HR: 0830h
AN: GP11C-0265 [PDF]
TI: Atmospheric Radiocarbon Production Between 30,000 and 50,000 Years B.P. and the Influence of
Geomagnetic Field Intensity Fluctuations
AU: * Chiu, T
EM: tcchiu@LDEO.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 205 New Core Lab, LDEO, 61 Rt. 9W, Palisades,
NY 10964 United States
AU: Fairbanks, R G
EM: fairbanks@LDEO.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 210 New Core Lab, LDEO, 61 Rt. 9W, Palisades,
NY 10964 United States
AU: Mortlock, R
EM: mortlock@LDEO.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 205 New Core Lab, LDEO, 61 Rt. 9W, Palisades,
NY 10964 United States
AU: Guilderson, T
EM: tguilderson@llnl.gov
AF: Center for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory, 7000 East Ave.,
Livermore, CA 94550 United States
AU: Bloom, A
EM: bloom@geology.cornell.edu
AF: Deapartment of Earth and Atmospheric Science, Snee Hall, Cornell University, Ithaca, NY 14853 United States
AB:
Near the limits of radiocarbon dating techniques, the Earth's geomagnetic field intensity transitioned from field strengths
about 20 percent higher than modern values to a near collapse of the geomagnetic field during the Laschamp excursion. The
production of $^{14}$C and all other cosmogenic radioisotopes increased dramatically during this transition. The time
interval between 52,000 and 38,000 years B.P. is also especially important to anthropologists due to the active migration
pathways of early man. However, the $^{14}$C production increase during this time interval produced a dramatic distortion in
the $^{14}$C radiometric clock that has confounded efforts to accurately date this interval.
Fossil corals provide a superior archive for measuring changes in $^{14}$C because they also contain the very accurate and
precise uranium series radiometric clocks, $^{238}$U/$^{234}$U/$^{230}$Th and $^{235}$U/$^{231}$Pa. We have developed a
procedure to date a single-piece coral sample by both $^{238}$U/$^{234}$U/$^{230}$Th and $^{235}$U/$^{231}$Pa methods using a
multiple-collector, magnetic sector, ICP mass spectrometry (Plasma 54). The configuration of our mass spectrometer
simplifies chemical preparation of samples and provides internal mass bias and detector gain corrections during analysis. The
corresponding $^{14}$C ages of corals were made on an accelerator mass spectrometer (AMS) at Lawrence Livermore National
Laboratory. Replicate radiocarbon analyses of progressively leached samples is an effective strategy for radiocarbon dating
samples older than eight half-lives.
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
DE: 1500 GEOMAGNETISM AND PALEOMAGNETISM
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2003 Fall Meeting