Constraining the effects of Mg:Ca ratio and temperature on
Transcript Constraining the effects of Mg:Ca ratio and temperature on
Constraining the effects of Mg:Ca ratio and temperature on nonbiogenic CaCO3 polymorph precipitation
Caroline E. Miller1, Uwe Balthasar1, Maggie Cusack1
School of Geographical Earth Sciences, University of Glasgow – [email protected]
ACS PRF Research Grant (PRF# 52963-ND8)
Non-biogenic CaCO3 precipitates can be grown in the lab.
Ocean chemistry has oscillated throughout Earth history, favouring the dominant non-biogenic
polymorphs of Calcium
CaCO3 is important because it is one of the most
marine environment, forming biogenically and non-biogenically.
Hyslop, K. (2014) www.marlin.ac.uk
Calcite and aragonite
but calcite is more
but because it is very
it is rarely
natural conditions, is vaterite.
M.Edulis shells contain both
aragonite and calcite polymorphs
Ooids from Great Salt Lake, Bahamas
CaCO3 precipitation experiments were
seawater Mg:Ca ratio
may cause shifts in original composition of non-biogenic marine
dominated by either calcite or aragonite (Morse et al., 2007) .
Higher temperature increases the growth rate of aragonite , while calcite growth slows
CaCO3 (based on Morse et al., 2007 &
(Burton & Walter, 1987).
Bots et al., 2011).
Temperature within seawater changes latitudinally.
Constant addition of NaHCO3 to
solutions of known Mg:Ca
Therefore, the spatial distributiono of polymorph formation may be influenced by bothsolution
ratio (1, 2 &3) were carried out at 20 C
& 30 C in still and shaken conditions
(shaking to mimic the natural
Sandberg (1983) proposed an ‘aragonite threshold’ where
below Mg:Ca ratio of 2 only calcite will precipitate ; above 2, aragonite also precipitates.
What is the effect of combining temperature and Mg:Ca ratio on CaCO3 polymorphs?
CaCO3 precipitates were analysed using
‘Aragonite-calcite seas’ are viewed as a global phenomenon where
Raman Spectroscopy and
conditions fluctuate over time. This does not consider latitudinal temperature variations .
Scanning Electron Microscope (SEM).
However, temperature further influences these proportions of crystals grown. 3
resulting from increased Mg:Ca ratios of 1,3 2 & 3, and temperature s of 20 & 30 C.
all experiments calcite,
aragonite and vaterite polymorphs were found to co-precipitate
Numbers of calcite
in shaken conditions.
at all Mg:Ca ratios (1, 2 & 3).
In order to mimic
Numbers of vaterite crystals are minor compared to
numbers of calcite and aragonite.
However, as these results demonstrate non-biogenicincrease
by temperature, these findings can be applied to biogenic polymorph forms.
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.
Biomineralising organisms live within these seawater conditions therefore could influence the
subsequent biomineralisation that occurs.
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 sediment3
producing organisms driven by tectonically forced shifts in seawater chemistry. Palaeogeography, Palaeoclimatology, Palaeoecology. 144, 3-19.