Sodium incorporation into inorganic CaCO3 and implications for biogenic carbonates

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+/Ca2+ 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/m2/s, while faster growth rate than 10−4.5 mol/m2/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/Ca2+ 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 [Ca2+]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/Ca2+ activity ratio of seawater. Estimated rates vary from 10−5.6 (planktic foraminifers) to above 10−4 (sea urchins) mol/m2/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.