A Comparison of U/Th and Rapid‐screen 14C Dates from Line Island Fossil Corals
Digital Object Identifier (DOI)
Geochemistry, Geophysics, Geosystems
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 14C 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 14C dating technique on shallow-water corals by comparing 44 rapid-screen 14C dates to both high-precision 14C dates and U/Th dates from mid- to late-Holocene fossil corals collected from the central tropical Pacific (2–4°N, 157–160°W). Our results show that 42 rapid-screen 14C 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 14C 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 14C 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 14C and U/Th dates and percent calcite values. Results illustrate that the rapid-screen 14C dates of Holocene-aged fossil corals are accurate for samples with less than 2% calcite. Application of this rapid-screen 14C 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.
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Grothe, Pamela R., Kim M. Cobb, Shari L. Bush, Hai Cheng, Guaciara M. Santos, John R. Southon, R. Lawrence Edwards, Daniel M. Deocampo, and Hussein R. Sayani. 2016. “A Comparison of U/Th and Rapid‐screen 14 C Dates from Line Island Fossil Corals.” Geochemistry, Geophysics, Geosystems 17 (3): 833–45. https://doi.org/10.1002/2015GC005893.