Hydrogenous Sediments
89
Obviously, in this instance of dolomite formation, the dolomite is closely associated with evaporation flats, tidal conditions, and unusually warm and saline water -
an environment unfriendly to higher organisms but favorable for recrystallisation.
Thus, we should not expect many macrofossils in sediments rich in this kind of
dolomite. Stromatolithic textures, however, may be preserved.
Experiments on dolomite formation suggest that the removal of the sulfate ion
from the interstitial waters (by reduction and precipitation as iron sulfide) is the
crucial step allowing dolomite formation. If so, the importance of the precipitation of
gypsum in evaporite lagoons may lie in the decrease of the sulfate concentration
rather than in the increase of the Mg/Ca ratio.
Microbial sulfate reduction in the uppermost few tens of meters of continental
margin sediments also seems to favor dolomitization. According to investigations
with deep-sea drilling cores, this occurs if accumulation rates are less than
500 m/million years and organic carbon contents are greater than 0.5 weight %.
Dolomite has also been found in deep-sea sediments deposited under anaerohic
conditions. This has been explained by bacterial sulfate reduction, removing this
obstacle to precipitation. Sulfate reduction also increases the alkalinity, providing
additional carbonate ions for precipitation. In general, diagenetic signals are strong in
hemipelagic sediments with suboxic processes. Under certain conditions, dolomitization of preformed carbonates may occur by dilution of interstitial seawater by freshwater, penetrating from nearby highs on tidal flats, as happens on the Bahaman
Islands, and on some coral reef islands.
3.8 Hydrogenous Sediments
With the question of dolomite formation, we have entered the evaporite environment.
Within continental margins, the bulk of hydrogenous sediments are evaporitic salt.
Strictly speaking, calcareous shells and skeletons are also hydrogenous, since they
originate in the water. However, we have called the minerals precipitated by organisms biogenous, and set them apart.
3.8.1 Marine Evaporites are those sediments which form on evaporation of seawater. Restriction of exchange with the open ocean, in a semi-enclosed basin, is
necessary to drive the salt content high enough for precipitation to begin. Such
restricted bodies of water are (1) coastal lagoons; (2) salt seas on the shelves; or (3)
early rift oceans in the deep sea. A special case is the Mediterranean, which was
partially isolated in the latest Miocene, 6 to 5 million years ago (see Sect. 9.5.2).
How much salt can be produced by evaporating a 1000-m- high column of seawater? Salt constitutes 3.5 % (or 35 %0) of the weight of the column, its density is
about 2.5 times that of water. Thus, we would obtain about 14 m salt. Most of this
would be table salt (halite) (see Table 3.1). The least soluble salts precipitate first:
calcium carbonate (aragonite), and calcium sulfate (gypsum). To precipitate halite,
the brine needs to be concentrated about tenfold. Many evaporites only contain carbonate and gypsum (or anhydrite), others have thick deposits of halite or, rarely, of
89
Obviously, in this instance of dolomite formation, the dolomite is closely associated with evaporation flats, tidal conditions, and unusually warm and saline water -
an environment unfriendly to higher organisms but favorable for recrystallisation.
Thus, we should not expect many macrofossils in sediments rich in this kind of
dolomite. Stromatolithic textures, however, may be preserved.
Experiments on dolomite formation suggest that the removal of the sulfate ion
from the interstitial waters (by reduction and precipitation as iron sulfide) is the
crucial step allowing dolomite formation. If so, the importance of the precipitation of
gypsum in evaporite lagoons may lie in the decrease of the sulfate concentration
rather than in the increase of the Mg/Ca ratio.
Microbial sulfate reduction in the uppermost few tens of meters of continental
margin sediments also seems to favor dolomitization. According to investigations
with deep-sea drilling cores, this occurs if accumulation rates are less than
500 m/million years and organic carbon contents are greater than 0.5 weight %.
Dolomite has also been found in deep-sea sediments deposited under anaerohic
conditions. This has been explained by bacterial sulfate reduction, removing this
obstacle to precipitation. Sulfate reduction also increases the alkalinity, providing
additional carbonate ions for precipitation. In general, diagenetic signals are strong in
hemipelagic sediments with suboxic processes. Under certain conditions, dolomitization of preformed carbonates may occur by dilution of interstitial seawater by freshwater, penetrating from nearby highs on tidal flats, as happens on the Bahaman
Islands, and on some coral reef islands.
3.8 Hydrogenous Sediments
With the question of dolomite formation, we have entered the evaporite environment.
Within continental margins, the bulk of hydrogenous sediments are evaporitic salt.
Strictly speaking, calcareous shells and skeletons are also hydrogenous, since they
originate in the water. However, we have called the minerals precipitated by organisms biogenous, and set them apart.
3.8.1 Marine Evaporites are those sediments which form on evaporation of seawater. Restriction of exchange with the open ocean, in a semi-enclosed basin, is
necessary to drive the salt content high enough for precipitation to begin. Such
restricted bodies of water are (1) coastal lagoons; (2) salt seas on the shelves; or (3)
early rift oceans in the deep sea. A special case is the Mediterranean, which was
partially isolated in the latest Miocene, 6 to 5 million years ago (see Sect. 9.5.2).
How much salt can be produced by evaporating a 1000-m- high column of seawater? Salt constitutes 3.5 % (or 35 %0) of the weight of the column, its density is
about 2.5 times that of water. Thus, we would obtain about 14 m salt. Most of this
would be table salt (halite) (see Table 3.1). The least soluble salts precipitate first:
calcium carbonate (aragonite), and calcium sulfate (gypsum). To precipitate halite,
the brine needs to be concentrated about tenfold. Many evaporites only contain carbonate and gypsum (or anhydrite), others have thick deposits of halite or, rarely, of