5.7.3.4 Evaporite Basins
In ocean areas with normal salinity, practically all
carbonate deposition is through biological precipitation. This is because the biological precipitation is
rather efficient so the sea water does not become
saturated with respect to the most common carbonate
minerals like aragonite and calcite. In areas with
somewhat higher salinity, however, for example in
the Persian Gulf, there may also be chemical precipitation of calcium carbonate. This is because there are
fewer organisms to precipitate carbonate and because
of evaporation concentrating the sea water. Here too
this precipitation may nevertheless be linked to
biological factors. Periods with algal blooms in the
surface waters involve photosynthesis and consumption of CO 2 . This raises the pH, creating oversaturation and thus favourable conditions for chemical
precipitation.
Carbonates make up only a small percentage of
the salt precipitated when seawater evaporates to
dryness. However, they are among the least soluble
of the common salts in restricted ocean basins where
the salinity is too low for the more soluble salts to
precipitate (e.g. NaCl). Carbonates together with
sulphates like gypsum or anhydrite often form thick
evaporite sequences. In highly saline basins with more
intense evaporation and restricted circulation, the
water will become enriched in Mg
2+ and dolomite
will form, perhaps also magnesite.
5.7.3.5 Carbonate Turbidites on Slopes
Skirting Carbonate Platforms
Carbonate sand and mud which is stirred up during
storms over shallow water may be transported over the
shelf edge to continue downslope as turbidites or
debris flows. Carbonate platforms may rise 2–3 km
above the surrounding seafloor and the slope may be
very steep (20–30
) because of the hard and stable
carbonate rocks.
5.7.3.6 Pelagic Carbonate
Pelagic carbonate deposits consist largely of planktonic organisms (coccolithophores, foraminifera,
pteropods etc.) which live in the upper part of the
water column, sinking to the bottom when they die.
How clean the carbonate deposits are depends on how
much other biological sedimentation there is, e.g.
from siliceous organisms like diatoms and radiolaria,
and how rapidly clastic sedimentation takes place.
Therefore, carbonate sediments commonly become
concentrated on top of submarine highs where the
siliciclastic sedimentation rates are low. This is often
observed on seismic lines where the reflections
become stronger on the top of submarine positive
tectonic structures.
In the deep ocean much of the pelagic carbonate
production is dissolved as the particles settle through
the water column. The sedimentation rate is a function
of productivity in the upper water layers minus solution as the dead organisms sink towards the seafloor.
The dissolution of skeletal material is due to
undersaturation of CaCO 3 in deep ocean water
because this cold water can dissolve more CO 2 than
warm surface water. The combination of low
temperatures and increasing hydrostatic pressure
with depth involves an increase in the pCO 2 and
decrease in the pH. The CO 2 is produced by respiration and decay of pelagic organisms in the deep ocean.
Below the depths where the rate of dissolution is equal
to the rate of sedimentation of carbonate, no carbonate
sediments accumulate. This is called the carbonate
compensation depth (CCD) and varies from 1–2 km
in cold water at higher latitude to 4–5 km in the warm
water equatorial regions.
Since the planktonic carbonate organisms are very
small, pelagic carbonate deposits form a fine-grained
ooze of clay- and silt-size particles, with occasional
larger fossil fragments. Large areas of the South
Atlantic and Pacific are covered by sediment
containing more than 50% CaCO 3 from planktonic
organisms. Foraminifera and coccolithophores form
the most important deep-sea carbonate deposits
(Fig. 5.14).
Coccolithophores live mainly in the photic zone.
In areas of high productivity, for example in the fjords
of Norway, the concentration of coccolithophores may
be several millions per litre, but 50,000–500,000 is a
more normal level. Although they consist of low-Mg
calcite, their size makes them relatively soluble in
cold water. In consequence, although production is
greatest at high latitudes, it is only at lower latitudes
that large quantities of coccolithophores are able to
accumulate on the seafloor.
Shallow, warm seas with little other carbonate production provide particularly favourable conditions
for the deposition of purer coccolith deposits. The
seas of northwest Europe in Cretaceous times were
a good example. The climate in the Mesozoic was
5 Carbonate Sediments
193
In ocean areas with normal salinity, practically all
carbonate deposition is through biological precipitation. This is because the biological precipitation is
rather efficient so the sea water does not become
saturated with respect to the most common carbonate
minerals like aragonite and calcite. In areas with
somewhat higher salinity, however, for example in
the Persian Gulf, there may also be chemical precipitation of calcium carbonate. This is because there are
fewer organisms to precipitate carbonate and because
of evaporation concentrating the sea water. Here too
this precipitation may nevertheless be linked to
biological factors. Periods with algal blooms in the
surface waters involve photosynthesis and consumption of CO 2 . This raises the pH, creating oversaturation and thus favourable conditions for chemical
precipitation.
Carbonates make up only a small percentage of
the salt precipitated when seawater evaporates to
dryness. However, they are among the least soluble
of the common salts in restricted ocean basins where
the salinity is too low for the more soluble salts to
precipitate (e.g. NaCl). Carbonates together with
sulphates like gypsum or anhydrite often form thick
evaporite sequences. In highly saline basins with more
intense evaporation and restricted circulation, the
water will become enriched in Mg
2+ and dolomite
will form, perhaps also magnesite.
5.7.3.5 Carbonate Turbidites on Slopes
Skirting Carbonate Platforms
Carbonate sand and mud which is stirred up during
storms over shallow water may be transported over the
shelf edge to continue downslope as turbidites or
debris flows. Carbonate platforms may rise 2–3 km
above the surrounding seafloor and the slope may be
very steep (20–30
) because of the hard and stable
carbonate rocks.
5.7.3.6 Pelagic Carbonate
Pelagic carbonate deposits consist largely of planktonic organisms (coccolithophores, foraminifera,
pteropods etc.) which live in the upper part of the
water column, sinking to the bottom when they die.
How clean the carbonate deposits are depends on how
much other biological sedimentation there is, e.g.
from siliceous organisms like diatoms and radiolaria,
and how rapidly clastic sedimentation takes place.
Therefore, carbonate sediments commonly become
concentrated on top of submarine highs where the
siliciclastic sedimentation rates are low. This is often
observed on seismic lines where the reflections
become stronger on the top of submarine positive
tectonic structures.
In the deep ocean much of the pelagic carbonate
production is dissolved as the particles settle through
the water column. The sedimentation rate is a function
of productivity in the upper water layers minus solution as the dead organisms sink towards the seafloor.
The dissolution of skeletal material is due to
undersaturation of CaCO 3 in deep ocean water
because this cold water can dissolve more CO 2 than
warm surface water. The combination of low
temperatures and increasing hydrostatic pressure
with depth involves an increase in the pCO 2 and
decrease in the pH. The CO 2 is produced by respiration and decay of pelagic organisms in the deep ocean.
Below the depths where the rate of dissolution is equal
to the rate of sedimentation of carbonate, no carbonate
sediments accumulate. This is called the carbonate
compensation depth (CCD) and varies from 1–2 km
in cold water at higher latitude to 4–5 km in the warm
water equatorial regions.
Since the planktonic carbonate organisms are very
small, pelagic carbonate deposits form a fine-grained
ooze of clay- and silt-size particles, with occasional
larger fossil fragments. Large areas of the South
Atlantic and Pacific are covered by sediment
containing more than 50% CaCO 3 from planktonic
organisms. Foraminifera and coccolithophores form
the most important deep-sea carbonate deposits
(Fig. 5.14).
Coccolithophores live mainly in the photic zone.
In areas of high productivity, for example in the fjords
of Norway, the concentration of coccolithophores may
be several millions per litre, but 50,000–500,000 is a
more normal level. Although they consist of low-Mg
calcite, their size makes them relatively soluble in
cold water. In consequence, although production is
greatest at high latitudes, it is only at lower latitudes
that large quantities of coccolithophores are able to
accumulate on the seafloor.
Shallow, warm seas with little other carbonate production provide particularly favourable conditions
for the deposition of purer coccolith deposits. The
seas of northwest Europe in Cretaceous times were
a good example. The climate in the Mesozoic was
5 Carbonate Sediments
193