MAGNESIAN CALCITE AS A PALEOENVIRONMENTAL INDICATOR

The magnesium content of calcite has been used as a proxy for the (Mg/Ca)_aq of seawater but factors such as (Mg/Ca)_aq, PCO₂, solid/solution ratio and precipitation rate potentially limit the power of this geochemical tool. To better understand the influence of these variables on the Mg content of calcite, thirty five experiments were conducted in which Mg-calcite was precipitated on calcite seed from solutions of constant chemical composition using the chemo-stat technique. Average Ca_(aq), Mg_(aq) and total alkalinity values ranged from 8.6 to 12.8, 5.2 to 94.4 and 11.6 to 76.3 mM, respectively. Average (Mg/Ca)_aq ratio ranged from 0.4 to 9.9, PCO₂ ranged from 0.05 to 0.50, and solid/solution ratio ranged from 10² to 10⁴ mg L^-1 for solution saturation states that ranged from 2.5 to 12.1 (with respect to calcite) and precipitation rates that ranged from 10^1.73 to 10^3.87 micromol m^-2 h^-1. Calcite overgrowths contained 0.7 to 22.8 mol % Mg, yielding Henderson-Kracek distribution coefficients (D) that ranged from 0.0164 to 0.0340.

A positive linear correlation (R²=0.9302) was observed between mol % Mg in the overgrowth and Mg/Ca_(aq) ratio while no dependence was observed on PCO₂, solid/solution ratio or precipitation rate. This suggests Mg/Ca_(aq) ratio is the principal factor controlling the Mg-content of calcite. Nevertheless, D values varied by a factor of two over the range of Mg/Ca_(aq) values investigated and this hampers the predictive capability of this geochemical proxy.

Previous work has shown that the structural attributes of growing crystals can lead to differential Mg incorporation in calcite. To test for this effect, eleven chemo-stat experiments were run under identical physicochemical conditions for various lengths of time (1 to 100 h) to determine if overgrowth % or crystal morphology/habit influence the incorporation of Mg in calcite. Distribution coefficients decreased systematically and crystal forms evolved from complex multifaceted rhombohedra to regular (104) forms as the overgrowth % decreased. At ~5% overgrowth, (104) rhombohedra displayed smooth faces with few to no macro-steps and D values increased again. These results suggest the incorporation of Mg in calcite is complex and may be influenced by both surface properties and crystal form but additional work is required to confirm these findings.