potentially by noncarbonate dilution in certain regions of the ocean.
While the term lysocline was originally used to
define a preservational boundary, it has also been
used in a fundamentally different sense to denote the
depth at which dissolution rates of carbonate on the
seafloor greatly accelerate. Whether these levels may
or may not coincide, and the nature of their relationship to the saturation horizon or ‘chemical
lysocline’, has been the subject of much discussion
and debate. One of the reasons for uncertainty in this
regard is the fact that both the carbonate content (%)
of a sediment sample and the preservation of the
calcareous microfossil assemblages there in can be
surprisingly poor indicators of the extent to which
dissolution has occurred. For example, the loss of
carbonate (L) from sediment, expressed as a weight
percentage of the total sediment, is given by:
L ¼ 100 1 À R o =R
ð
Þ
where R o and R are the initial and final values of the
noncarbonate (or residual) material. Thus, for a
sample initially containing 95% carbonate and a R o
value of 5%, 50% of the carbonate in the sample
must be dissolved in order to double the noncarbonate fraction and reduce the carbonate content
to 90%. Since the carbonate fraction of the pelagic
rain in the open ocean often approaches 95%, this
inherent insensitivity means that significant loss of
carbonate can occur before detectable changes in the
carbonate content are observed. As a consequence,
the carbonate lysocline, traditionally defined as the
level where the carbonate content of sediments begins to sharply decrease with water depth, may lie
deeper than the depth at which significant loss of
carbonate to dissolution actually begins to occur.
Dissolution leads to an increase in surface area
during the etching of carbonate skeletal material.
Etching produces roughness and widens pores,
leading to weakening and ultimately to breakage.
Because of their larger size, planktonic foraminifera
have usually been the subject of dissolution studies
that focus on the preservation state of the microfossils themselves. Planktonic foraminifera have a
wide range of morphologic characteristics that
2000
3000
4000
5000
_ 20
_ 10
0
10
20 0
20
40
60
80
100
Water depth (m)
Percent
Δ CO (Calcite)
_
2
3
GEOSECS Stn. 441
Indian Ocean
% Fragments
% CaCO 3
Saturation horizon
Lysocline
CCD
Figure 4 Comparison of carbonate saturation profile for the eastern equatorial Indian Ocean with measurements of foraminiferal
fragmentation and carbonate content (weight-%) from depth-distributed modern sediment samples in this region. The saturation
horizon with respect to calcite (DCO 3
2À ¼ 0) occurs locally in the water column at a depth of 3800 m. This level corresponds with both
the foraminiferal lysocline and carbonate lysocline as recognized in the sediments. The carbonate compensation depth (CCD) in this
region is found at a depth of approximately 5000 m. Increased foraminiferal fragmentation and decreases in sedimentary carbonate
content are the result of dissolution and carbonate loss below the lysocline. Carbonate saturation data are from GEOSECS Station
441 (512
0 S, 91147
0 E; Takahashi et al. 1980); modern sediment data are from Peterson and Prell (1985).
340 CALCIUM CARBONATES
While the term lysocline was originally used to
define a preservational boundary, it has also been
used in a fundamentally different sense to denote the
depth at which dissolution rates of carbonate on the
seafloor greatly accelerate. Whether these levels may
or may not coincide, and the nature of their relationship to the saturation horizon or ‘chemical
lysocline’, has been the subject of much discussion
and debate. One of the reasons for uncertainty in this
regard is the fact that both the carbonate content (%)
of a sediment sample and the preservation of the
calcareous microfossil assemblages there in can be
surprisingly poor indicators of the extent to which
dissolution has occurred. For example, the loss of
carbonate (L) from sediment, expressed as a weight
percentage of the total sediment, is given by:
L ¼ 100 1 À R o =R
ð
Þ
where R o and R are the initial and final values of the
noncarbonate (or residual) material. Thus, for a
sample initially containing 95% carbonate and a R o
value of 5%, 50% of the carbonate in the sample
must be dissolved in order to double the noncarbonate fraction and reduce the carbonate content
to 90%. Since the carbonate fraction of the pelagic
rain in the open ocean often approaches 95%, this
inherent insensitivity means that significant loss of
carbonate can occur before detectable changes in the
carbonate content are observed. As a consequence,
the carbonate lysocline, traditionally defined as the
level where the carbonate content of sediments begins to sharply decrease with water depth, may lie
deeper than the depth at which significant loss of
carbonate to dissolution actually begins to occur.
Dissolution leads to an increase in surface area
during the etching of carbonate skeletal material.
Etching produces roughness and widens pores,
leading to weakening and ultimately to breakage.
Because of their larger size, planktonic foraminifera
have usually been the subject of dissolution studies
that focus on the preservation state of the microfossils themselves. Planktonic foraminifera have a
wide range of morphologic characteristics that
2000
3000
4000
5000
_ 20
_ 10
0
10
20 0
20
40
60
80
100
Water depth (m)
Percent
Δ CO (Calcite)
_
2
3
GEOSECS Stn. 441
Indian Ocean
% Fragments
% CaCO 3
Saturation horizon
Lysocline
CCD
Figure 4 Comparison of carbonate saturation profile for the eastern equatorial Indian Ocean with measurements of foraminiferal
fragmentation and carbonate content (weight-%) from depth-distributed modern sediment samples in this region. The saturation
horizon with respect to calcite (DCO 3
2À ¼ 0) occurs locally in the water column at a depth of 3800 m. This level corresponds with both
the foraminiferal lysocline and carbonate lysocline as recognized in the sediments. The carbonate compensation depth (CCD) in this
region is found at a depth of approximately 5000 m. Increased foraminiferal fragmentation and decreases in sedimentary carbonate
content are the result of dissolution and carbonate loss below the lysocline. Carbonate saturation data are from GEOSECS Station
441 (512
0 S, 91147
0 E; Takahashi et al. 1980); modern sediment data are from Peterson and Prell (1985).
340 CALCIUM CARBONATES