Calibration of stalagmite trace elements with instrumental rainfall record from the Australian tropics

Stalagmites serve as valuable archives that significantly enhance our understanding of past climate and environmental changes. The trace element records preserved within stalagmites have been used to reconstruct past rainfall patterns at regional scale [1]. However, interpreting these geochemical proxies is challenging, as the functioning of the cave system, within its specific climatological and geological context, must be taken into account. Comparing instrumental climate measurements with these proxies from stalagmites that grew during the 20^th century provides an opportunity to investigate how stalagmite geochemistry responds to variations in rainfall.

In this study, we present results from a stalagmite collected from cave KNI-51, located in the Kimberley region of northeast Western Australia. Previous uranium–thorium disequilibrium dating of the stalagmite has yielded a high-precision age model (2 sd errors of ±1–2 years over much of the last century) and revealed rapid growth (1–2 mm/yr) [2], allowing for nearly annual resolution of geochemical records. We examined trace element variations related to historical annual rainfall fluctuations, retrieved from five stations near the cave area between 1915 and 2007. Comprehensive statistical analyses, accounting for stationarity and autocorrelation in the time series data, revealed significant correlations when comparing certain trace elements to both total annual rainfall and the rainfall recorded during the monsoon season (December to March). Notably, some trace elements exhibited a stronger response to rainfall occurring during the monsoon period. Furthermore, we applied rolling window correlation to assess the evolution and stability of these correlations over time, identifying intervals where the relationship between the time series appeared weaker or stronger.

The multi-annual calibration provided critical insights into how the stalagmite recorded rainfall variability through trace elements fluctuations and represents a key step in defining the response times of the cave and stalagmite "recording systems" to changes in climate and water balance in the Kimberley region. The disclosed correspondence between the instrumental rainfall record and the trace element signals encoded in the stalagmite demonstrates that rainfall time series can be successfully reconstructed from stalagmites. This marks an important milestone in the development of a calibrated trace element–rainfall transfer function, which can be applied to past stalagmite geochemical records.

[1]         S. F. Warken et al., “Reconstruction of late Holocene autumn/winter precipitation variability in SW Romania from a high-resolution speleothem trace element record,” Earth Planet. Sci. Lett., vol. 499, pp. 122–133, 2018, doi: https://doi.org/10.1016/j.epsl.2018.07.027.

[2]         R. F. Denniston et al., “Expansion and contraction of the indo-pacific tropical rain belt over the last three millennia,” Sci. Rep., vol. 6, pp. 1–9, 2016, doi: 10.1038/srep34485.