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

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.