Using Mg isotope ratios to trace Cenozoic weathering changes: A case study from the Chinese Loess Plateau

Abstract

The magnesium isotope system is a useful tracer of weathering processes due to its sensitivity to secondary mineral formation. We present elemental and isotopic data in bulk loess and paleosol sediments from the Luochuan profile in the Chinese Loess Plateau (CLP). Due to a relatively constant pre-weathering mineralogy, changes in the sediment chemistry with depth can be interpreted to reflect changes in weathering processes that are ultimately controlled by climate. We find that the Luochuan sediments are all enriched in light Mg relative to the upper continental crust (UCC: −0.22‰). The isotopically light signature is a result of three way mixing between Mg sourced from silicate, calcite and dolomite. Changes in δ26Mg between loess and paleosol reflect the higher weathering intensities during pedogenesis and corresponding loss of easily leachable calcite enriched in light Mg isotopes from the loess layers. This is confirmed by the results of a simple leaching procedure in which the leachate phase contains high Ca concentrations and isotopically light δ26Mg values of up to −2‰. Any isotopic fractionation of Mg imparted by isotopically heavy clay mineral formation appears to be insignificant, and is obscured by the presence of detrital dolomite that is resistant to acid leaching and is isotopically light. The isotopic composition of Mg at Luochuan broadly follows the magnetic susceptibility record in that loess layers are relatively isotopically light and have lower magnetic susceptibilities than their adjacent paleosol. As such, the changes in δ26Mg values with depth do correspond to changes in chemical weathering, however the driving factor behind these changing δ26Mg values is carbonate preservation which in itself is not a highly sensitive paleoclimate proxy.From a global perspective the δ26Mg value of Luochuan loess is similar to other loess sediments, which are often isotopically light relative to the UCC. We have recalculated the average δ26Mg value of global loess sediments to be −0.58±1.18‰. This reflects the fact that δ26Mg values in loess are highly heterogeneous, and are generally enriched in 24Mg compared to typical UCC rocks due to preservation of high amounts of isotopically light carbonate minerals. For this reason loess sediments cannot be used to approximate the isotopic composition of Mg in the Earth's crust.