315
9.2
Marine Environments of Carbonate Production and Accumulation
production should at least exceed 20 g m
-2
yr
-1
which is the average value of planktonic production in coastal waters. While global carbonate production on carbonate-poor shelves may be relatively high (4⋅10
12
mol yr
-1
; Milliman and Droxler
1996), carbonate accumulation in this environment
is negligible.
The rate of total neritic carbonate production in
the modern ocean is roughly 25⋅10
12
mol yr
-1
from
which 60 % (15⋅10
12
mol yr
-1
) accumulate as shallowwater carbonates. The difference, 10⋅10
12
mol yr
-1
, is
the contribution of the neritic environment to the
pelagic environment, either in the form of flux of
total dissolved inorganic carbon or particulate accumulation on continental slopes and in the deep
sea.
9.2.2
Pelagic Calcareous Sediments
Calcium carbonate is the most important biogenic
component in pelagic marine sediments which
cover an area of about 320⋅10
6
km
2
. Carbonate-rich
sediments (>30 % CaCO 3 ) form about 55 % of the
deposits on the continental slopes and the deepsea floor (Lisitzin 1996; Milliman 1993). A new
compilation on the distribution of carbonate-rich
pelagic sediments (Fig. 9.2b) has been recently
carried out by Archer (1996a).
The distribution of pelagic marine carbonates
is a result of a variety of influences. The most important components of the pelagic carbonate system affecting their formation and destruction are
shown in Figure 9.2b. These can be divided in external contributions such as riverine input and
CO 2 -exchange with the atmosphere and the lithosphere (hydrothermal venting), and internal processes. The main internal processes are those by
which CaCO 3 is produced and dissolved in the
ocean. In the pelagic realm carbonate is mainly
produced by planktonic organisms in ocean surface waters supersaturated with calcite and aragonite. Production of biogenic calcite or aragonite
comes from planktonic organisms like coccolithophorids, foraminifers and pteropods, as well as
calcareous dinoflagellates. The amount of the total production is determined by temperature, light,
and nutrient-conditions in the surface ocean. On
upper slopes, where food supply is high, part of
the carbonate production also originates from
benthic foraminifera and small mollusks. Estimates
of the global calcium carbonate production by
pelagic organisms are difficult to assess, because
the amount of production per unit of water volume
or area cannot be directly measured. In addition,
according to sparse data available from plankton
hauls and shallow floating trap catchments they
vary significantly between oligotrophic and eutrophic, and particularly between silicon-poor and silicon-rich surface waters (Fischer et al. 2004). Mean
global estimates are either derived from sediment
trap fluxes at about 1000 m water depth (Milliman
1993) or from theoretical approaches in which the
production required to explain the observed watercolumn profiles of alkalinity and total dissolved
inorganic carbon (Fig. 9.3) is calculated, taking the
modern ocean circulation pattern and residence
times of the different water masses into account.
However, these estimates differ by a factor of two
to three, whereby the sediment trap data and
surface-water properties suggest a mean global
calcium carbonate production of 23 or 60 to 90⋅10
12
mol yr
-1
, respectively (see comments and discussion in Morse and Mackenzie 1990; Milliman
and Droxler 1996; Wollast 1994).
How much of the carbonate produced in the
surface ocean finally accumulates on the sea floor
and is buried there, depends on the calcite dissolution in the water column during the settling of
particles and on the calcite dissolution within the
sediment. For slope sediments also dilution with
sedimentation of terrigeneous material affects the
carbonate content. Dissolution of pelagic carbonates is driven by the aragonite and carbonate
saturation levels in the deep-sea, controlled by
temperature and pressure conditions, as well as
by pH and alkalinity of circulating deep-water
masses (Berger 1976; Broecker and Peng 1982).
On its way to the deep sea the settling biogenic
carbonate reaches waters which are increasingly
undersaturated with respect to aragonite and
later calcite, due to the decrease in temperature,
increases of pressure, and the latter CO 2
resulting from the reoxidation of organic matter.
Increase of CO 2 due to benthic respiration also
affects CaCO 3 which reaches the sea floor. Early
diagenesis forces calcite dissolution in the upper
sediment column even at supralysoclinal and
lysoclinal water depths due to excess pore water
CO 2 originating from remineralization of
sedimentary organic matter (Archer 1991, 1994;
Jahnke et al. 1994; Martin and Sayles 1996). The
latter will be discussed in more detail in Section
9.4. Moreover, the ocean’s circulation also
changes the depth distribution of water masses
with different saturation levels of aragonite and
calcite. As a consequence, only a small fraction
9.2
Marine Environments of Carbonate Production and Accumulation
production should at least exceed 20 g m
-2
yr
-1
which is the average value of planktonic production in coastal waters. While global carbonate production on carbonate-poor shelves may be relatively high (4⋅10
12
mol yr
-1
; Milliman and Droxler
1996), carbonate accumulation in this environment
is negligible.
The rate of total neritic carbonate production in
the modern ocean is roughly 25⋅10
12
mol yr
-1
from
which 60 % (15⋅10
12
mol yr
-1
) accumulate as shallowwater carbonates. The difference, 10⋅10
12
mol yr
-1
, is
the contribution of the neritic environment to the
pelagic environment, either in the form of flux of
total dissolved inorganic carbon or particulate accumulation on continental slopes and in the deep
sea.
9.2.2
Pelagic Calcareous Sediments
Calcium carbonate is the most important biogenic
component in pelagic marine sediments which
cover an area of about 320⋅10
6
km
2
. Carbonate-rich
sediments (>30 % CaCO 3 ) form about 55 % of the
deposits on the continental slopes and the deepsea floor (Lisitzin 1996; Milliman 1993). A new
compilation on the distribution of carbonate-rich
pelagic sediments (Fig. 9.2b) has been recently
carried out by Archer (1996a).
The distribution of pelagic marine carbonates
is a result of a variety of influences. The most important components of the pelagic carbonate system affecting their formation and destruction are
shown in Figure 9.2b. These can be divided in external contributions such as riverine input and
CO 2 -exchange with the atmosphere and the lithosphere (hydrothermal venting), and internal processes. The main internal processes are those by
which CaCO 3 is produced and dissolved in the
ocean. In the pelagic realm carbonate is mainly
produced by planktonic organisms in ocean surface waters supersaturated with calcite and aragonite. Production of biogenic calcite or aragonite
comes from planktonic organisms like coccolithophorids, foraminifers and pteropods, as well as
calcareous dinoflagellates. The amount of the total production is determined by temperature, light,
and nutrient-conditions in the surface ocean. On
upper slopes, where food supply is high, part of
the carbonate production also originates from
benthic foraminifera and small mollusks. Estimates
of the global calcium carbonate production by
pelagic organisms are difficult to assess, because
the amount of production per unit of water volume
or area cannot be directly measured. In addition,
according to sparse data available from plankton
hauls and shallow floating trap catchments they
vary significantly between oligotrophic and eutrophic, and particularly between silicon-poor and silicon-rich surface waters (Fischer et al. 2004). Mean
global estimates are either derived from sediment
trap fluxes at about 1000 m water depth (Milliman
1993) or from theoretical approaches in which the
production required to explain the observed watercolumn profiles of alkalinity and total dissolved
inorganic carbon (Fig. 9.3) is calculated, taking the
modern ocean circulation pattern and residence
times of the different water masses into account.
However, these estimates differ by a factor of two
to three, whereby the sediment trap data and
surface-water properties suggest a mean global
calcium carbonate production of 23 or 60 to 90⋅10
12
mol yr
-1
, respectively (see comments and discussion in Morse and Mackenzie 1990; Milliman
and Droxler 1996; Wollast 1994).
How much of the carbonate produced in the
surface ocean finally accumulates on the sea floor
and is buried there, depends on the calcite dissolution in the water column during the settling of
particles and on the calcite dissolution within the
sediment. For slope sediments also dilution with
sedimentation of terrigeneous material affects the
carbonate content. Dissolution of pelagic carbonates is driven by the aragonite and carbonate
saturation levels in the deep-sea, controlled by
temperature and pressure conditions, as well as
by pH and alkalinity of circulating deep-water
masses (Berger 1976; Broecker and Peng 1982).
On its way to the deep sea the settling biogenic
carbonate reaches waters which are increasingly
undersaturated with respect to aragonite and
later calcite, due to the decrease in temperature,
increases of pressure, and the latter CO 2
resulting from the reoxidation of organic matter.
Increase of CO 2 due to benthic respiration also
affects CaCO 3 which reaches the sea floor. Early
diagenesis forces calcite dissolution in the upper
sediment column even at supralysoclinal and
lysoclinal water depths due to excess pore water
CO 2 originating from remineralization of
sedimentary organic matter (Archer 1991, 1994;
Jahnke et al. 1994; Martin and Sayles 1996). The
latter will be discussed in more detail in Section
9.4. Moreover, the ocean’s circulation also
changes the depth distribution of water masses
with different saturation levels of aragonite and
calcite. As a consequence, only a small fraction