Cave drip water solutes in south-eastern Australia: Constraining sources, sinks and processes

The impact of seasonal and event-based infiltration on transition metals (Cu, Ni, Co) in tropical cave drip water

RATIONALE

The first-row transition metals Cu, Ni, and Co show a strong binding affinity to natural organic matter. Compared to dissolved elements and stable water isotopes, they may be transported rapidly through the soil and host rock into caves in response to infiltration events. This study aims to assess the potential of transition metal ratios as indicators for infiltration changes in response to the seasonal and/or event-based rainfall variation.

METHODS

We developed a protocol to analyze Cu, Ni, and Co in the cave drip water using collision cell ICP-QMS without extensive sample pretreatment. The high Ca matrix leads to significant isobaric interferences on all isotope masses. Our method includes a correction of these matrix effects and yields results with comparable accuracy and reproducibility to other published methods. We applied this protocol to drip water samples from Larga Cave (Puerto Rico) covering at least two full annual cycles between 2014 and 2019 on a bimonthly scale.

RESULTS

The analysis of external reference materials yielded a reproducibility between 4.7% and 9.2% (relative standard deviation), validating the accuracy of the matrix correction method. The limit of detection is <0.04 ppb for Cu, <0.02 ppb for Ni, and <0.008 ppb for Co. The analysis of drip water samples from Larga Cave reveals pronounced changes of several orders of magnitude in all Element (El) to Ca, Cu/Ni, and Cu/Co ratios in response to seasonal infiltration changes. In addition, we observe a partly even stronger response after major tropical storms and heavy precipitation events of the period of record, for example, tropical storm "Bertha" (2014) and the category 5 hurricanes "Irma" and "Maria" (both 2017).

CONCLUSIONS

Transition metal ratios can be accurately measured in cave drip waters with high Ca matrix. At our tropical site, these are promising tracers of infiltration changes in response to changes in the amount of rainfall, providing the first step toward tropical cyclone reconstruction using trace elements in speleothems.

Application of Inductively Coupled Plasma Spectrometric Techniques and Multivariate Statistical Analysis in the Hydrogeochemical Profiling of Caves-Case Study Cloșani, Romania

The aim of the study was to develop the hydrogeochemical profiling of caves based on the elemental composition of water and silty soil samples and a multivariate statistical analysis. Major and trace elements, including rare earths, were determined in the water and soil samples. The general characteristics of water, anions content, inorganic and organic carbon fractions and nitrogen species (NO₃⁻ and NH₄⁺) were also considered. The ANOVA—principal component analysis (PCA) and two-way joining analysis were applied on samples collected from Cloșani Cave, Romania. The ANOVA-PCA revealed that the hydrogeochemical characteristics of Ca²⁺-HCO₃⁻ water facies were described by five factors, the strongest being associated with water-carbonate rock interactions and the occurrence of Ca, Mg and HCO₃⁻ (43.4%). Although organic carbon fractions have a lower influence (20.1%) than inorganic ones on water characteristics, they are involved in the chemical processes of nitrogen and of the elements involved in redox processes (Fe, Mn, Cr and Sn). The seasonal variability of water characteristics, especially during the spring, was observed. The variability of silty soil samples was described by four principal components, the strongest influence being attributed to rare earth elements (52.2%). The ANOVA-PCA provided deeper information compared to Gibbs and Piper diagrams and the correlation analysis.

Evidence of wet-dry cycles and mega-droughts in the Eemian climate of southeast Australia

Understanding past climate variability is critical to informing debate of likely impacts of global warming on weather and climate, and water resources. Here we present a near annual resolution reconstruction of climate developed from a speleothem that spans the Eemian [Marine Isotope Stage 5e (MIS 5e)] from 117,500 to 123,500 years BP—the most recent period in the Earth’s history when temperatures were similar to those of today. Using ²⁵ Mg, ⁸⁸Sr, and ¹³⁷Ba as proxies, we show the first indication of solar and teleconnection cyclic forcing of Eemian climate in southeast Australia, a region at present often affected by severe drought and bushfires. We find evidence for multi-centennial dry periods interpreted as mega-droughts, and highlight the importance of understanding the causes of these in the context of a rapidly warming world, where temperatures are now, or projected to exceed those of the Eemian.