Reconstruction of past climate variability during Marine Isotope Stage 3 and the Holocene using speleothems: Special emphasis on the application of the Sr isotope system
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Abstract
Speleothems are increasingly used as palaeoclimate archives due to their potential to reveal climatic and environmental patterns for all continental areas and climate zones. They can be precisely dated using the U-series disequilibrium method, allowing for the constraint of independent age depth models for proxy records, such as δ13C, δ18O and trace elements to reconstruct palaeoclimate variability. Additionally, Sr isotopes have been increasingly applied to speleothem samples in the recent years. The Sr isotope system is used to investigate hydrological processes in the karst, as well as differences in the weathering of soil components.
Strontium isotopes have traditionally been analysed by thermal ionisation mass spectrometry (TIMS), which requires chemical separation of Sr using ion-chromatographic resins and suffer from long measurement times. Progress in laser ablation systems (LA) and multi-collector inductively plasma mass spectrometry (MC-ICP-MS) have recently allowed the combination of these two techniques to measure Sr isotopes in several materials in-situ without any chemical separation. Furthermore, sample throughput is significantly increased with this method due to reduced analysis time. In the framework of this thesis, the LA-MC-ICP-MS technique was adapted for the Sr isotope analysis of speleothems. Since this technique has numerous potential applications besides speleothems, numerous carbonate and phosphate reference materials, as well as bioapatite samples were analysed. In order to improve the data availability of suitable reference materials, which are a basic requirement for reliable analyses, eight different reference materials have been analysed for Sr isotopes. Furthermore, a customised carbonate microanalytical reference material was characterised, improving the analytical precision and accuracy for further in-situ measurements of low-Sr carbonate samples, such as speleothems.
The analysis of a unique speleothem sample from Bunker Cave (Bu2) demonstrates the potential of Sr isotopes in a multi-proxy palaeoclimate reconstruction. Stalagmite Bu2 revealed two distinctive growth phases during the early Marine Isotope Stage 3, which has been often argued to be too cold and dry to enable speleothem growth in Central Europe. Based on the analysis of stable isotopes, trace elements and Sr isotopes, we were able to show that Central Europe was in a warm and humid climate state during this time, allowing speleothems to grow and soil to form. In addition, we show that these two growth phases differed significantly from each other, with the early phase being much more humid.
To evaluate the significance of a single stalagmite stable isotope record, four Holocene speleothem samples from the Hüttenbläserschachthöhle (HBSH) were analysed for stable isotopes, trace elements and Sr isotopes. The resulting dataset was compared to the nearby Bunker Cave and revealed significant differences in the stable isotope record. While similar trends were observed in the early Holocene in both cave systems, the most recent 7 – 6 ka revealed significant differences, especially in the δ13C record. Based on the additional analyses of trace elements and Sr isotopes, it was possible to attribute these differences to increased disequilibrium isotope fractionation in HBSH. Therefore, speleothem palaeoclimate records based on a single sample should be handled with care and a multi-proxy approach should be applied to enable trustworthy conclusions.