90
The mineralization of organic matter in the water
column of lakes depends on the oxygen supply to
bottom water. This and the destruction of organic
matter is usually high in regions of temperate, humid
climate. In summer, here cold, well-oxygenated river
water flows along the lake bottom to the outflow. In
addition, overtuming of the lake takes place in the
cold season. Both processes improve the oxygen supply of the hypolimnion and thus promote the mineralization of organic matter.
For these reasons, all the present-day, large, temperate
lakes of the northem hemisphere are oxic in their natural
state, for example the Great Lakes of North America, numerous smaller lakes of the Alpine region in Europe, and
even the 1600 m deep Lake Baikai in Siberia. As a result of
waste water injection, however, some ofthese lakes developed anoxie bottom waters (e.g., Lake Erie, Lake Zürich).
In contrast, lakes in zones of warm climate, which
reach some depth (about 100 m and more), tend to
develop a stable stratification throughout the year.
Furthermore, warm water lakes dissolve less oxygen
than cold water lakes. The limited oxygen supply to
the hypolimnion leads to the development of anoxie
bottom waters (eutrophie lakes) and thus favors the
preservation of organic matter.
Under these conditions both the open freshwater
lakes and the closed saline lake systems (usually during highstands) can produce oil shales with high contents of organic matter. Of course, the organic matter
should not be diluted by a too rapid sedimentation
rate of detrital clastics.
The 1500 m deep Lake Tanganyika in the East African rift
zone is an example of this lake type (Degens et al. 1971;
Cohen 1989; Cohen 1997). Its drainage area is semi-humid, and the lake has some outflow since the early 19th
century. Most of the river load settles in the upstream Lake
Kivu. Littoral sediments of Lake Tanganyika are rich in
carbonate inc1uding ooid shoals, stromatolites, and beach
rock. Anoxie conditions with some hydrogen sulfide in the
water prevail below a water depth of about 200 m. Sediments deposited in the shallower part ofthe lake contain 12% organic carbon, whereas the laminated anoxie sediments in deeper water reach 7-11 % organic carhon mainly
derived from diatoms. The sedimentation rate ofthe basinal
muds is ab out 0.5 mika; interbedded silty-sandy turbidites
are common.
A prominent exception to stratified tropical lakes is the
large but shallow Lake Victoria in the East African rift
zone.
Organic matter is generally preserved better in lake
sediments than in marine deposits for the following
reasons:
~ The sedimentation rate in lakes is usually much
higher (often by one to two orders of magnitude; see
below) than in open marine environments and thus
Chapter 2 Continental Sediments
protects part of the organic matter from being decomposed on the lake floor.
~ A low sulfate content in the pore water of many
lake sediments, in contrast to marine sediments, prevents strong activity of sulfate reducing bacteria that
simultaneously consume organic matter. Degradation
of organic matter must therefore be accomplished
mainly by the less efficient methane fermentation
(Demaison and Moore 1980; cf. Sect. 14.1).
For the same reason, sulfide supply is commonly limited
and cannot precipitate all the iron released from minerals as
pyrite; consequently, siderite nodules can form (as e.g.
found in the former lake stage of the Black Sea; Degens
and Stoffers 1980). Anoxie lake sediments are usually also
rich in carbonate.
As a result of changing climate, such lake sediments
may display vertical successions altemating between
(Fig. 2.33):
~ Kerogen-rich beds, coinciding with the more humid phase and high lake level.
~ Evaporitic beds, less rich in organic matter, associated with the more arid phase.
Successions ofthis type are common in many ancient
lake deposits, for example in the Triassie and in the
Eocene Green River Formation of North America
(see below).
In conclusion, lacustrine black shales in the ancient record indicate either a warm climate with small
seasonal variations in temperature and a lake basin
with limited inflow and outflow, or a completely
closed lake system during high water level. Lacustrine black shales normally contain limited amounts
of iron sulfide, whereas siderite nodules are common
in various lake deposits.
2.5.6 Successions of Lake Sediments
Lacustrine sediments are sensitive indicators of
changes in climate because climate in turn controls
weathering and sediment transport to the lakes. In the
case of hydrologically closed basins, changes in the
water budget of thc lake often cause drastic lake level
fluctuations (e.g. Scholz et al. 1998; cf. Sect. 7.6).
Lacustrine deposits therefore show, more than the
sediments of other depositional environments do,
characteristic vertical successions. They may display
rhythmic and/or cyclic sequences of different orders
(cf. Sects. 7.1, 7.6 and 7.9), with or without black
shales and/or evaporites. Outcrop sections or drilling
cores of formerly closed lake basins often show alternations between fluvial or lake margin clastics and
evaporites which record changes from wetter to more
arid conditions.
The mineralization of organic matter in the water
column of lakes depends on the oxygen supply to
bottom water. This and the destruction of organic
matter is usually high in regions of temperate, humid
climate. In summer, here cold, well-oxygenated river
water flows along the lake bottom to the outflow. In
addition, overtuming of the lake takes place in the
cold season. Both processes improve the oxygen supply of the hypolimnion and thus promote the mineralization of organic matter.
For these reasons, all the present-day, large, temperate
lakes of the northem hemisphere are oxic in their natural
state, for example the Great Lakes of North America, numerous smaller lakes of the Alpine region in Europe, and
even the 1600 m deep Lake Baikai in Siberia. As a result of
waste water injection, however, some ofthese lakes developed anoxie bottom waters (e.g., Lake Erie, Lake Zürich).
In contrast, lakes in zones of warm climate, which
reach some depth (about 100 m and more), tend to
develop a stable stratification throughout the year.
Furthermore, warm water lakes dissolve less oxygen
than cold water lakes. The limited oxygen supply to
the hypolimnion leads to the development of anoxie
bottom waters (eutrophie lakes) and thus favors the
preservation of organic matter.
Under these conditions both the open freshwater
lakes and the closed saline lake systems (usually during highstands) can produce oil shales with high contents of organic matter. Of course, the organic matter
should not be diluted by a too rapid sedimentation
rate of detrital clastics.
The 1500 m deep Lake Tanganyika in the East African rift
zone is an example of this lake type (Degens et al. 1971;
Cohen 1989; Cohen 1997). Its drainage area is semi-humid, and the lake has some outflow since the early 19th
century. Most of the river load settles in the upstream Lake
Kivu. Littoral sediments of Lake Tanganyika are rich in
carbonate inc1uding ooid shoals, stromatolites, and beach
rock. Anoxie conditions with some hydrogen sulfide in the
water prevail below a water depth of about 200 m. Sediments deposited in the shallower part ofthe lake contain 12% organic carbon, whereas the laminated anoxie sediments in deeper water reach 7-11 % organic carhon mainly
derived from diatoms. The sedimentation rate ofthe basinal
muds is ab out 0.5 mika; interbedded silty-sandy turbidites
are common.
A prominent exception to stratified tropical lakes is the
large but shallow Lake Victoria in the East African rift
zone.
Organic matter is generally preserved better in lake
sediments than in marine deposits for the following
reasons:
~ The sedimentation rate in lakes is usually much
higher (often by one to two orders of magnitude; see
below) than in open marine environments and thus
Chapter 2 Continental Sediments
protects part of the organic matter from being decomposed on the lake floor.
~ A low sulfate content in the pore water of many
lake sediments, in contrast to marine sediments, prevents strong activity of sulfate reducing bacteria that
simultaneously consume organic matter. Degradation
of organic matter must therefore be accomplished
mainly by the less efficient methane fermentation
(Demaison and Moore 1980; cf. Sect. 14.1).
For the same reason, sulfide supply is commonly limited
and cannot precipitate all the iron released from minerals as
pyrite; consequently, siderite nodules can form (as e.g.
found in the former lake stage of the Black Sea; Degens
and Stoffers 1980). Anoxie lake sediments are usually also
rich in carbonate.
As a result of changing climate, such lake sediments
may display vertical successions altemating between
(Fig. 2.33):
~ Kerogen-rich beds, coinciding with the more humid phase and high lake level.
~ Evaporitic beds, less rich in organic matter, associated with the more arid phase.
Successions ofthis type are common in many ancient
lake deposits, for example in the Triassie and in the
Eocene Green River Formation of North America
(see below).
In conclusion, lacustrine black shales in the ancient record indicate either a warm climate with small
seasonal variations in temperature and a lake basin
with limited inflow and outflow, or a completely
closed lake system during high water level. Lacustrine black shales normally contain limited amounts
of iron sulfide, whereas siderite nodules are common
in various lake deposits.
2.5.6 Successions of Lake Sediments
Lacustrine sediments are sensitive indicators of
changes in climate because climate in turn controls
weathering and sediment transport to the lakes. In the
case of hydrologically closed basins, changes in the
water budget of thc lake often cause drastic lake level
fluctuations (e.g. Scholz et al. 1998; cf. Sect. 7.6).
Lacustrine deposits therefore show, more than the
sediments of other depositional environments do,
characteristic vertical successions. They may display
rhythmic and/or cyclic sequences of different orders
(cf. Sects. 7.1, 7.6 and 7.9), with or without black
shales and/or evaporites. Outcrop sections or drilling
cores of formerly closed lake basins often show alternations between fluvial or lake margin clastics and
evaporites which record changes from wetter to more
arid conditions.