206 Imprint of Climatic Zonation on Marine Sediments
is reflected in the portions of the record which consist of finely laminated sediments,
with layers of millimeter dimensions (Sect. 3.9, Fig. 3.l3). These layers indicate the
absence of deposit feeders and other burrowers, hence they show the absence of
oxygen.
The increase in stagnation in the central Baltic during the last 50 years, then, could
be largely due to human activities, but it could also be a natural phenomenon which
will eventually reverse itself. This example illustrates a very general problem in
environmental research and engineering, namely that it is generally extremely difficult to keep apart the natural background fluctuations and the human influences.
The chemistry of anaerobic sediments is complex. Only a few aspects can be
mentioned here. The C02 concentration in the bottom-near water with very low 02
content is high: C02 is produced as the 02 is used up. C02 and water produce
carbonic acid, and the pH drops (to less than 7, compared with open ocean values
near 8). Consequently, calcareous shells are dissolved on the sea floor. High C02
content of interstitial waters also means that hydrogenous carbonates can form. Manganese is readily mobilized under conditions of oxygen deficiency, and migrates
upward in the sediment as Mn 2 +. It is kept on the sea floor both by oxidation and
precipitation as oxide (Mn02) and as manganese carbonate (MnC03), which forms
under conditions of anaerobism and high C02 concentrations. Iron carbonate may
also form, although iron is less mobile, being readily precipitated both as sulfide and
as oxide. When free oxygen is gone, bacteria produce sulfide by the reduction of
sulfate ion
(7.4)
Iron sulfide forms under these conditions; hence pyrite crystals (FeS2) are common in
black anaerobic sediments. If sulfate reduction occurs at the very surface of the
sediment, calcareous shells are preserved, because the destruction of sulfate greatly
increases alkalinity.
7.6.4 The Persian! Arabian Gulf As Arid Model of a Marginal Sea. None of the
conditions typical for the Baltic Sea develop in the Persian Gulf, because oxygen is
brought to the sea floor by sinking saline waters, and because fertility is generally
low. The incoming ocean sUlface waters are low in nutrients, as are warm surface
waters everywhere. Hence the Persian Gulf is nutrient-starved. One place where
anaerobic conditions can develop is in hypersaline lagoons, within the sediment.
Sulfate reduction then occurs and organic-rich layers with pyrite can develop. However, such layers should be readily distinguishable in the geologic record from the
stagnant basin layers in humid conditions.
The sediments of the Gulf proper are very different from those of the Baltic:
organic contents (0.6 to 1 %) are about five times lower, and carbonate content (> 50
%) is more than ten times higher. Benthic organisms exist at all depths and at all
times, any lamination is quickly destroyed by bioturbation. Mobilization of heavy
metals stops within the sediment because of the high oxygen supply in the mixed
layer.
is reflected in the portions of the record which consist of finely laminated sediments,
with layers of millimeter dimensions (Sect. 3.9, Fig. 3.l3). These layers indicate the
absence of deposit feeders and other burrowers, hence they show the absence of
oxygen.
The increase in stagnation in the central Baltic during the last 50 years, then, could
be largely due to human activities, but it could also be a natural phenomenon which
will eventually reverse itself. This example illustrates a very general problem in
environmental research and engineering, namely that it is generally extremely difficult to keep apart the natural background fluctuations and the human influences.
The chemistry of anaerobic sediments is complex. Only a few aspects can be
mentioned here. The C02 concentration in the bottom-near water with very low 02
content is high: C02 is produced as the 02 is used up. C02 and water produce
carbonic acid, and the pH drops (to less than 7, compared with open ocean values
near 8). Consequently, calcareous shells are dissolved on the sea floor. High C02
content of interstitial waters also means that hydrogenous carbonates can form. Manganese is readily mobilized under conditions of oxygen deficiency, and migrates
upward in the sediment as Mn 2 +. It is kept on the sea floor both by oxidation and
precipitation as oxide (Mn02) and as manganese carbonate (MnC03), which forms
under conditions of anaerobism and high C02 concentrations. Iron carbonate may
also form, although iron is less mobile, being readily precipitated both as sulfide and
as oxide. When free oxygen is gone, bacteria produce sulfide by the reduction of
sulfate ion
(7.4)
Iron sulfide forms under these conditions; hence pyrite crystals (FeS2) are common in
black anaerobic sediments. If sulfate reduction occurs at the very surface of the
sediment, calcareous shells are preserved, because the destruction of sulfate greatly
increases alkalinity.
7.6.4 The Persian! Arabian Gulf As Arid Model of a Marginal Sea. None of the
conditions typical for the Baltic Sea develop in the Persian Gulf, because oxygen is
brought to the sea floor by sinking saline waters, and because fertility is generally
low. The incoming ocean sUlface waters are low in nutrients, as are warm surface
waters everywhere. Hence the Persian Gulf is nutrient-starved. One place where
anaerobic conditions can develop is in hypersaline lagoons, within the sediment.
Sulfate reduction then occurs and organic-rich layers with pyrite can develop. However, such layers should be readily distinguishable in the geologic record from the
stagnant basin layers in humid conditions.
The sediments of the Gulf proper are very different from those of the Baltic:
organic contents (0.6 to 1 %) are about five times lower, and carbonate content (> 50
%) is more than ten times higher. Benthic organisms exist at all depths and at all
times, any lamination is quickly destroyed by bioturbation. Mobilization of heavy
metals stops within the sediment because of the high oxygen supply in the mixed
layer.