60th Annual Report on Research 2015

This project uses individual crystal counts derived from CaCO₃ precipitation experiments to investigate the influence of environmental factors such as the ratio of Mg:Ca, temperature, water agitation, and the presence of sulphate on the quantity and proportions of aragonite and calcite. Results will be interpreted in the context of the aragonite-calcite sea concept which describes secular oscillations of abiotic precipitation of aragonite and calcite throughout Earth history as a function of seawater composition. The aragonite-calcite sea concept in turn provides the environmental framework in which to assess the influence of seawater composition on the evolution of CaCO₃ biomineralization in marine organisms.

In the first phase of the project we initiated CaCO₃ precipitation via CO₂ degassing, the method used in the influential paper by Morse et al. (1997, Geology). Instead of a distinct Mg:Ca threshold that separates pure aragonite from calcite precipitation windows, as postulated in previous studies, we found that aragonite and calcite co-precipitate over a broad range of ambient Mg:Ca and temperature conditions with aragonite proportions increasing with temperature and Mg:Ca ratio. Our data suggest that instead of the traditional view of aragonite-calcite sea fluctuations through time there was a pronounced spatial pattern with aragonite being dominant in waters above 20º C throughout Earth history.

Throughout the second phase of this project we carried out experiments in which we precipitated CaCO₃ using continuous addition of NaHCO₃. By contrast to the degassing approach, the continuous addition experimental set-up guarantees an almost constant pCO₂ and a better control of pH and alkalinity. Constant addition experiments were carried out either as still or on an orbital shaker to investigate the effect of water agitation on CaCO₃ precipitation. The experiments were run at 20º C and 30º C, at Mg:Ca ratios of 1, 2, and 3, and with or without 5 mM sulphate (the latter at a Mg:Ca ratio of 1 only).

Our continuous addition experiments corroborate the general observation that aragonite and calcite co-precipitate with greater proportions of aragonite precipitating at higher Mg:Ca ratios and higher temperature. We find that the degassing experiments generally resulted in higher proportions of aragonite, which is best explained by the raised pCO₂ of this experimental set-up (Lee & Morse 2010, Geology). 

Quantifications of individual crystals enable us to portray changes in CaCO₃ polymorph proportions as differences in the number of nucleation events (as represented by the number of crystals) and crystal growth (as represented by crystal size). Preliminary analysis of our data for experiments conducted at a Mg:Ca ratio of 1 indicates that crystal growth is more strongly dependent on temperature than crystal nucleations (see blue (20º C) versus red (30º C) symbols in Figure 1). Shaken conditions (square symbols in Fig. 1) generally increases the aragonite proportion primarily through increasing aragonite nucleations while decreasing those of calcite (Figs. 1E-F).

The addition of sulphate is generally acknowledged to shift the CaCO₃ polymorph balance towards aragonite. In our experiments, adding 5 mM of sulphate increases nucleations for both polymorphs in still conditions while shaken conditions result in a decrease of aragonite nucleations whereas calcite appears to remain unaffected (Fig. 1B). The overall increase in aragonite proportions is primarily the result of an increased aragonite crystal growth (Fig. 1D).

Figure 1. Quantifications of CaCO₃ precipitation experiments at a Mg:Ca ratio of 1. A-B: Number of crystals per SEM image (size: ca. 0.23 mm²); C-D: average crystal size; E-F: Calcite/Aragonite proportions of crystal size against the number of crystals. Left column (A, C, E) without sulphate; right column (B, D, F) with sulphate.