286
Diekmann et al.
is achieved by deep and bottom water advection,
long-distance eolian dust fluxes, ice-rafting and turbidity currents (e. g. Stow 1994). Particularly in the
South Atlantic and Southern Ocean, deep and bottom water circulation most effectively control the
distribution of fine-grained terrigenous matter in the
pelagic realm (Faugeres et al. 1993; Stow 1994,
Petschick et al. 1996). Highest particle concentrations occur in near-bottom nepheloid suspension
layers (Biscaye and Eittreim 1977; McCave 1986).
As demonstrated in this volume, most paleoceanographers rely preferably on proxies inferred
from inorganic and organic biogenic sediment compounds to reconstruct the paleo-ocean. However,
the terrigenous fractions in deep-sea sediments may
serve as appropriate geological archives of past
environmental changes, especially in regions with
poor preservation of biogenic matter. Thus,
gravel concentrations can be used to estimate rates
of paleo ice-rafting (Heinrich 1988; Grobe and
Mackensen 1992; Warnkeetal.1992). Terrigenous
silt grain size properties are an important tool in
assessing paleo-current strengths (Ledbetter 1986;
Pudsey 1992; McCave et al. 1995; Diekmann and
Kuhn 1997) as well as paleo-wind activity and eolian
particle supply (Koopmann 1981; Tiedemann et al.
1989; Rea and Hovan 1995). Actually, most workers dealing with the terrigenous fraction of deepsea sediments focus their attention on clay mineralogy.
In the South Atlantic, long-term temporal variations of terrigenous clay mineral assemblages in
Jurassic to Eocene sediments were linked to distinct structural evolutions in the hinterland (Robert
1987), whereas compositional clay mineral variations in younger Cenozoic sediments may be attributed to climate changes in the source areas causing modifications of the prevailing weathering regimes (Robert 1980; Ehrmann and Mackensen
1992). During the Early Pliocene a determinant
control of ocean currents on clay mineral distribution probably commenced due to an intensification
of deep water circulation in the South Atlantic
(Robert 1980, Robert and Maillot 1983). Clay
mineralogical signatures reflecting deep water
circulation patterns are also observed in Late
Quaternary sediments of the northwestern North
Atlantic (Fagel et al. 1997).
In the South Atlantic the significance of clay
minerals, especially of kaolinite and chlorite, as tracers of deep and bottom water flow was first recognized in the Argentine Basin and the Vema Channel (Biscaye 1965; Chamley 1975) and was confirmed by subsequent workers (Melguen et at. 1978;
Jones 1984; Masse et al. 1996). Charnley (1975)
concluded that the strong latitudinal and reciprocal
distribution of kaolinite and chlorite, earlier described by Biscaye (1965), should characterize
advection of kaolinite-bearing North Atlantic Deep
Water to the south and chlorite-bearing Antarctic
Bottom Water to the north, respectively. Jones
(1984), however, questioned the role of North
Atlantic Deep Water as the main kaolinite carrier.
Detailed clay mineral investigations of surface
sediments from the entire South Atlantic and adjoining Southern Ocean carried out by Petschick et
at. (1996), nevertheless, support Charnley's conclusions concerning the intimate relationship between
the distribution of the kaolinite/chlorite-ratio and patterns of deep water advection. Diekmann et al.
(1996) adopted and applied the kaolinite/chlorite
proxy of Pets chick et al. (1996) to reconstruct glacial-interglacial contrasts of deep water mass extensions in the southeastern South Atlantic.
In this paper we will highlight our current findings with an additional discussion of accumulation
rates of terrigenous matter and clay minerals that
were not published so far. Furthermore, we present
new results from clay-mineralogical sediment core
investigations from the western South Atlantic, the
Mid-Atlantic Ridge and the eastern SW Indian
Ridge. In the first part we will show the relationships between modern deep water advection
and the kaolinite/chlorite distribution in surface
sediments. Then we will interpret downcore variations of the kaolinite/chlorite-ratio as tracers of
varying deep water mass configurations in response
to climate changes and address the role of topographic highs on temporal circulation changes. The
final part includes critical remarks and future perspectives regarding the limitations and necessary
refinements ofthe kaolinite/chlorite proxy.
Diekmann et al.
is achieved by deep and bottom water advection,
long-distance eolian dust fluxes, ice-rafting and turbidity currents (e. g. Stow 1994). Particularly in the
South Atlantic and Southern Ocean, deep and bottom water circulation most effectively control the
distribution of fine-grained terrigenous matter in the
pelagic realm (Faugeres et al. 1993; Stow 1994,
Petschick et al. 1996). Highest particle concentrations occur in near-bottom nepheloid suspension
layers (Biscaye and Eittreim 1977; McCave 1986).
As demonstrated in this volume, most paleoceanographers rely preferably on proxies inferred
from inorganic and organic biogenic sediment compounds to reconstruct the paleo-ocean. However,
the terrigenous fractions in deep-sea sediments may
serve as appropriate geological archives of past
environmental changes, especially in regions with
poor preservation of biogenic matter. Thus,
gravel concentrations can be used to estimate rates
of paleo ice-rafting (Heinrich 1988; Grobe and
Mackensen 1992; Warnkeetal.1992). Terrigenous
silt grain size properties are an important tool in
assessing paleo-current strengths (Ledbetter 1986;
Pudsey 1992; McCave et al. 1995; Diekmann and
Kuhn 1997) as well as paleo-wind activity and eolian
particle supply (Koopmann 1981; Tiedemann et al.
1989; Rea and Hovan 1995). Actually, most workers dealing with the terrigenous fraction of deepsea sediments focus their attention on clay mineralogy.
In the South Atlantic, long-term temporal variations of terrigenous clay mineral assemblages in
Jurassic to Eocene sediments were linked to distinct structural evolutions in the hinterland (Robert
1987), whereas compositional clay mineral variations in younger Cenozoic sediments may be attributed to climate changes in the source areas causing modifications of the prevailing weathering regimes (Robert 1980; Ehrmann and Mackensen
1992). During the Early Pliocene a determinant
control of ocean currents on clay mineral distribution probably commenced due to an intensification
of deep water circulation in the South Atlantic
(Robert 1980, Robert and Maillot 1983). Clay
mineralogical signatures reflecting deep water
circulation patterns are also observed in Late
Quaternary sediments of the northwestern North
Atlantic (Fagel et al. 1997).
In the South Atlantic the significance of clay
minerals, especially of kaolinite and chlorite, as tracers of deep and bottom water flow was first recognized in the Argentine Basin and the Vema Channel (Biscaye 1965; Chamley 1975) and was confirmed by subsequent workers (Melguen et at. 1978;
Jones 1984; Masse et al. 1996). Charnley (1975)
concluded that the strong latitudinal and reciprocal
distribution of kaolinite and chlorite, earlier described by Biscaye (1965), should characterize
advection of kaolinite-bearing North Atlantic Deep
Water to the south and chlorite-bearing Antarctic
Bottom Water to the north, respectively. Jones
(1984), however, questioned the role of North
Atlantic Deep Water as the main kaolinite carrier.
Detailed clay mineral investigations of surface
sediments from the entire South Atlantic and adjoining Southern Ocean carried out by Petschick et
at. (1996), nevertheless, support Charnley's conclusions concerning the intimate relationship between
the distribution of the kaolinite/chlorite-ratio and patterns of deep water advection. Diekmann et al.
(1996) adopted and applied the kaolinite/chlorite
proxy of Pets chick et al. (1996) to reconstruct glacial-interglacial contrasts of deep water mass extensions in the southeastern South Atlantic.
In this paper we will highlight our current findings with an additional discussion of accumulation
rates of terrigenous matter and clay minerals that
were not published so far. Furthermore, we present
new results from clay-mineralogical sediment core
investigations from the western South Atlantic, the
Mid-Atlantic Ridge and the eastern SW Indian
Ridge. In the first part we will show the relationships between modern deep water advection
and the kaolinite/chlorite distribution in surface
sediments. Then we will interpret downcore variations of the kaolinite/chlorite-ratio as tracers of
varying deep water mass configurations in response
to climate changes and address the role of topographic highs on temporal circulation changes. The
final part includes critical remarks and future perspectives regarding the limitations and necessary
refinements ofthe kaolinite/chlorite proxy.