3.4 Carbonates
tween 200 and 300 m. These reefs occur in the northeastern
Atlantic along a belt from the Iberian peninsula to Norway
and have grown in the Holocene on top of glacial deposits
or glacier-shaped sea floor (Henrich et al. 1996).
The rates of carbonate production (in terms of
kg/m 2 /a or cmlka) vary considerably in different regions and environments (cf. Sect. 10.2). Generally,
carbonate production is greater by about one order of
magnitude in tropical regions than in cooler areas,
but there are also examples of surprisingly high production in non-tropical seas, e.g. by coralline algae
(Lithothamnion sp.) building up reefs on shell beds
or rocks.
Ancient Carbonate-Producing Communities
The nature and properties of carbonate rocks are
strongly influenced by the evolution of carbonateproducing organisms and their different faunal assemblages through the Earth's history.
See, e.g., summary by James and Bourque (1992). As
frame-builders in the construction of reefs, stromatolites
played an important role in the Proterozoic; bryozoans,
strornatoporids and tabulate corals were abundant in the
lower Paleozoic, coralline sponges in the Permian, Triassic,
and Jurassic; crinoids were significant carbonate producers
on ramps with barrier shoals in the Triassic (Aigner 1985);
rudist bivalves formed large reef structures in the Cretaceous. This wide topic is not further treated here. Detailed
descriptions of the development and composition of carbonate buildups through geologic time are listed, e.g., by
Flügel and Flügel-Kahler (1995).
U sing the experience from modem environments, it
is now possible to discriminate between warm- and
cold-water carbonates also in the geological past. In
this case, the terms Chlorozoan, Chloralgal and
Foramol association mentioned above may be replaced by some other terms taking into account the
different biota of the past. For Permian carbonates,
for example, the terms Chlorosponge and
Chloraforam association (including green algae and
encrusting foraminifera) for warm environments or
Bryonoderm association (bryozoans and
echinoderms) for cold conditions have been proposed
(Beauchamp and Desrochers 1997). Non-tropical
carbonates were common in times when the Earth
was in an "icehouse state" (cf. Sect. 7.8).
Miocene examples of non-tropical temperate carbonates
have been described from Spain (Betzler et al. 1996) and
Austria (Nebelsick 1996). Permian cold-water carbonates
formed in the Permian shelf sea of Tasmania (Rao 1981),
where they alternate with tillites and silicic1astic shallowsea sediments rich in dropstones. Using criteria as mentioned above, temporal, latitudinal and bathymetric trends
similar to those of the modem world could be found in the
137
Permian along the northwestern margin of Pangea, i.e. in
the Canadian Arctic, Greenland, Svalbard, Barents Sea
(Beauchamp and Desrochers 1997). Siope carbonates along
subduction zones may have become incorporated into
accretionary wedges and masked by mechanical processes
and metamorphism. Nevertheless, more and more ancient
examples of non-tropical carbonates are identified.
3.4.3 Carbonate Mineralogy and
Non-Skeletal Grains
Organisms produce not only a variety of skeletal partides of different size and shape, but also carbonate
minerals of differing composition and thermodynamic stability. Tropical carbonates of the
chlorozoan association, rich in aragonite and highMg calcite, dissolve and recrystallize relatively easily
during early diagenesis in contrast to the temperate
and polar carbonates which are dominated by lowMg calcite. Thus, the warm-water carbonates tend to
form hardgrounds and to lithify earlier than cool-water carbonates.
N on-skeletal grains in carbonates comprise biochemically mediated carbonate particles (e.g. in association with bacteria, algae and seagrass), different
types of pellets as well as inorganically precipitated
carbonate minerals (mostly aragonite needles). An
often debated problem is the formation of micrite
(fine crystalline carbonate) which is a major constituent filling pore space in many reef structures.
Chemical precipitation of carbonate from supersaturated
sea water was observed in shallow parts ofthe Persian Gulf
and on the Great Bahama Bank (e.g., Shinn et al. 1989;
Milliman et al. 1993). In the Persian Gulf, these so-called
"whitings" are characterized by milky water containing
about 10 mg/I fine grained calcium carbonate, mostly aragonite. In the Bahamas, chemically precipitated aragonite
makes up more than 50% of the c1ay-size fraction of Iime
mud which is largely exported into deeper water.
Micrite rnay be produced either in situ via organic matrices (biomicrite, organOinicrite or automicrite, consisting of
high-Mg calcite; Reitner and Neuweiler 1995), or come
frorn other sources. Allomicrite is rich in aragonite, generated either by carbonate precipitation or mechanically
through reworking and grinding of skeletal carbonate as
weil as by boring and rasping organisms. Pellets are produced either by organisms (fecal pellets) or by reworking
of carbonate mud (lithic pellets). If their origin is not c1ear
they are referred to as peloids. Aggregates are composite
grains consisting of cemented smaller partic1es. (For detailed descriptions see, e.g., Bathurst 1975; Füchtbauer
1988; Tucker and Wright 1990; Rezak and Lavoie 1993).
Some of these grain types are rare or absent in cool and
low-salinity environments. Carbonate intraclasts are larger
(> 125 J.lm) and originate from reworking of serni-lithified
or lithified sediment.
The composition of reworked biogenic carbonate depends not only on the carbonate-producing organisms, but also on the grain-size fraction investigated
tween 200 and 300 m. These reefs occur in the northeastern
Atlantic along a belt from the Iberian peninsula to Norway
and have grown in the Holocene on top of glacial deposits
or glacier-shaped sea floor (Henrich et al. 1996).
The rates of carbonate production (in terms of
kg/m 2 /a or cmlka) vary considerably in different regions and environments (cf. Sect. 10.2). Generally,
carbonate production is greater by about one order of
magnitude in tropical regions than in cooler areas,
but there are also examples of surprisingly high production in non-tropical seas, e.g. by coralline algae
(Lithothamnion sp.) building up reefs on shell beds
or rocks.
Ancient Carbonate-Producing Communities
The nature and properties of carbonate rocks are
strongly influenced by the evolution of carbonateproducing organisms and their different faunal assemblages through the Earth's history.
See, e.g., summary by James and Bourque (1992). As
frame-builders in the construction of reefs, stromatolites
played an important role in the Proterozoic; bryozoans,
strornatoporids and tabulate corals were abundant in the
lower Paleozoic, coralline sponges in the Permian, Triassic,
and Jurassic; crinoids were significant carbonate producers
on ramps with barrier shoals in the Triassic (Aigner 1985);
rudist bivalves formed large reef structures in the Cretaceous. This wide topic is not further treated here. Detailed
descriptions of the development and composition of carbonate buildups through geologic time are listed, e.g., by
Flügel and Flügel-Kahler (1995).
U sing the experience from modem environments, it
is now possible to discriminate between warm- and
cold-water carbonates also in the geological past. In
this case, the terms Chlorozoan, Chloralgal and
Foramol association mentioned above may be replaced by some other terms taking into account the
different biota of the past. For Permian carbonates,
for example, the terms Chlorosponge and
Chloraforam association (including green algae and
encrusting foraminifera) for warm environments or
Bryonoderm association (bryozoans and
echinoderms) for cold conditions have been proposed
(Beauchamp and Desrochers 1997). Non-tropical
carbonates were common in times when the Earth
was in an "icehouse state" (cf. Sect. 7.8).
Miocene examples of non-tropical temperate carbonates
have been described from Spain (Betzler et al. 1996) and
Austria (Nebelsick 1996). Permian cold-water carbonates
formed in the Permian shelf sea of Tasmania (Rao 1981),
where they alternate with tillites and silicic1astic shallowsea sediments rich in dropstones. Using criteria as mentioned above, temporal, latitudinal and bathymetric trends
similar to those of the modem world could be found in the
137
Permian along the northwestern margin of Pangea, i.e. in
the Canadian Arctic, Greenland, Svalbard, Barents Sea
(Beauchamp and Desrochers 1997). Siope carbonates along
subduction zones may have become incorporated into
accretionary wedges and masked by mechanical processes
and metamorphism. Nevertheless, more and more ancient
examples of non-tropical carbonates are identified.
3.4.3 Carbonate Mineralogy and
Non-Skeletal Grains
Organisms produce not only a variety of skeletal partides of different size and shape, but also carbonate
minerals of differing composition and thermodynamic stability. Tropical carbonates of the
chlorozoan association, rich in aragonite and highMg calcite, dissolve and recrystallize relatively easily
during early diagenesis in contrast to the temperate
and polar carbonates which are dominated by lowMg calcite. Thus, the warm-water carbonates tend to
form hardgrounds and to lithify earlier than cool-water carbonates.
N on-skeletal grains in carbonates comprise biochemically mediated carbonate particles (e.g. in association with bacteria, algae and seagrass), different
types of pellets as well as inorganically precipitated
carbonate minerals (mostly aragonite needles). An
often debated problem is the formation of micrite
(fine crystalline carbonate) which is a major constituent filling pore space in many reef structures.
Chemical precipitation of carbonate from supersaturated
sea water was observed in shallow parts ofthe Persian Gulf
and on the Great Bahama Bank (e.g., Shinn et al. 1989;
Milliman et al. 1993). In the Persian Gulf, these so-called
"whitings" are characterized by milky water containing
about 10 mg/I fine grained calcium carbonate, mostly aragonite. In the Bahamas, chemically precipitated aragonite
makes up more than 50% of the c1ay-size fraction of Iime
mud which is largely exported into deeper water.
Micrite rnay be produced either in situ via organic matrices (biomicrite, organOinicrite or automicrite, consisting of
high-Mg calcite; Reitner and Neuweiler 1995), or come
frorn other sources. Allomicrite is rich in aragonite, generated either by carbonate precipitation or mechanically
through reworking and grinding of skeletal carbonate as
weil as by boring and rasping organisms. Pellets are produced either by organisms (fecal pellets) or by reworking
of carbonate mud (lithic pellets). If their origin is not c1ear
they are referred to as peloids. Aggregates are composite
grains consisting of cemented smaller partic1es. (For detailed descriptions see, e.g., Bathurst 1975; Füchtbauer
1988; Tucker and Wright 1990; Rezak and Lavoie 1993).
Some of these grain types are rare or absent in cool and
low-salinity environments. Carbonate intraclasts are larger
(> 125 J.lm) and originate from reworking of serni-lithified
or lithified sediment.
The composition of reworked biogenic carbonate depends not only on the carbonate-producing organisms, but also on the grain-size fraction investigated