A comparison of U/Th and rapid-screen super(14)C dates from Line Island fossil corals

• Published in: Geochemistry, geophysics, geosystems : G3 geophysics, geosystems : G3, 2016-03, Vol.17 (3), p.833-845
• Main Authors: Grothe, Pamela R, Cobb, Kim M, Bush, Shari L, Cheng, Hai, Santos, Guaciara M, Southon, John R, Lawrence Edwards, R, Deocampo, Daniel M, Sayani, Hussein R
• Format: Article
• Language: English
• ISSN: 1525-2027; 1525-2027
• DOI: 10.1002/2015GC005893

Time-consuming and expensive radiometric dating techniques limit the number of dates available to construct absolute chronologies for high-resolution paleoclimate reconstructions. A recently developed rapid-screen super(14)C dating technique reduces sample preparation time and per sample costs by 90%, but its accuracy has not yet been tested on shallow-water corals. In this study, we test the rapid-screen super(14)C dating technique on shallow-water corals by comparing 44 rapid-screen super(14)C dates to both high-precision super(14)C dates and U/Th dates from mid- to late-Holocene fossil corals collected from the central tropical Pacific (2-4 degree N, 157-160 degree W). Our results show that 42 rapid-screen super(14)C and U/Th dates agree within uncertainties, confirming closed-system behavior and ensuring chronological accuracy. However, two samples that grew 6500 years ago have calibrated super(14)C ages 1000 years younger than the corresponding U/Th ages, consistent with diagenetic alteration as indicated by the presence of 15-23% calcite. Mass balance calculations confirm that the observed dating discrepancies are consistent with super(14)C addition and U removal, both of which occur during diagenetic calcite recrystallization. Under the assumption that aragonite-to-calcite replacement is linear through time, we estimate the samples' true ages using the measured super(14)C and U/Th dates and percent calcite values. Results illustrate that the rapid-screen super(14)C dates of Holocene-aged fossil corals are accurate for samples with less than 2% calcite. Application of this rapid-screen super(14)C method to the fossil coral rubble fields from Kiritimati Island reveal significant chronological clustering of fossil coral across the landscape, with older ages farther from the water's edge. Key Points: * Rapid super(14)C dating is accurate for Holocene-aged fossil corals * Both super(14)C and U/Th coral dates are sensitive to secondary calcite * Concordant super(14)C and U/Th dates confirm closed system behavior