The primary purpose of using a model was to validate the proposed mechanism responsible for the mineral-fluid exchange in each reactor, quantify the rate of exchange and evaluate the impact of short-term exchange on geochemical proxies. Foraminiferal calcite is the foundation of many proxy-based reconstruction. Hence, understanding the mechanism of exchange and determination of rate of mineral-fluid exchange in foraminiferal calcite are critical for accurate estimation of the extent of post-formational alterations on carbonate-based proxies. The reaction rates estimated in most of the atom exchange experiments are under the basic assumption of homogeneous exchange at equilibrium (illustrated in the Appendix C, section C1) (e.g., Moller and
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However, in case of the biogenic calcites with intra-test chemical heterogeneity (Eggins et al., 2003; Eggins et al., 2004), such assumption may not always be applicable as the variable distribution of trace metals in the test can result intra-test variation in dissolution susceptibility. In this case, preferential dissolution and reprecipitation may not take place at equilibrium as some parts of the test (e.g. Mg and Sr-rich) become more soluble than the rests resulting a heterogeneous distribution of the 45Ca tracer within the solid. In this article, we utilized a simple box model (described in Appendix section C1, Fig. C3) that addressed the heterogeneous trace metal chemistry (i.e., Mg and Sr) of the initial solid. In this model the exchange takes place by partial dissolution of the solid by preferential removal of more soluble Mg and Sr-rich calcite and …show more content…
Here the precipitation flux from the fluid is only balanced by the dissolution fluxes from the exchanged solid reservoir and from the un-exchanged solids with low-Mg-Sr calcite. The model results were able to reproduce the observed increase in [Ca2+], [Mg2+], and [Sr2+] in the solution and incorporation of 45Ca in the solids (Fig. 4.11). A comparison between the model-fits using both the steady state and non-steady state simulation is done to justify importance of flux imbalance in the model in reproducing the observed solution chemistry (Appendix C. Fig.C4a and b). The sensitivity of rate of exchange to the back reaction is also tested in the model. It was demonstrated that the exchange rate required to reproduce the observed 45Ca data is higher if the exchanged solid is allowed entirely to re-equilibrate with the fluid (i.e., 100% back-reaction) as compared to the case when back-reaction is not allowed at all (Appendix. Fig. C4c and d). However, the influence of back-reaction appears to be more important on the solution chemistry after 9 days of
However, in case of the biogenic calcites with intra-test chemical heterogeneity (Eggins et al., 2003; Eggins et al., 2004), such assumption may not always be applicable as the variable distribution of trace metals in the test can result intra-test variation in dissolution susceptibility. In this case, preferential dissolution and reprecipitation may not take place at equilibrium as some parts of the test (e.g. Mg and Sr-rich) become more soluble than the rests resulting a heterogeneous distribution of the 45Ca tracer within the solid. In this article, we utilized a simple box model (described in Appendix section C1, Fig. C3) that addressed the heterogeneous trace metal chemistry (i.e., Mg and Sr) of the initial solid. In this model the exchange takes place by partial dissolution of the solid by preferential removal of more soluble Mg and Sr-rich calcite and …show more content…
Here the precipitation flux from the fluid is only balanced by the dissolution fluxes from the exchanged solid reservoir and from the un-exchanged solids with low-Mg-Sr calcite. The model results were able to reproduce the observed increase in [Ca2+], [Mg2+], and [Sr2+] in the solution and incorporation of 45Ca in the solids (Fig. 4.11). A comparison between the model-fits using both the steady state and non-steady state simulation is done to justify importance of flux imbalance in the model in reproducing the observed solution chemistry (Appendix C. Fig.C4a and b). The sensitivity of rate of exchange to the back reaction is also tested in the model. It was demonstrated that the exchange rate required to reproduce the observed 45Ca data is higher if the exchanged solid is allowed entirely to re-equilibrate with the fluid (i.e., 100% back-reaction) as compared to the case when back-reaction is not allowed at all (Appendix. Fig. C4c and d). However, the influence of back-reaction appears to be more important on the solution chemistry after 9 days of