 and A.D. 1700 correspond to periods of reduced solar activity (the Sporer and Maunder Minima, S and M, respectively). At these times the increased cosmic ray flux produces more l4C in the upper atmosphere. Combustion of fossil fuel has contaminated the atmosphere with l4C-free C02, hence the large negative departures shown since 1850 (from Eddy, 1977). Note that the ordinate is plotted with positive departures lowermost, corresponding to periods of reduced solar activity.

places thousands of kilometers apart, it is clear that the high-frequency variations have real geophysical significance and are not simply the result of noise in the radiocarbon chronology (de Jong et al., 1980).

By radiocarbon dating wood of known age from different regions of the world, a very consistent picture of the relationship between 14C age and calendar year age has been built up for the last -11,400 yr (Stuiver and Pearson, 1993; Pearson and Stuiver, 1993; Pearson et al., 1993; Kromer and Becker, 1993). This calibration is based on bi-decadal and decadal wood samples for most of the period, but for the last -500 yr a year-by-year analysis of wood has provided a very detailed comparison of calendar year and 14C ages (Stuiver, 1993). The 14C age is very close to the dendrochronological age (± 100 yr) for the last -2500 yr, but before that there was a systematic difference (14C underestimating true age) increasing to -1000 yr by -10,000 calendar yr B.P. (Fig. 3.9).

Suess Wiggles Radiocarbon

FIGURE 3.9 Bidecadal anomalies of AMC (%o) (left axis) in relation to the calendar year age of the wood samples analyzed. An anomaly of + l%o corresponds to the radiocarbon age underestimating the dendrochrono-logical age by 8 yr (right axis) (from Stuiver and Reimer, 1993).

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