59th Annual Report on Research 2014

Background

The CaCO₃ minerals aragonite and calcite form as common abiotic precipitates in seawater and represent the most abundant composition of the shells and skeletons of marine invertebrates. Oscillations in the main composition of abiotic CaCO₃ precipitation through deep time suggests that the past 540 million years can be subdivided into three intervals of ‘aragonite seas’ and two intervals of ‘calcite seas’ during which seawater conditions favored the abiotic precipitation of either aragonite or calcite. This aragonite/calcite sea hypothesis represents the main environmental context in which to assess the evolution of CaCO₃ biomineralization. Interpretation of the fossil record in the context of the established aragonite/calcite sea hypothesis, however, has produced contradicting results. An underlying assumption of the established aragonite / calcite sea hypothesis is that conditions are globally homogenous and thus change only through time, but not in space. Here we aim to amend the established aragonite / calcite sea concept by quantifying the influence of temperature on the type of CaCO₃ polymorph that will form from solution. Temperature is likely to shift the conditions at which different types of CaCO₃ polymorphs form by latitude and depth. Although a variety of parameters influence the type of CaCO₃ polymorph that will form from seawater, the main driving force is generally thought to be the ratio of Mg:Ca with a critical threshold between 1.3 and 2 separating exclusive calcite precipitation (at low Mg:Ca ratios) from a combination of high-Mg calcite and aragonite precipitation (at high Mg:Ca ratios).

Methods

CaCO₃ precipitation experiments were carried out at Mg:Ca ratios between 0.5 and 5.2, at temperatures of 15°, 20°, 25°, or 30°C, and at a fixed salinity of 35. CaCO₃ precipitation was initiated in two different ways: (1) by CO₂ degassing or (2) by constant addition of NaHCO₃. The constant addition experiments were further subdivided into solutions that remained still and solutions that were shaken on an orbital shaker at 80 rpm. For the degassing set-up, solutions were briefly bubbled with CO₂ before adding Na₂CO₃ immediately before the start of the experiment. For constant addition NaHCO₃ was added at a rate of 0.25 mM/h over 6 hours. For both experimental approaches nucleation occurred within 1-9 hours. Subsamples of the solution and of glass slides within the solution were collected every hour. The mineralogy of precipitates was determined using Raman spectroscopy and the distribution and morphology of crystals was documented using scanning electron microscopy. Of subsamples containing the first precipitates 5-10 images of the same magnification were taken containing a total of at least 100 crystals. These images were manually transformed into separate black & white images for calcite and aragonite crystals and the area coverage was calculated using imageJ.

Preliminary results The degassing experiments resulted in a broad field of aragonite and calcite co-precipitation in which the proportion of aragonite increases with temperature and Mg:Ca ratio (Figure 1). Almost exclusive calcite precipitation (<1% aragonite) was limited to a Mg:Ca ratio of 0.5 at temperatures below 20° C, and almost exclusive aragonite precipitation (>99% aragonite) at the upper spectrum of investigated Mg:Ca ratio and temperatures (Figure 1). Importantly, within the field of co-precipitation the proportions of CaCO₃ polymorphs can be expressed as a function of Mg:Ca ratio and temperature. Results from the constant addition set-up are currently being analyzed and suggest a similar situation of aragonite and calcite co-precipitation as a function of Mg:Ca ratio and temperature. Using the quantitative relationship of CaCO₃ polymorph proportions from the degassing experiments, it is possible to use published models of Phanerozoic Mg:Ca to estimate the proportion of aragonite that would have formed under similar conditions at a given temperature. The resulting graphs (Figure 2) provide a temperature-corrected perspective of aragonite / calcite seas that suggests that, contrary to the established view, abiotic aragonite could precipitate throughout most of the Phanerozoic in waters warmer than 20° C, whereas exclusive calcite precipitation was unlikely in such settings.

Figure 1. Results from the degassing experiments showing a broad zone of aragonite and calcite co-precipitation (in grey) with aragonite proportions increasing with temperature and Mg:Ca ratio.

Figure 2. Models of Phanerozoic Mg:Ca ratio expressed as percent of abiotic aragonite at a given temperature. Vertical axis as temperature (°C) and horizontal axis in millions of years. Black and white boxes labelled ‘aragonite’, ‘calcite’, and ‘?’ indicate the stratigraphic duration of traditional aragonite and calcite sea intervals. The models used are: A: Berner (2004, American Journal of Science); B: Demicco et al. (2005, Geology); C: Arvidson et al. (2013, Chemical Geology); D: Farkaš et al. (2007, Geochimica et Cosmochimica Acta).