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

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 δ26 Mg data from a monitored cave in Germany (Bunker Cave) including rain water (δ26 Mg: − 0.70 ± 0.14‰), soil water (δ26 Mg: − 0.51 ± 0.10‰) and drip waters (δ26 Mg: − 1.65 ± 0.08‰) sampled between November 2009 and May 2011. Field precipitation experiments, i.e., calcite precipitated on watch glasses (δ26 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 δ26 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 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.

► Dependency of δ26Mgrain water fractionation on temperature.
► δ26Mgsoil water is controlled by different leaching rates of Mg-bearing minerals.
► Clear correlation pattern between seasonal δ26Mgsoil water and δ26Mgdrip water.
► δ26 Mg of recent calcite precipitates are influenced notably by sampling artefacts.
► New insight on residence time and mixing patterns of soil and aquifer water.