Regions with coherent sedimentation patterns within
the oceans are extremely large compared with those on
land. Patterns of change in the oceans can thus be elucidated from relatively few sediment cores. Large regions
in the oceans have similar environmental conditions
(e.g., temperature, nutrients, and salinity), and most of
the ocean basins are being covered with a continual rain
of sediment. Deep-sea sediment regimes are also large
because they reflect the relative size of the depositional
processes, whether it is the region underneath continental
dust plumes or regions with coherent production of planktonic hard parts. On land, the primary variables
(precipitation, temperature, seasonality) vary on a much
smaller scale and thus must be sampled more densely. Terrestrial basins that systematically collect sediments are
also more rare than in the oceans.
Geophysical and geologic study of deep-sea
sediments
Marine geologists have difficulty accessing deep-sea
sediments, because the average ocean depth is ~3,800 m
and recovery of ocean bottom samples is technically
challenging and time consuming. While sampling is hard,
geophysical methods that use acoustic pulses are
much easier to perform at sea than on land. Sound propagates well through water and it is much easier to produce
and receive an acoustic pulse through the ocean than
through the land surface. Marine geologists are much
more dependent on geophysical methods than continental
geologists.
Marine geologists use acoustic methods to map the
topography of the seafloor (echo sounding or multibeam
bathymetry) and to map sediment layers beneath the seafloor (reflection seismology). Multibeam mapping uses
acoustic technology to emit a focused array of acoustic
pulses and uses the time for each pulse to return to calculate ocean floor depths in a swath perpendicular to the
ship. The travel time data (time for the echo to hit the bottom and return) can be converted into a topographic map
of the seafloor by using measured ocean sound velocity
to convert from time to depth (e.g., Caress and Chayes,
1996; Mosher, 2011). An example of the resultant map is
shown at the top of Figure 2, a survey around the IODP
drill site U1335 (Expedition 320/321 Scientists, 2010).
Seismic reflection measures the time for an echo to be
reflected back from sediment layers. The echoes occur
where sediment density and/or acoustic velocity change
and are typically caused by a change of sediment lithology. Rarely, acoustic interference causes composite echoes from closely spaced beds (Mayer, 1979). The echoes
from sediment layers produce a cross section of sediment
deposition through the sediments (Figure 2 bottom; Mayer
et al., 1986). The combination of swath mapping and seismic reflection allows marine geologists to interpret how
sediments have accumulated, allowing them to understand
the long-term development of a marine sediment sequence
(e.g., Sacchetti et al., 2013; Brothers et al., 2013).
Such information has limited use without ground truth
from sediment samples. Marine geologists use a variety
of techniques to sample near-surface sediments by lowering coring devices to the seafloor on a wire rope. Where
the ocean is greater than 3 km deep, it takes 2–6 h to lower
a corer to the bottom, collect a sample, and bring it back to
the ship. Sediment cores taken by wireline range in length
from 0.3 m to about 20 m long, sufficient to study relatively recent geologic events. Because of the effort needed
to collect these cores, the sediment cores are archived in
core repositories around the world (e.g., Lamont-Doherty
Earth Observatory core repository, http://www.ldeo.
columbia.edu/core-repository/collections). Locations of
many of these geologic samples can be plotted using the
online mapping and database tool GeoMapApp (http://
www.geomapapp.org/).
While wireline coring can collect near-surface sediments, drilling technology must be used to sample the
entire sediment column and the ocean crust (Duce et al.,
2011). The time needed to sample the entire sediment column may take days or weeks, depending on the thickness
of the sediments and the care taken to recover a full sediment column for scientific analysis. Once recovered, the
drilled sediment column can be correlated to the seismic
reflection profile to identify and date specific seismic
reflection horizons. With dated seismic horizons, it is possible to study regional variations in sedimentation by
using seismic reflection profiles (Mayer et al., 1986;
Tominaga et al., 2011).
Types of deep-sea sediments
Tens of thousands of archived deep-sea sediment samples
have made it possible to map different types of sediments,
linked to regional environmental conditions (Menard,
1964; Lisitzin, 1972; Dymond, 1981). Deep-sea sediments are often classified by the source of their major
components. For example, sediments consisting of ferromanganese oxyhydroxides precipitated from mid-ocean
ridge hydrothermal plumes are referred to as hydrothermal
sediments.
Based upon a source definition, there are four major
classes
of
deep-sea
sediment
components:
(a) terrigenous sediments, aluminosilicates from the continents, divided into hemipelagic (water-transported continental debris) and aeolian (windblown dust); (b) biogenic
sediments, primarily calcareous or siliceous hard parts of
organisms; (c) hydrothermal sediments, derived from
mid-ocean ridge hot springs; and (d) authigenic sediments, precipitated directly out of seawater. Most deepsea sediments are mixtures of the different components.
For example, hemipelagic sediments will also have a small
percentage of biogenic-sourced plankton tests.
The sediment types have typical sedimentation rates,
reflecting their relative supply to the pelagic environment
(Figure 3). Sedimentation rates are plotted against sediment C org content because the amount of C org is a primary
indicator of the rate of primary productivity in the surface
158
DEEP-SEA SEDIMENTS
the oceans are extremely large compared with those on
land. Patterns of change in the oceans can thus be elucidated from relatively few sediment cores. Large regions
in the oceans have similar environmental conditions
(e.g., temperature, nutrients, and salinity), and most of
the ocean basins are being covered with a continual rain
of sediment. Deep-sea sediment regimes are also large
because they reflect the relative size of the depositional
processes, whether it is the region underneath continental
dust plumes or regions with coherent production of planktonic hard parts. On land, the primary variables
(precipitation, temperature, seasonality) vary on a much
smaller scale and thus must be sampled more densely. Terrestrial basins that systematically collect sediments are
also more rare than in the oceans.
Geophysical and geologic study of deep-sea
sediments
Marine geologists have difficulty accessing deep-sea
sediments, because the average ocean depth is ~3,800 m
and recovery of ocean bottom samples is technically
challenging and time consuming. While sampling is hard,
geophysical methods that use acoustic pulses are
much easier to perform at sea than on land. Sound propagates well through water and it is much easier to produce
and receive an acoustic pulse through the ocean than
through the land surface. Marine geologists are much
more dependent on geophysical methods than continental
geologists.
Marine geologists use acoustic methods to map the
topography of the seafloor (echo sounding or multibeam
bathymetry) and to map sediment layers beneath the seafloor (reflection seismology). Multibeam mapping uses
acoustic technology to emit a focused array of acoustic
pulses and uses the time for each pulse to return to calculate ocean floor depths in a swath perpendicular to the
ship. The travel time data (time for the echo to hit the bottom and return) can be converted into a topographic map
of the seafloor by using measured ocean sound velocity
to convert from time to depth (e.g., Caress and Chayes,
1996; Mosher, 2011). An example of the resultant map is
shown at the top of Figure 2, a survey around the IODP
drill site U1335 (Expedition 320/321 Scientists, 2010).
Seismic reflection measures the time for an echo to be
reflected back from sediment layers. The echoes occur
where sediment density and/or acoustic velocity change
and are typically caused by a change of sediment lithology. Rarely, acoustic interference causes composite echoes from closely spaced beds (Mayer, 1979). The echoes
from sediment layers produce a cross section of sediment
deposition through the sediments (Figure 2 bottom; Mayer
et al., 1986). The combination of swath mapping and seismic reflection allows marine geologists to interpret how
sediments have accumulated, allowing them to understand
the long-term development of a marine sediment sequence
(e.g., Sacchetti et al., 2013; Brothers et al., 2013).
Such information has limited use without ground truth
from sediment samples. Marine geologists use a variety
of techniques to sample near-surface sediments by lowering coring devices to the seafloor on a wire rope. Where
the ocean is greater than 3 km deep, it takes 2–6 h to lower
a corer to the bottom, collect a sample, and bring it back to
the ship. Sediment cores taken by wireline range in length
from 0.3 m to about 20 m long, sufficient to study relatively recent geologic events. Because of the effort needed
to collect these cores, the sediment cores are archived in
core repositories around the world (e.g., Lamont-Doherty
Earth Observatory core repository, http://www.ldeo.
columbia.edu/core-repository/collections). Locations of
many of these geologic samples can be plotted using the
online mapping and database tool GeoMapApp (http://
www.geomapapp.org/).
While wireline coring can collect near-surface sediments, drilling technology must be used to sample the
entire sediment column and the ocean crust (Duce et al.,
2011). The time needed to sample the entire sediment column may take days or weeks, depending on the thickness
of the sediments and the care taken to recover a full sediment column for scientific analysis. Once recovered, the
drilled sediment column can be correlated to the seismic
reflection profile to identify and date specific seismic
reflection horizons. With dated seismic horizons, it is possible to study regional variations in sedimentation by
using seismic reflection profiles (Mayer et al., 1986;
Tominaga et al., 2011).
Types of deep-sea sediments
Tens of thousands of archived deep-sea sediment samples
have made it possible to map different types of sediments,
linked to regional environmental conditions (Menard,
1964; Lisitzin, 1972; Dymond, 1981). Deep-sea sediments are often classified by the source of their major
components. For example, sediments consisting of ferromanganese oxyhydroxides precipitated from mid-ocean
ridge hydrothermal plumes are referred to as hydrothermal
sediments.
Based upon a source definition, there are four major
classes
of
deep-sea
sediment
components:
(a) terrigenous sediments, aluminosilicates from the continents, divided into hemipelagic (water-transported continental debris) and aeolian (windblown dust); (b) biogenic
sediments, primarily calcareous or siliceous hard parts of
organisms; (c) hydrothermal sediments, derived from
mid-ocean ridge hot springs; and (d) authigenic sediments, precipitated directly out of seawater. Most deepsea sediments are mixtures of the different components.
For example, hemipelagic sediments will also have a small
percentage of biogenic-sourced plankton tests.
The sediment types have typical sedimentation rates,
reflecting their relative supply to the pelagic environment
(Figure 3). Sedimentation rates are plotted against sediment C org content because the amount of C org is a primary
indicator of the rate of primary productivity in the surface
158
DEEP-SEA SEDIMENTS