Paper No. 9
Presentation Time: 10:20 AM
A COMPARISON OF RAPID-SCREENING 14C DATES AND U-TH DATES FROM FOSSIL CORALS: IMPLICATIONS FOR PALEOCLIMATE RECONSTRUCTION
GROTHE, Pamela R.1, COBB, Kim M.
1, BUSH, Shari L.
2, DOS SANTOS, Guaciara
2, SOUTHON, John
3, CHENG, Hai
4 and EDWARDS, R. Lawrence
4, (1)School of Earth and Atmospheric Science, Georgia Institute of Technology, 311 Ferst Drive, Atlanta, GA 30332, (2)Dept. of Earth System Science - Keck AMS Lab, UC Irvine, Irvine, CA 92697, (3)Earth System Science, University of California, B321 Croul Hall, Irvine, CA 92697, (4)Department of Earth Sciences, University of Minnesota, 310 Pillsbury Dr. SE, Minneapolis, MN 55455, pamela.grothe@gatech.edu
Paleoclimate reconstructions from the central tropical Pacific provide much-needed targets for climate models that are used to simulate future projections of ENSO variability under enhanced greenhouse emissions. To date, paleo-ENSO records rely on rare but decades-long fossil coral sequences that date to the last 7,000 years (Cobb et al., 2013). By turning to abundant but shorter (5-10yrs-long) fossil coral rubble samples, we seek to produce a more statistically robust reconstruction of central tropical Pacific climate through the Holocene. To take full advantage of the rubble archive, we need to date hundreds of samples accurately and quickly. As a first step, we test a rapid-screening
14C dating method, developed at UC Irvine (Bush et al., in press), and compare its performance against high-precision
14C and U/Th dates. This method yields ages that differ by ±1.8% (1σ) from high-precision
14C dates for young (<10,000 years old) carbonates (Bush et al., in press) and is roughly 10 times faster than U/Th dates, making it well-suited to screen samples for time periods of interest.
We compare 69 rapid-screening 14C dates with new and published U/Th dates from fossil corals spanning the last 7,000 years from the Line Islands (~4N, 160W; Cobb et al., 2013). Results show that rapid-screening 14C dates are reproducible and differ from high-precision 14C dates by 1-4% (N=3). Sixty-seven of the 69 rapid-screening 14C dates agree with the U/Th dates within the relatively large uncertainties of the calibrated 14C dates. However, two samples with U/Th ages of ~6,500 years have 14C ages ~1000 years younger. Duplicate samples from these corals yield differing U/Th ages by ~300 years, indicating geochemical heterogeneity consistent with open-system behavior. Specifically, 14C addition (via precipitation of younger carbonates) and/or U removal (via preferential leaching of U during freshwater diagenesis) could explain the observed discrepancies. While the precision of the rapid-screening 14C dates translates into multi-century age errors for samples from the last millennium, this efficient dating technique could likely replace costly and time-consuming U/Th dates for older samples. At the very least, concordance between paired 14C and U/Th dates for fossil corals provides valuable confirmation of closed-system behavior.
