551
Answers to Problems
rates, particularly in areas of high primary bioproductivity. The stagnant basin and the productivity
model are used to illustrate favorable conditions
for organic matter preservation. In addition, high
sinking velocities, adsorption or organic matter to
mineral matter (clay) and small grain sizes of the
deposits, restricting exchange with the oxygencontaining ocean water after sedimentation,
contribute to organic matter preservation.
Answer 2
Molecular oxygen and seawater sulfate are the main
electron acceptors which mediate organic matter
metabolization during early diagenesis by aerobic
and anaerobic bacteria, respectively. Nitrate,
iron(III), and manganese(IV), often in that order of
significance, are additional electron acceptors in
organic matter respiration. Ultimately, methanogenesis uses carbon as electron acceptor at greater
sediment depth, when sulfate is depleted.
Answer 3
Not all parts of the biomass of decayed organisms
have the same chance of being preserved in the
fossil record. Water-soluble organic compounds
or organic biopolymers easily hydrolyzed into
small water-soluble monomers are rapidly recycled
already in the water column or early during
diagenesis. Examples are proteins and carbohydrates and their monomeric forms, sugars and
amino acids, respectively. Hydrophobic lipids
have a significantly higher preservation potential,
which increases with increasing molecular weight.
Reaction of organic matter with reduced sulfur
inorganic sulfur species (e.g. hydrogen sulfide)
formed during bacterial sulfate reduction, further
enhances the preservation potential by the
formation of high-molecular-weight organic material.
Answer 4
Biological markers are organic molecules which
preserve their carbon skeleton after the decay of
organisms and through the processes of
sedimentation, diagenesis and eventually catagenesis in a way that a correlation with a (group of)
precursor organism(s) is still possible. This relationship may reveal important information of the
environmental conditions at the time of sediment
deposition. Cyclic lipids like steroids and pentacyclic triterpenoids have proven particularly
useful as biological markers or molecular fossils.
Answer 5
A key question to be answered by organic geochemists is that of the marine or terrigenous origin
of organic matter in sediments and the extent of
mixing of the two components, respectively. A
number of bulk and molecular organic geochemical
parameters are available to address this question,
including elemental analysis, carbon stable isotope analysis, organic petrography and specific
biological markers for marine and terrestrial
organic matter. Sea surface temperatures can be
reconstructed by the analysis of the degree of
unsaturation of long-chain alkenones from haptophytes preserved in sediments (U 37
K' index) or by
the determination of the number of cyclo-pentane
rings in fossilized Crenarchaeotal membrane lipids
(TEX 86 index). Information on the sedimentary
methane cycle can be obtained by the combined
analysis of specific membrane lipids of the organisms involved in this process and by the determination of the carbon stable isotope signal of
lipids from methanotrophic microorganisms.
Subtle changes in molecular structure and their
sterical orientation reveals information of the
progress of diagenesis and catagenesis.
Chapter 5
Answer 1
A simplified calculation of diffusion distance, L =
(2Dt)
0.5
(Eq. 5.7), is made here for chloride: The
diffusion coefficient for Cl
-
at deep-sea temperature (4ºC) is D = 11 · 10
-10
m
2
s
-1
(see Chapter 3,
Table 3.1); the time is 10,000 years = 3.2 · 10
11
s;
thus, L = (2 · 11 · 10
-10
· 3.2 · 10
11
)
0.5
= 26 m. So, the
lowered salinity of the world ocean since the end
of the last glaciation can be traced in the pore
water profile of chloride down to about 26 m
below seafloor.
Answer 2
The following processes are energetically
feasible: 2, 3, and 5. Process 2 has, however, not
been detected in marine sediments and no microorganisms are known that can carry out the
Answers to Problems
rates, particularly in areas of high primary bioproductivity. The stagnant basin and the productivity
model are used to illustrate favorable conditions
for organic matter preservation. In addition, high
sinking velocities, adsorption or organic matter to
mineral matter (clay) and small grain sizes of the
deposits, restricting exchange with the oxygencontaining ocean water after sedimentation,
contribute to organic matter preservation.
Answer 2
Molecular oxygen and seawater sulfate are the main
electron acceptors which mediate organic matter
metabolization during early diagenesis by aerobic
and anaerobic bacteria, respectively. Nitrate,
iron(III), and manganese(IV), often in that order of
significance, are additional electron acceptors in
organic matter respiration. Ultimately, methanogenesis uses carbon as electron acceptor at greater
sediment depth, when sulfate is depleted.
Answer 3
Not all parts of the biomass of decayed organisms
have the same chance of being preserved in the
fossil record. Water-soluble organic compounds
or organic biopolymers easily hydrolyzed into
small water-soluble monomers are rapidly recycled
already in the water column or early during
diagenesis. Examples are proteins and carbohydrates and their monomeric forms, sugars and
amino acids, respectively. Hydrophobic lipids
have a significantly higher preservation potential,
which increases with increasing molecular weight.
Reaction of organic matter with reduced sulfur
inorganic sulfur species (e.g. hydrogen sulfide)
formed during bacterial sulfate reduction, further
enhances the preservation potential by the
formation of high-molecular-weight organic material.
Answer 4
Biological markers are organic molecules which
preserve their carbon skeleton after the decay of
organisms and through the processes of
sedimentation, diagenesis and eventually catagenesis in a way that a correlation with a (group of)
precursor organism(s) is still possible. This relationship may reveal important information of the
environmental conditions at the time of sediment
deposition. Cyclic lipids like steroids and pentacyclic triterpenoids have proven particularly
useful as biological markers or molecular fossils.
Answer 5
A key question to be answered by organic geochemists is that of the marine or terrigenous origin
of organic matter in sediments and the extent of
mixing of the two components, respectively. A
number of bulk and molecular organic geochemical
parameters are available to address this question,
including elemental analysis, carbon stable isotope analysis, organic petrography and specific
biological markers for marine and terrestrial
organic matter. Sea surface temperatures can be
reconstructed by the analysis of the degree of
unsaturation of long-chain alkenones from haptophytes preserved in sediments (U 37
K' index) or by
the determination of the number of cyclo-pentane
rings in fossilized Crenarchaeotal membrane lipids
(TEX 86 index). Information on the sedimentary
methane cycle can be obtained by the combined
analysis of specific membrane lipids of the organisms involved in this process and by the determination of the carbon stable isotope signal of
lipids from methanotrophic microorganisms.
Subtle changes in molecular structure and their
sterical orientation reveals information of the
progress of diagenesis and catagenesis.
Chapter 5
Answer 1
A simplified calculation of diffusion distance, L =
(2Dt)
0.5
(Eq. 5.7), is made here for chloride: The
diffusion coefficient for Cl
-
at deep-sea temperature (4ºC) is D = 11 · 10
-10
m
2
s
-1
(see Chapter 3,
Table 3.1); the time is 10,000 years = 3.2 · 10
11
s;
thus, L = (2 · 11 · 10
-10
· 3.2 · 10
11
)
0.5
= 26 m. So, the
lowered salinity of the world ocean since the end
of the last glaciation can be traced in the pore
water profile of chloride down to about 26 m
below seafloor.
Answer 2
The following processes are energetically
feasible: 2, 3, and 5. Process 2 has, however, not
been detected in marine sediments and no microorganisms are known that can carry out the