329
related to concentrations instead of activities.
Compilations of apparent solubility constants are
available e.g. by Mehrbach et al. (1973). In several
studies, a further dependence of R d on the calcium
carbonate content (respectively the surface area)
in the sediment is considered. (for a more detailed
overview of this subject see Zeebe and WolfGladrow 2000).
The most extensively used reaction order for
modeling calcite dissolution is 4.5 and 4.2 for
aragonite (as suggested by Keir 1980). A more
extensive re-evaluation of this topic has been
provided by Cai et al. (1995). However, the discussion concerning the “correct” reaction order still
continues. The values of k d reported so far range
over several orders of magnitude from 0.005-0.16 % d
-1
(Berelson et al. 1994; Hales and Emerson 1996, 1997a)
up to laboratory values of 10-1000 % d
-1
(Keir 1980,
1983)
2
. The reason for this huge discrepancy is not
clearly known. Important and regionally variable
factors, however, may be the grain size and thus the
surface area of calcium carbonate crystals in the
sediments or adsorbed coatings like phosphate ions
protecting calcium carbonate grains from corrosive
pore waters (Jahnke et al. 1994; Hales and Emerson
1997a). In contrast, Hales and Emerson (1997b)
found evidence that in-situ pH measurements in
pore waters of calcite rich deep-sea sediments are
more consistent with a first-order instead of 4.5
th
order dependence. Applied to their data, they rewrote Equation 9.17 to
R d = 38 (1- Ω)
1
(9.19)
In contrast, the study of calcite dissolution
kinetics in CaCO 3 -poor sedi-ments of the
equatorial Atlantic, Adler et al. (2001) again
favored higher reaction orders. In this sense, the
observed dissolution rate constants are highly
variable, which seems to be mainly dependent on
differences in the physical (e.g. surface area) and
chemical properties (high/low Mg-calcite) of the
calcite mineral phase.
It is also important to consider where in the
sediment dissolution occurs. Metabolically produced CO 2 released immediately at the sedimentwater interface is probably much less effective for
carbonate dissolution than in deeper sediment
strata, because neutralization with bottom water
CO 3
2might occur instead of dissolution. If the
particulate organic matter is more rapidly mixed
down, i.e. by bioturbation, and oxidized in deeper
sediment strata, the CO 2 released into the pore
waters can probably more effectively dissolve
carbonates (Martin and Sayles 1996).
It is generally reported that dissolution at and
above the saturation horizon is solely attributed
to the oxidation of organic matter. The importance
of oxidation-related dissolution decreases with
increasing undersaturation of the bottom water,
however, the efficiency by which C org oxidation
drives CaCO 3 dissolution increases with increasing undersaturation of the bottom water (Martin
and Sayles 1996). In the same sense, Berelson et
al. (1994) suggested that the undersaturation of
the bottom water is more important for better
soluble calcite phases (high k-values), whereas
for more resistant calcite phases (low k-values),
the rain of C org to the seafloor, and thus mineralization, becomes the driving force for calcite
dissolution (Fig. 9.7). This can be understood as
an alteration process, where dissolution starts at
the sediment-water interface, right after deposition of the calcite phase, and becomes more
resistant to dissolution during burial. In the zone
of oxic respiration the removal of organic
coatings around calcite grains might also play an
important role since they help to expose larger
surface areas to the pore water, thus triggering
dissolution processes.
In summary, a closer evaluation of Fig. 9.7
reveals:
(1) It is obvious that for low oxygen fluxes the
relative importance of bottom water undersaturation is the driving force for CaCO 3 dissolution, meaning that the y-axis intercept is
exceptionally determined by k d and Ω.
(2) It is implicit from Eq. 9.17 that a higher
degree of undersaturation increases the differences of dissolution fluxes between the chosen
values of k d (distance between hatched and solid
lines).
(3) Higher oxygen fluxes (higher amount of
metabolically released CO 2 ) are obviously more
efficient in carbonate dissolution when bottom
waters are stronger undersaturated (lower Ω).
(2) The unit % d -1 originates from experimental studies
(e.g. Morse 1978; Keir 1980) and is used in most studies dealing with carbonate dissolution in marine
sediments. The use is, however, not always consistent
regarding the units of the dissolution rate and the parameters used in the equation and should, therefore,
generally be evaluated with caution.
9.4
Carbonate Reservoir Sizes and Fluxes Between Particulate and Dissolved Reservoirs
related to concentrations instead of activities.
Compilations of apparent solubility constants are
available e.g. by Mehrbach et al. (1973). In several
studies, a further dependence of R d on the calcium
carbonate content (respectively the surface area)
in the sediment is considered. (for a more detailed
overview of this subject see Zeebe and WolfGladrow 2000).
The most extensively used reaction order for
modeling calcite dissolution is 4.5 and 4.2 for
aragonite (as suggested by Keir 1980). A more
extensive re-evaluation of this topic has been
provided by Cai et al. (1995). However, the discussion concerning the “correct” reaction order still
continues. The values of k d reported so far range
over several orders of magnitude from 0.005-0.16 % d
-1
(Berelson et al. 1994; Hales and Emerson 1996, 1997a)
up to laboratory values of 10-1000 % d
-1
(Keir 1980,
1983)
2
. The reason for this huge discrepancy is not
clearly known. Important and regionally variable
factors, however, may be the grain size and thus the
surface area of calcium carbonate crystals in the
sediments or adsorbed coatings like phosphate ions
protecting calcium carbonate grains from corrosive
pore waters (Jahnke et al. 1994; Hales and Emerson
1997a). In contrast, Hales and Emerson (1997b)
found evidence that in-situ pH measurements in
pore waters of calcite rich deep-sea sediments are
more consistent with a first-order instead of 4.5
th
order dependence. Applied to their data, they rewrote Equation 9.17 to
R d = 38 (1- Ω)
1
(9.19)
In contrast, the study of calcite dissolution
kinetics in CaCO 3 -poor sedi-ments of the
equatorial Atlantic, Adler et al. (2001) again
favored higher reaction orders. In this sense, the
observed dissolution rate constants are highly
variable, which seems to be mainly dependent on
differences in the physical (e.g. surface area) and
chemical properties (high/low Mg-calcite) of the
calcite mineral phase.
It is also important to consider where in the
sediment dissolution occurs. Metabolically produced CO 2 released immediately at the sedimentwater interface is probably much less effective for
carbonate dissolution than in deeper sediment
strata, because neutralization with bottom water
CO 3
2might occur instead of dissolution. If the
particulate organic matter is more rapidly mixed
down, i.e. by bioturbation, and oxidized in deeper
sediment strata, the CO 2 released into the pore
waters can probably more effectively dissolve
carbonates (Martin and Sayles 1996).
It is generally reported that dissolution at and
above the saturation horizon is solely attributed
to the oxidation of organic matter. The importance
of oxidation-related dissolution decreases with
increasing undersaturation of the bottom water,
however, the efficiency by which C org oxidation
drives CaCO 3 dissolution increases with increasing undersaturation of the bottom water (Martin
and Sayles 1996). In the same sense, Berelson et
al. (1994) suggested that the undersaturation of
the bottom water is more important for better
soluble calcite phases (high k-values), whereas
for more resistant calcite phases (low k-values),
the rain of C org to the seafloor, and thus mineralization, becomes the driving force for calcite
dissolution (Fig. 9.7). This can be understood as
an alteration process, where dissolution starts at
the sediment-water interface, right after deposition of the calcite phase, and becomes more
resistant to dissolution during burial. In the zone
of oxic respiration the removal of organic
coatings around calcite grains might also play an
important role since they help to expose larger
surface areas to the pore water, thus triggering
dissolution processes.
In summary, a closer evaluation of Fig. 9.7
reveals:
(1) It is obvious that for low oxygen fluxes the
relative importance of bottom water undersaturation is the driving force for CaCO 3 dissolution, meaning that the y-axis intercept is
exceptionally determined by k d and Ω.
(2) It is implicit from Eq. 9.17 that a higher
degree of undersaturation increases the differences of dissolution fluxes between the chosen
values of k d (distance between hatched and solid
lines).
(3) Higher oxygen fluxes (higher amount of
metabolically released CO 2 ) are obviously more
efficient in carbonate dissolution when bottom
waters are stronger undersaturated (lower Ω).
(2) The unit % d -1 originates from experimental studies
(e.g. Morse 1978; Keir 1980) and is used in most studies dealing with carbonate dissolution in marine
sediments. The use is, however, not always consistent
regarding the units of the dissolution rate and the parameters used in the equation and should, therefore,
generally be evaluated with caution.
9.4
Carbonate Reservoir Sizes and Fluxes Between Particulate and Dissolved Reservoirs