Description: In order to study Strontium (Sr) partitioning and isotope fractionation of Sr and Calcium (Ca) in calcite we performed precipitation experiments decoupling temperature and precipitation rate (R∗). Calcite was precipitated at 12.5, 25.0 and 37.5 °C by diffusing NH3 and CO2 gases into aqueous solutions closely following the experimental setup of Lemarchand et al (2004). The precipitation rate (R∗) for every sample was determined applying the initial rate method and from the specific surface area of almost all samples for each reaction. The order of reaction with respect to Ca2+ ions was determined to be one and independent of T. However, the order of reaction with respect to HCO3- changed from three to one as temperature increases from 12.5, 25 °C and 37.5 °C. Strontium incorporated into calcite (expressed as DSr= [Sr/Ca] calcite/ [Sr/Ca] solution) was found to be R∗ and T dependent. As a function of increasing R∗ the Δ88/86Sr-values become more negative and as temperature increases the Δ88/86Sr values also increase at constant R∗. The DSr and Δ88/86Sr-values are correlated to a high degree and depend only on R∗ being independent of temperature, complexation and varying initial ratios. Latter observation may have important implications for the study of diagenesis, the paleo-sciences and the reconstruction of past environmental conditions. Calcium isotope fractionation (Δ44/40Ca) was also found to be R∗ and T dependent. For 12.5 and 25.0 °C we observe a general increase of the Δ44/40Ca values as a function of R∗ (Lemarchand et al type behavior, Lemarchand et al (2004)). Whereas at 37.5 °C a significant decreasing Δ44/40Ca is observed relative to increasing R∗ (Tang et al type behavior, Tang et al. (2008)). In order to reconcile the discrepant observations we suggest that the temperature triggered change from a Ca2+-NH3-aquacomplex covalent controlled bonding to a Ca2+-H2O-aquacomplex van-der-Waals controlled bonding caused the change in sign of the R∗ - Δ44/40Ca slope due to the switch of an equilibrium type of isotope fractionation related to the covalent bonding during lower temperatures to a kinetic type of isotope fractionation at higher temperatures. This is supported by the observation that the Δ44/40Ca ratios are independent from the [Ca]: [DIC] ratio at 12.5 and 25°C but highly dependent at 37.5°C. Our observations imply the chemical fluid composition and temperature dependent complexation controls the amount and direction of Ca isotope fractionation in contrast to the Sr isotopes which do not show any change of its fractionation behaviour as a function of complexation in the liquid phase.

Description: In order to study Strontium (Sr) partitioning and isotope fractionation of Sr and Calcium (Ca) in aragonite we performed precipitation experiments decoupling temperature and precipitation rates (R∗, μmol/m2.h) in the interval of about 2.3 to 4.5 μmol/m2.h. Aragonite is the only pure solid phase precipitated from a stirred solutions exposed to an atmosphere of NH3 and CO2 gases throughout the spontaneous decomposition of (NH4)2CO3. The order of reaction with respect to Ca ions is one and independent of temperature. However, the order of reaction with respect to the dissolved inorganic carbon (DIC) is temperature dependent and decreases from three via two to one as temperature increases from 12.5 and 25.0 to 37.0 °C, respectively. Strontium distribution coefficient (DSr) increases with decreasing temperature. However, R∗ responds differently depending on the initial Sr/Ca concentration and temperature: at 37.5 °C DSr increase as a function of increasing R∗ but decrease for 12.5 and 25 °C. Not seen at 12.5 and 37.5 °C but at 25°C the DSr-R∗ gradient is also changing sign depending on the initial Sr/Ca ratio. Magnesium (Mg) adsorption coefficient between aragonite and aqueous solution (DMg) decreases with temperature but increases with R∗ in the range of 2.4 to 3.8 μmol/m2.h. Strontium isotope fractionation (Δ88/86Sraragonite-aq) follows the kinetic type of fractionation and become increasingly negative as a function of R∗ for all temperatures. In contrast Ca isotope fractionation (Δ44/40Caaragonite-aq) shows a different behavior than the Sr isotopes. At low temperatures (12.5 and 25°C) Ca isotope fractionation (Δ44/40Caaragonite-aq) becomes positive as a function of R∗. In contrast, at 37.5°C and as a function of increasing R∗ the Δ44/40Caaragonite-aq show a Sr type like behavior and becomes increasingly negative. Concerning both the discrepant behavior of DSr as a function of temperature as well as for the Ca isotope fractionation as a function of temperature we infer that the switch of sign in the trace element partitioning as well as in the direction of the Ca isotope fractionation is probably due to the switch of complexation from a Ca2+-NH3 complexation at and below 25 °C to an Ca2+-H2O aquacomplex at 37.5 °C. The DSr - Δ88/86Srcalcite-aq correlation for calcite is independent of temperature in contrast to aragonite. We interpreted the strong DSr-temperature dependency of aragonite, the smaller range of Sr isotope fractionation as well as the shallower Δ88/86Srcalcite-aq-R∗ gradients to be a consequence of the increased aragonite solubility and the “Mg blocking effect”. In contrast to Sr the Ca isotope fractionation values in calcite and aragonite depend both on the complexation in solution and independent on polymorphism.

Description: Trace elements incorporation of certain trace elements like strontium (Sr) and magnesium (Mg) among others and their isotope composition in different CaCO3 polymorphs (e.g. calcite and aragonite) as archives provide valuable for proxy information, that can be used as important tools for reconstructing the paleo- environmental conditions of the oceans throughout time. However, data on Sr incorporation into inorganic precipitated CaCO3 (calcite and aragonite), Mg incorporation into aragonite and Sr isotopic fractionation during minerals formation are still very rare. In addition, literature values available concerning Ca isotopic fractionation between calcite and aqueous solution are discrepant to a certain extent, while on the other hand the data available concerning Ca isotopic fractionation between inorganic precipitated aragonite and aqueous solution are also scarce. In order to overcome this lag of information in this study calcite and aragonite were precipitated at three different temperatures (12.5, 25.0 and 37.5±0.2 °C), different precipitation rates (R*) and solution composition by diffusing NH3 and CO2 gases into aqueous solutions containing trace elements and NH3 ions. For the kinetic study we used the initial rate method to solve the rate equations (rate law) with R* values in the range between 2.5 to 4.5 μmol/m2.h. We find that both calcite and aragonite have exactly the same order of reaction only differing in their activation energy (114 kJ/mol for calcite and 149 kJ/mol for aragonite) and rate constants at 25 °C (80.6*10-4 for calcite and 17.3*10-4 mM-2.h-1 for aragonite). The order of reaction with respect to Ca2+ ions is ≈ 1 and temperature dependent, while the order of reaction with respect to HCO3- ions is temperature dependent decreasing from 3 via 2 to 1 as temperature increases from 12.5 via 25.0 to 37.5°C, respectively. Calcium isotope fractionation for both calcite and aragonite (Δ44/40Ca) was found to be R* and temperature dependent. For 12.5 and 25.0 °C we observe a general increase of the Δ44/40Ca values as a function of R*, whereas at 37.5 °C decreasing Δ44/40Ca values are observed relative to increasing R*. It is suggested that the temperature triggered change from a Ca2+-NH3-aquacomplex covalent controlled bonding to a Ca2+-H2O-aquacomplex van-der-Waals controlled bonding caused the change in sign of the R* - Δ44/40Ca slope due to the switch of an equilibrium type of isotope fractionation related to the covalent bonding during lower temperatures to a kinetic type of isotope fractionation at higher temperatures. This behavior of Ca is in sharp contrast to the Sr isotopes which do not show any change of its fractionation behaviour as a function of complexation in the liquid phase. For both polymorphs of CaCO3 as a function of increasing R* the Δ88/86Sr-values become more negative and as temperature increases the Δ88/86Sr values also increase at constant rate. However effect oft R* on the Δ88/86Sr values is more significant in calcite than in aragonite. Magnesium incorporated into aragonite (expressed as DMg= [Mg/Ca] aragonite/ [Mg/Ca] solution) increases with decreasing temperature and also increases with increasing R* and as temperature increases the R* effect decreases. Later behavior is opposite to Mg in calcite (as temperature increases DMg also increases) as already known from earlier studies. Strontium incorporated into both calcite and aragonite (expressed as DSr= [Sr/Ca] solid/ [Sr/Ca] solution) was found to be R* and temperature dependent. Rate effect is more dominant over temperature effect in calcite, while on the other hand temperature effect is more dominant over rate effect in the case of aragonite. In calcite DSr increases with increasing R* and decreasing temperature. In aragonite also DSr increases with decreasing temperature. However concerning R* it responds differently: at 37.5°C DSr as R* increases DSr values increase, but decrease at 12.5°C. At 25.0°C, both behaviors are detected depending on the molar [Sr]/[Ca] ratio of the reacting solution (0.005 or 0.01). In the frame of a qualitative model to explain our trace element and isotope observations we speculate that increasing Mg2+ -concentrations control the material flux back (R*detach) from the crystal to the solution to a large extend. As a consequence R* values for aragonite tend to be lower than for calcite as observed from our data. Hence, Sr incorporation into aragonite is affected as function of temperature to a higher degree when compared to the R* effect. This is also reflect on the Δ88/86Sr values and decreasing the R* effect when compared to the temperature effect. Moreover concerning Ca isotope fractionation, the switch of direction in Ca isotope fractionation above ~25°C may be either due to the Mg2+ blocking effect or due to the switch of complexation from NH3 at and below 25 °C to H2O complexation at 37.5 °C. Plotting DSr versus Δ88/86Sr may be used as a proxy to reconstruct precipitation rates of calcite and of precipitation temperature of inorganic aragonite. Latter correlation may also have important implications for the verification of CaCO3 diagenesis.

Description: To study the incorporation of magnesium (Mg) and strontium (Sr) in calcite precipitated over synthetic calcite seeds (overgrowths) as a function of the precipitation rate (R*, μmol/m2 h), we performed precipitation experiments wherein temperature and precipitation rates were decoupled at intervals of approximately 3.63–5.22 μmol/m2 h. In most sample reactions, high‑magnesium calcite (HMC) overgrowths co-precipitated with aragonite from the stirred solutions exposed to an atmosphere of NH3 and CO2 gases throughout the spontaneous decomposition of (NH4)2CO3. The percentage of aragonite in the solid CaCO3 increased with both temperature and dissolved inorganic carbon (DIC). The order of reaction with respect to the [DIC] is temperature dependent and is 1.9, 2.4, and 2.9 at temperatures of 12.5, 25.0, and 37.5 °C, respectively. The magnesium distribution coefficient (DMg) increases significantly with increasing R*, temperature, and Mg/Ca ratio in the fluid. The strontium distribution coefficient (DSr) increases with R* and with increasing MgCO3 concentrations in the calcite overgrowths. However, it is independent of temperature.

Description: Highlights • Sr isotope composition in different minerals is an excellent proxy, that can be used to reconstruct the environmental conditions of their precipitation. In order to study Strontium (Sr) isotope fractionation during the precipitation of strontianite (SrCO3) as a function of the specific precipitation rate (R*) and temperature (T), strontianite was precipitated at 12.5, 25.0 and 37.5 °C by diffusing NH3 and CO2 gases into aqueous solutions. The specific precipitation rate R* (mol/cm2.h) for every sample was determined by applying the initial rate method. The mean isotope difference between bulk solution and precipitate (∆88/86Srstrontianite-solution) was found to be −0.279 ± 0.005‰ (2σmean) independent of both rate and temperature. Hence, Sr isotope fractionation in strontianite is completely different from that in calcite and aragonite, where a strong dependency from both rate and temperature can be observed. The latter is interpreted to reflect the competition between Sr2+ and Ca2+ ions for incorporation into the calcium carbonate crystal lattice, which is absent during the precipitation of pure strontianite. The isotope difference between strontianite and bulk solution then simply reflects the intermolecular forces in the aqueous solutions as well as the kinetic effect. The difference in the (∆88/86Srstrontianite-solution between experiments then reflects the dehydration energy of Sr ions in the adsorption layer of SrCO3.