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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 20th 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. 

The 1257 CE eruption of Mt. Samalas in Indonesia is argued to be the largest of the last two millennia in terms of global volcanic aerosol forcing, with a reduction in insolation of more than 30 W/m2 (Sigl et al., 2015, Nature, 523). Large volcanic eruptions are tied to short-term climatic shifts, including changes to monsoon rainfall (Ridley et al., 2015, Nature Geoscience, 8). In order to investigate the impact of this eruption on the Indian summer monsoon in Nepal, we analyzed at ultra-high resolution the carbon and oxygen isotopes of a fast-growing, precisely-dated aragonite stalagmite from Siddha cave in the Pokhara Valley of central Nepal (28.0˚ N, 84.1˚ E; ~850 m.a.s.l.). We micromilled the stalagmite in ~40 µm-wide traverses during the interval through the Mt. Samalas eruption (a total of 261 analyses). Studies near Siddha cave and in Kathmandu, 130 km to the southeast, reveal that amount effects of oxygen isotopes in precipitation in this region are weak, and so we rely on carbon isotopes as a proxy for rainfall. Carbon isotopes define sinusoids that appear to represent annual cycles of rainfall associated with the summer monsoon and winter dry season. The average magnitude of these cycles is ~0.3 to 0.6‰. While some ambiguities exist, the number of seasonal cycles (18-21) is within error of the years of growth for this interval as determined by U/Th dating (26±8 years). To investigate the impact of the eruption on regional hydroclimate, we detrended the carbon isotope data and then calculated anomalies in the wet and dry seasons relative to the mean of those values. The most prominent feature of the time series is two large positive isotope anomalies separated by a moderate negative isotope anomaly. We interpret these to reflect disruptions to both the monsoon and dry season precipitation regimes by aerosol forcing from Mt. Samalas. If true, then these results reveal somewhat surprising an anomalously wet monsoon season in the first year after the eruption and that seasonal sinusoids return to their pre-eruption pattern after only two years following the eruption. In order to better understand these results, we investigate this interval using the Last Millennium Ensemble, a state-of-the-art suite of climate model simulations conducted by the National Center for Atmospheric Research with the Community Earth System Model. 

Stalagmites and climate models reveal ITCZ shifts drove concurrent changes in Australian tropical cyclone and monsoon rainfall. 

Abstract. Close coupling of Iberian hydroclimate and North Atlantic seasurface temperature (SST) during recent glacial periods has been identifiedthrough the analysis of marine sediment and pollen grains co-deposited on thePortuguese continental margin. While offering precisely correlatable records,these time series have lacked a directly dated, site-specific record ofcontinental Iberian climate spanning multiple glacial cycles as a point ofcomparison. Here we present a high-resolution, multi-proxy (growth dynamicsandδ¹³C, δ¹⁸O, and δ²³⁴Uvalues) composite stalagmite record of hydroclimate from two caves in westernPortugal across the majority of the last two glacial cycles (∼220ka).At orbital and millennial scales, stalagmite-based proxies for hydroclimateproxies covaried with SST, with elevated δ¹³C,δ¹⁸O, and δ²³⁴U values and/or growth hiatusesindicating reduced effective moisture coincident with periods of lowered SSTduring major ice-rafted debris events, in agreement with changes inpalynological reconstructions of continental climate. While in many cases thePortuguese stalagmite record can be scaled to SST, in some intervals themagnitudes of stalagmite isotopic shifts, and possibly hydroclimate, appearto have been somewhat decoupled from SST.