Constraining the effects of Mg:Ca ratio and temperature on non- biogenic CaCO 3 polymorph precipitation Caroline E. Miller 1, Uwe Balthasar 1, Maggie Cusack 1 School of Geographical Earth Sciences, University of Glasgow – Ocean chemistry has oscillated throughout Earth history, favouring the dominant non-biogenic polymorphs of Calcium Carbonate (CaCO 3 ) -calcite or aragonite. One other polymorph found within the laboratory setting, but because it is very unstable it is rarely found in natural conditions, is vaterite. CaCO 3 is important because it is one of the most widely distributed minerals in the marine environment, forming biogenically and non-biogenically. Calcite Aragonite Calcite and aragonite are both found within the geological record, but calcite is more stable at ambient pressure and temperature. 1 Ooids from Great Salt Lake, Bahamas 5mm Non-biogenic CaCO 3 precipitates can be grown in the lab. Vaterite ACS PRF Research Grant (PRF# ND8) M.Edulis shells contain both aragonite and calcite polymorphs Hyslop, K. (2014)

‘Aragonite-calcite seas’ are viewed as a global phenomenon where conditions fluctuate over time. This does not consider latitudinal temperature variations. Higher temperature increases the growth rate of aragonite, while calcite growth slows (Burton & Walter, 1987). Temperature within seawater changes latitudinally. Therefore, the spatial distribution of polymorph formation may be influenced by both temperature and Mg:Ca ratio. Question: What is the effect of combining temperature and Mg:Ca ratio on CaCO 3 polymorphs? Fluctuations in the seawater Mg:Ca may cause shifts in original composition of non-biogenic marine carbonates to be dominated by either calcite or aragonite (Morse et al., 2007). Sandberg (1983) proposed an ‘aragonite threshold’ where below Mg:Ca ratio of 2 only calcite will precipitate ; above 2, aragonite also precipitates. CaCO 3 precipitation experiments were designed to investigate Mg:Ca ratio and temperature on non-biogenic CaCO 3 (based on Morse et al., 2007 & Bots et al., 2011). Constant addition of NaHCO 3 to solutions of known Mg:Ca ratio (1, 2 &3) were carried out at 20 o C & 30 o C in still and shaken conditions (shaking to mimic the natural environment). 2 CaCO 3 precipitates were analysed using Raman Spectroscopy and Scanning Electron Microscope (SEM). Still solution Shaken solution

3 Increased Mg:Ca ratio on CaCO 3 precipitates (still conditions) The same trends in CaCO 3 polymorphs caused by Mg:Ca ratio are also present when temperature is increased. Numbers of vaterite crystals are minor compared to numbers of calcite and aragonite. Results are presented for the number of crystals precipitated as proportions of CaCO 3 resulting from increased Mg:Ca ratios of 1, 2 & 3, and temperature s of 20 & 30 o C. In order to mimic the natural environment, all experimental scenarios were repeated with the addition of water movement. More crystals precipitate in still conditions than shaken. More vaterite crystals precipitate in shaken conditions. Fewer calcite & aragonite crystals are precipitated in shaken conditions. Considering temperature alongside Mg:Ca ratio in a range of conditions that mimic the natural environment allows a realistic framework which can be applied to conditions today. These results show that increased Mg:Ca ratio influences the polymorph proportions precipitated. However, temperature further influences these proportions of crystals grown. Therefore, Mg:Ca ratio cannot be investigated in isolation without considering the influence of temperature. These findings are based on non-biogenic precipitates. However, as these results demonstrate non-biogenic polymorphs can be influenced by temperature, these findings can be applied to biogenic polymorph forms. Biomineralising organisms live within these seawater conditions therefore could influence the subsequent biomineralisation that occurs. References Bots, P., Benning, L.G., Rickaby, R.E.M. & Shaw, S. (2011) The role of So4 in the switch from calcite to aragonite seas. Geology. 39, Burton, E.A. & Walter, L. M. (1987) Relative precipitation rates of aragonite and Mg-calcite from seawater: Temperature or carbonate ion control? Geology. 15, Morse, J.W., Arvidson, R.S. & Luttge, A. (2007) calcium carbonate formation and dissolution. Chemical Reviews. 107, Sandberg, P.A. (1983) An oscillating trend in Phanerozoic non-skeletal carbonate mineralogy. Nature. 305 (1), Stanley, S.M. & Hardie, L.A. (1998) Secular oscillations in the carbonate mineralogy of reef-building and sediment-producing organisms driven by tectonically forced shifts in sediment- producing organisms driven by tectonically forced shifts in seawater chemistry. Palaeogeography, Palaeoclimatology, Palaeoecology. 144, In all experiments calcite, aragonite and vaterite polymorphs were found to co-precipitate at all Mg:Ca ratios (1, 2 & 3). Numbers of calcite crystals precipitated decrease at higher Mg:Ca ratios The number of aragonite and vaterite crystals precipitated increase at higher Mg:Ca. Fewer crystals precipitate at higher Mg:Ca Fewer crystals precipitate in total at higher temperatures Numbers of calcite crystals decrease but more aragonite and vaterite crystals precipitate at 30 o C. Influence of temperature (still conditions) Influence of water movement (shaking conditions)