Magnesium isotope fractionation in biogenic and abiogenic carbonates: implications for paleoenvironmental proxies

Geochemical variations in marine biogenic carbonates that are preserved in the geological record serve as proxies of past environmental change. However, interpreting most proxies is complicated by biologically-mediated vital effects, highlighting the need to develop new tools for reconstructing paleenvironmental change. Recently, magnesium (Mg) isotope variability in carbonates has been explored extensively to determine its utility as a paleoenvironmental proxy. We review the results of these works, which have yielded valuable information on the factors affecting Mg isotope fractionation between carbonates and solution (Delta Mg-26(carb-sol)) in biogenic and abiogenic carbonate minerals. Strong evidence exists for a mineralogical control on Delta Mg-26(carb-sol), with the negative offset from 0 parts per thousand, following the sequence aragonite < dolomite < magneseit e gb similar to 3 mol/mol) and saturation states (Omega > similar to 3) that are similar to seawater suggest that Delta Mg-26(carb-sol) has a temperature dependence of similar to 0.01 parts per thousand degrees C-1 and is insensitive to precipitation rate. In contrast, a significant precipitation rate dependence is observed in calcites precipitated from solutions with relatively low Mg/Ca ratios (