Lithium isotopes and partition coefficients in inorganic carbonates: Proxy calibration for weathering reconstruction

Abstract

We report Li measurements from cave-analogue carbonate-precipitation experiments in order to: i) assess the expected isotope fractionation factors applicable to speleothem growth, and ii) contribute to the wider understanding of lithium incorporation in carbonates. The experimental setup closely mimics natural processes (e.g. precipitation driven by CO2 degassing, low ionic strength solution, thin solution film) but within a laboratory setting that allows for controlled growth conditions (temperature, pCO2, drip rate, carbonate saturation index and the composition of the initial solution). For the main batch of calcite growth experiments our average 1000lnαcalcite-solution is -8.5±2(2σ). This low sensitivity of 1000lnα to in-cave growth conditions is encouraging for weathering intensity reconstruction using speleothem samples. At each temperature, growth rate was varied independently with calcite saturation indices ranging between 0.1 and 0.6, with no significant impact on either D(Li) or 1000lnα. For the full range of growth conditions (considering all temperatures and calcite saturation indices) we observe a small but significant decrease in 1000lnα, which we do not attribute to temperature. Based on an inter-study comparison we define LogDLi = 0.09(±0.04)×GR-3.2(±0.2), where GR is growth rate in units of 10-8mmolcm-2s-1. This is similar to the previously defined relationship over a reduced range of growth rates. Over this study’s full range of growth rates, 1000lnα = -0.13(±0.04)×GR-7.1(±0.3) is significant at the ρ=0.018 level, consistent with a surface entrapment control. Future experiments should test factors such as growth rate and pH, independently of other variables, to further assess their role in defining 1000lnα. A subset of 20 °C experiments, with increasing [Mg]solution, provides 1000lnα for calcite, high-magnesian calcite (HMC) and aragonite. Our values of 1000lnα for these three mineralogies range between -7.3 and -10.7. For HMC and aragonite in particular there is close agreement with other published values of 1000lnα for CaCO3 grown in laboratory and in natural settings. In agreement with measurements in bulk carbonates, benthic foraminifera and brachiopods, this suggests that differences in 1000lnα between carbonate mineralogies may not always be as large as suggested by some earlier studies.