Effect of growth rate on nickel and cobalt incorporation in aragonite

The incorporation of Ni and Co in aragonite was experimentally investigated as a function of mineral growth rate using the constant addition technique at 25 °C and 1 bar pCO₂. The distribution coefficients of Ni and Co (i.e., D_Me=[Formula presented]) exhibit a linear correlation with mineral growth rate in the range −9.1 ≤ Log r_p ≤ −7.5 mol/m²/s. The obtained results suggest a stronger dependency of D_Ni to growth rate compared to that observed for D_Co. These dependencies can be described as: Log D_Ni = 1.247 (±0.152) Log r_p + 7.448 (±1.212); R² = 0.82 Log D_Co = 0.312 (±0.047) Log r_p + 1.664 (±0.383); R² = 0.81 The D_Me-aragonite values for both Ni and Co are systematically lower than unity and their increase with increasing growth rate is in agreement with the incorporation of elements incompatible with the host mineral structure. Using the dependency of D_Ni and D_Co to the saturation indices (SI) of the reactive solution with respect to aragonite, the D_Me under equilibrium were estimated. These estimates yield equilibrium Log D_Me values of −3.9 and − 1.0 for Ni and Co, respectively. These experimentally defined D_eq values are 1.3 to 4.3 orders of magnitude lower compared to theoretical estimates that were earlier published in the literature. Similar to other incompatible elements, the observed increase of D_Ni and D_Co values with degree of saturation, points towards incorporation related to the density of defect sites on the mineral surface. Finally, the dependencies recorded in this study suggest that D_Ni and D_Co have the potential to be used as a proxy for saturation degree of the reactive solution.