9
Marine Carbonates: Their Formation and Destruction
334
water saturation conditions the global estimate
extending the Berelson et al. (1994) approach is in
agreement to that of Mackenzie et al. (1993). Based
on Eq. 9.22 and assuming an total mineralization of
55 · 10
12
mol C yr
-1
as well as an average contribution
of 15 · 10
12
mol yr
-1
from non-respiratory dissolution
(Archer 1996b) we may come up with a total
alkalinity flux of about 100 · 10
12
mol yr
-1
as a best
current estimate.
In combination with the flux of dissolved
organic carbon (DOC), which may add another
20 · 10
12
mol yr
-1
(Otto 1996; Table 9.7), a total
carbon release from deep-sea sediments of
about 120 · 10
12
mol yr
-1
seems to be the best
recent approximation regarding all sources of
uncertainty.
Summary
The main subjects addressed in Chapter 9 are
listed below:
• The major site of marine carbonate accumulation is the neritic environment, including coral
reefs, banks and continental shelves, and
pelagic calcite-rich sediments. In total, about
35⋅10 12 mol CaCO 3 accumulate per annum in
the marine realm.
• Based on budget calculations of calcium carbonate, reservoir sizes in the world ocean and
exchange fluxes between reservoirs the carbonate system is not in steady state.
• However, calcium carbonate budget calculations are strongly biased by inexact estimations of calcite production in the surface ocean
and of the dissolution of pelagic biogenic calcite in the water column and in sediments
above the calcite lysocline. In addition, the uncertainty is enhanced by the difficulty to estimate dissolved inorganic carbon release from
sediments.
• The total carbon release from deep-sea sediments is estimated to be about 120⋅10 12 mol yr -1 ,
but is subject to great uncertainty due to the
complexity of processes controlling carbon
remobilization.
• Both bottom water undersaturation and organic matter decay are responsible for calcium
carbonate dissolution in the sediments at more
or less equal levels.
• The efficiency of calcium carbonate dissolution by metabolic CO 2 strongly depends on the
organic carbon / calcium carbonate rain ratio at
the sediment surface, the oxidation rate of
organic matter (and the depth horizon, where
oxidation occurs), as well as the saturation
state of bottom water (Ω) and the dissolution
rate constant k d .
9.5
Problems
Problem 1
Where would you expect more CaCO 3 production:
On the continental shelves, on the slopes, or in
the deep sea?
Problem 2
Explain the difference between CaCO 3 production
and CaCO 3 accumulation. Discuss the difference
between both in connection with the processes of
CaCO 3 dissolution in the water column and in
sediment.
Problem 3
At which water depth would you expect the calcite
compensation depth (CCD) in waters of high latitudes and in which water depth in waters of low latitudes? Explain your answer.
Problem 4
Which carbonate species has the highest concentrations in sea-water? How much of the total
calcium concentration in normal sea-water exists
as CaSO 4 complex? Under which conditions is this
complex rather insignificant in anoxic pore-water?
Problem 5
CaCO 3 dissolution fluxes from sediment to ocean
bottom water can be estimated from C org mineralization rates. What are the geochemical processes
behind this correlation? Which calcite dissolution
rate would you expect for sediments in an upwelling area, and which for deep-sea sediments?
Marine Carbonates: Their Formation and Destruction
334
water saturation conditions the global estimate
extending the Berelson et al. (1994) approach is in
agreement to that of Mackenzie et al. (1993). Based
on Eq. 9.22 and assuming an total mineralization of
55 · 10
12
mol C yr
-1
as well as an average contribution
of 15 · 10
12
mol yr
-1
from non-respiratory dissolution
(Archer 1996b) we may come up with a total
alkalinity flux of about 100 · 10
12
mol yr
-1
as a best
current estimate.
In combination with the flux of dissolved
organic carbon (DOC), which may add another
20 · 10
12
mol yr
-1
(Otto 1996; Table 9.7), a total
carbon release from deep-sea sediments of
about 120 · 10
12
mol yr
-1
seems to be the best
recent approximation regarding all sources of
uncertainty.
Summary
The main subjects addressed in Chapter 9 are
listed below:
• The major site of marine carbonate accumulation is the neritic environment, including coral
reefs, banks and continental shelves, and
pelagic calcite-rich sediments. In total, about
35⋅10 12 mol CaCO 3 accumulate per annum in
the marine realm.
• Based on budget calculations of calcium carbonate, reservoir sizes in the world ocean and
exchange fluxes between reservoirs the carbonate system is not in steady state.
• However, calcium carbonate budget calculations are strongly biased by inexact estimations of calcite production in the surface ocean
and of the dissolution of pelagic biogenic calcite in the water column and in sediments
above the calcite lysocline. In addition, the uncertainty is enhanced by the difficulty to estimate dissolved inorganic carbon release from
sediments.
• The total carbon release from deep-sea sediments is estimated to be about 120⋅10 12 mol yr -1 ,
but is subject to great uncertainty due to the
complexity of processes controlling carbon
remobilization.
• Both bottom water undersaturation and organic matter decay are responsible for calcium
carbonate dissolution in the sediments at more
or less equal levels.
• The efficiency of calcium carbonate dissolution by metabolic CO 2 strongly depends on the
organic carbon / calcium carbonate rain ratio at
the sediment surface, the oxidation rate of
organic matter (and the depth horizon, where
oxidation occurs), as well as the saturation
state of bottom water (Ω) and the dissolution
rate constant k d .
9.5
Problems
Problem 1
Where would you expect more CaCO 3 production:
On the continental shelves, on the slopes, or in
the deep sea?
Problem 2
Explain the difference between CaCO 3 production
and CaCO 3 accumulation. Discuss the difference
between both in connection with the processes of
CaCO 3 dissolution in the water column and in
sediment.
Problem 3
At which water depth would you expect the calcite
compensation depth (CCD) in waters of high latitudes and in which water depth in waters of low latitudes? Explain your answer.
Problem 4
Which carbonate species has the highest concentrations in sea-water? How much of the total
calcium concentration in normal sea-water exists
as CaSO 4 complex? Under which conditions is this
complex rather insignificant in anoxic pore-water?
Problem 5
CaCO 3 dissolution fluxes from sediment to ocean
bottom water can be estimated from C org mineralization rates. What are the geochemical processes
behind this correlation? Which calcite dissolution
rate would you expect for sediments in an upwelling area, and which for deep-sea sediments?