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Stable lithium isotopes (δ7Li) of CaCO3 minerals have increasingly been used as a tracer for changes in silicate weathering processes. However, there is limited understanding of the influence of physical and chemical conditions on δ7Li values of CaCO3 minerals during their formation in aqueous solutions. Here, we examined Li isotope fractionation in inorganic calcite and aragonite precipitation experiments with systematic manipulations of solution pH and concentrations of total dissolved inorganic carbon species ([DIC] ≈ [HCO3−] + [CO32−]) and calcium ion (Ca2+). Calcite and aragonite samples had δ7Li values lower than those of dissolved Li in solutions by about 3‰ and 16‰, respectively, indicating preferential uptake of the lighter 6Li isotopes. Aragonite consistently had δ7Li values lower than those of calcite by ∼13‰, likely due to differences in Li coordination and thereby the strength of bonds formed by/with Li within the respective mineral structure. We observed no statistically significant changes in aragonite nor calcite δ7Li values in response to changing solution pH, [DIC], [Ca2+], and CaCO3 precipitation rates, indicating our solution chemistry manipulations imposed little effect on Li isotope fractionation. These findings lead us to argue that the observed Li isotope fractionations in calcite and aragonite with respect to dissolved Li in solutions are dominated by equilibrium isotope effects, and that kinetic effects for δ7Li values in CaCO3 are either non-existent or too small to be expressed under our experimental conditions. 

Calcite is known to incorporate a range of non-constituent ions during its precipitation from aqueous solutions. Their concentrations (measured as E/Ca ratios, where E denotes the elemental forms of non-constituent ions) in calcite formed in seawater can serve as useful tools for paleoceanographic studies. But this requires concrete understanding of the incorporation patterns and their dependence to environmental factors at the time of mineral precipitation. Here, we present Na/Ca, K/Ca, S/Ca, and B/Ca ratios of inorganic calcite samples generated in laboratory experiments using Mg-free artificial seawater with systematic manipulations of pH, [DIC], and [Ca2+]. The three parameters were varied both individually (the pH, DIC, and Ca experimental series) and in tandem (the pH-Ca and DIC-Ca series) to form calcites under variable versus near-constant precipitation rates (denoted as R). All measured E/Ca ratios showed a robust positive linear dependence to changes in [Ca2+] in the Ca, pH-Ca, and DIC-Ca series, irrespective of changes in R. While K/Ca and S/Ca ratios changed almost exclusively with [Ca2+], Na/Ca and B/Ca ratios showed an additionally strong increase with increasing pH and a more moderate increase with rising [DIC], when R changed accordingly in the pH and DIC series. While R-driven kinetic effects and/or formation of certain cation–anion pairs may be important for the elemental uptake in calcite under some circumstances, these mechanisms or processes cannot fully account for the observed trends in every experimental series for all E/Ca ratios considered here. We propose that the observed E/Ca trends can be comprehensively explained by simultaneously considering the nonequivalent influence of changes in solution [Ca2+] and [CO32−] on step-specific kink formation dynamics and the size difference between the respective non-constituent ions (K+, Na+, SO42−, and B(OH)4− and B(OH)3) relative to Ca2+ and CO32− that constitute the calcite lattice. 

The δ7Li of marine carbonates has been interpreted as an archive of the evolution of seawater δ7Li, and therefore continental weathering, through geological time. However, little is known about the incorporation of Li into calcium carbonate minerals and, consequently, the controls on Li partitioning (DLi) and isotopic fractionation (Δ7Lisolid-fluid) associated with Li incorporation. Crucially, we lack a fundamental understanding of how Li partitioning and Δ7Lisolid-fluid change in response to the chemical and physical conditions of crystal formation. Here, we present DLi and Δ7Lisolid-fluid data from a series of inorganic calcite precipitation experiments where temperature, and solution pH and dissolved inorganic carbon (DIC) were independently varied. We find DLi values in the range 0.8–1.5 × 10−3, which show no relationship with temperature, a strong positive correlation with pH, and a weak positive correlation with DIC. At face value, these patterns are inconsistent with the results of previous precipitation studies. However, the correlations with pH and DIC are consistent with a strong precipitation rate control on DLi that aligns well with previous data, with a likely secondary influence from the incorporation of Li-HCO30 ion pairs from solution. We find Δ7Lisolid-fluid values in the range −6 to −2 ‰, which show no relationship with temperature or pH, and a weak positive correlation with DIC and crystal precipitation rate. These results do not agree with previously published data. Considered alongside previously published data, we observe no consistent relationship between Δ7Lisolid-fluid and any reported physical or chemical experimental parameter, highlighting the need for substantial further work to determine whether systematic controls on Li isotopic fractionation exist in carbonate minerals, and whether they may be environmentally significant. 

The sulfur over calcium ratio (S/Ca) in foraminiferal shells was recently proposed as a new and independent proxy for reconstructing marine inorganic carbon chemistry. This new approach assumes that sulfur is incorporated into CaCO3 predominantly in the form of sulfate (SO42−) through lattice substitution for carbonate ions (CO32–), and that S/Ca thus reflects seawater [CO32–]. Although foraminiferal growth experiments validated this approach, field studies showed controversial results suggesting that the potential impact of [CO32–] may be overwritten by one or more parameters. Hence, to better understand the inorganic processes involved, we here investigate S/Ca values in inorganically precipitated CaCO3 (S/Ca(cc)) grown in solutions of CaCl2 − Na2CO3 − Na2SO4 − B(OH)3 − MgCl2. Experimental results indicate the dependence of sulfate partitioning in CaCO3 on the carbon chemistry via changing pH and suggest that faster precipitation rates increase the partition coefficient for sulfur. The S/Ca ratios of our inorganic calcite samples show positive correlation with modelled [CaSO40](aq), but not with the concentration of free SO42− ions. This challenges the traditional model for sulfate incorporation in calcite and implies that the uptake of sulfate potentially occurs via ion-ion pairs rather than being incorporated as single anions. Based on the [Ca2+] dependence via speciation, we here suggest a critical evaluation of this potential proxy. As sulfate complexation seems to control sulfate uptake in inorganic calcite, application as a proxy using foraminiferal calcite may be limited to periods for which seawater chemistry is well-constrained. As foraminiferal calcite growth is modulated by inward Ca2+ flow to the site of calcification coupled to outward H+ pumping, sulfate incorporation as CaSO40 ion-pair in the foraminifer’s shell also provides a mechanistic link for the observed relationship between S/Ca(cc) and [CO32–].