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

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 (delta(26)mg) from a monitored cave in NW Germany (Bunker Cave). The data set includes delta Mg-26 values of loess-derived soil above the cave (-1.0 +/- 0.5 parts per thousand), soil water (-1.2 +/- 0.5 parts per thousand), the carbonate hostrock (-3.8 +/- 0.5 parts per thousand), drip-water in the cave (-1.8 +/- 0.2 parts per thousand), speleothem low-Mg calcite (stalactites, stalagmites; -4.3 +/- 0.6 parts per thousand), cave loam (-0.6 +/- 0.1 parts per thousand) and runoff water (-1.8 +/- 0.1 parts per thousand) 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 alpha(Mg-cc-Mg(aq)) = -2.4 parts per thousand. A similar Mg-isotope fractionation (1000ln alpha(Mg-cc-Mg(aq)) approximate to -2.parts per thousand) is obtained by abiogenic precipitation experiments carried out at aqueous Mg/Ca ratios and temperatures close to cave conditions. Accordingly, Mg-26 discrimination during low-Mg calcite formation in caves is highly related to inorganic fractionation effects, which may comprise dehydration of Mg2+ 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. (C) 2010 Elsevier Ltd. All rights reserved.