90
Sources and Composition of Marine Sediments
the valuable (and very soluble) potassium salts. This sequence of mineral precipitation was experimentally established by Usiglio in 1849.
An evaporite basin, besides having restricted access to the open ocean, must lie
within an arid climate. New seawater must be delivered from time to time. This
seawater must be concentrated by evaporation. If only gypsum is to form, a concentration beyond threefold must be prevented, by new incursions of seawater, or else
halite must be removed during or after each evaporation cycle (Fig. 3.11). If only
halite is to form, left-over brine from a gypsum-precipitating basin must be available,
or the halite must be leached from elsewhere and brought into the basin by nonmarine
waters. Differential preservation, then, and serial fractionation are the key processes
in controlling the chemistry of salt deposits. In lagoonal settings, a periodic covering
of existing salts, by wind or flood deposits, may be necessary to prevent redissolution
by the next invasion of ocean water.
Only very few modern examples for submarine evaporite formation are known:
coastal lagoons of Ojo de Liebre in Baja California (Mexico) or the 20-km-Iong
drowned river valley Bocana de Virrila (Peru), where gypsum is precipitated when
salinities reach 160 %0 by evaporation and halite at more than 320 %0 (type a in Fig.
3.11).
The formation of diagenetic dolomite and gypsum/anhydrite in coastal "sabkha"
environments was discussed in the last section. Features typical for this environment,
such as big gypsum crystals, nodular or intensely deformed anhydrite, and algal mats,
are well known in ancient evaporite series of the Permian period (Lower Clear Formation, Texas; Zechstein, Northwestern Europe). Also, such features were discovered
in uppermost Miocene sediments, underlying the Mediterranean, by deep-sea drilling
(see Fig. 3.12).
3.8.2 Phosphorites. As is true for nonskeletal calcium carbonate, marine phosphorites are in the boundary line between hydrogenous and biogenous origin. After
all, phosphorus is intimately associated with the life cycle on Earth, in the sea as well
as on land. It is an essential component of every living cell. In fact, it is likely that the
availability of phosphorus to photosynthetic organisms ultimately controls the fertility of the ocean, and hence also the formation of biogenous deposits.
Phosphorites deserve special attention for the reason of this tie-in to ocean fertility,
but also because of their economic value (Sect. 10.3.1).
3.8.3 Iron Compounds are abundant in both oceanic margin sediments and in the
deep sea, since iron is one of the most abundant elements on Earth. On the slopes, the
high supply of organic matter commonly leads to oxy gen deficiency, and to sulfate
reduction by marine bacteria, in the uppermost sediment layers. This process results
in H2S formation, and in the precipitation of iron sulfide (pyrite). In the deep sea, on
the other hand, oxygen is generally plentiful, and essentially all iron occurs in its
oxidized form, as iron-oxide/hydroxide (goethite), Here it is especially associated
with manganese deposits (Sect. 10.4).
The reduction of sulfate in anaerobic sediments. and the associated precipitation of
iron sulfide, is a geochemical process of major importance, which bears on the
Sources and Composition of Marine Sediments
the valuable (and very soluble) potassium salts. This sequence of mineral precipitation was experimentally established by Usiglio in 1849.
An evaporite basin, besides having restricted access to the open ocean, must lie
within an arid climate. New seawater must be delivered from time to time. This
seawater must be concentrated by evaporation. If only gypsum is to form, a concentration beyond threefold must be prevented, by new incursions of seawater, or else
halite must be removed during or after each evaporation cycle (Fig. 3.11). If only
halite is to form, left-over brine from a gypsum-precipitating basin must be available,
or the halite must be leached from elsewhere and brought into the basin by nonmarine
waters. Differential preservation, then, and serial fractionation are the key processes
in controlling the chemistry of salt deposits. In lagoonal settings, a periodic covering
of existing salts, by wind or flood deposits, may be necessary to prevent redissolution
by the next invasion of ocean water.
Only very few modern examples for submarine evaporite formation are known:
coastal lagoons of Ojo de Liebre in Baja California (Mexico) or the 20-km-Iong
drowned river valley Bocana de Virrila (Peru), where gypsum is precipitated when
salinities reach 160 %0 by evaporation and halite at more than 320 %0 (type a in Fig.
3.11).
The formation of diagenetic dolomite and gypsum/anhydrite in coastal "sabkha"
environments was discussed in the last section. Features typical for this environment,
such as big gypsum crystals, nodular or intensely deformed anhydrite, and algal mats,
are well known in ancient evaporite series of the Permian period (Lower Clear Formation, Texas; Zechstein, Northwestern Europe). Also, such features were discovered
in uppermost Miocene sediments, underlying the Mediterranean, by deep-sea drilling
(see Fig. 3.12).
3.8.2 Phosphorites. As is true for nonskeletal calcium carbonate, marine phosphorites are in the boundary line between hydrogenous and biogenous origin. After
all, phosphorus is intimately associated with the life cycle on Earth, in the sea as well
as on land. It is an essential component of every living cell. In fact, it is likely that the
availability of phosphorus to photosynthetic organisms ultimately controls the fertility of the ocean, and hence also the formation of biogenous deposits.
Phosphorites deserve special attention for the reason of this tie-in to ocean fertility,
but also because of their economic value (Sect. 10.3.1).
3.8.3 Iron Compounds are abundant in both oceanic margin sediments and in the
deep sea, since iron is one of the most abundant elements on Earth. On the slopes, the
high supply of organic matter commonly leads to oxy gen deficiency, and to sulfate
reduction by marine bacteria, in the uppermost sediment layers. This process results
in H2S formation, and in the precipitation of iron sulfide (pyrite). In the deep sea, on
the other hand, oxygen is generally plentiful, and essentially all iron occurs in its
oxidized form, as iron-oxide/hydroxide (goethite), Here it is especially associated
with manganese deposits (Sect. 10.4).
The reduction of sulfate in anaerobic sediments. and the associated precipitation of
iron sulfide, is a geochemical process of major importance, which bears on the