Controls of temperature and mineral growth rate on lithium and sodium incorporation in abiotic aragonite

Abstract

The use of Li/Ca, Na/Ca and Li/Mg ratios in biogenic aragonite exhibits high potential for reconstructing environmental parameters such as temperature and/or salinity. To date however, only a little is known about the mechanisms controlling the incorporation of monovalent metals such as Li+ and Na+ in aragonite. In this study, the effects of temperature and growth rate on Li and Na incorporation into abiotically precipitated aragonite were experimentally investigated. The results for aragonite overgrowths at 5, 15 and 25 °C and for the surface normalized growth rate range 10–8.6 ≤ rp ≤ 10–7.1 (mol/m2/s) suggest that apparent distribution coefficients (i.e., DMe+=CMe+CCaaragonitemMe+mCasolution) of Li and Na are mainly controlled by mineral growth rate, whereas temperature has a minor effect. The combined effect of growth rate and temperature on DLi and DNa can be described as:LogDLi=0.836±0.028Logrp−0.026±0.002T+2.958±0.221;R2=0.97LogDNa=0.456±0.030Logrp−0.018±0.002T−0.253±0.234;R2=0.90where Log rp is the growth rate in mol/m2/s and T is the temperature in degrees Celsius.The DLi and DNa values increase at increasing mineral growth rate, but also decrease as a function of temperature in experiments with similar normalized growth rate. These observations suggest that the incorporation of Li and Na in abiotic aragonite is controlled by the density of mineral surface defect sites that are correlated with the degree of saturation of the reactive solution with respect to aragonite. Interestingly, no correlation between Li/Mg in the aragonite overgrowths and temperature of formation was observed in this study. This difference of Li/Mg and temperature correlations between abiotic aragonite and natural biogenic samples is intriguing and underlines the need for robust understanding of elemental incorporation during carbonate mineral formation, and proxy calibrations specific for growth conditions (e.g., abiotic versus biogenic). The results of this study do not support the use of Li/Mg as a temperature proxy in abiotic aragonite.