ESTABLISHING A BASELINE FOR CONTROLS OF MG/CA SIGNATURES IN CALCITE: INFLUENCE OF SALINITY

Many paleoclimate models assume temperature is the predominant driver of Mg/Ca in biogenic calcite. Efforts to validate and apply these environmental proxy models are dogged by the 50 year question of whether Mg/Ca signatures in calcites are influenced by seawater salinity. Previous studies of foraminifera concluded salinity has only a small influence on Mg signatures in calcified biominerals compared to temperature (Lea, 1999). However, the issue recently reemerged with a culturing study reporting that Mg/Ca ratios in foram tests correlate more strongly with salinity than temperature (Ferguson et al., 2008, EPSL).

To establish an inorganic baseline for how salinity influences Mg uptake in calcite, we tested the hypothesis that higher salinity environments promote Mg uptake by promoting desolvation of Mg ion relative to Ca. Experiments measured Mg/Ca in calcites grown in NaCl or KCl solutions of different ionic strengths (0.05 – 0.3). In situ fluid cell AFM monitored the formation of these Mg-calcite overgrowths onto seed crystals in solutions with carefully controlled chemistries. Kinetic measurements found the growth rate by step propagation increases with increasing solution ionic strength. SIMS analyses of these overgrowths show that higher ionic strength solutions promote the uptake of Mg into these inorganic calcites by up to 40%. Calcite incorporates significantly more Mg when grown in NaCl solutions compared to KCl, for otherwise similar solution chemistries.

Analyses of the data show the decrease in Mg content with ionic strength is primarily a thermodynamic effect. This may arise from competition of the background salts with Mg for the calcite surface. For the range of salinity investigated here, the data suggest reductions in the kinetic barrier to desolvating Mg relative to Ca are small. Our analyses indicate higher ionic strength solutions have the effect of increasing the effective supersaturation of growth solutions as a result of the lower amount of Mg that is incorporated into the calcite (lower solubility). This explanation is consistent with the measured increase in step growth rate in higher ionic strength solutions. These findings are also consistent with a previous study (Davis et al., 2000, Science) showing that decreases in velocity with increasing Mg content are primarily a thermodynamic effect.