『Abstract
　The potential of magnesium isotope records from cave carbonate archives (speleothems) has been documented but remains underexplored. This is due to the limited knowledge regarding the complex suite of physico-chemical and biological disequilibrium fractionation processes affecting meteoric fluids in the soil zone, the carbonate hostrock and calcite precipitation in the cave. This study presents δ²⁶Mg data from a monitored cave in Germany (Bunker Cave) including rain water (δ²⁶Mg: -0.70±0.14％), soil water (δ²⁶Mg: -0.51±0.10％) and drip waters (δ²⁶Mg: -1.65±0.08％) sampled between November 2009 and May 2011. Field precipitation experiments, i.e., calcite precipitated on watch glasses (δ²⁶Mg: -3.56±0.26％); May 2006 to June 2010), were found to be of limited use. This is because of experimental, crystallographic and sampling artefacts. Conversely,variations in soil and drip water δ²⁶Mg over time are predominantly related to seasonal variations in water availability and air temperature affecting the subtle weathering ratio between Mg-bearing clay minerals in the soil, here mainly chlorite and montmorillonite, and the low-Mg calcite hostrock. Bunker Cave δ²⁶Mgdrip water values display a significant dependency on the air temperature outside the cave. This is because air temperature influences CO2 levels in the soil and hence rock-water interaction. For fast drip sites, the direct correlation of δ²⁶Mgsoil water and δ²⁶Mgdrip water documents a relative short residence time of the fluid in the carbonate aquifer and thus limited isotope equilibration and mixing of different reservoirs. This result is encouraging and adds new evidence to the poorly understood hydro-geochemistry of carbonate aquifers. Slow (seepage flow) drip sites display an annual δ²⁶Mgdrip water pattern that is geochemically unrelated to that of the soil water. Further research, including laboratory experiments, must focus on the complex fractionation between drip water and speleothem calcite Mg isotope record.

Keywords: Magnesium isotopes; Speleothems; Hydrogeochemistry; Calcite; Weathering』

1. Introduction
2. Cave setting
3. Monitoring, materials and analytical methods
　3.1. Monitoring program in Bunker Cave and materials analyzed
　　3.1.1. Rain and soil water
　　3.1.2. Drip site characteristics
　　3.1.3. Watch glass precipitation experiments
　　3.1.4. Carbon and oxygen isotope data from drip water and calcite precipitation experiments
　　3.1.5. Soil and drip water chemistry
　3.2. Tritium age dating of water samples
　3.3. Magnesium isotope analysis
4. Description and interpretation of δ²⁶Mg analytical results
　4.1. Rain water
　　4.1.1. Description
　　4.1.2. Interpretation
　4.2. Soil water
　　4.2.1. Description
　　4.2.2. Interpretation
　4.3. Drip water
　　4.3.1. Description
　　4.3.2. Interpretation
　4.4. Watch glass calcite precipitates
　　4.4.1. Description
　　4.4.2. Interpretation
5. Discussion
　5.1. Magnesium isotope values of rain water
　5.2. Magnesium isotope fractionation in the soil zone
　5.3. Field precipitation experiments - potentials and pitfalls
　5.4. Relevance of monitoring data for the interpretation of δ²⁶Mg time series data from speleothems
6. Conclusions
Acknowledgements
References