Significant century to millennial scale fluctuations in the atmospheric concentration of radiocarbon (Δ14C) have now been observed in many pre-Holocene records. These have been attributed to changes in terrestrial or solar magnetic fields, and/or changes in the carbon cycle, particularly ocean circulation. Not all of these records agree, however, leading us to the conclusion that some or all of them may have issues with the calendar age timescale, may have suffered from alteration, or may have complications resulting from indeterminate reservoir or dead carbon effects. Previously, we presented a continuous record of atmospheric Δ14C from 45 to 11 ka B.P. based on TIMS U, Th and Pa measurements and AMS 14C ages of a stalagmite (GB-89-24-1) from a cave on Grand Bahama (Beck et al., 2001). This record revealed elevated Δ14C for the duration of growth, particularly so at circa 44 ka cal BP. Also observed in this record was a general decline in Δ14C between 26 and 11 ka B.P. from ~ 700 to ~ 100 ‰, which is considered too large to be solely a result of changes in the Earth's magnetic field intensity. Instead, we suggested that this steep decline was also related to redistribution of 14C during a mode of ocean circulation much different to the present day.
Here, we focus on efforts to reproduce and explain the millennial and sub-millennial Δ14C variations observed in this record, using an additional Bahamas speleothem sample (GB89-25-3). A robust chronology for this new stalagmite has been obtained using MC-ICPMS U and Th isotope measurements with precisions comparable to AMS 14C measurements for similar sample sizes. Comparison of high-resolution laser-ablation trace-element profiles between these two stalagmites has significantly improved our ability to constrain the age models for these speleothems. We present new data demonstrating significant coherence in the Δ14C recorded in these two stalagmites, although some disparities are also observed that may be linked to fluctuations in dead carbon fraction.
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