2
Physical Properties of Marine Sediments
64
300,000 years ago. Only terrigenous turbidites
causing strong reflections below 6 m depth were
deposited.
Transferred to the Parasound seismogram section in Figure 2.23 this interpretation means that
the first prominent reflection horizon in the levees
can also be attributed to the Toba Ash layer (75 ka).
The following transparent zone indicates completely
pelagic sediments deposited on the levees during
the inactive time of the channel (< 240 ka). The
suspension cloud was probably trapped between
the channel walls so that only very thin, finegrained terrigenous turbidites were deposited on
the terrace but did not reach the levee crests. The
high reflectivity part below the transparent zone in
the levees is probably caused by terrigenous
turbidites deposited here more than 300,000 years
ago during the active time of the channel.
2.6.2
Narrow-Beam Parasound
Echosounder Recordings
Finally, some examples of Parasound echosounder
recordings from different terrigenous and biogenic
provinces are presented to illustrate that characteristic sediment compositions can already be
recognized from such remote sensing surveys
prior to the sediment core retrieval.
The first example was recorded on the Rio
Grande Rise in the western South Atlantic during
RV Meteor cruise M29/2 (Bleil et al. 1994) and is
typical for a calcareous environment (Fig. 2.25a).
It is characterized by a strong sea bottom reflector
caused by coarse-grained foraminiferal sands.
Signal penetration is low and reaches only about
20 m.
In contrast, the second example from the
Conrad Rise in the Antarctic Ocean, recorded
during RV Polarstern cruise ANT XI/4 (Kuhn, pers.
communication), displays a typical biogenic
siliceous environment (Fig. 2.25b). The diatomaceous sediment coverage seems to be very
transparent, and the Parasound signal penetrates
to about 160 m depth. Reflection horizons are
caused by calcareous foraminiferal and nannofossil layers deposited during a retreat of the
Polar Frontal Zone during interglacial stages.
The third example recorded in the Weddell Sea/
Antarctic Ocean during RV Polarstern cruise ANT
XI/4 as well (Kuhn, pers. communication) indicates a deep sea environment with clay sediments
(Fig. 2.25c). Signal penetration again is high
(about 140 m), and reflection horizons are very
sharp and distinct. Zones with upward curved
reflection horizons might possibly be indicators
for pore fluid migrations.
The fourth example from the distal Bengal
Fan (Hübscher et al. 1997) was recorded during
RV Sonne cruise SO125 and displays terrigenous
features and sediments (Fig. 2.25d). Active and
older abandoned channel levee systems are characterized by a diffuse reflection pattern which
indicates sediments of coarse-grained turbidites.
Signal penetration in such environments is rather
low and reaches about 30 - 40 m.
Acknowledgment
We thank the captains, crews and scientists onboard of RV Meteor, RV Polarstern and RV Sonne
for their efficient cooperation and help during the
cruises in the South Atlantic, Antarctic and Indian
Ocean. Additionally, special thanks are to M.
Richter who provided a lot of unpublished electrical resistivity data and to F. Pototzki, B. Pioch
and C. Hilgenfeld who gave much technical
assistance and support during the development of
the full waveform logging system. V. Spieß
developed the ParaDIGMA data acquisition
system for digital recording of the Parasound
echosounder data and the program system for
their processing and display. His help is greatly
appreciated, too. H. Villinger critically read an
early draft and improved the manuscript by many
helpful discussions. The research was funded by
the Deutsche Forschungsgemeinschaft (Special
Research Project SFB 261 at Bremen University,
contribution no. 251 and project no. Br 1476/2-1+2)
and by the Federal Minister of Education, Science,
Research and Technology (BMBF), grant no.
03G0093C.
Appendix
A:
Physical Properties of Sediment
Grains and Sea Water
The density (ρ g ) and bulk modulus (K g ) of the
sediment grains are the most important physical
properties which characterize the sediment type -
terrigenous, calcareous and siliceous - and
composition. Additionally, they are required as
input parameters for wave propagation modeling,
Physical Properties of Marine Sediments
64
300,000 years ago. Only terrigenous turbidites
causing strong reflections below 6 m depth were
deposited.
Transferred to the Parasound seismogram section in Figure 2.23 this interpretation means that
the first prominent reflection horizon in the levees
can also be attributed to the Toba Ash layer (75 ka).
The following transparent zone indicates completely
pelagic sediments deposited on the levees during
the inactive time of the channel (< 240 ka). The
suspension cloud was probably trapped between
the channel walls so that only very thin, finegrained terrigenous turbidites were deposited on
the terrace but did not reach the levee crests. The
high reflectivity part below the transparent zone in
the levees is probably caused by terrigenous
turbidites deposited here more than 300,000 years
ago during the active time of the channel.
2.6.2
Narrow-Beam Parasound
Echosounder Recordings
Finally, some examples of Parasound echosounder
recordings from different terrigenous and biogenic
provinces are presented to illustrate that characteristic sediment compositions can already be
recognized from such remote sensing surveys
prior to the sediment core retrieval.
The first example was recorded on the Rio
Grande Rise in the western South Atlantic during
RV Meteor cruise M29/2 (Bleil et al. 1994) and is
typical for a calcareous environment (Fig. 2.25a).
It is characterized by a strong sea bottom reflector
caused by coarse-grained foraminiferal sands.
Signal penetration is low and reaches only about
20 m.
In contrast, the second example from the
Conrad Rise in the Antarctic Ocean, recorded
during RV Polarstern cruise ANT XI/4 (Kuhn, pers.
communication), displays a typical biogenic
siliceous environment (Fig. 2.25b). The diatomaceous sediment coverage seems to be very
transparent, and the Parasound signal penetrates
to about 160 m depth. Reflection horizons are
caused by calcareous foraminiferal and nannofossil layers deposited during a retreat of the
Polar Frontal Zone during interglacial stages.
The third example recorded in the Weddell Sea/
Antarctic Ocean during RV Polarstern cruise ANT
XI/4 as well (Kuhn, pers. communication) indicates a deep sea environment with clay sediments
(Fig. 2.25c). Signal penetration again is high
(about 140 m), and reflection horizons are very
sharp and distinct. Zones with upward curved
reflection horizons might possibly be indicators
for pore fluid migrations.
The fourth example from the distal Bengal
Fan (Hübscher et al. 1997) was recorded during
RV Sonne cruise SO125 and displays terrigenous
features and sediments (Fig. 2.25d). Active and
older abandoned channel levee systems are characterized by a diffuse reflection pattern which
indicates sediments of coarse-grained turbidites.
Signal penetration in such environments is rather
low and reaches about 30 - 40 m.
Acknowledgment
We thank the captains, crews and scientists onboard of RV Meteor, RV Polarstern and RV Sonne
for their efficient cooperation and help during the
cruises in the South Atlantic, Antarctic and Indian
Ocean. Additionally, special thanks are to M.
Richter who provided a lot of unpublished electrical resistivity data and to F. Pototzki, B. Pioch
and C. Hilgenfeld who gave much technical
assistance and support during the development of
the full waveform logging system. V. Spieß
developed the ParaDIGMA data acquisition
system for digital recording of the Parasound
echosounder data and the program system for
their processing and display. His help is greatly
appreciated, too. H. Villinger critically read an
early draft and improved the manuscript by many
helpful discussions. The research was funded by
the Deutsche Forschungsgemeinschaft (Special
Research Project SFB 261 at Bremen University,
contribution no. 251 and project no. Br 1476/2-1+2)
and by the Federal Minister of Education, Science,
Research and Technology (BMBF), grant no.
03G0093C.
Appendix
A:
Physical Properties of Sediment
Grains and Sea Water
The density (ρ g ) and bulk modulus (K g ) of the
sediment grains are the most important physical
properties which characterize the sediment type -
terrigenous, calcareous and siliceous - and
composition. Additionally, they are required as
input parameters for wave propagation modeling,