NANOSCALE EFFECTS OF STRONTIUM ON CALCITE GROWTH: A BASELINE FOR UNDERSTANDING BIOMINERALIZATION IN THE ABSENCE OF VITAL EFFECTS

The Sr/Ca and Mg/Ca ratios in calcium carbonate biominerals have become widely used proxies to interpret paleoenvironment temperatures. Previous studies show that the form and chemistry of biogenic carbonates are affected by factors such as temperature, salinity, water turbidity and dissolved oxygen. To complicate these relationships, complex biological processes, termed 'vital effects,' also influence the compositions of skeletal materials during mineral formation. Quantifying molecular-scale controls on mineralization in the absence of 'vital effects' is essential to sorting out the principles that control compositional signatures of biogenic carbonates.

Here we use in situ atomic force microscopy to observe directly the molecular scale effects of Sr on layer growth of abiotic calcite and couple these insights with quantitative measurements of the kinetics and thermodynamics of growth. Strontium inhibits calcite growth by different mechanisms for positive (larger, geometrically more open kink sites) and negative (smaller and more shielded kink sites) surface coordination environments that characterize calcite step edge directions. Low concentrations of strontium enhance the rate of calcite growth through changes in kinetics. A new conceptual model is introduced to explain this behavior. Higher concentrations of strontium inhibit and ultimately stop calcite growth by a step blocking mechanism. The critical supersaturation required to initiate growth increases with increasing levels of strontium. At higher supersaturations, strontium causes growth rates to increase to levels greater than those for the pure system. The step blocking model proposed by Cabrera and Vermilyea (1958) does not predict this experimental behavior.

Preliminary evidence indicates that strontium is preferentially incorporated into the positive step directions, and impurity concentrations are not homogeneous throughout the structure. This study demonstrates that strontium and magnesium have different surface interaction mechanisms, showing the importance of understanding growth processes at the nanoscale. We reiterate the significance of interpreting impurity signatures within the framework of step-specific interactions that occur during mineralization.