Sodium incorporation into inorganic CaCO3 and implications for biogenic carbonates. (1st December 2021)

• Record Type: Journal Article
• Authors: Devriendt, L.S.; Mezger, E.M.; Olsen, E.K.; Watkins, J.M.; Kaczmarek, K.; Nehrke, G.; de Nooijer, L.J.; Reichart, G.-J.
• Is Part Of: Geochimica et cosmochimica acta. Volume 314(2021)
• Page Start: 294
• Page End: 312
• Publication Date: 2021-12-01
• Subjects: Na/Ca -- Sodium -- Calcite -- CaCO3 -- Crystal growth rate -- Salinity -- Calcium concentration -- Foraminifer -- Marine carbonates -- Mineral growth rate; Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2021.07.024
• Languages: English
• ISSNs: 0016-7037

Abstract

Abstract: The sodium content of biogenic carbonates shows potential as a palaeoceanographic proxy for salinity and/or calcium concentration but the incorporation of Na + into inorganic and biogenic calcite is poorly understood. Taxonomic and conspecific variations in the sensitivity of carbonate Na/Ca to seawater Na + /Ca 2+ and salinity point to a biological influence on Na + partitioning and/or covariations with other environmental parameters. One major unknown of the biological control during calcification is the rate of mineral precipitation, which has a strong control on trace-element partitioning in inorganic carbonate systems. We conducted inorganic CaCO3 precipitation experiments where the effect of solution composition and crystal growth rate on Na + uptake by carbonate crystals are independently assessed. Calcite crystals were precipitated at rates varying from 10 −6.5 to 10 −4.5 mol/m 2 /s, while faster growth rate than 10 −4.5 mol/m 2 /s resulted in the coprecipitation of aragonite and vaterite. For a given crystal growth rate, calcite Na/Ca increases by 0.22% per % increase in solution (Na + ) 2 /Ca 2+ activity ratio. However, calcite Na/Ca increases up to fivefold per order of magnitude increase in crystal growth rate, suggesting crystal growth rate and precursor phases are likely dominant controls on marine carbonate Na/Ca. We use these results in the framework of the DePaolo (2011) model for trace element uptake by calcite to assess the origin of variable (Na/Ca)foraminifer sensitivities to [Ca 2+ ]seawater and salinity. Last, maximum mineral growth rates are estimated for a range of marine carbonates based on known carbonate Na/Ca and the (Na + ) 2 /Ca 2+ activity ratio of seawater. Estimated rates vary from 10 −5.6 (planktic foraminifers) to above 10 −4 (sea urchins) mol/m 2 /s. Such high mineral growth rates imply high degrees of oversaturation with respect to calcite (10 to >100), supporting the idea that elemental partitioning and isotopic fractionation recorded in marine biogenic carbonates are controlled by kinetic rather than equilibrium exchanges.