HR: 1330h
AN: V32C-1045    [PDF]
TI: The Enigma of the Ubiquity of $^{14}$C in Organic Samples Older Than 100 ka
AU: * Baumgardner, J R
EM: baumgardner@lanl.gov
AF: Los Alamos National Laboratory, MS B216, Los Alamos, NM 87545 United States
AU: Humphreys, D R
EM: drhumph@swcp.com
AF: Institute for Creation Research, P.O. Box 2667, El Cajon, CA 92021 United States
AU: Snelling, A A
EM: aasnelling@ozemail.com.au
AF: Geo-Research Pty Ltd, P.O. Box 1208, Springwood, Qld 4127 Australia
AU: Austin, S A
EM: saustin@icr.edu
AF: Institute for Creation Research, P.O. Box 2667, El Cajon, CA 92021 United States
AB: An early expectation of the accelerator mass spectrometry (AMS) technique was that the dramatically improved AMS precision relative to beta counting methods would roughly double the range for $^{14}$C dating from about 40 ka (7 times the half-life) to about 80 ka (14 times the half-life). Very soon it was discovered, however, that organic samples throughout the Phanerozoic record commonly display $^{14}$C levels hundreds of times the AMS detection threshold. Thought to be contamination, AMS researchers mounted a major campaign to identify the source or sources of this $^{14}$C. Although a few minor sources of contamination were identified and corrected, the majority of the $^{14}$C in samples that should have been $^{14}$C dead given their location in the geological record appeared to be intrinsic to the samples themselves. Studies on $in situ$ production of $^{14}$C from U and Th decay indicated this source to be too feeble to account for the elevated $^{14}$C levels being detected. This situation motivated us to investigate this phenomenon in a more systematic fashion. We selected ten coal samples from the DOE Coal Sample Bank at Penn State University. These samples had been carefully collected and preserved in argon since collection. Our samples were selected for geographical diversity and to span a significant fraction of the geological record. They included three Eocene, three Cretaceous, and four Pennsylvanian coals. They were analyzed using AMS with multiple runs for high precision. After subtracting a standard background correction of 0.08 percent modern carbon (pMC), the results for these ten samples ranged from 0.10$\pm$0.03 pMC to 0.46$\pm$0.03 pMC with a mean of 0.25 and standard deviation of 0.11. In terms of radiocarbon ages, the range is between 44 ka and 57 ka with a mean of 50 ka. When averaged by geological interval, we obtained mean values of 0.26 pMC (49 ka) for Eocene, 0.021 pMC (51 ka) for Cretaceous, and 0.027 pMC (49 ka) for Pennsylvanian. These results match closely AMS determinations for coal reported in the radiocarbon literature by other investigators. This study indicates that $^{14}$C levels in coal are indeed well above the background correction commonly used today and that there is little if any dependence of $^{14}$C level on position in the geological column. We conclude the phenomenon of intrinsic $^{14}$C in organic material throughout the Phanerozoic record is genuine and persists as an unresolved research question.
DE: 1035 Geochronology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting