Accurate chronological construction for two young stalagmites from the tropical South Pacific

Abstract

Modern to Holocene tropical Pacific stalagmites are commonly difficult to date with the U-series, the most commonly used dating method for speleothems. When U-series does not provide robust age models, due to multiple sources of 230Th or little U, radiocarbon is, potentially, the best alternative. The 14C content of two stalagmites (Pu17 and Nu16) collected from Pouatea and Nurau caves in the Cook Island Archipelago of the South Pacific were measured to obtain accurate chronology for their most modern parts. The bomb-pulse soil continuum modelling indicates that bomb radiocarbon in Pu17 onsets in 1956 and reaches its maximum in 1966 CE, suggesting a fast transfer of atmospheric carbon to the stalagmite of < 1 year. The modelling for Pu17 suggests a 20% contribution from C1 - an instantaneous carbon source, which renders possible an immediate transfer of atmospheric signal into the cave. Nu16 shows a slower transfer of atmospheric carbon to the stalagmite than Pu17, with bomb radiocarbon onsetting in 1957 CE and peaking in 1972 CE. The less negative δ13C values in Nu16 than Pu17, and also the modelling corroborated this, which points out no contribution from the instantaneous carbon source. The radiocarbon age models and laminae counting age models were then spliced to achieve a single master chronology for the top part of each stalagmite. This study is an example of 14C age modelling combined with visible physical and chemical laminae counting and how it can improve the accuracy and precision of dating for otherwise hard-to-date tropical Pacific speleothems. Such accurate and precise age models pave the way to obtain sub-annually resolved paleoclimate records by further improving the calibration of climate proxy data with the current and instrumental weather parameters.

The Ernesto Cave, northern Italy, as a candidate auxiliary reference section for the definition of the Anthropocene series

“…Remarkably, Pu17 stalagmite shows a sharp and early bomb peak in 1966, with a delay of just 2.5 years with respect to the atmospheric signal. On the other hand, stalagmites characterised by thick soil cover display much damped bomb 14 C rising and much lower peak pMC values (Markowska et al, 2019: Faraji et al, 2022. Overall, the 14 C record in ER77 is one of the most prominent and little-delayed records compared to over 20 stalagmite records published worldwide.…”

“…The comparison with other stalagmite records worldwide clarifies the role of soil and aquifer in smoothing the atmospheric bomb-pulse 14 C signal. Two recent papers employed a new unsaturated zone C model which considers C decomposition as a continuum, to better understand 14 C dynamics (Markowska et al, 2019: Faraji et al, 2022. In particular, the study from two Cook Islands stalagmites from the South Pacific characterised by patchy soil cover and limited rock overburden that allowed a fast transmission of the atmospheric signal shows the highest peak pMC value (134.4) and the earliest inflection points (1956 and1957 CE) documented worldwide in speleothems (Faraji et al, 2022 preprint).…”