HR: 17:00h
AN: PP54B-05    [Abstracts]
TI: Atmospheric Radiocarbon Record for the Past 50,000 Years via Uranium-series and Radiocarbon Dating of Pristine Fossil Corals
AU: * Chiu, T
EM: tcchiu@LDEO.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964 United States
AU: Fairbanks, R G
EM: fairbanks@LDEO.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964 United States
AU: Cao, L
EM: lcao@LDEO.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964 United States
AU: Mortlock, R A
EM: mortlock@LDEO.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964 United States
AU: Kaplan, A
EM: alexeyk@LDEO.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964 United States
AB: Changes in the earth's geomagnetic field intensity, solar variability, and the earth's carbon cycle control past changes in the atmospheric radiocarbon content. Of these three, the field intensity is the dominant factor. We present an atmospheric Δ¹⁴C record for the past 50,000 years using pristine fossil corals from Kiritimati, Barbados, Araki and Santo Islands. High-precision, high-accuracy dating by ²³⁰Th/²³⁴U/²³⁸U and redundant ²³¹Pa/²³⁵U methods anchor the timing of this Δ¹⁴C record. We observe a general consistency between our Δ¹⁴C record and the Δ¹⁴C output from carbon cycle models based on the global ¹⁴C production estimates and fixed or modern carbon cycle parameters, but Δ¹⁴C values derived from corals older than 30,000 years BP are considerably higher compared to carbon cycle-modeled Δ¹⁴C values. A linear component of the long-term Δ¹⁴C trend may be due to inaccuracies in the generally accepted ¹⁴C decay constant. Analysis of the published measurements of ¹⁴C half-life, combined with new measurements of the ¹⁴C beta spectrum, suggests that the commonly accepted value may be under-estimated. The uncertainty in the ¹⁴C decay constant may be large enough to shift the computed atmospheric Δ¹⁴C to overlap the Δ¹⁴C model estimates. We do not observe the large Δ¹⁴C fluctuations computed from radiocarbon measurements made in Caricao Basin cores (Hughen et al., 2004) which we believe are artifacts attributable to inaccuracies in the GISP2 age model as applied to the Cariaco Basin sediment record. These findings serve to highlight the need for a new measurement of the radiocarbon half-life.
UR: http://www.radiocarbon.LDEO.columbia.edu
DE: 1040 Radiogenic isotope geochemistry
DE: 1105 Quaternary geochronology
DE: 1115 Radioisotope geochronology
DE: 4900 PALEOCEANOGRAPHY (0473, 3344)
DE: 4916 Corals (4220)
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005