260
Dittert et al.
biostratigraphic events. Especially, ifthe variance
of the carbonate content is low, the CaC0 3 -MAR
will be dominated by the SR changes, which depend on the resolution ofthe age model and which
are mostly difficult to reproduce the sediment accumulation variability with time.
Two other processes are at work which could
bias the interpretations based on the CaC0 3 -content. The first of these consists in the winnowing
of sediment by currents which carry away the fine
material and thereby enrich the coarse material
(mainly shells). This raises the CaC0 3 -content (Wu
et al. 1990). The other is deposition by turbidity and
boundary currents. As currents often originate along
the continental margins, the debris they carry is
usually very low in CaCO)' To avoid the impacts
of these processes, quiet zones on the sea-floor
must be carefully chosen as the localities for such
studies.
Regarding all these potential complications, how
should one carry out a reliable quantification of
calcite dissolution in the deep-sea? Significant
progress has been made in modelling the diagenesis
of CaCO J in sediments, on diagenetic scales of
centimeters (Emerson and Bender 1981; Archer et
al. 1989; Keir 1990; Hales et al. 1994), basin-wide
scales (Emerson and Archer 1990), and global
scales (Keir 1990; Archer and Maier-Reimer
1994). The models have reached the point where
even small variations ofthe carbonate distribution
on the sea-floor might be interpretable or serve to
differentiate model formulations and assumptions.
Archer (1996) converted available sedimentary data
into a format suitable for validating models of
CaC0 3 -dynamics in the ocean. He related the distribution of sedimentary calcite in the deep-sea to
a new gridded field of water column !lCot in an
attempt to reveal regional variations in calcite preservation and thus the shape of the calcite transition zone. As a result, the transition zone has been
found thicker (i.e., has a greater contrast in !lCO,2between the high- and low-calcite sediments) in the
western Pacific and in the Atlantic Oceans than it
is in the eastern equatorial Pacific. This pattern is
consistent with the model's response to varying
rates of dilution caused by terrigenous material. In
low latitudes, calcite can be preserved to -30 llmoll
kgC0,z-, whereas calcite is depleted from higher
latitude sediments by a rate of -10 llmol/kgCOt.
This gradient is smaller than the glaciallinterglacial
shift as required by the "rain ratio model" for generating lower atmospheric pC0 2 (Archer and
Maier-Reimer 1994). This implies that the model
requires an application of conditions in glacial times
which have no analog in today's ocean. This conclusion rules out that the modern ocean carbonate
system not necessarily validates models of
CaCOJ-dynamics in the past. This is especially true
for the preservation events at the onset of each
interglaciation and for the dissolution events at the
onset of each glaciation, recorded in dissolution
records in the deep of the Indian and Pacific
Ocean, which require the additional examination of
compensation processes in the carbonate system
on different time scales.
The Use of Calcareous Micro- and
Nannoplankton as Dissolution Proxies
Comparative Analysis of Dissolution Proxies
Derivedfrom Planktic and Benthic
Foraminifera
The preservation potential of planktic foraminifera
strongly depends on the internal wall structure,
which consists of small, anhedral crystals on the
proximal side, larger crystals toward the distal side,
and in some partially deep-living species, large crystals, forming the calcite crust (Be et al. 1975). The
shell containing the largest crystals is the most resistant one. In some cases, a very smooth distal
calcite layer ("cortex"; e.g. Pulleniatina
obliquiloculata) covers the outside of the test. It
retards dissolution for some time, protecting the
underlying crust.
Berger (1967, 1968) investigated samples from
a mooring (Peterson 1966) as well as plankton and
sediment samples. He finds that carbonate dissolution changes species diversity, test size distribution, content of damaged shells, and average particle weight of an assemblage. Moreover, he establishes the ranking of planktic foraminiferal species
with respect to their preservation potential. Investigations on the shell calcite show that the ratio of
elements such as Na, Mg, Sr, F, V, U versus Ca
Dittert et al.
biostratigraphic events. Especially, ifthe variance
of the carbonate content is low, the CaC0 3 -MAR
will be dominated by the SR changes, which depend on the resolution ofthe age model and which
are mostly difficult to reproduce the sediment accumulation variability with time.
Two other processes are at work which could
bias the interpretations based on the CaC0 3 -content. The first of these consists in the winnowing
of sediment by currents which carry away the fine
material and thereby enrich the coarse material
(mainly shells). This raises the CaC0 3 -content (Wu
et al. 1990). The other is deposition by turbidity and
boundary currents. As currents often originate along
the continental margins, the debris they carry is
usually very low in CaCO)' To avoid the impacts
of these processes, quiet zones on the sea-floor
must be carefully chosen as the localities for such
studies.
Regarding all these potential complications, how
should one carry out a reliable quantification of
calcite dissolution in the deep-sea? Significant
progress has been made in modelling the diagenesis
of CaCO J in sediments, on diagenetic scales of
centimeters (Emerson and Bender 1981; Archer et
al. 1989; Keir 1990; Hales et al. 1994), basin-wide
scales (Emerson and Archer 1990), and global
scales (Keir 1990; Archer and Maier-Reimer
1994). The models have reached the point where
even small variations ofthe carbonate distribution
on the sea-floor might be interpretable or serve to
differentiate model formulations and assumptions.
Archer (1996) converted available sedimentary data
into a format suitable for validating models of
CaC0 3 -dynamics in the ocean. He related the distribution of sedimentary calcite in the deep-sea to
a new gridded field of water column !lCot in an
attempt to reveal regional variations in calcite preservation and thus the shape of the calcite transition zone. As a result, the transition zone has been
found thicker (i.e., has a greater contrast in !lCO,2between the high- and low-calcite sediments) in the
western Pacific and in the Atlantic Oceans than it
is in the eastern equatorial Pacific. This pattern is
consistent with the model's response to varying
rates of dilution caused by terrigenous material. In
low latitudes, calcite can be preserved to -30 llmoll
kgC0,z-, whereas calcite is depleted from higher
latitude sediments by a rate of -10 llmol/kgCOt.
This gradient is smaller than the glaciallinterglacial
shift as required by the "rain ratio model" for generating lower atmospheric pC0 2 (Archer and
Maier-Reimer 1994). This implies that the model
requires an application of conditions in glacial times
which have no analog in today's ocean. This conclusion rules out that the modern ocean carbonate
system not necessarily validates models of
CaCOJ-dynamics in the past. This is especially true
for the preservation events at the onset of each
interglaciation and for the dissolution events at the
onset of each glaciation, recorded in dissolution
records in the deep of the Indian and Pacific
Ocean, which require the additional examination of
compensation processes in the carbonate system
on different time scales.
The Use of Calcareous Micro- and
Nannoplankton as Dissolution Proxies
Comparative Analysis of Dissolution Proxies
Derivedfrom Planktic and Benthic
Foraminifera
The preservation potential of planktic foraminifera
strongly depends on the internal wall structure,
which consists of small, anhedral crystals on the
proximal side, larger crystals toward the distal side,
and in some partially deep-living species, large crystals, forming the calcite crust (Be et al. 1975). The
shell containing the largest crystals is the most resistant one. In some cases, a very smooth distal
calcite layer ("cortex"; e.g. Pulleniatina
obliquiloculata) covers the outside of the test. It
retards dissolution for some time, protecting the
underlying crust.
Berger (1967, 1968) investigated samples from
a mooring (Peterson 1966) as well as plankton and
sediment samples. He finds that carbonate dissolution changes species diversity, test size distribution, content of damaged shells, and average particle weight of an assemblage. Moreover, he establishes the ranking of planktic foraminiferal species
with respect to their preservation potential. Investigations on the shell calcite show that the ratio of
elements such as Na, Mg, Sr, F, V, U versus Ca