cal. ka BC

FIGURE 3.10 The atmospheric AI4C record of the last 30,000 yr derived from both dendrochronologically based wood samples and corals dated by 230Th/234U, with a 400-yr correction for the reservoir age of tropical surface waters. The coral data are shown as circles with 2ct error bars (from Stuiver and Braziunas, 1993).

versus calendar year age record of the German oak/pine chronology, and the AMS-dated varved sediments from the Cariaco Basin (Fig. 3.11). Once this section of the varved record was firmly anchored to the tree-ring series, the older varves could then be assigned calendar ages and compared to their radiocarbon age, derived from the AMS dates on forams (Fig. 3.12). In this way, the varved sediment record has been used to extend the radiocarbon calibration back to 15,000 (calendar) yr, and with farther varve studies it should be possible to push this back even farther in time. The results so far clearly support conclusions (based on the very sparse coral data) that radiocarbon and calendar ages diverged in pre-Holocene time. This evidence conflicts with several lake varve sediment studies suggesting that before -12,000 calendar yr B.P., the 14C ages and calendar ages may have converged (Wohlfarth et ai, 1995; Wohlfarth, 1996). However, in toto the lake varve studies provide an inconsistent picture, possibly due to redeposition of older material (which, when dated, makes the varve appear older than it is) or to errors in constructing the varve chronologies (Hajdas et al., 1995a, 1995b; Zolitschka, 1996b). As independent studies of corals from Barbados, the South Pacific, and New Guinea are all strongly supportive of systematically higher levels of atmospheric 14C in the period before 11,000 calendar yr B.P., this seems to fit well with the hypothesis that past geomagnetic field variations imposed a first-order control on 14C

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