Accurate dating of stalagmites from low seasonal contrast tropical Pacific climate using Sr 2D maps, fabrics and annual hydrological cycles

David P Mattey,John C Hellstrom,Andrew Lorrey,Andrea Borsato,Silvia Frisia,Adam Hartland,Mohammadali Faraji,Alan Greig

doi:10.1038/s41598-021-81941-x

Abstract

Tropical Pacific stalagmites are commonly affected by dating uncertainties because of their low U concentration and/or elevated initial 230Th content. This poses problems in establishing reliable trends and periodicities for droughts and pluvial episodes in a region vulnerable to climate change. Here we constrain the chronology of a Cook Islands stalagmite using synchrotron µXRF two-dimensional mapping of Sr concentrations coupled with growth laminae optical imaging constrained by in situ monitoring. Unidimensional LA-ICP-MS-generated Mg, Sr, Ba and Na variability series were anchored to the 2D Sr and optical maps. The annual hydrological significance of Mg, Sr, Ba and Na was tested by principal component analysis, which revealed that Mg and Na are related to dry-season, wind-transported marine aerosols, similar to the host-rock derived Sr and Ba signatures. Trace element annual banding was then used to generate a calendar-year master chronology with a dating uncertainty maximum of ± 15 years over 336 years. Our approach demonstrates that accurate chronologies and coupled hydroclimate proxies can be obtained from speleothems formed in tropical settings where low seasonality and problematic U–Th dating would discourage the use of high-resolution climate proxies datasets.

Highlights

The South Pacific islands are highly vulnerable to the effects of both climate variability driven by El Niño–Southern Oscillation (ENSO)[1] activity and climate change
Chronologies built upon annual trace element cycles that can be precisely compared with the physical aspects of layering in speleothems are the most reliable, because peaks and troughs along sampling transects at high spatial resolution can be visually superimposed to fabrics[16,17,23,24]
Following the protocol outlined in Nagra et al.[13] we employed principal component analysis (PCA) to construct a robust chronology, and we developed an aridity index to constrain inter-annual variability

Summary

Introduction

The South Pacific islands are highly vulnerable to the effects of both climate variability driven by El Niño–Southern Oscillation (ENSO)[1] activity and climate change. Seasonal chemical cycles have confirmed the annual nature of stalagmite fluorescent laminae from monsoon Asia and indicate trace elements are a mechanism to improve dating in speleothems lacking visual or fluorescent lamination[20] These studies used unidimensional laser ablation ICP-MS (LA-ICP-MS) tracts along the stalagmite vertical growth direction and assumed that the horizontal (annual) growth layers capture changes in parent-water chemistry in a laterally continuous and homogeneous manner. In this case, peaks and troughs identifying high-frequency trace element cycles along speleothem vertical growth axis transects (the time dimension) should show the same pattern and number. Annual laminae-specific heterogeneities due to crystal fabrics and presence of nano-particulate can only be detected by two-dimensional, continuous, high resolution trace element m apping[25], which can be performed by using Synchrotron radiation-based micro X-Ray fluorescence over entire speleothem s labs[26,27]

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Conclusion