The primary marine calcium carbonate polymorph is thought to have altered between calcite and aragonite throughout the Phanerozoic, on timescales of between 100 and 200 million years, giving rise to the terms calcite and aragonite seas. Aragonite seas are characterized by aragonite and high Mg calcite forming as the main precipitates while in calcite seas low Mg calcite is the common precipitate.
Past studies of the factors that have a control over the precipitation of carbonate minerals from seawater have observed that the precipitation of calcium carbonate is strongly influenced by factors such as Mg:Ca, temperature and pCO2. At present, aqueous Mg:Ca is thought to be the main driving force behind the oscillation between calcite and aragonite seas, with the alternations occurring at a Mg:Ca threshold of ~2 (Morse et al, 1997). Change in seawater Mg:Ca is driven by seafloor spreading (Stanley and Hardie, 1999), with periods of high oceanic spreading rates being coincident with calcite seas and greenhouse conditions due to increased atmospheric CO2. This therefore raises interesting questions regarding the link between calcite and aragonite seas and global climate.
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There is also a general consensus within the existing literature that temperature is a significant factor in determining calcium carbonate precipitation (Morse et al, 1997, Morse et al, 1993, Burton &Walter, 1987). The Mg:Ca ratio at which different polymorphs of calcium carbonate form is strongly dependent on temperature (Morse et al, 1997). It has also been observed that aragonite precipitation rates increase with temperature, with aragonite precipitation rates being more rapid than those of calcite at higher temperatures. In contrast, at lower temperatures calcite precipitation rates have been observed as being approximately equal to those of aragonite (Burton and Walter, 1987).
Studies have also raised questions regarding the effect of sulphate on calcium carbonate precipitation, although little work has been carried out. It has been claimed that sulphate is an inhibitor of calcite and aragonite, with it having a greater inhibitive effect on calcite (Walter, 1986). More recently, it has been observed that an increase in dissolved sulphate reduces the Mg:Ca at which aragonite becomes the dominant polymorph. Sulphate was also found to result in a decrease in calcite precipitation rate relative to aragonite, with the incorporation of sulphate having little effect on aragonite structure (Bots et al, 2011). Sulphate has also been observed to have had an effect on crystal morphology and that its incorporation has negative effects regarding both calcite and aragonite structure (Diaz et al, 2010).
This paper therefore sets out to report the combined effects of Mg:Ca, temperature and sulphate on calcium carbonate precipitation.
Five experiments were carried out at five different temperatures (10Â°C, 15Â°C, 20Â°C, 25Â°C, 30Â°C) using artificial sweater solutions to observe the effects of temperature, Mg:Ca and sulphate on calcium carbonate precipitation.
Four artificial seawater solutions were created prior to running each experiment - two at a Mg:Ca of 1.5 (representative of Silurian (calcite) sea conditions (Lowenstein et al, 2003) and two at a Mg:Ca of 5.1 (representative of Modern (aragonite) sea conditions). In both instances one solution contained sulphate while the second remained sulphate free. The initial solution (500ml) for each solution at a Mg:Ca = 1.5 contained approximately 2.4257g CaCl2, 5.0319g MgCl2 and 0.7153g Na2CO3, while at a Mg:Ca = 5.1 each solution was composed of approximately 0.7557g CaCl2, 5.3694g MgCl2 and 0.7153g Na2CO3. In the solutions in which sulphate was added 0.5682g and 2.0056g Na2SO4 were also added to the 1.5 and 5.1 solutions respectively, resulting in a concentration of 29mM - the concentration of modern seawater. The appropriate mass of NaCl was also added to bring the salinity of each solution up to 35g/l. Each solution was also bubbled with CO2 for 20 seconds, and solutions were left in an incubator at a set temperature overnight prior to running the experiments.
50ml of each solution was added to a labeled petri dish containing 4 glass coverslips and placed in the incubator. Samples were removed at one hour intervals over six hours in total. Coverslips were rinsed in Millipore water and titration experiments were then carried out on each solution with the resulting data was used to calculate total alkalinity and later run through an Excel based macro in order to calculate other variables such as pCO2.
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Crystal morphology was observed using scanning electron microscopy of the six hour samples, while Raman spectroscopy was used to determine the chemical composition of the precipitates and identify the time period in which calcite or aragonite initially begins to precipitate. Samples were gold coated prior to SEM analysis.
Using images taken at 320x magnification, crystal outlines were traced and polymorph size and abundance analysis was calculated using ImageJ.
Scanning electron microscopy revealed that calcite formed as crystals with a rhombohedra-like morphology with well defined crystal faces, while aragonite formed as crystals with a needle-like, branching morphology. A combination of Raman spectroscopy and scanning electron microscopy reveal that at a Mg:Ca = 1.5 at temperatures of 25Â°C and 30Â°C after six hours both calcite and aragonite had precipitated, while at 20Â°C calcite was the sole polymorph to precipitate. At the lower temperatures of 10Â°C and 15Â°C precipitates were scarce, and due to the small size of the existing crystals Raman spectroscopy is unable to confirm with certainty that these precipitates are definitively calcite or aragonite. At a Mg:Ca = 1.5 at 30Â°C, calcium carbonate was found to have started precipitating at 3 hours in the absence and presence of sulphate, while at a Mg:Ca = 5.1 aragonite did not precipitate until 6 hours in the absence of sulphate. At 25Â°C both calcite and aragonite precipitated after 3 hours in the absence of sulphate, while in samples containing sulphate precipitation does not appear to have begun until after 4 hours. At 20Â°C precipitates can only be said to have precipitated with certainty in the 6 hour samples. In contrast to samplesat a Mg:Ca = 1.5, at a Mg:Ca = 5.1 after six hours precipitates were far less abundant with aragonite appearing to be the sole calcium carbonate polymorph. At both 25Â°C and 30Â°C there were aragonite precipitates, while at 10Â°C, 15Â°C and 20Â°C there is no evidence of any calcium carbonate precipitation having taken place.
Figure 1. CaCO3 polymorphs as a function of temperature and Mg:Ca, A= sulphate absent, B= sulphate present. Red squares = aragonite, blue squares = calcite, blue/red squares = aragonite and calcite
This data is illustrated in Figure 1(A), showing the polymorphs that have precipitated as a function of temperature and Mg:Ca, while Figure 1(B) is the same figure but representing the solutions in which sulphate was present. As shown in Figure 1, the presence of sulphate produces similar results in terms of polymorphs present, with the noticeable exception of calcite at 10Â°C and a Mg:Ca = 1.5, where a solitary calcite crystal was discovered. However, despite the apparent similarities shown in Figure 1 sulphate has a significant effect on both the ratio of aragonite to calcite and crystal size.
Figure 2. Percentage aragonite relative to calcite in samples + or - sulphate after 6h. Red bars represent % aragonite, blue bars represent % calcite
AFigure 2 shows the percentage of aragonite relative to calcite and vice versa at temperatures between 20Â°C and 30Â°C after a time period of 6 hours, in both the presence and absence of sulphate. As can be seen from the figure, the presence of sulphate appears to slightly favour the precipitation of aragonite over calcite. This is evident at both 30Â°C (72.05% aragonite in the presence of sulphate in comparison to 51.05% in its absence) and to a lesser extent at 25Â°C (92.63% aragonite in the presence of sulphate in comparison to 85.01% in its absence). At 20Â°C SEM analysis suggests that precipitates were ~ 100% calcite, both in the presence and absence of sulphate, although the discovery of a solitary aragonite with Raman spectroscopy suggests that there are aragonite crystals present with the addition of sulphate, though these are scarce.
BSulphate also appears to have an inhibitory effect on crystal size (Figure 3). Image J analysis of crystal size for six hour samples at 20Â°C, 25Â°C, and 30Â°C showed that sulphate has had the most significant effect at 20Â°C, with an average reduction in calcite size of 39.37% (65 um2 to 21.41 um2). Six hour samples at 30Â°C showed an average reduction in aragonite size of 36.59% (821.96 um2 to 555.69 um2) while average calcite size was reduced by 32.39% (227.3 um2 to 144.12 um2). The effects of sulphate at 25Â°C were less significant, with calcite crystals reducing in size by an average of 15.99% (368.83 um2 to 290.36 um2) and aragonite crystals undergoing a 21.27% (126.25 um2 to 106.06 um2) size reduction.
Figure 3. The effect of sulphate on crystal size (um2) at Mg:Ca = 1.5 after 6h. Red columns are representative of calcite, blue columns are representative of calcite.
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The results of this study, along with previous work such as that of Morse et al (1997) support the assumption that calcium carbonate polymorph formation is dependent on both Mg:Ca and temperature. Aragonite precipitates in warm seawater at modern day Mg:Ca ratios ( known "aragonite sea" conditions) while calcite precipitates at lower Mg:Ca ratios (1.5 - known "calcite sea" conditons) at temperatures between 20Â°C and 30Â°C. However, unlike the Morse et al (1997) experiments precipitates are scarce at temperatures below 20Â°C. This could be a result of the time period that precipitation is left to occur in. It is possible that allowing precipitation to occur for longer than six hours could result in increased precipitates at lower temperatures.
At the higher Mg:Ca ratio of 5.1, aragonite is the sole precipitate. However, precipitates are less abundant and no precipitation occurs at temperatures below 25Â°C. This is in agreement with previous research regarding the effects of magnesium on calcium carbonate. Bots et al. (2011) notes that with increasing magnesium in solution the rate of substitution of magnesium into calcite increases. With this increase, calcite stability decreases and an increase in aragonite to calcite ratio is observed. Therefore, a with a higher Mg:Ca aragonite is expected to be the dominant polymorph. Berner (1975) also notes that the presence of magnesium in solution inhibits the precipitation of calcite due to the fact that it is adsorped on the surface of calcite more easily and to a greater extent than it is on the surface of aragonite. They claim that magnesium has little to no effect on the rate of aragonite precipitation. Morse et al (1997) also observed that aragonite formed at higher temperatures and higher Mg:Ca ratios.
There is little past research regarding the effects of sulphate on calcium carbonate precipitation, but indications are that dissolved sulphate can have a significant effect on the polymorph that forms. Bots et al. (2011) observed that sulphate incorporation into calcite resulted in a change in calcite structure, while also causing an increase in the solubility of calcite and decreasing the precipitation rate of calcite in comparison to aragonite (Walter, 1986, cited in Bots et al. 2011). It was observed that the incorporation of sulphate into aragonite was 2-4 times lower than the incorporation of sulphate into calcite. In contrast with its effects on calcite, sulphate was observed to have little effect on aragonite structure. Bots et al. (2011) claim that with sulphate having less influence on aragonite structure than it does on calcite structure, aragonite becomes more stable than calcite. This is a result of the differing structure of calcite and aragonite, which therefore affects the free energy of each polymorph differently and as a result their stability (Diaz et al, 2010). In a study by Diaz et al. (2010), it was observed that the crystal lattice energy was less for calcite than that of aragonite and they claim that this suggests that the substitution of sulphate ions for carbonate ions was unfavorable regarding aragonite structure. Diaz et al. (2010) also note that calcite can incorporate sulphate more actively than aragonite, and note that sulphate promotes the precipitation of aragonite while having an inhibitory effect on the transformation of aragonite to calcite, as aforementioned by Bots et al (2011).
These findings are somewhat reflected in the results of this study, as shown in Figure 2. At both 25Â°C and 30Â°C the presence of sulphate in solution appears to favour the precipitation of aragonite over calcite. It is possible that this is a result of the aforementioned effects of sulphate on calcite and aragonite structure. However, the indication is that sulphate incorporation has also been shown to have a significant effect on aragonite crystals too, with significant reductions in crystal size at 25Â°C and 30Â°C (Figure 3). Therefore, aragonite may be the more stable polymorph in the presence of sulphate but its addition can still have an inhibitory effect on its precipitation.
Associations have been made between aragonite sea periods and times of decreased mid ocean spreading rates. Low spreading rates contribute towards low hydrothermal brine fluxes and lower Mg:Ca ratios, increasing the MgSO4 composition of seawater and resulting in the precipitation of MgSO4 evaporites (Stanley and Hardie, 1999)â€¦ tbc
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