270 Paleoceanography - the Deep-Sea Record
cycle. The sharp drop in the Atlantic after 10 million years ago has a strong regional
component, and reflects (among other things) the increase in production of North
Atlantic Deep Water discussed in connection with the "silica switch" (Sect. 9.5.5).
The overall similarity in the CCD fluctuations of Pacific and Atlantic indicates
that the chemical climate of the ocean is changing on a global scale throughout.
Comparison with sea level reconstructions and (5 18 0 stratigraphy suggests that
periods of high sea level are characterized by a shallow CCD and by warm high
latitudes; periods of low sea level by a deep CCD and cold high latitudes (and cold
deep waters). Why should a relatively warm ocean have a shallower CCD than a cold
one? Is not cold water less favorable to the preservation of carbonate than warm
water?
9.7.3 Possible Causes of CCD Fluctuations. A simple hypothesis linking sea level
to CCD fluctuations is the concept of "basin-shelf -fractionation". The shelf, being
shallow, is the favored place for carbonate to accumulate, because of the correlation
between solubility and pressure. Flooded shelves, then, are carbonate traps, and
remove CaC03 from the ocean so that the deep-sea floor starves. Conversely, bared
shelve supply carbonate to the deep sea. Thus, it is not the temperature which is
important, but the sea level. Temperature happens to be correlated with sea level for
various reasons (decrease of albedo and increase in pC02 during flooding of shelves;
see Sect. 9.2.2).
There is good reason to believe that the mass balance hypothesis of CCD fluctuations does not suffice. We must, in addition, employ a more subtle argument, based
on internal cycling of carbonate within the deep ocean basins. Remember that the
biological productivity of the ocean is responsible for precipitating carbonate. Removal of solids from a solution lowers saturation. High productivity, then, results in an
undersaturated ocean with a shallow CCD. In this model, we can read the CCD
fluctuations as productivity fluctuations, with fertility high in the Eocene and
Miocene, low in the Oligocene. There is other independent evidence that productivity
was low in the Oligocene, supporting this concept.
CCD fluctuations also are closely associated with the history of erosion on the
deep sea floor. The dissolution of carbonate itself is a form of erosion, of course.
Compilations of sedimentation rates and hiatuses (gaps in the record) show that both
these stratigraphic parameters fluctuated consideraby through time. However, the
relationships between these fluctuations and those of the CCD are still obscure. One
problem is that hiatuses may be "produced" during drilling, whenever recovery is
difficult. This happens, for example, in the chert-rich sediments of Eocene Age.
9.8 Cretaceous Oceans: a Question of Oxygenation
9.S.1 A "Stagnant" Ocean? When the Deep Sea Drilling Project recovered mid-Cretaceous sediments in the Atlantic Ocean, it was found that some of these were very
rich in organic matter, indicating either high organic supply to the sea floor, or
reduced losses of organics due to low oxygen content in deep water at the time, or
cycle. The sharp drop in the Atlantic after 10 million years ago has a strong regional
component, and reflects (among other things) the increase in production of North
Atlantic Deep Water discussed in connection with the "silica switch" (Sect. 9.5.5).
The overall similarity in the CCD fluctuations of Pacific and Atlantic indicates
that the chemical climate of the ocean is changing on a global scale throughout.
Comparison with sea level reconstructions and (5 18 0 stratigraphy suggests that
periods of high sea level are characterized by a shallow CCD and by warm high
latitudes; periods of low sea level by a deep CCD and cold high latitudes (and cold
deep waters). Why should a relatively warm ocean have a shallower CCD than a cold
one? Is not cold water less favorable to the preservation of carbonate than warm
water?
9.7.3 Possible Causes of CCD Fluctuations. A simple hypothesis linking sea level
to CCD fluctuations is the concept of "basin-shelf -fractionation". The shelf, being
shallow, is the favored place for carbonate to accumulate, because of the correlation
between solubility and pressure. Flooded shelves, then, are carbonate traps, and
remove CaC03 from the ocean so that the deep-sea floor starves. Conversely, bared
shelve supply carbonate to the deep sea. Thus, it is not the temperature which is
important, but the sea level. Temperature happens to be correlated with sea level for
various reasons (decrease of albedo and increase in pC02 during flooding of shelves;
see Sect. 9.2.2).
There is good reason to believe that the mass balance hypothesis of CCD fluctuations does not suffice. We must, in addition, employ a more subtle argument, based
on internal cycling of carbonate within the deep ocean basins. Remember that the
biological productivity of the ocean is responsible for precipitating carbonate. Removal of solids from a solution lowers saturation. High productivity, then, results in an
undersaturated ocean with a shallow CCD. In this model, we can read the CCD
fluctuations as productivity fluctuations, with fertility high in the Eocene and
Miocene, low in the Oligocene. There is other independent evidence that productivity
was low in the Oligocene, supporting this concept.
CCD fluctuations also are closely associated with the history of erosion on the
deep sea floor. The dissolution of carbonate itself is a form of erosion, of course.
Compilations of sedimentation rates and hiatuses (gaps in the record) show that both
these stratigraphic parameters fluctuated consideraby through time. However, the
relationships between these fluctuations and those of the CCD are still obscure. One
problem is that hiatuses may be "produced" during drilling, whenever recovery is
difficult. This happens, for example, in the chert-rich sediments of Eocene Age.
9.8 Cretaceous Oceans: a Question of Oxygenation
9.S.1 A "Stagnant" Ocean? When the Deep Sea Drilling Project recovered mid-Cretaceous sediments in the Atlantic Ocean, it was found that some of these were very
rich in organic matter, indicating either high organic supply to the sea floor, or
reduced losses of organics due to low oxygen content in deep water at the time, or