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J. W.Morse
genic carbonates strongly dominate. It is convenient to divide modern sedimentary
carbonates into those found in relatively shallow waters and those being deposited on
deep ocean basins. Sources, mineralogy and diagenetic processes are generally quite
different in these environments. In this chapter, we will not be dealing with deep ocean
sediments. In shallow water marine sediments, aragonite is usually the most common
carbonate mineral, followed by high-magnesian calcite (e.g. Cao.87Mgo.13C03) and minor amounts of low-Mg calcite.
The pK (= -logK) values for calcite and aragonite solubility products (Eq. 7.29) are
respectively at 25°C, 8.48 and 8.30. The solubility of high magnesian calcites is a complex and controversial topic (e.g. abiotic and biotic high magnesian calcites of the same
composition appear to have different solubilities; see Morse and Mackenzie 1990 for
extensive discussion of this topic).
CaC03 ~ Ca 2 + + CO;Ksp = aCa2+acoj(7.29)
Observations of the composition of pore waters in carbonate-rich shallow water
sediments have demonstrated that no single value for the calcium carbonate ion activity product widely occurs. This results in a moderate range of saturation states relative to aragonite (Fig. 7.4; Morse et al. 1985) that have been demonstrated to represent
conditions of "dynamic equilibrium" between the pore waters and carbonates
(Bernstein and Morse 1985). For equilibrium with aragonite in a S = 35 pore water at
25°C with normal Ca 2 + concentration (aCa2+ == 2 X 10- 3 ), the following approximate carbonic acid system parameter values at a pH of 7.2 (fairly typical of anoxic sediments)
are total alkalinity about 2 x that of sea water (5 meq kg-I), and Pc0 2 about 10 matm.
acoj- would then be about 2.5 x 10- 6 •
7.2.3.2
Iron Sulphide Minerals
Just as with the carbonates, there are several sulphide minerals that are found in modern sediments. However, unlike the sedimentary carbonate minerals, they are dominantly of authigenic rather than biogenic origin. Sedimentary sulphides are usually
divided into acid volatile sulphide (AVS) and pyrite (FeS2)' which is the thermodynamically stable phase.
AVS comprises an "operationally defined" group of what are generally believed to
be metastable iron sulphide minerals plus dissolved H2S species. Often, but with major exceptions, AVS is confined to a relatively small portion «10%) of total sedimentary sulphides. It can be ephemeral. Spatially, AVS often appears with a maximum in
the top few cm of anoxic sediments and disappears rapidly with depth to close to undetectable concentrations within the top 20 cm or less. Temporally, it can exhibit major changes on a seasonal basis in the upper few cm of sediments. AVS has received
considerable attention, because it is one of the most chemically reactive components
of anoxic sediments during early diagenesis. AVS is readily oxidized within hours or
less when anoxic sediments are exposed to oxic waters. The iron sulphide minerals
amorphous-FeS, mackinawite (FeO.9S) and griegite (Fe3S4) that are generally purported
to be the major components of AVS have also traditionally (e.g. Berner 1984) been held
to be precursor phases necessary for the formation of the dominant and thermody-
J. W.Morse
genic carbonates strongly dominate. It is convenient to divide modern sedimentary
carbonates into those found in relatively shallow waters and those being deposited on
deep ocean basins. Sources, mineralogy and diagenetic processes are generally quite
different in these environments. In this chapter, we will not be dealing with deep ocean
sediments. In shallow water marine sediments, aragonite is usually the most common
carbonate mineral, followed by high-magnesian calcite (e.g. Cao.87Mgo.13C03) and minor amounts of low-Mg calcite.
The pK (= -logK) values for calcite and aragonite solubility products (Eq. 7.29) are
respectively at 25°C, 8.48 and 8.30. The solubility of high magnesian calcites is a complex and controversial topic (e.g. abiotic and biotic high magnesian calcites of the same
composition appear to have different solubilities; see Morse and Mackenzie 1990 for
extensive discussion of this topic).
CaC03 ~ Ca 2 + + CO;Ksp = aCa2+acoj(7.29)
Observations of the composition of pore waters in carbonate-rich shallow water
sediments have demonstrated that no single value for the calcium carbonate ion activity product widely occurs. This results in a moderate range of saturation states relative to aragonite (Fig. 7.4; Morse et al. 1985) that have been demonstrated to represent
conditions of "dynamic equilibrium" between the pore waters and carbonates
(Bernstein and Morse 1985). For equilibrium with aragonite in a S = 35 pore water at
25°C with normal Ca 2 + concentration (aCa2+ == 2 X 10- 3 ), the following approximate carbonic acid system parameter values at a pH of 7.2 (fairly typical of anoxic sediments)
are total alkalinity about 2 x that of sea water (5 meq kg-I), and Pc0 2 about 10 matm.
acoj- would then be about 2.5 x 10- 6 •
7.2.3.2
Iron Sulphide Minerals
Just as with the carbonates, there are several sulphide minerals that are found in modern sediments. However, unlike the sedimentary carbonate minerals, they are dominantly of authigenic rather than biogenic origin. Sedimentary sulphides are usually
divided into acid volatile sulphide (AVS) and pyrite (FeS2)' which is the thermodynamically stable phase.
AVS comprises an "operationally defined" group of what are generally believed to
be metastable iron sulphide minerals plus dissolved H2S species. Often, but with major exceptions, AVS is confined to a relatively small portion «10%) of total sedimentary sulphides. It can be ephemeral. Spatially, AVS often appears with a maximum in
the top few cm of anoxic sediments and disappears rapidly with depth to close to undetectable concentrations within the top 20 cm or less. Temporally, it can exhibit major changes on a seasonal basis in the upper few cm of sediments. AVS has received
considerable attention, because it is one of the most chemically reactive components
of anoxic sediments during early diagenesis. AVS is readily oxidized within hours or
less when anoxic sediments are exposed to oxic waters. The iron sulphide minerals
amorphous-FeS, mackinawite (FeO.9S) and griegite (Fe3S4) that are generally purported
to be the major components of AVS have also traditionally (e.g. Berner 1984) been held
to be precursor phases necessary for the formation of the dominant and thermody-