80
Lakes with Significant Proportions of Biogenie
Sediments
Many lakes or parts of them exhibit a transitional
stage between over-supplied and sediment-starved
conditions (Fig. 2.29c through e). The contribution
of biogenic skeletal components and organic matter
to lake sediments is strongly influenced by the chemistry oflake water and nutrient supply. Waters rich in
earth alkali ions, particularly calcium, favor the
growth of lime-secreting organisms, whereas lakes
poor in calcium ions but relatively rich in dissolved
silica and nutrients (such as phosphorus and nitrate)
allow a high production of biogenic opaline silica.
U sually both biogenic carbonate and opaline silica
are found in lake sediments, but in widely varying
fractions of the total sediment.
In so-called hard-water lakes which, at least seasonally, are oversaturated with respect to calcium
carbonate, calcareous shells of organisms and inorganically precipitated carbonate play a significant
part. This is particularly important in the littoral
zone, even in lakes which are well supplied with detrital clastics (Fig. 2.29c). Charophytes and bluegreen
algae extract CO 2 from lake water for photosynthesis
and thus cause precipitation of calcium carbonate at
their surface. Successive layers of calcite coatings
form different types of crusts and nodules (oncoids),
associated with reworked, sand-sized carbonate.
During the warm season calcium carbonate is also
precipitated chemically in the form of tiny micritic
crystals of low-Mg calcite. This occurs in the
epilimnion over the entire lake basin which becomes
markedly oversaturated as a result of increased temperature and CO 2 consumption by phytoplankton
(Fig. 2.29d). The tiny calcite particles are partially
redissolved in the hypolimnion which remains
undersaturated with respect to calcium carbonate,
due to the release of CO 2 from decaying organic matter and a lower pH. The remainder of the micritic
calcite (lutite) generates a thin, light-colored layer in
the profundal lake zone which is later (in fall and
winter time) overlain by clayey material and settling
organic debris (Kelts and Hsü 1978). In spring a high
production of diatoms may contribute to the formation of such nonglacial annual varves which are a
few tenths to a few millimeters thick (Fig. 2.28al).
Such varves are well developed and preserved in
lakes with little benthic bottom life and limited influx
of terrigenous material.
The oxygen content ofthe hypolimnion (as well as
terrigenous input) may vary from lake to lake or in
the same lake with time (Fig. 2.2ge). The bottom sediments of such lakes therefore often show transitional
facies between bioturbated marly silts and clays,
marls, and light-colored, highly calcareous muds
(seekreide), or between sediments poor and rich in
organic matter. Muds containing considerable
Chapter 2 Continental Sediments
amounts of organic detritus but still showing some
bioturbation are called gyttja. Well laminated,
organic-rich muds are referred to as sapropel which
frequently contains more than 10% organic carbon
(see also below).
Soft-water lakes poor in calcium but sufficiently
rich in nutrients, particularly phosphorus and nitrate,
may accumulate sediments rich in opaline silica derived largely from diatom frustules. Such lakes are
common in regions of cool climate when input of
detrital material is low.
A prominent example is the deep, relatively old Lake
Baikai in Siberia, which exists since the
Oligocene/Miocene and has accumulated up to about 2000
m of sediments (e.g. Moore et al. 1997). These mainly consist of deltaic material shed from the east and
diatomaceous muds. In areas where the sedimentation rate
is low «3 cmlka). ferric iron crusts have been found at the
lake floor (Deike et al. 1997). These crusts also contain
Mn, Ca, and P and rnay alternate with layers in which vivianite (iron-II phosphate) was detected.
Arctic lakes, which are not associated with glaciers,
commonly are poor in nutrients and organic production and
often receive !ittle detrital material (Kipphut 1988). Lakes
in arid polar regions may even precipitate salts such as hydrated sodium sulfate and calcium chloride (Müller 1988).
The sedimentation rate of soft-water lakes is often
comparatively low (on the order of 10 cmlka). Shallow, poorly drained lakes with the growth of peat
tend to become acidic and develop reducing conditions.
Post-glacial Lake Evolution
The post-glacial development of a relatively small,
glacier-shaped model lake in humid temperate climate is shown in Fig. 2.30. The overall tendency is
the evolution from a relatively deep, large, waterfilled basin to a small pond and finally dry land. At
the beginning of this development, the lake floor may
be covered by thick glacial till (as observed in periAlpine lakes). The first lake sediments after the retreat of ice are sandy prodelta foresets and silty glacial varves deposited by turbidity currents in the
profundal zone. These sediments and their relatively
high sedimentation rates reflect ongoing ice action
and reworking of older glacial deposits by meltwater
in the drainage area. Vegetation is absent or sparse in
the surrounding land and, due to the still unfavorable
climatic conditions and low nutrient supply, the organic productivity of the lake is also very low at this
stage. The rapidly accumulating clastic sediments are
therefore poor in organic matter.
In a second stage, vegetation starts to spread out
over the land and reduces the input of detrital clastics
into the lake. Simultaneously, dissolution of carbonates and chemical weathering of silicates in the drain-
Lakes with Significant Proportions of Biogenie
Sediments
Many lakes or parts of them exhibit a transitional
stage between over-supplied and sediment-starved
conditions (Fig. 2.29c through e). The contribution
of biogenic skeletal components and organic matter
to lake sediments is strongly influenced by the chemistry oflake water and nutrient supply. Waters rich in
earth alkali ions, particularly calcium, favor the
growth of lime-secreting organisms, whereas lakes
poor in calcium ions but relatively rich in dissolved
silica and nutrients (such as phosphorus and nitrate)
allow a high production of biogenic opaline silica.
U sually both biogenic carbonate and opaline silica
are found in lake sediments, but in widely varying
fractions of the total sediment.
In so-called hard-water lakes which, at least seasonally, are oversaturated with respect to calcium
carbonate, calcareous shells of organisms and inorganically precipitated carbonate play a significant
part. This is particularly important in the littoral
zone, even in lakes which are well supplied with detrital clastics (Fig. 2.29c). Charophytes and bluegreen
algae extract CO 2 from lake water for photosynthesis
and thus cause precipitation of calcium carbonate at
their surface. Successive layers of calcite coatings
form different types of crusts and nodules (oncoids),
associated with reworked, sand-sized carbonate.
During the warm season calcium carbonate is also
precipitated chemically in the form of tiny micritic
crystals of low-Mg calcite. This occurs in the
epilimnion over the entire lake basin which becomes
markedly oversaturated as a result of increased temperature and CO 2 consumption by phytoplankton
(Fig. 2.29d). The tiny calcite particles are partially
redissolved in the hypolimnion which remains
undersaturated with respect to calcium carbonate,
due to the release of CO 2 from decaying organic matter and a lower pH. The remainder of the micritic
calcite (lutite) generates a thin, light-colored layer in
the profundal lake zone which is later (in fall and
winter time) overlain by clayey material and settling
organic debris (Kelts and Hsü 1978). In spring a high
production of diatoms may contribute to the formation of such nonglacial annual varves which are a
few tenths to a few millimeters thick (Fig. 2.28al).
Such varves are well developed and preserved in
lakes with little benthic bottom life and limited influx
of terrigenous material.
The oxygen content ofthe hypolimnion (as well as
terrigenous input) may vary from lake to lake or in
the same lake with time (Fig. 2.2ge). The bottom sediments of such lakes therefore often show transitional
facies between bioturbated marly silts and clays,
marls, and light-colored, highly calcareous muds
(seekreide), or between sediments poor and rich in
organic matter. Muds containing considerable
Chapter 2 Continental Sediments
amounts of organic detritus but still showing some
bioturbation are called gyttja. Well laminated,
organic-rich muds are referred to as sapropel which
frequently contains more than 10% organic carbon
(see also below).
Soft-water lakes poor in calcium but sufficiently
rich in nutrients, particularly phosphorus and nitrate,
may accumulate sediments rich in opaline silica derived largely from diatom frustules. Such lakes are
common in regions of cool climate when input of
detrital material is low.
A prominent example is the deep, relatively old Lake
Baikai in Siberia, which exists since the
Oligocene/Miocene and has accumulated up to about 2000
m of sediments (e.g. Moore et al. 1997). These mainly consist of deltaic material shed from the east and
diatomaceous muds. In areas where the sedimentation rate
is low «3 cmlka). ferric iron crusts have been found at the
lake floor (Deike et al. 1997). These crusts also contain
Mn, Ca, and P and rnay alternate with layers in which vivianite (iron-II phosphate) was detected.
Arctic lakes, which are not associated with glaciers,
commonly are poor in nutrients and organic production and
often receive !ittle detrital material (Kipphut 1988). Lakes
in arid polar regions may even precipitate salts such as hydrated sodium sulfate and calcium chloride (Müller 1988).
The sedimentation rate of soft-water lakes is often
comparatively low (on the order of 10 cmlka). Shallow, poorly drained lakes with the growth of peat
tend to become acidic and develop reducing conditions.
Post-glacial Lake Evolution
The post-glacial development of a relatively small,
glacier-shaped model lake in humid temperate climate is shown in Fig. 2.30. The overall tendency is
the evolution from a relatively deep, large, waterfilled basin to a small pond and finally dry land. At
the beginning of this development, the lake floor may
be covered by thick glacial till (as observed in periAlpine lakes). The first lake sediments after the retreat of ice are sandy prodelta foresets and silty glacial varves deposited by turbidity currents in the
profundal zone. These sediments and their relatively
high sedimentation rates reflect ongoing ice action
and reworking of older glacial deposits by meltwater
in the drainage area. Vegetation is absent or sparse in
the surrounding land and, due to the still unfavorable
climatic conditions and low nutrient supply, the organic productivity of the lake is also very low at this
stage. The rapidly accumulating clastic sediments are
therefore poor in organic matter.
In a second stage, vegetation starts to spread out
over the land and reduces the input of detrital clastics
into the lake. Simultaneously, dissolution of carbonates and chemical weathering of silicates in the drain-