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 growthrate, 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 carbonatesbased 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.
Alle informatie in het Integrated Marine Information System (IMIS) valt onder het VLIZ Privacy beleid
