Magnesium isotope time-series analyses of dolostone cave dripwater and speleothems: Proxy calibration and application

• Applicant: Dr. Sylvia Riechelmann
• Subject Area: Palaeontology ; Mineralogy, Petrology and Geochemistry
• Term: from 2017 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 374585249

Palaeoclimate data provide the background against which models, forecasting future climate changes, are calibrated. Well-dated, high-resolution speleothem deposits are instrumental in this context, but caves and thus speleothems are affected by a complex spectrum of processes, which take place in the atmosphere, the soil zone, the karst zone or the cave itself. Monitoring programmes performed in caves worldwide shed light on these processes. Recent studies have shown that the non-traditional Magnesium (Mg) isotope system is instrumental in understanding processes taking place in the soil and karst zone. This is because Mg isotope ratios provide information on rock-water interaction, water residence time and weathering processes. However, research focused mainly on caves formed in limestone (Mg-poor carbonate) caves up to now. On the other hand, dolostone (Mg-rich carbonate) caves were nearly completely unattended so far.Here, I propose to analyse the Mg isotopic composition of drip waters and speleothems (calcite and aragonite) collected in dolostone caves. All sample material needed for the proposed project is already available at the Ruhr-University Bochum due to monitoring programmes performed in the past few years. Samples are from German as well as Moroccan caves and thus, different climate settings. Soil (Mg-bearing silicates), host rock (dolostone/limestone) and speleothem material will first be analysed regarding the Mg concentration. Subsequently all samples, including drip waters with known Mg concentration due to the monitoring programmes, will be analysed for their Mg isotope composition. The obtained data will be set in context with temperature and rainfall amount data of nearby meteorological stations in order to interpret the Mg isotope variability.The objectives of this proposal include (i) the documentation of the Mg isotope variability of dolostone cave drip waters as a function of changes in the silicate versus carbonate weathering ratio and thus, changes in temperature and rainfall amount, (ii) the evaluation of a dominant factor (temperature or rainfall amount) influencing the silicate versus carbonate weathering ratio in different climate belts, (iii) the identification of the Mg isotope variability in aragonite and calcite stalagmites as changes in the silicate versus carbonate weathering ratio and thus, changes in temperature and rainfall amount and (iv) testing the influence of prior calcite precipitation (PCP) on the Mg isotope composition. These aims lead to the verification of the following working hypothesis: In case of dolostone caves, the Mg isotopes system is an adequate proxy for past climate reconstructions and the process PCP. The proposed project will lead to an advanced level of understanding of the Mg isotope system not only in dolostone caves but also in general and thus, in palaeoclimate research.

• DFG Programme: Research Grants