Kaolinite and Chlorite as Tracers of Modern and Late Quaternary Deep Water Circulation
307
The important role of northern-source deep
water production on the deep water mass configurations in the South Atlantic and adjoining Southern Ocean is supported by our reconstructions of
deep water mass configurations based on the
kaolinite/chlorite proxy. Cross-spectral analyses
between the parameters kaolinite/chlorite ratio,
%-kaolinite, and %-chlorite with the SPECMAP
0 18 0 stack (Imbrie et al. 1984) demonstrate that
variations in deep water advection in the ACC
region are nearly in tune with global ice volume
(Fig. 7, Table 2). Actually, these clay mineral parameters are also nearly in tune with the '%NADW
index' (Atlantic-Pacifico 13 C fractionation index of
Raymo et al. 1990) (Fig. 8).
However, the slight lead of minimal %-chlorite
and the slight lag of maximal %-kaolinite to the
maximal kaolinite/chlorite-ratio (Fig. 8) may
indicate that advection of chlorite-bearing southemsource deep water responds slightly earlier to
changes of abyssal circulation than kaolinitebearing northern-source deep water. The low
statististical significance of the clay mineralogical
phase relationships, however, limits the validity of
this assertion. In sediment cores PS1768-8 and
PS 1786-1 the maxima of kaolinite/chlorite ratio in
the Holocene interval correspond to the Holocene
climate optimum of the southern hemisphere
(±9 ka) that leads the climate optimum ofthe northern hemisphere by some 3 ka (Salinger 1981;
Gersonde et al. 1996). In both sediment cores variations of kaolinite/chlorite are mainly controlled by
fluctuations of%-chlorite, also suggesting an earlier response of southern-source deep water to
changing patterns of abyssal circulation.
The Kaolinite/Chlorite Proxy: Limitations
and Perspectives
Deep water circulation in the South Atlantic and
Southern Ocean forms an integral part of the interhemispheric heat conveyer that responds to global climate changes (e.g. Imbrie et al. 1992; Berger
and Wefer 1996). A better knowledge of paleo-deep
water circulation thus helps to unravel causes and
feedbacks between climatic and oceanographic
changes.
Usually deep water proxies are deduced
from the geochemistry and faunal assemblages of
benthic foraminifers or carbonate preservation indices. We have demonstrated the application ofthe
kaolinite/chlorite-ratio as an alternative proxy of
deep water mass distribution in the South Atlantic
and the adjoining Southem Ocean that is even suitable in regions with poor carbonate preservation.
Latitudinal and water depth-related patterns of
the kaolinite/chlorite-ratio in surface sediments
correspond to the modern pattern of deep water
mass distribution. Kaolinite originates from lowlatitudes and traces advection of northern-source
deep water (NADW) to the south. Chlorite from
the southern high-latitudes is exported via northward advecting southern-source water masses
(AABW and CPDW). Latitudinal gradients of
the kaolinite/chlorite-ratio persisted throughout the
Late Quaternary, and downcore variations of the
kaolinite/chlorite-ratio reflect meridional shifts of
deep water mass extensions during glacial-interglacial cycles.
Similar to other deep water proxies, such
as stable carbon isotopes in benthic foraminifera
(Mackensen and Bickert this volume), the kaolinite!
chlorite proxy has limitations. A premise for the
interpretation of temporal fluctuations of the
kaolinite/chlorite-ratio is that the input of the tracer
minerals kaolinite and chlorite to the ocean varies
concomitantly in the opposite source regions of both
clay minerals. The latter effect is indicated by
downcore covariations of kaolinite and chlorite AR
in the investigated sediment cores. Only in that case,
the relative supply of kaolinite and chlorite in the
pelagic realm can be attributed to changing modes
of deep water advection.
Another premise is a minor contribution of direct dust input to the deep sea through long-distance
eolian transport that may overprint particle fluxes
in the water masses. In the ACC region, the important role of deep water advection on sediment
transport and deposition is underlined by the high
spatial and temporal variability of accumulation
rates of terrigenous matter. Characteristics of
paleo- 230 Th ex fluxes also indicate current-controlled reworking and redeposition ('winnowing and focusing') of sedimentary particles in the ACC region (Frank et al. this volume).
307
The important role of northern-source deep
water production on the deep water mass configurations in the South Atlantic and adjoining Southern Ocean is supported by our reconstructions of
deep water mass configurations based on the
kaolinite/chlorite proxy. Cross-spectral analyses
between the parameters kaolinite/chlorite ratio,
%-kaolinite, and %-chlorite with the SPECMAP
0 18 0 stack (Imbrie et al. 1984) demonstrate that
variations in deep water advection in the ACC
region are nearly in tune with global ice volume
(Fig. 7, Table 2). Actually, these clay mineral parameters are also nearly in tune with the '%NADW
index' (Atlantic-Pacifico 13 C fractionation index of
Raymo et al. 1990) (Fig. 8).
However, the slight lead of minimal %-chlorite
and the slight lag of maximal %-kaolinite to the
maximal kaolinite/chlorite-ratio (Fig. 8) may
indicate that advection of chlorite-bearing southemsource deep water responds slightly earlier to
changes of abyssal circulation than kaolinitebearing northern-source deep water. The low
statististical significance of the clay mineralogical
phase relationships, however, limits the validity of
this assertion. In sediment cores PS1768-8 and
PS 1786-1 the maxima of kaolinite/chlorite ratio in
the Holocene interval correspond to the Holocene
climate optimum of the southern hemisphere
(±9 ka) that leads the climate optimum ofthe northern hemisphere by some 3 ka (Salinger 1981;
Gersonde et al. 1996). In both sediment cores variations of kaolinite/chlorite are mainly controlled by
fluctuations of%-chlorite, also suggesting an earlier response of southern-source deep water to
changing patterns of abyssal circulation.
The Kaolinite/Chlorite Proxy: Limitations
and Perspectives
Deep water circulation in the South Atlantic and
Southern Ocean forms an integral part of the interhemispheric heat conveyer that responds to global climate changes (e.g. Imbrie et al. 1992; Berger
and Wefer 1996). A better knowledge of paleo-deep
water circulation thus helps to unravel causes and
feedbacks between climatic and oceanographic
changes.
Usually deep water proxies are deduced
from the geochemistry and faunal assemblages of
benthic foraminifers or carbonate preservation indices. We have demonstrated the application ofthe
kaolinite/chlorite-ratio as an alternative proxy of
deep water mass distribution in the South Atlantic
and the adjoining Southem Ocean that is even suitable in regions with poor carbonate preservation.
Latitudinal and water depth-related patterns of
the kaolinite/chlorite-ratio in surface sediments
correspond to the modern pattern of deep water
mass distribution. Kaolinite originates from lowlatitudes and traces advection of northern-source
deep water (NADW) to the south. Chlorite from
the southern high-latitudes is exported via northward advecting southern-source water masses
(AABW and CPDW). Latitudinal gradients of
the kaolinite/chlorite-ratio persisted throughout the
Late Quaternary, and downcore variations of the
kaolinite/chlorite-ratio reflect meridional shifts of
deep water mass extensions during glacial-interglacial cycles.
Similar to other deep water proxies, such
as stable carbon isotopes in benthic foraminifera
(Mackensen and Bickert this volume), the kaolinite!
chlorite proxy has limitations. A premise for the
interpretation of temporal fluctuations of the
kaolinite/chlorite-ratio is that the input of the tracer
minerals kaolinite and chlorite to the ocean varies
concomitantly in the opposite source regions of both
clay minerals. The latter effect is indicated by
downcore covariations of kaolinite and chlorite AR
in the investigated sediment cores. Only in that case,
the relative supply of kaolinite and chlorite in the
pelagic realm can be attributed to changing modes
of deep water advection.
Another premise is a minor contribution of direct dust input to the deep sea through long-distance
eolian transport that may overprint particle fluxes
in the water masses. In the ACC region, the important role of deep water advection on sediment
transport and deposition is underlined by the high
spatial and temporal variability of accumulation
rates of terrigenous matter. Characteristics of
paleo- 230 Th ex fluxes also indicate current-controlled reworking and redeposition ('winnowing and focusing') of sedimentary particles in the ACC region (Frank et al. this volume).