Organic Carbon and Carbonate as Paleoproductivity Proxies
325
Reconstructing Tropical Atlantic
Paleoproductivity
The application as well as problems related to organic carbon and carbonate as paleoproductivity
proxies will be illustrated by comparing two sediment cores: one from the highly productive eastern Atlantic upwelling area off Angola and the
other from the Ceara Rise (Fig. 2), a topographic
high in the oligotrophic western equatorial Atlantic
north of Brazil. The eastern Atlantic sediment core
GeoB 1016-3 (11°46'SII1°41'E; 3411m water
depth) is characterized by high and strongly varying organic carbon accumulation rates, whereas the
western Atlantic core GeoB 1523-1 (03°50'NI
41 °37'W; 3292m water depth) shows the opposite
with low and very slightly varying organic carbon
accumulation rates (Fig. 7b). In contrast, CaC0 3
accumulation was high at the Ceara Rise and comparatively low at the eastern Atlantic upwelling site
(Fig. 7c). Below, we will discuss the factors influencing the differences in organic carbon and carbonate deposition between the two core sites. We
will compare the three paleoproductivity equations
from MUller and Suess (1979), Stein (1986) and
Sarnthein et al. (1992), and will evaluate how
paleoproductivity can be estimated from carbonate accumulation rates as proposed by Brummer
and van Eijden (1992).
Organic Carbon as Paleoproductivity
Indicator
Organic carbon is the most straightforward
paleoproductivity indicator since it is the basic element of all organisms. Its application as a
paleoproductivity proxy has been extensively discussed and several quantitative empirical equations
have been proposed for sediments underlying oxic
bottom waters (MUller and Suess 1979; Stein 1986;
Sarnthein et al. 1987, 1988, 1992; Vasileva 1987;
Berger et at. 1989b) and anoxic bottom waters
(Brumsack 1980; Bralower and Thierstein 1984).
The observation that the distribution of organic
carbon contents in marine sediments matches the
pattern of primary production (e.g. Berger and
Herguera 1992) is the basis for using organic carbon as an indicator of paleoproductivity. The dichotomy of the tropical South Atlantic with respect
to primary production, for instance, is reflected in
the distribution of sedimentary organic carbon. An
E-W transect of surficial sediment organic carbon
contents in the tropical Atlantic reveals a strong
contrast with high values up to 5 weight-% at the
African continental slope and comparatively low
values in the pelagic ocean and the South American continental margin (Fig. 8). A comparison of
the organic carbon accumulation rates between the
two sediment cores (Fig. 7b) indicates that the
general pattern of high productivity in the eastern
coastal upwelling region and low productivity in the
oligotrophic western Atlantic has prevailed at least
throughout the last 300 ka (ka = 1000 years) and
that it was even more pronounced during cold climatic stages.
Other proxies of paleoproductivity or nutrient
utilization such as barium (Schneider et al. 1997),
carbonate (RUhlemann et at. 1996), and 8 15 N
(Holmes et al. 1997) confirm that the contrast of
organic carbon accumulation between the two
sediment cores is primarily caused by differences
in paleoproductivity. Modem primary production is
three times higher at the location of core GeoB
1016-3 (150 gC m· 2 a· l ) than at the location of core
GeoB 1523-1 (50 gC m· 2 a· l ; Fig. 2), whereas average Holocene (0-6 ka) organic carbon accumulation rates are five times higher in core GeoB
1016-3. Stronger organic carbon degradation at the
depositional environment of core GeoB 1523-1
probably caused the difference in the rates of
paleoproductivity and organic carbon accumulation. In the following, the factors that contribute to
a better preservation of organic carbon at station
GeoB 1016-3 will be discussed.
Factors Affecting the Preservation of
Organic Carbon in the Tropical Atlantic
Due to its higher primary production and stronger
seasonality, the eastern Atlantic displays a much
higher export production than the western Atlantic. To estimate modern export production in the
eastern and the tropical-subtropical South Atlantic we determined the seasonalities from flux data
se;s of two sedimenttraps representing the different productivity systems: one from the eastern
325
Reconstructing Tropical Atlantic
Paleoproductivity
The application as well as problems related to organic carbon and carbonate as paleoproductivity
proxies will be illustrated by comparing two sediment cores: one from the highly productive eastern Atlantic upwelling area off Angola and the
other from the Ceara Rise (Fig. 2), a topographic
high in the oligotrophic western equatorial Atlantic
north of Brazil. The eastern Atlantic sediment core
GeoB 1016-3 (11°46'SII1°41'E; 3411m water
depth) is characterized by high and strongly varying organic carbon accumulation rates, whereas the
western Atlantic core GeoB 1523-1 (03°50'NI
41 °37'W; 3292m water depth) shows the opposite
with low and very slightly varying organic carbon
accumulation rates (Fig. 7b). In contrast, CaC0 3
accumulation was high at the Ceara Rise and comparatively low at the eastern Atlantic upwelling site
(Fig. 7c). Below, we will discuss the factors influencing the differences in organic carbon and carbonate deposition between the two core sites. We
will compare the three paleoproductivity equations
from MUller and Suess (1979), Stein (1986) and
Sarnthein et al. (1992), and will evaluate how
paleoproductivity can be estimated from carbonate accumulation rates as proposed by Brummer
and van Eijden (1992).
Organic Carbon as Paleoproductivity
Indicator
Organic carbon is the most straightforward
paleoproductivity indicator since it is the basic element of all organisms. Its application as a
paleoproductivity proxy has been extensively discussed and several quantitative empirical equations
have been proposed for sediments underlying oxic
bottom waters (MUller and Suess 1979; Stein 1986;
Sarnthein et al. 1987, 1988, 1992; Vasileva 1987;
Berger et at. 1989b) and anoxic bottom waters
(Brumsack 1980; Bralower and Thierstein 1984).
The observation that the distribution of organic
carbon contents in marine sediments matches the
pattern of primary production (e.g. Berger and
Herguera 1992) is the basis for using organic carbon as an indicator of paleoproductivity. The dichotomy of the tropical South Atlantic with respect
to primary production, for instance, is reflected in
the distribution of sedimentary organic carbon. An
E-W transect of surficial sediment organic carbon
contents in the tropical Atlantic reveals a strong
contrast with high values up to 5 weight-% at the
African continental slope and comparatively low
values in the pelagic ocean and the South American continental margin (Fig. 8). A comparison of
the organic carbon accumulation rates between the
two sediment cores (Fig. 7b) indicates that the
general pattern of high productivity in the eastern
coastal upwelling region and low productivity in the
oligotrophic western Atlantic has prevailed at least
throughout the last 300 ka (ka = 1000 years) and
that it was even more pronounced during cold climatic stages.
Other proxies of paleoproductivity or nutrient
utilization such as barium (Schneider et al. 1997),
carbonate (RUhlemann et at. 1996), and 8 15 N
(Holmes et al. 1997) confirm that the contrast of
organic carbon accumulation between the two
sediment cores is primarily caused by differences
in paleoproductivity. Modem primary production is
three times higher at the location of core GeoB
1016-3 (150 gC m· 2 a· l ) than at the location of core
GeoB 1523-1 (50 gC m· 2 a· l ; Fig. 2), whereas average Holocene (0-6 ka) organic carbon accumulation rates are five times higher in core GeoB
1016-3. Stronger organic carbon degradation at the
depositional environment of core GeoB 1523-1
probably caused the difference in the rates of
paleoproductivity and organic carbon accumulation. In the following, the factors that contribute to
a better preservation of organic carbon at station
GeoB 1016-3 will be discussed.
Factors Affecting the Preservation of
Organic Carbon in the Tropical Atlantic
Due to its higher primary production and stronger
seasonality, the eastern Atlantic displays a much
higher export production than the western Atlantic. To estimate modern export production in the
eastern and the tropical-subtropical South Atlantic we determined the seasonalities from flux data
se;s of two sedimenttraps representing the different productivity systems: one from the eastern