RADIOCARBON CALIBRATION USING HIGH PRECISION 230TH/234U/238U AND 14C MEASUREMENTS ON PRISTINE CORALS

Of practical importance to a wide range of scientific disciplines is the radiocarbon calibration. Radiocarbon ages spanning the last 11,900 years are calibrated to calendar years by making radiocarbon age determinations on tree rings of known age. Over this time interval, fluctuations in the atmospheric 14C were largely produced by changes in the solar magnetic field. For the age interval between 12,000 years and 50,000 yrs BP, radiocarbon ages are calibrated by less accurate methods, such as varved sediments, rapidly accumulating marine sediments, 230Th/234U/238U dating of spelothems, and 230Th/234U/238U dating of corals. In addition to solar contributions prevalent in the Holocene, carbon cycle variations contribute to atmospheric 14C fluctuations in the pre-Holocene. Combining published radiocarbon calibration data greater than 12,000 yrs. BP results in an unusable calibration data set because offsets and contradictions are the norm among these various data sets. Our contributions to the international radiocarbon calibration effort are the high precision 230Th/234U/238U and 14C analyses of a suite of unusually high quality coral samples cored from drowned reefs offshore several islands in the tropical Atlantic and Pacific Oceans. We have overlapped and extended the tree-ring radiocarbon calibration from 11,000 to 50,000 yrs. BP in enough detail to be used as a stand-alone radiocarbon calibration of the highest quality. We have measured a radiocarbon anomaly produced during the Laschamp geomagnetic excursion that confirms the K/Ar date on the Laschamp flow at 47.4 +/- 1.9 kyrs BP reported by Hall and York (1978). This age is between 15% and 24% older than recent Laschamp age estimates based on ice core and marine core chronologies. The age offset between uncalibrated radiocarbon years and 230Th/234U/238U calendar years exceeds 9200 years during the Laschamp excursion. The radioactive decay of the Laschamp 14C anomaly, compounded by several smaller anomalies, results in the long-term convergence of radiocarbon and calendar years. Conveniently, the large 14C production anomaly produced during the Laschamp excursion "spiked" the carbon reservoirs with sufficient radiocarbon to effectively extend the useful range of radiocarbon dating to eight half-lives.