5.3 Hemipelagic and Pelagic Sediments
calcium hydrogen carbonate and by smaller contributors, for example hydrothermal sources along midoceanic ridges. The deficiency in biogenic calciu~
carbonate consumption is balanced by the oceamc
circulating system which recyc1es p~rt ?f the e~tracted material. Calcium carbonate lS d1ssolved m
the deep sea by cold bottom currents coming from
high-latitude regions (in the present oceans by Antarctic and Arctic bottom waters).
Prior to their downwelling, these water masses flow
poleward as surface currents. On their way to high latitudes, they become depleted in calcium ~~rbonate .but rich
in free CO2 due to near-surface productJV1ty, coohng, and
uptake of CO2 from the atmosphere. Later, after they have
sunk into intermediate depths or to the sea floor, they can
take up CO2 from the decomposition of organic matter on
their way toward the equator (cf Fig. S.le). Thus, they
become undersaturated with respect to calcium carbonate
and therefore dissolve carbonate shells settling through
deep waters or already resting on the sea floor.
Aragonite shells and thin skeletons of calcite are
more readily dissolved than robust calcitic shells.
Carbonate wrapped into fecal pellets has a better
chance of being preserved than do free floating
shells.
From mapping the sea floor and testing its surface
sediments, one can find two boundaries (Figs. 5.3b
and 5.4a):
(1) A contour zone on the sea floor, where dissolution of less stable shells (consisting of aragonite)
starts to reduce significantly the normal carbonate
content of the sediment found at higher elevations.
This boundary is referred to as the lysocline.
(2) The second, deeper boundary, is known as the
calcite compensation depth, or CCD. Here, the
planktonic calcium carbonate supply rate is balance~
by the dissolution rate. Below this depth, the sed1ment should be free of calcium carbonate.
In the present-day oceans the CCD is observed generallyat depths between 3.5 and 5.5 km. It is usually
depressed in low-latitude regions and below zones. of
equatorial divergence (upwelling) due to the h1gh
production of calcium carbonate, and it is rai~ed in
high-latitude areas with low carbonate productlOn. A
higher CCD is also found along western continental
margins, where coastal upwelling is widespread. In
this case dissolution of carbonate near or at the sea
floor ma~ be enhanced by decomposition of or~anic
matter, which releases CO 2 provided by the h1ghly
fertile photic zone (see below).
Production and Dissolution of Carbonate:
aDynamie Equilibrium
193
For a limited geologic time period which is not affected by major c1imatic or other global changes, the
position of the lysoc1ine and CCD can be expected to
remain constant. These boundaries then signify that a
kind of steady-state condition is being maintained
between (biogenic) precipitation of calcium carbonate retrieval of calcium carbonate from the ocean by
dis~olution, and supply of calcium carbona~e fr?m
outside sources. Figure 5.4e shows such a sltuatlOn
for the sediments of a mid-oceanic spreading center.
With continuing accretion of new oceanic crnst, the
ridge crest moves away from it~ former pos~tion
(from location I to 2) and the coohng crnst subs1des.
As a result, the depositional area sinks below the
CCD and the calcareous sediments sitting directly on
pillo~ lava or basaltic flows are overlain by light
gray or red siliceous sediments and later by red c1ay.
In the history of many ocean basins, however, the
CCD (inc1uding the lysoc1ine) did not maintain a
constant position, particularly when long time periods are considered. Biogenic carbonate production in
shallow waters and carbonate dissolution in deep
waters may be considered as a system being in a dynamic equilibrium. The steady-state model mentioned above implies a raising or lowering of the
CCD, if one or several·of the parameters controlling
the system are changed.
Decreasing oceanic circulation, for example due
to a more balanced c1imate between the poles and the
equator, can lead to a drop in the overall fertili~ of
the oceans and thus to lower carbonate productlOn.
Ihis in turn will reduce the need for carbonate dissolution in deep waters to keep the carbonate budget
balanced and consequently cause a drop of the CCD.
A similar result can be achieved by an increase in the
input of hydrogen carbonate from the ~ontinents. or
by dissolution of emerged carbonate bUlldups durmg
low sea-Ievel stands.
In contrast, intensified oceanic circulation and
mineralization of organic matter can enhance fertility
and thereby raise the CCD. Ihe same effect may be
generated by increased carbonate production on
shelves and in reefal structures. The problem in interpreting the situation in the geological past is that
some of these factors can counteract each other.
From information gathered by deep-sea drilling, the CCD
has moved up and down several times (by as much as 2 to
3 km) in the last 100 Ma (e.g., Hay 1987; Seibold and
Berger 1993; Berger and Wefer 1996). Sometimes these
variations were synchronous in the three major ocean basins sometimes the CCD behaved quite differently in each.
Th~ CCD was very low around the Cretaceous/Tertiary
boundary in the Pacific Ocean, while it stayed relatively
high in the Atlantic and Indian Oceans. A CCD ~arke~ly
higher than at present was found for the early TertJary, W1th
calcium hydrogen carbonate and by smaller contributors, for example hydrothermal sources along midoceanic ridges. The deficiency in biogenic calciu~
carbonate consumption is balanced by the oceamc
circulating system which recyc1es p~rt ?f the e~tracted material. Calcium carbonate lS d1ssolved m
the deep sea by cold bottom currents coming from
high-latitude regions (in the present oceans by Antarctic and Arctic bottom waters).
Prior to their downwelling, these water masses flow
poleward as surface currents. On their way to high latitudes, they become depleted in calcium ~~rbonate .but rich
in free CO2 due to near-surface productJV1ty, coohng, and
uptake of CO2 from the atmosphere. Later, after they have
sunk into intermediate depths or to the sea floor, they can
take up CO2 from the decomposition of organic matter on
their way toward the equator (cf Fig. S.le). Thus, they
become undersaturated with respect to calcium carbonate
and therefore dissolve carbonate shells settling through
deep waters or already resting on the sea floor.
Aragonite shells and thin skeletons of calcite are
more readily dissolved than robust calcitic shells.
Carbonate wrapped into fecal pellets has a better
chance of being preserved than do free floating
shells.
From mapping the sea floor and testing its surface
sediments, one can find two boundaries (Figs. 5.3b
and 5.4a):
(1) A contour zone on the sea floor, where dissolution of less stable shells (consisting of aragonite)
starts to reduce significantly the normal carbonate
content of the sediment found at higher elevations.
This boundary is referred to as the lysocline.
(2) The second, deeper boundary, is known as the
calcite compensation depth, or CCD. Here, the
planktonic calcium carbonate supply rate is balance~
by the dissolution rate. Below this depth, the sed1ment should be free of calcium carbonate.
In the present-day oceans the CCD is observed generallyat depths between 3.5 and 5.5 km. It is usually
depressed in low-latitude regions and below zones. of
equatorial divergence (upwelling) due to the h1gh
production of calcium carbonate, and it is rai~ed in
high-latitude areas with low carbonate productlOn. A
higher CCD is also found along western continental
margins, where coastal upwelling is widespread. In
this case dissolution of carbonate near or at the sea
floor ma~ be enhanced by decomposition of or~anic
matter, which releases CO 2 provided by the h1ghly
fertile photic zone (see below).
Production and Dissolution of Carbonate:
aDynamie Equilibrium
193
For a limited geologic time period which is not affected by major c1imatic or other global changes, the
position of the lysoc1ine and CCD can be expected to
remain constant. These boundaries then signify that a
kind of steady-state condition is being maintained
between (biogenic) precipitation of calcium carbonate retrieval of calcium carbonate from the ocean by
dis~olution, and supply of calcium carbona~e fr?m
outside sources. Figure 5.4e shows such a sltuatlOn
for the sediments of a mid-oceanic spreading center.
With continuing accretion of new oceanic crnst, the
ridge crest moves away from it~ former pos~tion
(from location I to 2) and the coohng crnst subs1des.
As a result, the depositional area sinks below the
CCD and the calcareous sediments sitting directly on
pillo~ lava or basaltic flows are overlain by light
gray or red siliceous sediments and later by red c1ay.
In the history of many ocean basins, however, the
CCD (inc1uding the lysoc1ine) did not maintain a
constant position, particularly when long time periods are considered. Biogenic carbonate production in
shallow waters and carbonate dissolution in deep
waters may be considered as a system being in a dynamic equilibrium. The steady-state model mentioned above implies a raising or lowering of the
CCD, if one or several·of the parameters controlling
the system are changed.
Decreasing oceanic circulation, for example due
to a more balanced c1imate between the poles and the
equator, can lead to a drop in the overall fertili~ of
the oceans and thus to lower carbonate productlOn.
Ihis in turn will reduce the need for carbonate dissolution in deep waters to keep the carbonate budget
balanced and consequently cause a drop of the CCD.
A similar result can be achieved by an increase in the
input of hydrogen carbonate from the ~ontinents. or
by dissolution of emerged carbonate bUlldups durmg
low sea-Ievel stands.
In contrast, intensified oceanic circulation and
mineralization of organic matter can enhance fertility
and thereby raise the CCD. Ihe same effect may be
generated by increased carbonate production on
shelves and in reefal structures. The problem in interpreting the situation in the geological past is that
some of these factors can counteract each other.
From information gathered by deep-sea drilling, the CCD
has moved up and down several times (by as much as 2 to
3 km) in the last 100 Ma (e.g., Hay 1987; Seibold and
Berger 1993; Berger and Wefer 1996). Sometimes these
variations were synchronous in the three major ocean basins sometimes the CCD behaved quite differently in each.
Th~ CCD was very low around the Cretaceous/Tertiary
boundary in the Pacific Ocean, while it stayed relatively
high in the Atlantic and Indian Oceans. A CCD ~arke~ly
higher than at present was found for the early TertJary, W1th