Sediment dissolution and the carbonate
compensation depth
Biogenic silica must be buried relatively quickly to prevent dissolution, since all ocean waters are undersaturated
with respect to biogenic opal (Ragueneau et al., 2000).
Slowly accumulating sediments do not preserve
biogenic opal.
The solubility of calcite increases with increasing pressure, decreasing temperature, and decreasing dissolved
carbonate ion, [CO 3 ]
¼ (Boudreau et al., 2010). Deepocean water temperature is low and relatively constant,
but pressure causes the calcite solubility product constant
to double between 2,000 and 5,000 m water depth
(Boudreau et al., 2010). Deeper parts of the ocean lack
CaCO 3 because of elevated CaCO 3 dissolution. The calcite compensation depth (CCD) is defined as the depth
where the rate of dissolution matches the CaCO 3 particulate rain, and all CaCO 3 is dissolved before burial. Since
the CCD depends upon both production and dissolution,
the CCD has changed as environmental conditions have
changed (Figure 9; Pälike and Expedition 320/321 Shipboard Scientists, 2012).
Shallow ocean waters are supersaturated with respect to
CaCO 3 , but there is a depth, referred to as the lysocline,
where the increasing effects of CaCO 3 dissolution become
apparent, e.g., the depth below which dissolutionsusceptible foraminifera rapidly disappear. This depth
occasionally matches the depth where undersaturation
occurs, but it may also show where the cumulative effects
of dissolution are easily measured. The lysocline and CCD
are at different depths in different ocean basins because
they experience different levels of CaCO 3 production
and because the different basins have different deep water
[CO 3 ]
¼ levels. In the Atlantic, the lysocline ranges from
4 to 5 km deep, and the CCD is 5–6 km deep (Figure 7;
Biscaye et al., 1976). In contrast, the Pacific Ocean
lysocline ranges from <3 to 4 km deep, and the CCD is
typically about 4,500 m deep (Berger et al., 1976). The
Atlantic has much better CaCO 3 preservation than the
Pacific or Indian Ocean because deep waters in the basin
are flushed by high [CO 3 ]
¼ water North Atlantic Deep
Water sourced from the North Atlantic surface ocean
rather than from lower [CO 3 ]
¼ water sourced from the
Antarctic.
Early diagenesis
The chemical environment within sediments is significantly different from that of the abyssal ocean, so that
some minerals will dissolve and new minerals can form.
The changes are referred to as early diagenesis. Oxidation
of organic matter is a major driver of early diagenesis,
leading to reduction and remobilization of Fe and Mn
and the formation of oxide-rich surface sediment layers
(Froelich et al., 1979; Finney et al., 1988). Other early diagenetic reactions include the formation of calcite overgrowths on foraminifera and formation of diagenetic
dolomite (CaMgCO 3 ). Diagenetic calcite overgrowths
can severely affect the chemical composition of foraminiferal tests and thus cause major errors in proxy estimates of
past temperatures and other properties (e.g., Pearson et al.,
2001).
Near-bottom sediment movement and sediment
focusing
The cycle of elements to the seafloor is further complicated by near-seafloor particle dynamics. The particle rain
from surface waters can fall to the seafloor but then be
resuspended back into the lower water column before final
incorporation into sediments (McCave, 2010). In the abyssal realm, Walsh et al. (1988) found that there was a
“rebound” sediment flux caught in sediment traps up to
500 m above the bottom. Chemical analysis of the nearbottom sediment traps showed that the additional flux is
intermediate in composition between particle rain and
the surface sediment, suggesting that the recycled particulate flux includes degrading particles resuspended before
they were completely incorporated into the sediment.
During the resuspension process, currents and tides can
cause sediments to be transported laterally, and sediments
will preferentially fill in low topography. Over time, in
depositional environments, the infill will tend to flatten
seafloor topography because sedimentation rates are
higher in abyssal valleys (Tominaga et al., 2011). The
amount of drape versus sediment infill appears to depend
upon topography and rate of sedimentation. The high
topography of the slow-spreading mid-Atlantic Ridge
results in bare ridges with little sediment drape and much
basin infill (Ruddiman, 1972). In contrast, the subdued
abyssal hill topography in the tropical Pacific results in
high amounts of sediment drape and only small differences between sediment cover on abyssal hills versus valleys (Tominaga et al., 2011). As can be seen in Figure 2,
seafloor topography still mimics the topography of basaltic basement, even though the sediment column is many
times thicker than the original basement topography.
Summary and conclusions
Deep-sea sediments deposit slowly and are distinctive
chemically from typical terrestrial or continental margin
sediments. Because such a small fraction of clays is deposited in the deep ocean compared to continental margins,
deep-sea sediments can have large fractions of biogenic
debris or metal oxides precipitated from hydrothermal
plumes or directly from seawater. Deep-sea sediments also
steadily accumulate, making them an excellent archive of
Earth’s environmental change.
Acknowledgments
I thank G. Ross Heath for his review of the manuscript.
This study was supported in part by NSF grant
OCE-0962184.
DEEP-SEA SEDIMENTS
167
compensation depth
Biogenic silica must be buried relatively quickly to prevent dissolution, since all ocean waters are undersaturated
with respect to biogenic opal (Ragueneau et al., 2000).
Slowly accumulating sediments do not preserve
biogenic opal.
The solubility of calcite increases with increasing pressure, decreasing temperature, and decreasing dissolved
carbonate ion, [CO 3 ]
¼ (Boudreau et al., 2010). Deepocean water temperature is low and relatively constant,
but pressure causes the calcite solubility product constant
to double between 2,000 and 5,000 m water depth
(Boudreau et al., 2010). Deeper parts of the ocean lack
CaCO 3 because of elevated CaCO 3 dissolution. The calcite compensation depth (CCD) is defined as the depth
where the rate of dissolution matches the CaCO 3 particulate rain, and all CaCO 3 is dissolved before burial. Since
the CCD depends upon both production and dissolution,
the CCD has changed as environmental conditions have
changed (Figure 9; Pälike and Expedition 320/321 Shipboard Scientists, 2012).
Shallow ocean waters are supersaturated with respect to
CaCO 3 , but there is a depth, referred to as the lysocline,
where the increasing effects of CaCO 3 dissolution become
apparent, e.g., the depth below which dissolutionsusceptible foraminifera rapidly disappear. This depth
occasionally matches the depth where undersaturation
occurs, but it may also show where the cumulative effects
of dissolution are easily measured. The lysocline and CCD
are at different depths in different ocean basins because
they experience different levels of CaCO 3 production
and because the different basins have different deep water
[CO 3 ]
¼ levels. In the Atlantic, the lysocline ranges from
4 to 5 km deep, and the CCD is 5–6 km deep (Figure 7;
Biscaye et al., 1976). In contrast, the Pacific Ocean
lysocline ranges from <3 to 4 km deep, and the CCD is
typically about 4,500 m deep (Berger et al., 1976). The
Atlantic has much better CaCO 3 preservation than the
Pacific or Indian Ocean because deep waters in the basin
are flushed by high [CO 3 ]
¼ water North Atlantic Deep
Water sourced from the North Atlantic surface ocean
rather than from lower [CO 3 ]
¼ water sourced from the
Antarctic.
Early diagenesis
The chemical environment within sediments is significantly different from that of the abyssal ocean, so that
some minerals will dissolve and new minerals can form.
The changes are referred to as early diagenesis. Oxidation
of organic matter is a major driver of early diagenesis,
leading to reduction and remobilization of Fe and Mn
and the formation of oxide-rich surface sediment layers
(Froelich et al., 1979; Finney et al., 1988). Other early diagenetic reactions include the formation of calcite overgrowths on foraminifera and formation of diagenetic
dolomite (CaMgCO 3 ). Diagenetic calcite overgrowths
can severely affect the chemical composition of foraminiferal tests and thus cause major errors in proxy estimates of
past temperatures and other properties (e.g., Pearson et al.,
2001).
Near-bottom sediment movement and sediment
focusing
The cycle of elements to the seafloor is further complicated by near-seafloor particle dynamics. The particle rain
from surface waters can fall to the seafloor but then be
resuspended back into the lower water column before final
incorporation into sediments (McCave, 2010). In the abyssal realm, Walsh et al. (1988) found that there was a
“rebound” sediment flux caught in sediment traps up to
500 m above the bottom. Chemical analysis of the nearbottom sediment traps showed that the additional flux is
intermediate in composition between particle rain and
the surface sediment, suggesting that the recycled particulate flux includes degrading particles resuspended before
they were completely incorporated into the sediment.
During the resuspension process, currents and tides can
cause sediments to be transported laterally, and sediments
will preferentially fill in low topography. Over time, in
depositional environments, the infill will tend to flatten
seafloor topography because sedimentation rates are
higher in abyssal valleys (Tominaga et al., 2011). The
amount of drape versus sediment infill appears to depend
upon topography and rate of sedimentation. The high
topography of the slow-spreading mid-Atlantic Ridge
results in bare ridges with little sediment drape and much
basin infill (Ruddiman, 1972). In contrast, the subdued
abyssal hill topography in the tropical Pacific results in
high amounts of sediment drape and only small differences between sediment cover on abyssal hills versus valleys (Tominaga et al., 2011). As can be seen in Figure 2,
seafloor topography still mimics the topography of basaltic basement, even though the sediment column is many
times thicker than the original basement topography.
Summary and conclusions
Deep-sea sediments deposit slowly and are distinctive
chemically from typical terrestrial or continental margin
sediments. Because such a small fraction of clays is deposited in the deep ocean compared to continental margins,
deep-sea sediments can have large fractions of biogenic
debris or metal oxides precipitated from hydrothermal
plumes or directly from seawater. Deep-sea sediments also
steadily accumulate, making them an excellent archive of
Earth’s environmental change.
Acknowledgments
I thank G. Ross Heath for his review of the manuscript.
This study was supported in part by NSF grant
OCE-0962184.
DEEP-SEA SEDIMENTS
167