Constraining the effects of Mg:Ca ratio and temperature on non- biogenic CaCO 3 polymorph...
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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 – [email protected]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# 52963-ND8) M.Edulis shells contain both aragonite and calcite polymorphs Hyslop, K. (2014) www.marlin.ac.uk
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
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 [email protected] 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# 52963-ND8) M.Edulis shells contain both aragonite and
calcite polymorphs Hyslop, K. (2014) www.marlin.ac.uk
Slide 2
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
Slide 3
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, 331-334. Burton, E.A.
& Walter, L. M. (1987) Relative precipitation rates of
aragonite and Mg-calcite from seawater: Temperature or carbonate
ion control? Geology. 15, 111-114. Morse, J.W., Arvidson, R.S.
& Luttge, A. (2007) calcium carbonate formation and
dissolution. Chemical Reviews. 107, 342-381. Sandberg, P.A. (1983)
An oscillating trend in Phanerozoic non-skeletal carbonate
mineralogy. Nature. 305 (1), 19-22. 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, 3-19. 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)