458
Holmes eta!.
Surface Sediments
It is evident that sinking particles reflect changes
in euphotic zone processes, but this signal must be
passed on to the underlying sediments in order to
use sedimentary parameters to study past upper
ocean nutrient dynamics and paleoproductivity.
Holmes et al. (1996, 1998a) found that in the
Angola and Cape Basins, surface sediment 8'5N
values show spatial variations which can be related
to surface water nitrate concentrations and relative utilization. Shelf and upper slope sediments
exhibited 8'5N values between 4.6 and 6.3%0, while
offshore, 8'5N values increased to up to 11.8%0
(Fig. 9a and b). This nearshore to offshore increase
in '5N/'4N can be explained by the high nearshore
nutrient concentrations at the coast which are
brought to the surface by coastal upwelling and in
the case ofthe Zaire Fan transect, also by nutrient
input from the Zaire River outflow (Calvert and
Price 1971; Emery et al. 1973). Near the coast,
nitrate concentrations are as high as 30 mM and
decrease to undetectable levels offshore (Chapman
and Shannon 1985; SNEC II 1986).
In Fig. 9c and d, bulk surface sedimentary8''N
is plotted against the natural log ofhistoricaliy averaged [N0l"J,mface (Conkright et al. 1994). In Fig.
9c, the two samples from the Zaire River and the
four shallowest samples from the Zaire Fan are not
included because ofthe large amount of terrestrial
material in these sediments (seen as light 8 13 C;
Muller et a1. 1994). Although the general trend of
increasing 8 '5N values with decreasing
[NO;l,url'ace is clear, there are differences in slope
among the transects. The slope of the regression
line fitted to 8'5N and In[N03·l should approximate
the fractionation factor (a) associated with nitrate
uptake by phytoplankton if the particulate nitrogen
produced sinks rapidly and is deposited on the sea
floor directly below (Altabet and Francois 1994).
Thus, the dissimilar slopes could indicate different
a between sites, but are likely a combined effect
ofthis and varying residence times of the particles
in the water column. The latter process would effectively cause the value of a to be underestimated
if the particulate nitrogen was transported some
distance from its site of production. Additionally,
advection of new nitrate to surface waters, especially where nitrate concentrations are low, can
serve to further dampen the spatial increase in
8'5N. Indeed, the slopes of the regression lines for
the Angola Basin (1.0%0) and the southern part of
the Cape Basin (2.0%0) are lower than field estimates for a of 5 - 9%0 (Wada 1980; Altabet and
Francois 1994). Thus, in hydrographicallycomplex
environments such as this, the reconstruction of
paleo-nitrate concentrations from sedimentary8'5N
is complicated, although estimates of relative nitrate utilization are possible.
As mentioned above, 8'1C values show that
nearshore sediments in the Zaire Fan contain significant amounts of terrigenous material. However,
the 8'5N signal does not show the mixing between
terrestrial matter entering the Zaire Fan from the
Zaire River and sediment of marine origin (Fig. 9a).
The enrichment in 15N of the two sediment samples from within the Zaire estuary relative to the
Zaire Fan sediments may be due to the uptake by
phytoplankton in the estuary of1 5 N-enriched NO l -
caused by denitrification. Alternatively, the terrestrial material ofthe Zaire River may simply have a
heavier nitrogen isotopic composition than the
marine endmember, a possibility that cannot be
excluded in view ofthe wide range of8'5N signatures found in terrestrial systems (-17 to +30%0,
from Sweeney et al. 1978). Unfortunately, no soil
8' 5N data is available to allow evaluation of the terrestrial endmember. A clear mixing trend between
the terrestrial and marine systems may be masked
by the predominant influence of the high nearshore
nutrient concentrations, which lead to 15N-depleted
particulate nitrogen near the coast.
Late Quaternary Changes in Nitrate
Utilization
Sedimentary 15N/'4N ratios in two gravity cores,
one off the Zaire River at around 6 0 S (core GeoB
1008-3) and one in the middle Angola Basin, near
12°S (core GeoB 1016-3) were measured by
Holmes et al. (1997) and range from 5.0 to 9.5%0
(Fig. lOa and b). As seen in Fig. 10 low 8 15 N coincides with low SST and high organic carbon
(OC), and during periods of high 8 15 N, SST is also
generally high and OC low. Low SST indicates the
presence of recently upwelled, nutrient-rich water,
Holmes eta!.
Surface Sediments
It is evident that sinking particles reflect changes
in euphotic zone processes, but this signal must be
passed on to the underlying sediments in order to
use sedimentary parameters to study past upper
ocean nutrient dynamics and paleoproductivity.
Holmes et al. (1996, 1998a) found that in the
Angola and Cape Basins, surface sediment 8'5N
values show spatial variations which can be related
to surface water nitrate concentrations and relative utilization. Shelf and upper slope sediments
exhibited 8'5N values between 4.6 and 6.3%0, while
offshore, 8'5N values increased to up to 11.8%0
(Fig. 9a and b). This nearshore to offshore increase
in '5N/'4N can be explained by the high nearshore
nutrient concentrations at the coast which are
brought to the surface by coastal upwelling and in
the case ofthe Zaire Fan transect, also by nutrient
input from the Zaire River outflow (Calvert and
Price 1971; Emery et al. 1973). Near the coast,
nitrate concentrations are as high as 30 mM and
decrease to undetectable levels offshore (Chapman
and Shannon 1985; SNEC II 1986).
In Fig. 9c and d, bulk surface sedimentary8''N
is plotted against the natural log ofhistoricaliy averaged [N0l"J,mface (Conkright et al. 1994). In Fig.
9c, the two samples from the Zaire River and the
four shallowest samples from the Zaire Fan are not
included because ofthe large amount of terrestrial
material in these sediments (seen as light 8 13 C;
Muller et a1. 1994). Although the general trend of
increasing 8 '5N values with decreasing
[NO;l,url'ace is clear, there are differences in slope
among the transects. The slope of the regression
line fitted to 8'5N and In[N03·l should approximate
the fractionation factor (a) associated with nitrate
uptake by phytoplankton if the particulate nitrogen
produced sinks rapidly and is deposited on the sea
floor directly below (Altabet and Francois 1994).
Thus, the dissimilar slopes could indicate different
a between sites, but are likely a combined effect
ofthis and varying residence times of the particles
in the water column. The latter process would effectively cause the value of a to be underestimated
if the particulate nitrogen was transported some
distance from its site of production. Additionally,
advection of new nitrate to surface waters, especially where nitrate concentrations are low, can
serve to further dampen the spatial increase in
8'5N. Indeed, the slopes of the regression lines for
the Angola Basin (1.0%0) and the southern part of
the Cape Basin (2.0%0) are lower than field estimates for a of 5 - 9%0 (Wada 1980; Altabet and
Francois 1994). Thus, in hydrographicallycomplex
environments such as this, the reconstruction of
paleo-nitrate concentrations from sedimentary8'5N
is complicated, although estimates of relative nitrate utilization are possible.
As mentioned above, 8'1C values show that
nearshore sediments in the Zaire Fan contain significant amounts of terrigenous material. However,
the 8'5N signal does not show the mixing between
terrestrial matter entering the Zaire Fan from the
Zaire River and sediment of marine origin (Fig. 9a).
The enrichment in 15N of the two sediment samples from within the Zaire estuary relative to the
Zaire Fan sediments may be due to the uptake by
phytoplankton in the estuary of1 5 N-enriched NO l -
caused by denitrification. Alternatively, the terrestrial material ofthe Zaire River may simply have a
heavier nitrogen isotopic composition than the
marine endmember, a possibility that cannot be
excluded in view ofthe wide range of8'5N signatures found in terrestrial systems (-17 to +30%0,
from Sweeney et al. 1978). Unfortunately, no soil
8' 5N data is available to allow evaluation of the terrestrial endmember. A clear mixing trend between
the terrestrial and marine systems may be masked
by the predominant influence of the high nearshore
nutrient concentrations, which lead to 15N-depleted
particulate nitrogen near the coast.
Late Quaternary Changes in Nitrate
Utilization
Sedimentary 15N/'4N ratios in two gravity cores,
one off the Zaire River at around 6 0 S (core GeoB
1008-3) and one in the middle Angola Basin, near
12°S (core GeoB 1016-3) were measured by
Holmes et al. (1997) and range from 5.0 to 9.5%0
(Fig. lOa and b). As seen in Fig. 10 low 8 15 N coincides with low SST and high organic carbon
(OC), and during periods of high 8 15 N, SST is also
generally high and OC low. Low SST indicates the
presence of recently upwelled, nutrient-rich water,