CONTROLS ON ISOTOPIC AND ELEMENTAL INCORPORATION OF LITHIUM IN INORGANIC CALCIUM CARBONATES

Abstract

There is increasing interest in using the δ7Li value of marine biogenic CaCO3 (i.e., foraminifera tests) as a tracer for secular changes in silicate weathering, an important process in sequestering atmospheric CO2. However, culture studies have suggested the possibility of additional controls on Li incorporation in foraminiferal CaCO3, indicating a need to better understand the physical and chemical controls on Li incorporation in inorganic CaCO3 without the complications of vital effects from a biological system. To this end, the present study conducted inorganic calcite and aragonite precipitation experiments with systematic manipulations of solution chemistry parameters (pH, [DIC], and [Ca2+]) using a pH-stat system. Both calcite and aragonite samples had lower δ7Li values than the δ7Li value of dissolved Li in the experimental solution, indicating preferential uptake of the lighter isotope (6Li) in carbonate minerals. This is presumably due to the tetrahedral coordination of Li bound with water molecules in solution resulting in the strongest bonds. Aragonite consistently had δ7Li values lower than calcite, likely due to differences in Li coordination within the crystal lattice and the resulting bond strength. There was an observed effect on the Li isotopic fractionation in aragonite samples with changes in pH, [DIC], and [Ca2+]. Additionally, a significant relationship was found between the Li isotopic fractionation in calcite samples and pH. Despite being statistically significant, these relationships were weak and resulted in a minimal overall effect on fractionation compared to the experimental uncertainty. There was no clear relationship observed between precipitation rate and fractionation, suggesting fractionation is dominated by equilibrium effects rather than kinetic effects. Overall, the present study has implications for the use of Li in carbonate minerals as a proxy tool for silicate weathering. In particular, while precipitation rate and solution chemistry may not dramatically influence Li isotope fractionation in inorganic calcite, biological vital effects may play a significant role and should be further evaluated in order to confidently use Li isotopes for silicate weathering reconstructions.