Magnesium-isotope fractionation during low-Mg calcite precipitation in a limestone cave – Field study and experiments

• Published in: Geochimica et cosmochimica acta 2010-08, Vol.74 (15), p.4346-4364
• Main Authors: Immenhauser, A., Buhl, D., Richter, D., Niedermayr, A., Riechelmann, D., Dietzel, M., Schulte, U.
• Format: Article
• Language: English
• Subjects: Calcite; Caves; Fractionation; Isotopes; Magnesium; Mathematical models; Precipitation; Soil (material)
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/j.gca.2010.05.006

The chemical and isotopic composition of speleothem calcite and particularly that of stalagmites and flowstones is increasingly exploited as an archive of past environmental change in continental settings. Despite intensive research, including modelling and novel approaches, speleothem data remain difficult to interpret. A possible way foreword is to apply a multi-proxy approach including non-conventional isotope systems. For the first time, we here present a complete analytical dataset of magnesium isotopes (δ 26Mg) from a monitored cave in NW Germany (Bunker Cave). The data set includes δ 26Mg values of loess-derived soil above the cave (−1.0 ± 0.5‰), soil water (−1.2 ± 0.5‰), the carbonate hostrock (−3.8 ± 0.5‰), dripwater in the cave (−1.8 ± 0.2‰), speleothem low-Mg calcite (stalactites, stalagmites; −4.3 ± 0.6‰), cave loam (−0.6 ± 0.1‰) and runoff water (−1.8 ± 0.1‰) in the cave, respectively. Magnesium-isotope fractionation processes during weathering and interaction between soil cover, hostrock and solute-bearing soil water are non-trivial and depend on a number of variables including solution residence times, dissolution rates, adsorption effects and potential neo-formation of solids in the regolith and the carbonate aquifer. Apparent Mg-isotope fractionation between dripwater and speleothem low-Mg calcite is about 1000ln α Mg-cc-Mg(aq) = −2.4‰. A similar Mg-isotope fractionation (1000ln α Mg-cc-Mg(aq) ≈ −2.1‰) is obtained by abiogenic precipitation experiments carried out at aqueous Mg/Ca ratios and temperatures close to cave conditions. Accordingly, 26Mg discrimination during low-Mg calcite formation in caves is highly related to inorganic fractionation effects, which may comprise dehydration of Mg 2+ prior to incorporation into calcite, surface entrapment of light isotopes and reaction kinetics. Relevance of kinetics is supported by a significant negative correlation of Mg-isotope fractionation with the precipitation rate for inorganic precipitation experiments.