9
Marine Carbonates: Their Formation and Destruction
324
9.4
Carbonate Reservoir Sizes
and Fluxes between
Particulate and Dissolved
Reservoirs
Present-day production of calcium carbonate in the
pelagic ocean is calculated to be in the order of 6 to
9 billion tons (bt) per year (60-90⋅10
12
mol yr
-1
), from
which about 1.1 to 2 bt (11 to 20⋅10
12
mol yr
-1
)
accumulate in sediments (Tables 9.1 and 9.6). Together with the accumulation in shallow waters of
14.5⋅10
12
mol yr
-1
(Table 9.1, Fig. 9.5), the total carbonate accumulation in the world ocean amounts to
3.5 bt per year. This latter number is twice as much
calcium as is brought into the ocean by rivers and
hydrothermal activity (1.6 bt, Wollast 1994).
Wollast (1994) and Milliman and Droxler (1996)
therefore consider the carbonate system of the
modern ocean to be in non-steady state conditions,
because production is not equal to the input of
Ca
2+
and HCO 3
-
by rivers and hydrothermal vents
(Fig. 9.5). Consequently, the marine carbonate system is at a stage of imbalance, or the output controlled by calcite sedimentation or input conveiled
by dissolution has been overestimated or underestimated, respectively. On the other hand, one or
more input sources may have been not detected so
far. Published calcium carbonate budgets vary
strongly, because the various authors have used
different data sets and made different assumptions
with respect to the production and accumulation of
carbonates (e.g. Table 9.6), as well as for the
sources of dissolved calcium and carbonate in marine waters (see summaries in Milliman 1993;
Milliman and Droxler 1996; Wollast 1994).
Table 9.4 Model calculation applying the computer program PHREEQC (Parkhurst 1995) to a sample of cold surface
water of the ocean from higher latitudes. The constant of the solubility product for calcite is accordingly corrected for
temperature.
Model of cold surface seawater
input concentrations:
dissolved constituents from Nordstrom et al. 1979, cf. Tabel 15.1
boundary conditions:
temperature
2 °C
pCO 2
280 µatm
(i.e. log pCO 2 = -3.55)
log k calcite
-8,34
(at 2 °C and 1 atm pressure)
input situation without calcite-carbonate-equilibrium:
pH
8,23
sum of carbonate species (TIC)
2.28 mmol/l
sum of calcium species
10.63 mmol/l
SI calcite
0,43
(i.e. Ω calcite = 2.7)
reactions:
calcite supersaturation constant at SI = 0.50
PHREEQC model results:
pH
8,30
sum of carbonate species (TIC)
2.30 mmol/l
sum of calcium species
10.65 mmol/l
SI calcite
0,50
(i.e. Ω calcite = 3.2)
Marine Carbonates: Their Formation and Destruction
324
9.4
Carbonate Reservoir Sizes
and Fluxes between
Particulate and Dissolved
Reservoirs
Present-day production of calcium carbonate in the
pelagic ocean is calculated to be in the order of 6 to
9 billion tons (bt) per year (60-90⋅10
12
mol yr
-1
), from
which about 1.1 to 2 bt (11 to 20⋅10
12
mol yr
-1
)
accumulate in sediments (Tables 9.1 and 9.6). Together with the accumulation in shallow waters of
14.5⋅10
12
mol yr
-1
(Table 9.1, Fig. 9.5), the total carbonate accumulation in the world ocean amounts to
3.5 bt per year. This latter number is twice as much
calcium as is brought into the ocean by rivers and
hydrothermal activity (1.6 bt, Wollast 1994).
Wollast (1994) and Milliman and Droxler (1996)
therefore consider the carbonate system of the
modern ocean to be in non-steady state conditions,
because production is not equal to the input of
Ca
2+
and HCO 3
-
by rivers and hydrothermal vents
(Fig. 9.5). Consequently, the marine carbonate system is at a stage of imbalance, or the output controlled by calcite sedimentation or input conveiled
by dissolution has been overestimated or underestimated, respectively. On the other hand, one or
more input sources may have been not detected so
far. Published calcium carbonate budgets vary
strongly, because the various authors have used
different data sets and made different assumptions
with respect to the production and accumulation of
carbonates (e.g. Table 9.6), as well as for the
sources of dissolved calcium and carbonate in marine waters (see summaries in Milliman 1993;
Milliman and Droxler 1996; Wollast 1994).
Table 9.4 Model calculation applying the computer program PHREEQC (Parkhurst 1995) to a sample of cold surface
water of the ocean from higher latitudes. The constant of the solubility product for calcite is accordingly corrected for
temperature.
Model of cold surface seawater
input concentrations:
dissolved constituents from Nordstrom et al. 1979, cf. Tabel 15.1
boundary conditions:
temperature
2 °C
pCO 2
280 µatm
(i.e. log pCO 2 = -3.55)
log k calcite
-8,34
(at 2 °C and 1 atm pressure)
input situation without calcite-carbonate-equilibrium:
pH
8,23
sum of carbonate species (TIC)
2.28 mmol/l
sum of calcium species
10.63 mmol/l
SI calcite
0,43
(i.e. Ω calcite = 2.7)
reactions:
calcite supersaturation constant at SI = 0.50
PHREEQC model results:
pH
8,30
sum of carbonate species (TIC)
2.30 mmol/l
sum of calcium species
10.65 mmol/l
SI calcite
0,50
(i.e. Ω calcite = 3.2)