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RESEARCH PRODUCT

Hydrogeochemistry and fractionation pathways of Mg isotopes in a continental weathering system: Lessons from field experiments

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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 δ 26  Mg data from a monitored cave in Germany (Bunker Cave) including rain water (δ 26  Mg: − 0.70 ± 0.14‰), soil water (δ 26  Mg: − 0.51 ± 0.10‰) and drip waters (δ 26  Mg: − 1.65 ± 0.08‰) sampled between November 2009 and May 2011. Field precipitation experiments, i.e., calcite precipitated on watch glasses (δ 26  Mg: − 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 δ 26  Mg 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 δ 26 Mg drip water values display a significant dependency on the air temperature outside the cave. This is because air temperature influences CO 2 levels in the soil and hence rock-water interaction. For fast drip sites, the direct correlation of δ 26 Mg soil water and δ 26 Mg drip 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 δ 26 Mg drip 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.