9
Marine Carbonates: Their Formation and Destruction
312
ticles which have their origin in biotic and abiotic
processes, or by massive reefs and platforms built
up by skeleton-forming organisms. On a global
scale, these shallow-water carbonates in the modern environment are mainly constituted by particles of skeletal origin. However, aside from the
corals, the understanding of the physico-chemical
and vital factors affecting the biomineral composition of shallow platform calcareous sediments in
warm waters is still incomplete. Shallow environment precipitates form ooids and aragonitic needle
muds, whereby the former involve primarily
abiotic processes and the latter have both an
abiotic and biotic source.
For long time, the classical picture of shallow
water carbonates was suggesting that most of
their formation was restricted to tropical and subtropical regions within the 22
°
C isotherm of annual
mean surface water temperatures (e.g. Berger and
Seibold 1993), but it now has become evident that
a significant amount of carbonate can also be
formed as so-called 'cool-water' carbonate banks
and reefs in temperate and cold latitudes (review
by James 1997; Freiwald 2002). In the temperate
and cold-water zones, particularly shelf and upper
slope areas with only very low inputs of
terrigenous sediments are covered by cool-water
carbonate bioherms (Fig. 9.1a). Different from the
warm-water environment, where the major portion
of skeletal carbonate is predominantly formed by
an association of hermatypic corals and green
algae referred to as 'Photozoan Association'
(James 1997), the cool-water carbonates can be
composed of molluscs, foraminifers, echinoderms,
bryozoans, barnacles, ostracods, sponges, worms,
ahermatypic corals and coralline algae. For
differentiation from the warm-water Photozoan
Association, the group of organisms forming coolwater carbonates in shallow waters that are colder
than 20
°
C is defined by James 1997 as Heterozoan
Association (see also Fig. 9.1). According to
Freiwald (2002) the largest coral reef provinces
occur at greater water depths under cold and dark
conditions usually below the storm wave base from
the high to low latitudes of both hemispheres. In
the North Atlantic the dominant reef-forming corals
belong to scleractinian species. A first attempt to
calculate the amount of CaCO 3 produced by coldwater reefs on global carbonate production was
provided by Lindberg & Mienert (2005). Their
estimate is in the order of 4 to 12 % of that of
tropical reefs which results in a tentative estimate >
1 % of total marine carbonate production. Since
these first estimates are relying on data from the
Norwegian shelf only, it is difficult to come up
with really reliable global carbonate production
esti-mates for cold-water reefs at the moment.
Therefore the estimates of Wollast (1994); Milliman & Droxler (1996), and Vecsei (2004) for
worldwide shelf areas are considered here. For
budget considerations it seems feasible to separate shal-low-water carbonates according to
Milliman (1993) into coral reefs, carbonate
platforms which consist of non-reef habitats,
such as banks and embayments dominated by
the sedimentation of biogenic and abiogenic
calcareous particles, and shelves which can be
further subdivided into car-bonate-rich and carbonate-poor shelves (Table 9.1 adopted from
Milliman (1993) and Milliman and Droxler (1996),
taking into account Vescei (2004) estimates.
Reefs
Hermatypic coralalgal reefs and their fore-reef
sections occupy an area of about 0.35 ⋅10
6
km
2
and
are considered as the most productive carbonate
environments in modern times. Carbonate
accretion is the result of corals and green algae
and, to a minor extent also of benthic foraminifera.
Measured on a global scale, the mean calcium
carbonate accumulation in coral reefs is in the
order of 900-2700 g CaCO 3 m
-2
yr
-1
. The total
present-day global CaCO 3 production by coral
reefs then is 6.5-8.3 ⋅10
12
mol yr
-1
from which
about 7⋅10
12
mol yr
-1
accumulate, while the rest
(up to 1.5 ⋅10
12
mol yr
-1
) undergoes physical
erosion and offshore transport, as well as
biological destruction (Milliman 1993).
Carbonate platforms
These platforms are the second important tropical
to subtropical environment where high amounts of
carbonate are produced and accumulated at water
depths shallower than 50 m. The areal extension is
about 0.8⋅10
6
km
2
. In contrast to reefs, on
carbonate platforms production is mainly carried
out by benthic red/green algae, mollusks and
benthic foraminifera. Estimates of biotic and, to a
much lesser extent, abiotic carbonate production on
platforms range between 300-500 g CaCO 3 m
-2
yr
-1
,
which amounts to 4⋅10
12
mol yr
-1
on a global scale.
Accumulation of platform carbonate is difficult to
assess because a lot of it is dissolved or can be
found as exported material in several 10 to 100 m
Marine Carbonates: Their Formation and Destruction
312
ticles which have their origin in biotic and abiotic
processes, or by massive reefs and platforms built
up by skeleton-forming organisms. On a global
scale, these shallow-water carbonates in the modern environment are mainly constituted by particles of skeletal origin. However, aside from the
corals, the understanding of the physico-chemical
and vital factors affecting the biomineral composition of shallow platform calcareous sediments in
warm waters is still incomplete. Shallow environment precipitates form ooids and aragonitic needle
muds, whereby the former involve primarily
abiotic processes and the latter have both an
abiotic and biotic source.
For long time, the classical picture of shallow
water carbonates was suggesting that most of
their formation was restricted to tropical and subtropical regions within the 22
°
C isotherm of annual
mean surface water temperatures (e.g. Berger and
Seibold 1993), but it now has become evident that
a significant amount of carbonate can also be
formed as so-called 'cool-water' carbonate banks
and reefs in temperate and cold latitudes (review
by James 1997; Freiwald 2002). In the temperate
and cold-water zones, particularly shelf and upper
slope areas with only very low inputs of
terrigenous sediments are covered by cool-water
carbonate bioherms (Fig. 9.1a). Different from the
warm-water environment, where the major portion
of skeletal carbonate is predominantly formed by
an association of hermatypic corals and green
algae referred to as 'Photozoan Association'
(James 1997), the cool-water carbonates can be
composed of molluscs, foraminifers, echinoderms,
bryozoans, barnacles, ostracods, sponges, worms,
ahermatypic corals and coralline algae. For
differentiation from the warm-water Photozoan
Association, the group of organisms forming coolwater carbonates in shallow waters that are colder
than 20
°
C is defined by James 1997 as Heterozoan
Association (see also Fig. 9.1). According to
Freiwald (2002) the largest coral reef provinces
occur at greater water depths under cold and dark
conditions usually below the storm wave base from
the high to low latitudes of both hemispheres. In
the North Atlantic the dominant reef-forming corals
belong to scleractinian species. A first attempt to
calculate the amount of CaCO 3 produced by coldwater reefs on global carbonate production was
provided by Lindberg & Mienert (2005). Their
estimate is in the order of 4 to 12 % of that of
tropical reefs which results in a tentative estimate >
1 % of total marine carbonate production. Since
these first estimates are relying on data from the
Norwegian shelf only, it is difficult to come up
with really reliable global carbonate production
esti-mates for cold-water reefs at the moment.
Therefore the estimates of Wollast (1994); Milliman & Droxler (1996), and Vecsei (2004) for
worldwide shelf areas are considered here. For
budget considerations it seems feasible to separate shal-low-water carbonates according to
Milliman (1993) into coral reefs, carbonate
platforms which consist of non-reef habitats,
such as banks and embayments dominated by
the sedimentation of biogenic and abiogenic
calcareous particles, and shelves which can be
further subdivided into car-bonate-rich and carbonate-poor shelves (Table 9.1 adopted from
Milliman (1993) and Milliman and Droxler (1996),
taking into account Vescei (2004) estimates.
Reefs
Hermatypic coralalgal reefs and their fore-reef
sections occupy an area of about 0.35 ⋅10
6
km
2
and
are considered as the most productive carbonate
environments in modern times. Carbonate
accretion is the result of corals and green algae
and, to a minor extent also of benthic foraminifera.
Measured on a global scale, the mean calcium
carbonate accumulation in coral reefs is in the
order of 900-2700 g CaCO 3 m
-2
yr
-1
. The total
present-day global CaCO 3 production by coral
reefs then is 6.5-8.3 ⋅10
12
mol yr
-1
from which
about 7⋅10
12
mol yr
-1
accumulate, while the rest
(up to 1.5 ⋅10
12
mol yr
-1
) undergoes physical
erosion and offshore transport, as well as
biological destruction (Milliman 1993).
Carbonate platforms
These platforms are the second important tropical
to subtropical environment where high amounts of
carbonate are produced and accumulated at water
depths shallower than 50 m. The areal extension is
about 0.8⋅10
6
km
2
. In contrast to reefs, on
carbonate platforms production is mainly carried
out by benthic red/green algae, mollusks and
benthic foraminifera. Estimates of biotic and, to a
much lesser extent, abiotic carbonate production on
platforms range between 300-500 g CaCO 3 m
-2
yr
-1
,
which amounts to 4⋅10
12
mol yr
-1
on a global scale.
Accumulation of platform carbonate is difficult to
assess because a lot of it is dissolved or can be
found as exported material in several 10 to 100 m