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Constraining sulfur incorporation in calcite using inorganic precipitation experiments
Karancz, S.; Uchikawa, J.; de Nooijer, L.J.; Wolthers, M.; Conner, K.A.; Hite, C.G.; Zeebe, R.E.; Sharma, S.K.; Reichart, G.-J. (2024). Constraining sulfur incorporation in calcite using inorganic precipitation experiments. Geochim. Cosmochim. Acta 381: 116-130. https://dx.doi.org/10.1016/j.gca.2024.07.034
In: Geochimica et Cosmochimica Acta. Elsevier: Oxford,New York etc.. ISSN 0016-7037; e-ISSN 1872-9533, meer
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| Author keywords |
Calcite growth experiment; Sulfate incorporation; Ion-pair formation; Inorganic carbon chemistry; Proxy |
| Auteurs | | Top |
- Karancz, S., meer
- Uchikawa, J.
- de Nooijer, L.J., meer
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- Wolthers, M.
- Conner, K.A.
- Hite, C.G.
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- Zeebe, R.E.
- Sharma, S.K.
- Reichart, G.-J., meer
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| Abstract |
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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