Organic Carbon and Carbonate as Paleoproductivity Proxies
337
dominated eastern coastal upwelling areas (e.g.
Miiller et al. 1983; Lyle 1988; Sarnthein et al. 1988;
Schneider et al. 1996).
We attribute the decrease ofpaleoproductivity
during the cold climatic stages - which might have
occurred globally in the oligotrophic open ocean -
to a reduced nutrient content in central and intermediate waters (Francois et al. 1990; Mulitza et al.
1998a; Mulitza et al. this volume). A redistribution
of nutrients was observed in the glacial ocean
(Boyle 1988b; Duplessy et al. 1988; Mix et al.
1991) with low-nutrient intermediate waters and
high-nutrient deep-waters, in contrast to the
present-day ocean where nutrients are enriched in
intermediate waters. In the Pacific, however, these
changes were much smaller than observed in
records obtained from the North Atlantic (Mix et
al. 1991). Since intermediate waters are the primary source of nutrients for new production in the
oligotrophic areas, a lowering of their nutrient concentration is a likely explanation for a widespread
decreasing paleo-productivity in the cold stages
(Francois et al. 1990). The pronounced increase in
paleoproductivity appearing at the same time in the
high-productivity areas was obviously sustained by
intensified wind-driven upwelling.
Additionally, inverse organic carbon deposition
in the tropical Atlantic could have been cansed by
a wind-driven lowering of the nutricline in the West,
connected with a synchronous shallowing in the
East (Riihlemann et al. 1996). At present, mixed
layer depth is at maximum during August to
October when the zonal windstress is most intense
and surface waters, imported from the eastern
Atlantic, are piled up in the West. In April, when
the windstress is at minimum, thermocline and
nutricline shoal in the West and deepen in the East.
In analogy to the modern seasonal variation of
the thermocline and nutricline depths (Longhurst
1993), the insolation-controlled intensification of the
zonal windstress during cold substages (McIntyre
et al. 1989) could have caused a shallowing ofthe
nutricline in the eastern Atlantic, whereas the
nutricline in the western Atlantic was deepened.
The increase in the slope of the nutricline might
have enhanced paleoproductivity - and hence organic carbon accumulation - in the East and at the
same time lowered it in the West.
Conclusions
The application of organic carbon and carbonate
as paleoproductivity indicators has been assessed
by comparing two sediment cores obtained from
high and low productivity areas. Organic carbon reflects the production of all organisms and is thus
not confined to certain areas as are other tracers,
such as CaCO J dominating in the open ocean or
biogenic opal which is characteristic for upwelling
areas and polar regions. The agreement between
organic carbon accumulation and other proxies of
paleoproductivity, such as barium (Gingele and
Dahmke 1994, Schneider et al. 1997), biogenic opal
(Lyle et al. 1988; Abrantes et al. 1994; Berger et
al. 1994), 8 15 N (Holmes et al. 1997), and 8\3C
(Schneider et al. 1994) supports the reliability of organic carbon as an estimator of paleoproductivity
in upwelling areas. Particularly, continental slope
sediments at the western margins ofthe continents
frequently contain several percent of organic carbon. In the oligotrophic ocean, however, organic
carbon accumulation is problematic as a tracer of
paleoproductivity, because only a very small fraction is finally preserved in the sedimentary record.
In contrast, calcium carbonate is fairly well preserved above the lysocline in the oligotrophic open
ocean where it is the predominant biogenic component. There, carbonate accumulation basically
reflects primary production. This environment covers about 80% of the total ocean area (Berger et
al. 1989b) of which about one third is situated above
the calcite lysocline depth. The estimation of
paleoproductivity on the basis of carbonate, however, is an indirect method relying on the modern
relationships of the C o,/CaC0 3 flux and of the primary production/C"g flux which might have been
different in ancient oligotrophic regions. To reduce
the uncertainty in estimating paleoproductivity from
carbonate, a direct empirical relationship between
primary production and carbonate accumulation in
the pelagic ocean should be established. In summary, we confirm to use calcium carbonate as a
paleoproductivity proxy in the low productivity open
ocean areas. There it is more reliable than organic
carbon which is severely decomposed. Generally,
it is reasonable to compare several proxies to as-
337
dominated eastern coastal upwelling areas (e.g.
Miiller et al. 1983; Lyle 1988; Sarnthein et al. 1988;
Schneider et al. 1996).
We attribute the decrease ofpaleoproductivity
during the cold climatic stages - which might have
occurred globally in the oligotrophic open ocean -
to a reduced nutrient content in central and intermediate waters (Francois et al. 1990; Mulitza et al.
1998a; Mulitza et al. this volume). A redistribution
of nutrients was observed in the glacial ocean
(Boyle 1988b; Duplessy et al. 1988; Mix et al.
1991) with low-nutrient intermediate waters and
high-nutrient deep-waters, in contrast to the
present-day ocean where nutrients are enriched in
intermediate waters. In the Pacific, however, these
changes were much smaller than observed in
records obtained from the North Atlantic (Mix et
al. 1991). Since intermediate waters are the primary source of nutrients for new production in the
oligotrophic areas, a lowering of their nutrient concentration is a likely explanation for a widespread
decreasing paleo-productivity in the cold stages
(Francois et al. 1990). The pronounced increase in
paleoproductivity appearing at the same time in the
high-productivity areas was obviously sustained by
intensified wind-driven upwelling.
Additionally, inverse organic carbon deposition
in the tropical Atlantic could have been cansed by
a wind-driven lowering of the nutricline in the West,
connected with a synchronous shallowing in the
East (Riihlemann et al. 1996). At present, mixed
layer depth is at maximum during August to
October when the zonal windstress is most intense
and surface waters, imported from the eastern
Atlantic, are piled up in the West. In April, when
the windstress is at minimum, thermocline and
nutricline shoal in the West and deepen in the East.
In analogy to the modern seasonal variation of
the thermocline and nutricline depths (Longhurst
1993), the insolation-controlled intensification of the
zonal windstress during cold substages (McIntyre
et al. 1989) could have caused a shallowing ofthe
nutricline in the eastern Atlantic, whereas the
nutricline in the western Atlantic was deepened.
The increase in the slope of the nutricline might
have enhanced paleoproductivity - and hence organic carbon accumulation - in the East and at the
same time lowered it in the West.
Conclusions
The application of organic carbon and carbonate
as paleoproductivity indicators has been assessed
by comparing two sediment cores obtained from
high and low productivity areas. Organic carbon reflects the production of all organisms and is thus
not confined to certain areas as are other tracers,
such as CaCO J dominating in the open ocean or
biogenic opal which is characteristic for upwelling
areas and polar regions. The agreement between
organic carbon accumulation and other proxies of
paleoproductivity, such as barium (Gingele and
Dahmke 1994, Schneider et al. 1997), biogenic opal
(Lyle et al. 1988; Abrantes et al. 1994; Berger et
al. 1994), 8 15 N (Holmes et al. 1997), and 8\3C
(Schneider et al. 1994) supports the reliability of organic carbon as an estimator of paleoproductivity
in upwelling areas. Particularly, continental slope
sediments at the western margins ofthe continents
frequently contain several percent of organic carbon. In the oligotrophic ocean, however, organic
carbon accumulation is problematic as a tracer of
paleoproductivity, because only a very small fraction is finally preserved in the sedimentary record.
In contrast, calcium carbonate is fairly well preserved above the lysocline in the oligotrophic open
ocean where it is the predominant biogenic component. There, carbonate accumulation basically
reflects primary production. This environment covers about 80% of the total ocean area (Berger et
al. 1989b) of which about one third is situated above
the calcite lysocline depth. The estimation of
paleoproductivity on the basis of carbonate, however, is an indirect method relying on the modern
relationships of the C o,/CaC0 3 flux and of the primary production/C"g flux which might have been
different in ancient oligotrophic regions. To reduce
the uncertainty in estimating paleoproductivity from
carbonate, a direct empirical relationship between
primary production and carbonate accumulation in
the pelagic ocean should be established. In summary, we confirm to use calcium carbonate as a
paleoproductivity proxy in the low productivity open
ocean areas. There it is more reliable than organic
carbon which is severely decomposed. Generally,
it is reasonable to compare several proxies to as-