Changes in the geomagnetic field intensity, solar variability, and the internal changes of the carbon cycle are believed to be the three controlling factors of past atmospheric radiocarbon (C-14) concentrations (denoted as Delta C-14). Of these three, it is believed that the field intensity is the dominant factor. We analyze an atmospheric Delta C-14 record spanning the past 50,000 years based on previously-published Th-230/U-234/U-238 and C-14 dates of fossil corals from Kiritimati, Barbados, Araki and Santo Islands, and identify the role of the Laschamp geomagnetic field excursion on the long term trend of the Delta C-14 record. There is a general consistency between the coral Delta C-14 record and the Delta C-14 output from carbon cycle models based on the global C-14 production estimates. High-precision, high-accuracy Th-230/U-234/U-238 dates and redundant Pa-231/U-235 dates anchor the timing of this Delta C-14 record. We propose that a significant fraction of the long-term Delta C-14 trend may be due to inaccuracies in the generally accepted C-14 decay constant. The uncertainty in estimating the shape of C-14 beta spectrum below 20 keV leads to one of the greatest errors in decay constant estimates. Once the C-14 half-life is validated via redundant techniques, Delta C-14 records will provide a better opportunity to examine the roles of carbon cycle and C-14 production influences. (c) 2006 Elsevier Ltd. All rights reserved.
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