Alkhatib, Mahmoud (2016) Trace element and isotope measurement on artificially precipitated calcium carbonate. (PhD/ Doctoral thesis), Christian-Albrechts-Universität zu Kiel, Kiel, Germany, 99 pp.

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Abstract

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.

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