100
4.5 Comparison of the early diagenesis in freshwater and marine
sediments
Organic matter deposited in freshwater and marine sediments is usually reduced by
02, Mn oxide, NO;, Fe oxide and SO42". After consumption of these oxidants,
methane fermentation is a dominating process. However, different processes
dominate mineralization of organic matter in each system. This may be attributed to
differences in ionic composition and input of different organic matter in each system.
Quality and quantity of organic matter play an important role in the mineralization
of organic substances. The more labile components reach the sediment, the faster
mineralization processes take place. The flux of organic matter depends on the
primary productivity in the supemating water. As much more nutrients are discharged
into freshwater compared to marine systems, most freshwater sediments have high
organic carbon contents. As a result, 02 is rapidly consumed in the surface layer in
freshwater sediments, anoxic conditions dominate these sediments (J~rgensen 1983;
Carignan and Nriagu 1985; Schwedhelm et al. 1988; Vuynovich 1989; Williams
1992). In contrast, marine sediments have normally low organic carbon contents.
Furthermore, labile organic particles in the deep sea are usually oxidized by 02 in
seawater before they reach the sediment surface. The depth of 02 penetration is
several centimeters compared to a few millimeter in freshwater sediments.
SO~ 2" reduction is a dominant anoxic reduction process in marine sediments,
whereas low sulfate concentrations generally limit the rate of sulfate reduction in
freshwater sediments. Because of the rapid consumption of SO4" in freshwater
sediments (between 0 and 20 cm depth, major rivers of Germany), methane
fermentation plays an important role in the mineralization processes.
Because of low concentrations of NO3" in marine sediments, NO3" reduction does
not play a significant role in the mineralization of organic matter (Balzer 1989).
However, the research both in marine (Sagemann et al 1994) and in freshwater
sediments (Neckar River) show the relationship between denitrification rate and
temperature, resulting from bacterial activity.
In marine sediments, the zones of aerobic oxidation, NOr reduction, Mn oxide
reduction, Fe oxide reduction, and SO4" reduction can be spatially separated
(Froelich et al. 1979). This is not true in freshwater sediments because of the rapid
decomposition of organic matter.
Marine sediments are a site of decomposition and nutrient regeneration from
organic matter. They provide essential nutrients for biomass formation and play an
important role in cycling of C, N, and P in the ocean. In freshwater environments, the
rapid decomposition of organic matter in anoxic sediments leads to a strong release of
NH4 + and PO4 3" into the porewater. High concentrations of NH4 § (2 - 16 mM) were
measured in freshwater sediments compared to 0.03 mM in marine sediments
(Froelich et al. 1979). A large scale release of NH, § and Fe 2§ into the supemating
water could cause an O2-depletion, which in extreme cases could result in an
ecological catastrophe.
4.5 Comparison of the early diagenesis in freshwater and marine
sediments
Organic matter deposited in freshwater and marine sediments is usually reduced by
02, Mn oxide, NO;, Fe oxide and SO42". After consumption of these oxidants,
methane fermentation is a dominating process. However, different processes
dominate mineralization of organic matter in each system. This may be attributed to
differences in ionic composition and input of different organic matter in each system.
Quality and quantity of organic matter play an important role in the mineralization
of organic substances. The more labile components reach the sediment, the faster
mineralization processes take place. The flux of organic matter depends on the
primary productivity in the supemating water. As much more nutrients are discharged
into freshwater compared to marine systems, most freshwater sediments have high
organic carbon contents. As a result, 02 is rapidly consumed in the surface layer in
freshwater sediments, anoxic conditions dominate these sediments (J~rgensen 1983;
Carignan and Nriagu 1985; Schwedhelm et al. 1988; Vuynovich 1989; Williams
1992). In contrast, marine sediments have normally low organic carbon contents.
Furthermore, labile organic particles in the deep sea are usually oxidized by 02 in
seawater before they reach the sediment surface. The depth of 02 penetration is
several centimeters compared to a few millimeter in freshwater sediments.
SO~ 2" reduction is a dominant anoxic reduction process in marine sediments,
whereas low sulfate concentrations generally limit the rate of sulfate reduction in
freshwater sediments. Because of the rapid consumption of SO4" in freshwater
sediments (between 0 and 20 cm depth, major rivers of Germany), methane
fermentation plays an important role in the mineralization processes.
Because of low concentrations of NO3" in marine sediments, NO3" reduction does
not play a significant role in the mineralization of organic matter (Balzer 1989).
However, the research both in marine (Sagemann et al 1994) and in freshwater
sediments (Neckar River) show the relationship between denitrification rate and
temperature, resulting from bacterial activity.
In marine sediments, the zones of aerobic oxidation, NOr reduction, Mn oxide
reduction, Fe oxide reduction, and SO4" reduction can be spatially separated
(Froelich et al. 1979). This is not true in freshwater sediments because of the rapid
decomposition of organic matter.
Marine sediments are a site of decomposition and nutrient regeneration from
organic matter. They provide essential nutrients for biomass formation and play an
important role in cycling of C, N, and P in the ocean. In freshwater environments, the
rapid decomposition of organic matter in anoxic sediments leads to a strong release of
NH4 + and PO4 3" into the porewater. High concentrations of NH4 § (2 - 16 mM) were
measured in freshwater sediments compared to 0.03 mM in marine sediments
(Froelich et al. 1979). A large scale release of NH, § and Fe 2§ into the supemating
water could cause an O2-depletion, which in extreme cases could result in an
ecological catastrophe.