form highly insoluble metal sulfide precipitates with metal ions (Lee and Kittrick
1984a,b; Carignan and Nriagu 1985; Carignan and Tessier 1985; Gendron et al. 1986;
Wallmann 1990).
It is noticeable that many precipitation/dissolution reactions are kinetically
inhibited. Supersaturation of minerals is often observed in porewater before
precipitation of solid phase actually occurs (Nriagu and Dell 1974; Postma 1981;
Holdren and Armstrong 1986). High temperature, presence of nucleating surfaces,
and biological activities can dramatically enhance the kinetics (Morel and Hering
1993). In addition, the existence of organics and organic colloids may lead to a
supersaturation of sulfides (Boulegue et al. 1982).
Under changes of physicochemical conditions (e.g. Eh-pH), the equilibrium of
precipitation/dissolution is disturbed. This leads to a redistribution of heavy metals
and phosphorus between sediments and porewater, and can influence the mobility of
these components (Kersten et al. 1985; Wallmann 1990; Gambrell et al. 1991;
Calmano et al. 1992).
The cycling of heavy metals and nutrients in sediments is controlled by
precipitation/dissolution of minerals, but the mobility and bioavailability may be
related to the organic and inorganic complexation. Porewater studies reflect that
heavy metal concentrations are significantly higher than could be predicted from
metal-sulfide solubility (Elderfield et al. 1981; Carignan and Nriagu 1985). In
laboratory experiments, Salomons et al. (1987) and Wallmarm (1992b) reported that
inorganic complexes (e.g. with sulfide and chloride) can explain the supersaturation
of metal-sulfides. Organic complexing materials such as humic substances are
important because of their high affinity to heavy metals (Elderfield 1981; van den
Berg and Dharmvanij 1984; Douglas et al. 1986). Previous studies show that organic
colloids (e.g. humic substances) increase with depth as a result of the degradation of
organic matter (Orem et al. 1986; Chin and Gschwend 1991). This may enhance the
mobility and bioavailability of heavy metals during the early diagenesis (Elderfield
1981; Douglas et al. 1986).
2.3 Methods used in the study of early diagenesis
Until now, three kinds of studies have been carried out to study diagenetic processes.
The first are laboratory experiments (core incubation). For this method, sediment
cores are brought into the laboratory, where water and biological debris (e.g. plant,
shrimps) are added on the top of the sediments. The release of nutrients, dissolved
organic carbon (DOC), and heavy metals is then measured in the water column. This
method provides a direct measurement of the degradation processes under changes of
physicochemical conditions. It is relatively simple and inexpensive, and can be
conducted with a wide variety of parameters such as temperature, input of organic
matter, redox potential, chemical composition of the water (Matisoff et al. 1981;
Gerringa 1990; Wu et al. 1992; Sondergaard et al. 1992; Matsunaga et al. 1993;
Morse and Arakaki 1993; Sagemaun et al. 1994). Radiotracers are often used to study
redox reaction mechanisms (Winfrey and Zeikus 1977; Gurmarsson and Rrnnow
1984a,b; Carignan and Nriagu 1985; Carignan and Tessier 1985; Gendron et al. 1986;
Wallmann 1990).
It is noticeable that many precipitation/dissolution reactions are kinetically
inhibited. Supersaturation of minerals is often observed in porewater before
precipitation of solid phase actually occurs (Nriagu and Dell 1974; Postma 1981;
Holdren and Armstrong 1986). High temperature, presence of nucleating surfaces,
and biological activities can dramatically enhance the kinetics (Morel and Hering
1993). In addition, the existence of organics and organic colloids may lead to a
supersaturation of sulfides (Boulegue et al. 1982).
Under changes of physicochemical conditions (e.g. Eh-pH), the equilibrium of
precipitation/dissolution is disturbed. This leads to a redistribution of heavy metals
and phosphorus between sediments and porewater, and can influence the mobility of
these components (Kersten et al. 1985; Wallmann 1990; Gambrell et al. 1991;
Calmano et al. 1992).
The cycling of heavy metals and nutrients in sediments is controlled by
precipitation/dissolution of minerals, but the mobility and bioavailability may be
related to the organic and inorganic complexation. Porewater studies reflect that
heavy metal concentrations are significantly higher than could be predicted from
metal-sulfide solubility (Elderfield et al. 1981; Carignan and Nriagu 1985). In
laboratory experiments, Salomons et al. (1987) and Wallmarm (1992b) reported that
inorganic complexes (e.g. with sulfide and chloride) can explain the supersaturation
of metal-sulfides. Organic complexing materials such as humic substances are
important because of their high affinity to heavy metals (Elderfield 1981; van den
Berg and Dharmvanij 1984; Douglas et al. 1986). Previous studies show that organic
colloids (e.g. humic substances) increase with depth as a result of the degradation of
organic matter (Orem et al. 1986; Chin and Gschwend 1991). This may enhance the
mobility and bioavailability of heavy metals during the early diagenesis (Elderfield
1981; Douglas et al. 1986).
2.3 Methods used in the study of early diagenesis
Until now, three kinds of studies have been carried out to study diagenetic processes.
The first are laboratory experiments (core incubation). For this method, sediment
cores are brought into the laboratory, where water and biological debris (e.g. plant,
shrimps) are added on the top of the sediments. The release of nutrients, dissolved
organic carbon (DOC), and heavy metals is then measured in the water column. This
method provides a direct measurement of the degradation processes under changes of
physicochemical conditions. It is relatively simple and inexpensive, and can be
conducted with a wide variety of parameters such as temperature, input of organic
matter, redox potential, chemical composition of the water (Matisoff et al. 1981;
Gerringa 1990; Wu et al. 1992; Sondergaard et al. 1992; Matsunaga et al. 1993;
Morse and Arakaki 1993; Sagemaun et al. 1994). Radiotracers are often used to study
redox reaction mechanisms (Winfrey and Zeikus 1977; Gurmarsson and Rrnnow