Terrestrial Organic Matter in Marine Sediments
559
eastern Subtropical Atlantic is deduced from
increased CIN elemental ratios (10-12) and
palynological evidence in sediment trap samples off
Cape Blanc (Wefer and Fischer 1993). Terrestrial
biomarkers are reported from shallow water trap
samples from the Guinea Basin (Andersen 1996).
High concentrations of n-alkanes (C29-31), collected in boreal winter, are related to the seasonal
development of the winter dust plume close to the
equator.
The modem distribution ofterrigenous OM was
investigated along the Equatorial African margin
(Westerhausen et al. 1993). Quantification of
terrigenous vs. marine organic carbon and further
subdivision of the eolian- vs. river-borne fraction
was conducted on the basis of organic geochemical
data using elemental, isotopic, and molecular
records. The authors conclude that terrigenous
organic carbon reaches proportions> 60% ofTOC
on the shelves off Liberia, Ivory Coast and Gabon,
whereas proportions drastically decrease below
20% along the upper continental slope. TOC in
deep-sea sediments beyond an offshore distance
of200 km was considered to be entirely of marine
production. Admixture of river-borne OM was only
identified in front of river systems.
Results and Discussion
We extended the working area of Westerhausen
et ai. (1993) toward the basin to evaluate the transport of terrigenous OM to the pelagic Equatorial
Atlantic below the Equatorial Divergence. Surface
sediments covering the area between 8°-29°W and
6 0 S to 2°N (positions specified in Fig. Sa) were
analyzed for their TOC content and composition.
Differentiation between marine and terrigenous
organic carbon was delineated from CIN, Hydrogen Index, T mox' 013C o'g' and microscopic (macerals
and palynomorphs) evidence. Methodological procedures are described in Wagner (1998) and
Dupont and Agwu (1991, 1992). Results of each
individual parameter, supplemented by literature
data (Diester-Haass and Milller 1979; Milller and
Suess 1979; Verardo and McIntyre 1994; Westerhausen et aI., 1993; Fischer et ai. 1998; unnamed
positions in Fig. Sa), are shown in spatial distribution maps (Figs. 5-8, 10). Interpolation between positions was conducted using either "kriging" or
"minimum curvator" methods with 0.5° spacing.
Steps between isolines were chosen rather large
to highlight the main lateral gradients between the
continental shelves and the pelagic basins.
Figure 5b shows the TOC distribution in surface
sediments of the central-eastern Equatorial Atlantic. Highest concentrations are observed at shallow depths along the African shelf and upper slope
reflecting the continental upwelling cells off! vory
Coast/Ghana (Verstraete 1992) and Guinea/Senegal (Schemainda et ai. 1975). The steep gradient
in TOC from the shelf/slope break to concentrations < 0.5% in the pelagic basins is attributed to
decreasing flux rates while remineralization of OM
increases as a function of water depth and continental distance (Milller and Suess 1979; Emerson
and Hedges 1988). However, the current set of
data suggests that TOC remains slightly elevated
(0.5-1.0%) within a NE-SW heading belt from the
shelves of Guinea and Liberia to the central Equatorial Atlantic. It has to be noted that elevated TOC
contents in the deep Brasil Basin (positions 2903
and 2904, Fig. Sa) are apparently influenced by
reworked material (Schulz et ai. 1995). Enhanced
marine organic fluxes below the high productive
Equatorial Divergence are not recorded by elevated
TOC contents in surface sediments, as was anticipated.
First information on the composition of OM
preserved in surface sediments may be carefully
deduced from the spatial distribution ofCIN, HI,
and Tmax (Fig. 6a-c). CIN is low, never exceeding
a ratio of 15 (Fig. 6a). A broad area revealing ratios exceeding lOis identified along an E-W heading zone following the African continent. The C/
N-ratio drastically drops to low levels at the shelf/
slope break in the north and in deeper settings in
the south. Identification ofterrigenous OM based
on such CIN-ratios is highly speculative (see discussion above and Westerhausen et ai. 1993). The
high degree of OM degradation in surface
sediments is also evident from low HI, rarely exceeding 150 mgHC/gTOC (Fig. 6b). Slightly enhanced preservation of hydrogen-enriched (marine) OM is suggested below the center of the
Equatorial Divergence probably reflecting increased marine flux rates. A dominant immature
559
eastern Subtropical Atlantic is deduced from
increased CIN elemental ratios (10-12) and
palynological evidence in sediment trap samples off
Cape Blanc (Wefer and Fischer 1993). Terrestrial
biomarkers are reported from shallow water trap
samples from the Guinea Basin (Andersen 1996).
High concentrations of n-alkanes (C29-31), collected in boreal winter, are related to the seasonal
development of the winter dust plume close to the
equator.
The modem distribution ofterrigenous OM was
investigated along the Equatorial African margin
(Westerhausen et al. 1993). Quantification of
terrigenous vs. marine organic carbon and further
subdivision of the eolian- vs. river-borne fraction
was conducted on the basis of organic geochemical
data using elemental, isotopic, and molecular
records. The authors conclude that terrigenous
organic carbon reaches proportions> 60% ofTOC
on the shelves off Liberia, Ivory Coast and Gabon,
whereas proportions drastically decrease below
20% along the upper continental slope. TOC in
deep-sea sediments beyond an offshore distance
of200 km was considered to be entirely of marine
production. Admixture of river-borne OM was only
identified in front of river systems.
Results and Discussion
We extended the working area of Westerhausen
et ai. (1993) toward the basin to evaluate the transport of terrigenous OM to the pelagic Equatorial
Atlantic below the Equatorial Divergence. Surface
sediments covering the area between 8°-29°W and
6 0 S to 2°N (positions specified in Fig. Sa) were
analyzed for their TOC content and composition.
Differentiation between marine and terrigenous
organic carbon was delineated from CIN, Hydrogen Index, T mox' 013C o'g' and microscopic (macerals
and palynomorphs) evidence. Methodological procedures are described in Wagner (1998) and
Dupont and Agwu (1991, 1992). Results of each
individual parameter, supplemented by literature
data (Diester-Haass and Milller 1979; Milller and
Suess 1979; Verardo and McIntyre 1994; Westerhausen et aI., 1993; Fischer et ai. 1998; unnamed
positions in Fig. Sa), are shown in spatial distribution maps (Figs. 5-8, 10). Interpolation between positions was conducted using either "kriging" or
"minimum curvator" methods with 0.5° spacing.
Steps between isolines were chosen rather large
to highlight the main lateral gradients between the
continental shelves and the pelagic basins.
Figure 5b shows the TOC distribution in surface
sediments of the central-eastern Equatorial Atlantic. Highest concentrations are observed at shallow depths along the African shelf and upper slope
reflecting the continental upwelling cells off! vory
Coast/Ghana (Verstraete 1992) and Guinea/Senegal (Schemainda et ai. 1975). The steep gradient
in TOC from the shelf/slope break to concentrations < 0.5% in the pelagic basins is attributed to
decreasing flux rates while remineralization of OM
increases as a function of water depth and continental distance (Milller and Suess 1979; Emerson
and Hedges 1988). However, the current set of
data suggests that TOC remains slightly elevated
(0.5-1.0%) within a NE-SW heading belt from the
shelves of Guinea and Liberia to the central Equatorial Atlantic. It has to be noted that elevated TOC
contents in the deep Brasil Basin (positions 2903
and 2904, Fig. Sa) are apparently influenced by
reworked material (Schulz et ai. 1995). Enhanced
marine organic fluxes below the high productive
Equatorial Divergence are not recorded by elevated
TOC contents in surface sediments, as was anticipated.
First information on the composition of OM
preserved in surface sediments may be carefully
deduced from the spatial distribution ofCIN, HI,
and Tmax (Fig. 6a-c). CIN is low, never exceeding
a ratio of 15 (Fig. 6a). A broad area revealing ratios exceeding lOis identified along an E-W heading zone following the African continent. The C/
N-ratio drastically drops to low levels at the shelf/
slope break in the north and in deeper settings in
the south. Identification ofterrigenous OM based
on such CIN-ratios is highly speculative (see discussion above and Westerhausen et ai. 1993). The
high degree of OM degradation in surface
sediments is also evident from low HI, rarely exceeding 150 mgHC/gTOC (Fig. 6b). Slightly enhanced preservation of hydrogen-enriched (marine) OM is suggested below the center of the
Equatorial Divergence probably reflecting increased marine flux rates. A dominant immature