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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.
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Constraining sulfur incorporation in calcite using inorganic precipitation experiments
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–].
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- PAR ID:
- 10537614
- Publisher / Repository:
- Elsevier
- Date Published:
- Journal Name:
- Geochimica et Cosmochimica Acta
- Volume:
- 381
- Issue:
- C
- ISSN:
- 0016-7037
- Page Range / eLocation ID:
- 116 to 130
- Subject(s) / Keyword(s):
- Calcite growth experiment Sulfate incorporation Ion-pair formation Inorganic carbon chemistry Proxy
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1016/j.gca.2024.07.034
