Constraining the effects of Mg:Ca ratio and temperature on

Report
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
Carbonate
(CaCO
)
-calcite
or
aragonite.
3
Calcite
CaCO3 is important because it is one of the most
widely
distributed
minerals
in
the
Aragonite
marine environment, forming biogenically and non-biogenically.
Hyslop, K. (2014) www.marlin.ac.uk
Vaterite
One otherare
polymorph
found
within
laboratory
setting,
Calcite and aragonite
both found
within
the the
geological
record,
but calcite is more
but because it is very
unstable
it is rarely
found
intemperature.
natural conditions, is vaterite.
stable
at ambient
pressure
and
M.Edulis shells contain both
aragonite and calcite polymorphs
Ooids from Great Salt Lake, Bahamas
5mm
1
CaCO3 precipitation experiments were
designed
Fluctuations
into
theinvestigate
seawater Mg:Ca ratio
may cause shifts in original composition of non-biogenic marine
and temperature
Still
carbonates toonbenon-biogenic
dominated by either calcite or aragonite (Morse et al., 2007) .
Higher temperature increases the growth rate of aragonite , while calcite growth slows
solution
CaCO3 (based on Morse et al., 2007 &
(Burton & Walter, 1987).
Bots et al., 2011).
Temperature within seawater changes latitudinally.
Constant addition of NaHCO3 to
Shaken
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
temperature
and
Mg:Ca
ratio.
o
& 30 C in still and shaken conditions
(shaking to mimic the natural
environment).
Sandberg (1983) proposed an ‘aragonite threshold’ where
Question:
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).
2
Influence
of
water
movement
(shaking
conditions)
Results
are
presented
for
the
number
of
crystals
precipitated
as
proportions
of
CaCO
However, temperature further influences these proportions of crystals grown. 3
These results
show
that
increased
Mg:Ca
ratio
influences
the
polymorph
proportions
precipitated.
o
Influence
of
temperature
(still
conditions)
Increased
Mg:Ca
ratio
on
CaCO
precipitates
(still
conditions)
resulting from increased Mg:Ca ratios of 1,3 2 & 3, and temperature s of 20 & 30 C.
Fewer
calcite
&
aragonite
crystals
are
precipitated
More
crystals
precipitate
in
still
conditions
than
Therefore,
Mg:Ca
ratio
cannot
be
investigated
in
isolation
without
considering
the
influence
of
The Insame
trends
in
CaCO
polymorphs
caused
by
Mg:Ca
ratio
are
also
present
when
temperature
is
3
all experiments calcite,
aragonite and vaterite polymorphs were found to co-precipitate
Numbers of calcite
crystals
decrease
in shaken conditions.
shaken.
temperature.
increased.
at all Mg:Ca ratios (1, 2 & 3).
o
but
more
aragonite
and
vaterite
crystals
precipitate
at
30
C.
More
vaterite
crystals
precipitate
in
shaken
conditions.
In order to mimic
the
natural
environment,
all
experimental
scenarios
were
repeated
Numbers of vaterite crystals are minor compared to
The
number
of
aragonite
and
These findings
are
based
on
non-biogenic
precipitates.
Numbers
of
calcite
crystals
with
the
addition
of
water
movement.
Fewer
crystals
precipitate
Fewer
crystals
precipitate
in
total
at
numbers of calcite and aragonite.
vateriteatcrystals
precipitated
precipitated
decrease
at
higher
Mg:Ca
higher
Mg:Ca
higher
temperatures
at higher
Mg:Ca.
ratios
However, as these results demonstrate non-biogenicincrease
polymorphs
can
be
influenced
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.
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 sediment3
producing organisms driven by tectonically forced shifts in seawater chemistry. Palaeogeography, Palaeoclimatology, Palaeoecology. 144, 3-19.

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