2.5 Lake Sediments
marine salt deposits have been discovered (Neev and
Emery 1966; Zak 1997). Besides rock salt (halite )
these evaporites also contain some gypsum,
anhydrite, camallite, polyhalite, and potash salts. The
lake is fed by the river Jordan which partially drains
regions with young volcanic rocks. In addition, minor
ephemeral rivers enter the basin from the sides, and a
number of springs discharge high-concentrated salt
water into the lake.
The salt content of the lake (ab out 330 g/l, density
1.242 g/cm J ) is eight to nine times higher than that of
normal sea water, but the Dead Sea brine strongly
differs in its composition from sea water. NaCl
makes up only one third of the total salt content,
whereas MgC1 2 reaches about one half. The brine is
remarkably rich in Ca and K (Mg>Na>Ca>K) as well
as Br (-5 g/l), but almost devoid of sulfate.
The origin of the complex Dead Sea brine has been controversially discussed by many authors. According to the present stage of knowledge (summary by Zak 1997) the chemical composition of the brine results from mixing of two
other brines and meteoric water:
(1) A highly saline diagenetic Ca-CI brine resulting from
evaporated sea water which is modified by the exchange of
marine Mg for Ca of the surrounding carbonate rocks. This
leads to Ca>HCOJ+S04 in deep-seated groundwater
emerging in springs.
(2) A highly concentrated metamorphic brine ofMg-Ca-KCI composition which is interpreted as the result of incongruent alteration and dissolution of hydrous marine
evaporite minerals such as carnallite and gypsum. This
brine appears in valley-floor springs.
(3) Meteoric water of CI-S04-(HC03) nature in which airborne sea salts and other constituents are somewhat enriched by evaporation. These waters enter the basin via
rivers or shallow groundwater.
The Dead Sea brine may be characteristic for continental rifts in arid regions where the basin was temporarily
connected with the ocean. The brine is stratified with a
more or less stable pycnocline; the upper water body is significantly affected by seasonal and episodic phenomena,
such as unusual freshwater inflow after heavy rain. The
Dead Sea is not completely barren of life; it contains
bluegreen algae, some planktic microorganisms, and several types ofbacteria.
At present aragonite, gypsum, and locally some halite
precipitate in the Dead Sea. The brine is almost saturated with respect to halite, but in the past widely
extended halite layers formed episodically. Most of
the gypsum is dissolved in the anoxie hypolimnion
by bacterial sulfate reduction. The generation of hydrogen sulfide leads to the formation of iron sulfide.
Thus, dark, mainly ca1careous mud with some gypsum, halite, and organic matter is deposited. Precipitation of halite in the form of "salt reefs" is taking
place in the shallow southem part of the lake, where
"end brines" from evaporation pans mix with Dead
Sea brines (Beyth 1980).
89
In the late Pleistocene (60-13 ka B.P.) the Jordan
Rift was occupied by a large lake (220 km in length,
cf. Fig. 11.18a). Its (fluctuating) water level stood
>200 m above the present Dead Sea level, and its
salinity was lower than that of the present brine. Its
sediments (Lisan Fm.) largely display annual varves
consisting of altemating thin laminae of aragonite
and gypsum (Niemi et al. 1997).
Other specific examples. The sediments of ancient
lake deposits have been studied in numerous cases all
over the world. Only a few of these can be mentioned
here and in the following section.
An unusually thick and voluminous nonmarine salt deposit
has accumulated in a halfgraben of the Basin and Range
province in Arizona (Faulds et al. 1997). The synrift
evaporites (up to 2500 m halite, with intercalated and capping anhydrite) fonned in an intracontinental playa collecting Na- and CI-rich water from a large drainage area.
Another specific example is the history of a paleolake
generated by a large Miocene meteorite impact in Southern
Gennany (Ries). The crater was first occupied by a playa
lake where algal biohenns, dolomite, gypsum, and also
some oil shales were deposited (Wolff and Füchtbauer
1976). Later, it evolved into a fresh-water lake and finally
fell dry.
Further examples are described, e.g., by Picard and
High (1981), Talbot and Kelts (1989), Talbot and Allen
(1996).
2.5.5 Black Shale Deposition in Lakes
Organic production in lakes is normally high due to
sufficient nutrient supply. Even highly saline waters
are not devoid of organisms (Larsen 1980). Although
only a few species exist, these can grow in large
quantities due to the abundance of nutrients and a
high temperature. Bluegreen algae and bacteria in
particular, but also some' planktonic organisrns,
copepods, nematods, crustacea, and higher plants
manage to live under these conditions and produce
and decompose organic matter (Eugster and Hardie
1978; cf. Sect. 10.3).
Recent measurements of primary productivity, for example
in the Great Salt Lake, Utah, have shown that salt lakes
represent some of the most productice ecosystems (Eugster
1985). Phytoplankton productivity as weil as the growth
rate of algal mats can be very high. Because most of this
production is destroyed by grazing higher organisms and
microbial decomposition, only a small fraction of this production is eventually deposited together with the inorganic
sediment fraction (cf. Sect. 10.5). However, this fraction is
much greater than in open-marine basins and sufficient for
the fonnation of organic-rich oil shales prior to the precipitation of salts. Even within evaporite sequences, black layers rich in organic carbon and some pyrite are encountered.
marine salt deposits have been discovered (Neev and
Emery 1966; Zak 1997). Besides rock salt (halite )
these evaporites also contain some gypsum,
anhydrite, camallite, polyhalite, and potash salts. The
lake is fed by the river Jordan which partially drains
regions with young volcanic rocks. In addition, minor
ephemeral rivers enter the basin from the sides, and a
number of springs discharge high-concentrated salt
water into the lake.
The salt content of the lake (ab out 330 g/l, density
1.242 g/cm J ) is eight to nine times higher than that of
normal sea water, but the Dead Sea brine strongly
differs in its composition from sea water. NaCl
makes up only one third of the total salt content,
whereas MgC1 2 reaches about one half. The brine is
remarkably rich in Ca and K (Mg>Na>Ca>K) as well
as Br (-5 g/l), but almost devoid of sulfate.
The origin of the complex Dead Sea brine has been controversially discussed by many authors. According to the present stage of knowledge (summary by Zak 1997) the chemical composition of the brine results from mixing of two
other brines and meteoric water:
(1) A highly saline diagenetic Ca-CI brine resulting from
evaporated sea water which is modified by the exchange of
marine Mg for Ca of the surrounding carbonate rocks. This
leads to Ca>HCOJ+S04 in deep-seated groundwater
emerging in springs.
(2) A highly concentrated metamorphic brine ofMg-Ca-KCI composition which is interpreted as the result of incongruent alteration and dissolution of hydrous marine
evaporite minerals such as carnallite and gypsum. This
brine appears in valley-floor springs.
(3) Meteoric water of CI-S04-(HC03) nature in which airborne sea salts and other constituents are somewhat enriched by evaporation. These waters enter the basin via
rivers or shallow groundwater.
The Dead Sea brine may be characteristic for continental rifts in arid regions where the basin was temporarily
connected with the ocean. The brine is stratified with a
more or less stable pycnocline; the upper water body is significantly affected by seasonal and episodic phenomena,
such as unusual freshwater inflow after heavy rain. The
Dead Sea is not completely barren of life; it contains
bluegreen algae, some planktic microorganisms, and several types ofbacteria.
At present aragonite, gypsum, and locally some halite
precipitate in the Dead Sea. The brine is almost saturated with respect to halite, but in the past widely
extended halite layers formed episodically. Most of
the gypsum is dissolved in the anoxie hypolimnion
by bacterial sulfate reduction. The generation of hydrogen sulfide leads to the formation of iron sulfide.
Thus, dark, mainly ca1careous mud with some gypsum, halite, and organic matter is deposited. Precipitation of halite in the form of "salt reefs" is taking
place in the shallow southem part of the lake, where
"end brines" from evaporation pans mix with Dead
Sea brines (Beyth 1980).
89
In the late Pleistocene (60-13 ka B.P.) the Jordan
Rift was occupied by a large lake (220 km in length,
cf. Fig. 11.18a). Its (fluctuating) water level stood
>200 m above the present Dead Sea level, and its
salinity was lower than that of the present brine. Its
sediments (Lisan Fm.) largely display annual varves
consisting of altemating thin laminae of aragonite
and gypsum (Niemi et al. 1997).
Other specific examples. The sediments of ancient
lake deposits have been studied in numerous cases all
over the world. Only a few of these can be mentioned
here and in the following section.
An unusually thick and voluminous nonmarine salt deposit
has accumulated in a halfgraben of the Basin and Range
province in Arizona (Faulds et al. 1997). The synrift
evaporites (up to 2500 m halite, with intercalated and capping anhydrite) fonned in an intracontinental playa collecting Na- and CI-rich water from a large drainage area.
Another specific example is the history of a paleolake
generated by a large Miocene meteorite impact in Southern
Gennany (Ries). The crater was first occupied by a playa
lake where algal biohenns, dolomite, gypsum, and also
some oil shales were deposited (Wolff and Füchtbauer
1976). Later, it evolved into a fresh-water lake and finally
fell dry.
Further examples are described, e.g., by Picard and
High (1981), Talbot and Kelts (1989), Talbot and Allen
(1996).
2.5.5 Black Shale Deposition in Lakes
Organic production in lakes is normally high due to
sufficient nutrient supply. Even highly saline waters
are not devoid of organisms (Larsen 1980). Although
only a few species exist, these can grow in large
quantities due to the abundance of nutrients and a
high temperature. Bluegreen algae and bacteria in
particular, but also some' planktonic organisrns,
copepods, nematods, crustacea, and higher plants
manage to live under these conditions and produce
and decompose organic matter (Eugster and Hardie
1978; cf. Sect. 10.3).
Recent measurements of primary productivity, for example
in the Great Salt Lake, Utah, have shown that salt lakes
represent some of the most productice ecosystems (Eugster
1985). Phytoplankton productivity as weil as the growth
rate of algal mats can be very high. Because most of this
production is destroyed by grazing higher organisms and
microbial decomposition, only a small fraction of this production is eventually deposited together with the inorganic
sediment fraction (cf. Sect. 10.5). However, this fraction is
much greater than in open-marine basins and sufficient for
the fonnation of organic-rich oil shales prior to the precipitation of salts. Even within evaporite sequences, black layers rich in organic carbon and some pyrite are encountered.