278
Dittert et al.
wards higher values with respect to both the open
ocean and the continental margin.
In contrast, most of the parameters dealing with
planktic foraminifera are only qualified to determine
the position ofthe lysocline. No matter whether it
concerns the modification of grain size distribution
of the sand fraction (Fig. 9, upper), the modification of organism assemblages (Fig. 10, upper), the
variation of planktic foraminifera assemblages (Fig.
10, lower), the ratio of dissolution susceptible to
resistant planktic foraminifera (Figs. Ila,b,f), the
ratio of radiolaria to planktic foraminifera (Fig. 11 c),
the ratio of benthic to planktic foraminifera (Fig.
11 d), or the fragmentation of planktic foraminifera
(F igs. 11 e, g), the calcite lysocline can be positioned
at about 4,300 m for the open ocean transects and
at about 4, I 00 m water depth for the continental
margin transect. The reasons why the continental
margin situation cannot be distinguished are diverse.
If there were intermediate water currents they
might blowout lighter particles, i.e. finer grain-size
fractions, fragments, and radiolaria. This would bias
the results of grain-size investigations as well as the
results of micropaleontological examinations towards less dissolution (Diester-Haass and MUlier
1979). In a similar way, most samples below 4,675
m water depth (Fig. II) present a distorted picture
of less dissolution. We ascribe this to the fact that
the number of organisms to be investigated is too
small due to the effects of dissolution, i.e. the total
decreases under the minimum that is required for
statistical relevance (CLIMAP 1984).
In order to establish BDX and CEX as global
dissolution proxies, we will apply these parameters
on further realms which include continental margin situations outside the upwelling areas and regions with modified bottom water influence.
Conclusions
Carbonate dissolution in the deep ocean was determined by using sea water carbonate ion data,
bulk sediment parameters, and calcareous microand nannoplankton parameters as dissolution proxies. Investigation areas were the open ocean regime specified by two transects into the western
Brazil Basin and the western Cape Basin. The
continental margin realm is characterized by one
transect into the eastern Cape Basin. Carbonate
ion contents were measured between 5 m and
5,075 m water depth. The sediment surface samples are derived from water depths between 1,007
m and 5,213 m.
We can conclude that all parameters are capable of distinguishing the area above from the area
below the calcite lysocline. Beyond that, some
parameters are suited to distinguish the upper continental margin of the coastal ocean within an
upwelling area controlled by enormous productivity, high export and rapid sedimentation. In detail,
these are the carbonate ion content of the water
column and the weight percentage of sediment
CaCO) -content (Fig. 8), the rain ratio (Fig. 9, lower;
Fig. I3c), the Globigerina bulloides Dissolution
Index (BDX; Fig. 13a), and the Calcidiscus
leptoporus - Emiliania huxleyi Dissolution Index
(CEX; Fig. l3b).
Regarding the three different oceanographic
regimes, only the carbonate ion content and the
percentage of sediment carbonate content put us
in the position to determine top, bottom, and thickness of the transition zone. If these parameters are
not available, a combination ofBDX, CEX and rain
ratio (Fig. 13) gives the best approach to the authentic conditions.
According to the investigated transects, the top
of the calcite transition zone can be set to about
4,300 m in the open ocean realm ofthe Brazil- and
the western Cape Basin. It reflects the modern
boundary between the North Atlantic Deep
Water and the corrosive Antarctic Bottom Water,
subsequently leading to sublysoclinal dissolution.
The thickness of the transition zones amounts to
about 800 m. With respect to the continental margin ofthe eastern Cape Basin, the lysocline is situated at about 4,100 m water depth; the transition
zone thickness becomes >900 m. It reflects the
high amount of organic matter buried with CaCq,
induced by coastal upwelling processes of
the Benguela Current, subsequently leading to
sublysoclinal dissolution as well as to supralysoclinal
dissolution.
Dittert et al.
wards higher values with respect to both the open
ocean and the continental margin.
In contrast, most of the parameters dealing with
planktic foraminifera are only qualified to determine
the position ofthe lysocline. No matter whether it
concerns the modification of grain size distribution
of the sand fraction (Fig. 9, upper), the modification of organism assemblages (Fig. 10, upper), the
variation of planktic foraminifera assemblages (Fig.
10, lower), the ratio of dissolution susceptible to
resistant planktic foraminifera (Figs. Ila,b,f), the
ratio of radiolaria to planktic foraminifera (Fig. 11 c),
the ratio of benthic to planktic foraminifera (Fig.
11 d), or the fragmentation of planktic foraminifera
(F igs. 11 e, g), the calcite lysocline can be positioned
at about 4,300 m for the open ocean transects and
at about 4, I 00 m water depth for the continental
margin transect. The reasons why the continental
margin situation cannot be distinguished are diverse.
If there were intermediate water currents they
might blowout lighter particles, i.e. finer grain-size
fractions, fragments, and radiolaria. This would bias
the results of grain-size investigations as well as the
results of micropaleontological examinations towards less dissolution (Diester-Haass and MUlier
1979). In a similar way, most samples below 4,675
m water depth (Fig. II) present a distorted picture
of less dissolution. We ascribe this to the fact that
the number of organisms to be investigated is too
small due to the effects of dissolution, i.e. the total
decreases under the minimum that is required for
statistical relevance (CLIMAP 1984).
In order to establish BDX and CEX as global
dissolution proxies, we will apply these parameters
on further realms which include continental margin situations outside the upwelling areas and regions with modified bottom water influence.
Conclusions
Carbonate dissolution in the deep ocean was determined by using sea water carbonate ion data,
bulk sediment parameters, and calcareous microand nannoplankton parameters as dissolution proxies. Investigation areas were the open ocean regime specified by two transects into the western
Brazil Basin and the western Cape Basin. The
continental margin realm is characterized by one
transect into the eastern Cape Basin. Carbonate
ion contents were measured between 5 m and
5,075 m water depth. The sediment surface samples are derived from water depths between 1,007
m and 5,213 m.
We can conclude that all parameters are capable of distinguishing the area above from the area
below the calcite lysocline. Beyond that, some
parameters are suited to distinguish the upper continental margin of the coastal ocean within an
upwelling area controlled by enormous productivity, high export and rapid sedimentation. In detail,
these are the carbonate ion content of the water
column and the weight percentage of sediment
CaCO) -content (Fig. 8), the rain ratio (Fig. 9, lower;
Fig. I3c), the Globigerina bulloides Dissolution
Index (BDX; Fig. 13a), and the Calcidiscus
leptoporus - Emiliania huxleyi Dissolution Index
(CEX; Fig. l3b).
Regarding the three different oceanographic
regimes, only the carbonate ion content and the
percentage of sediment carbonate content put us
in the position to determine top, bottom, and thickness of the transition zone. If these parameters are
not available, a combination ofBDX, CEX and rain
ratio (Fig. 13) gives the best approach to the authentic conditions.
According to the investigated transects, the top
of the calcite transition zone can be set to about
4,300 m in the open ocean realm ofthe Brazil- and
the western Cape Basin. It reflects the modern
boundary between the North Atlantic Deep
Water and the corrosive Antarctic Bottom Water,
subsequently leading to sublysoclinal dissolution.
The thickness of the transition zones amounts to
about 800 m. With respect to the continental margin ofthe eastern Cape Basin, the lysocline is situated at about 4,100 m water depth; the transition
zone thickness becomes >900 m. It reflects the
high amount of organic matter buried with CaCq,
induced by coastal upwelling processes of
the Benguela Current, subsequently leading to
sublysoclinal dissolution as well as to supralysoclinal
dissolution.