RADIOCARBON CALIBRATION VIA U/TH/PA ON PRISTINE CORALS

Cosmic ray production of ¹⁴C in the upper atmosphere varies as a function of solar magnetic activity and changes in the shielding effects of the Earth’s geomagnetic field. The amount of ¹⁴C that remains in the atmosphere depends upon its decay rate and its exchange with the much larger carbon reservoirs of the ocean and biosphere. Measurements of ¹⁴C/¹²C in tree rings for the period 0 to 11,900 years before present provide a detailed calibration of radiocarbon years to calendar years. For the time interval 12,000 to 45,000 yrs. before present, ¹⁴C/¹²C analyses of varved sediments and paired ¹⁴C/¹²C and ²³⁰Th/²³⁴U age determinations of fossil corals have extended the tree-ring radiocarbon calibration to calendar years, albeit sparingly. Existing data illustrate large and sometimes sudden changes in atmospheric ¹⁴C/¹²C over the past 45,000 years compared to the 1950 reference value. Not all records are in agreement, however. Wire-line drill cores recovered from Holocene and late Pleistocene submerged coral reefs in the Caribbean and central Pacific contain pristine aragonite corals suitable for high precision radiocarbon and ²³⁸U/²³⁴U/²³⁰Th age dating as well as ²³⁵U/²³¹Pa age dating. The entire useful range of radiocarbon dating (0 to 45,000 yrs before present) is represented by samples recovered in these cores. Development of ²³⁸U/²³⁴U/²³⁰Th and ²³⁵U/²³¹Pa age dating techniques using a multi-collector magnetic sector ICP mass spectrometer simplifies the laboratory procedures for high precision age dating of corals, enabling detailed calibration of the radiocarbon time scale over the past 45,000 yrs. Originally developed for studies of the carbon cycle and solar magnetic and geomagnetic phenomena, the data provide an excellent radiocarbon calibration data set. Accelerator Mass Spectrometer (AMS) ¹⁴C/¹²C determinations have been made on the calibration samples at the Lawrence Livermore National Laboratory. The radiocarbon calibration program is available for interactive conversion of radiocarbon ages to calendar ages and is maintained at http://radiocarbon.ldeo.columbia.edu/ and updated twice per year.