388
Romero et al.
open ocean. We believe that the large regional differences seen in the diatom export fluxes at water
depths ranging between 500-2000 m can be used
as good representatives of diverse productivity
regimes (Fig. 1, Table 3). Within coastal upwelling
areas, temporal differences are seen in the diatom
exportation rates at time scales of days-months.
For example, off Cape Blanc and off Peru, the
quasi-constant input of nutrients results in an intense, almost permanent diatom export flux with low
seasonality (Fig. 2; Schrader and Sorknes 1991;
Lange et al. 1998). On the other hand, at the Walvis
Ridge (Treppke et al. 1996b) and in other upwelling
areas like off Portugal (e.g. Abrantes and Sancetta
1985; Abrantes 1988) and off southern California
(Sautter and Sancetta 1992; Lange et al. 1997),
marked seasonal upwelling pulses result in concomitant marked fluctuations in the export of diatoms.
However, the relationship between opal sediment accumulation and either total primary productivity or diatom productivity in surface waters is not
simple. Nelson et al. (1995) suggested that regional
differences in silica preservation (enhanced by
low surface water temperature, formation of aggregates during diatom blooms or export via fecal
pellets and rapid transit to the seafloor, trace elements which combine with diatom silica in ways
that decrease its dissolution rate) dominate over
regional productivity differences in producing the
known geographic distribution of opal sediments.
Furthermore, cell size, silicification of the diatom
frustule and the species composition of the diatom
assemblage play an important role in determining
whether the siliceous material of a given species
will reach the seafloor and preserve in the sedimentary record (Nelson et al. 1995). In several areas
of the oceans, the effect of preservation removes
information from the most productive season thus
leaving the sediment with no evidence of original
variability (e.g. Sancetta 1989a, 1992). This is the
case for the Cape Verde area and the equatorial
Atlantic presented here where small, lightly-silicified
diatoms of the Nitzschia bicapitata group dominate in the water column during times of high production but do not preserve in the surface
sediments (Table 3). In contrast, diatoms in the
surface sediments underneath some coastal
upwelling systems record the high-productivity season ofthe surface waters which is represented by
moderately robust taxa readily preserved in the
sediment: for example, off the Portuguese coast
(Abrantes and Sancetta 1985; Abrantes 1988), off
Peru (Schrader and Sorknes 1991), offMauretania
(Lange et al. 1998), and for laminated sediments
off Peru (Brodie and Kemp 1994), off California
(Grimm et al. 1996; Lange et al. 1996), off Namibia
(Lange unpubl obs), and within the Gulf of
California (Sancetta 1995; Pike 1996). It is evident,
that in order to use the fossil record to interpret past
oceanographic/climatic conditions it is necessary to
understand the nature of the seasonal representation in the sediments. Caution is adviced for
paleoproductivity studies which often interpret formation of opal-rich sediments as evidence of high
diatom productivity. Combining biogenic opal with
other proxies of export production (e.g. organic
carbon, barite, benthic foraminifers, etc.) seems to
be the best way to achieve a coherent reconstruction of past surface ocean history.
Acknowledgments
We are grateful to the officers and crews ofRN
"Meteor" for competent assistance in recovery of
mooring assays, and retrieval of sediment samples.
W. H. Berger and F. M. H. Reid reviewed the first
version of this manuscript. C. Sancetta reviewed
the final version and her helpful comments are
greatly acknowledged. We are also indebted to
M. Scholz, V. Diekamp and C. Slickers for
laboratory work for total, opal and organic carbon
determinations. This research was funded by
the Deutsche Forschungsgemeinschaft (Sonderforschungsbereich 261 at Bremen University, Contribution no. 190), and by the Marine Life Research
Group and the Geosciences Research Division of
the Scripps Institution of Oceanography. Data
are available under www.pangaea.de/Projects/
SFB26 1.
Romero et al.
open ocean. We believe that the large regional differences seen in the diatom export fluxes at water
depths ranging between 500-2000 m can be used
as good representatives of diverse productivity
regimes (Fig. 1, Table 3). Within coastal upwelling
areas, temporal differences are seen in the diatom
exportation rates at time scales of days-months.
For example, off Cape Blanc and off Peru, the
quasi-constant input of nutrients results in an intense, almost permanent diatom export flux with low
seasonality (Fig. 2; Schrader and Sorknes 1991;
Lange et al. 1998). On the other hand, at the Walvis
Ridge (Treppke et al. 1996b) and in other upwelling
areas like off Portugal (e.g. Abrantes and Sancetta
1985; Abrantes 1988) and off southern California
(Sautter and Sancetta 1992; Lange et al. 1997),
marked seasonal upwelling pulses result in concomitant marked fluctuations in the export of diatoms.
However, the relationship between opal sediment accumulation and either total primary productivity or diatom productivity in surface waters is not
simple. Nelson et al. (1995) suggested that regional
differences in silica preservation (enhanced by
low surface water temperature, formation of aggregates during diatom blooms or export via fecal
pellets and rapid transit to the seafloor, trace elements which combine with diatom silica in ways
that decrease its dissolution rate) dominate over
regional productivity differences in producing the
known geographic distribution of opal sediments.
Furthermore, cell size, silicification of the diatom
frustule and the species composition of the diatom
assemblage play an important role in determining
whether the siliceous material of a given species
will reach the seafloor and preserve in the sedimentary record (Nelson et al. 1995). In several areas
of the oceans, the effect of preservation removes
information from the most productive season thus
leaving the sediment with no evidence of original
variability (e.g. Sancetta 1989a, 1992). This is the
case for the Cape Verde area and the equatorial
Atlantic presented here where small, lightly-silicified
diatoms of the Nitzschia bicapitata group dominate in the water column during times of high production but do not preserve in the surface
sediments (Table 3). In contrast, diatoms in the
surface sediments underneath some coastal
upwelling systems record the high-productivity season ofthe surface waters which is represented by
moderately robust taxa readily preserved in the
sediment: for example, off the Portuguese coast
(Abrantes and Sancetta 1985; Abrantes 1988), off
Peru (Schrader and Sorknes 1991), offMauretania
(Lange et al. 1998), and for laminated sediments
off Peru (Brodie and Kemp 1994), off California
(Grimm et al. 1996; Lange et al. 1996), off Namibia
(Lange unpubl obs), and within the Gulf of
California (Sancetta 1995; Pike 1996). It is evident,
that in order to use the fossil record to interpret past
oceanographic/climatic conditions it is necessary to
understand the nature of the seasonal representation in the sediments. Caution is adviced for
paleoproductivity studies which often interpret formation of opal-rich sediments as evidence of high
diatom productivity. Combining biogenic opal with
other proxies of export production (e.g. organic
carbon, barite, benthic foraminifers, etc.) seems to
be the best way to achieve a coherent reconstruction of past surface ocean history.
Acknowledgments
We are grateful to the officers and crews ofRN
"Meteor" for competent assistance in recovery of
mooring assays, and retrieval of sediment samples.
W. H. Berger and F. M. H. Reid reviewed the first
version of this manuscript. C. Sancetta reviewed
the final version and her helpful comments are
greatly acknowledged. We are also indebted to
M. Scholz, V. Diekamp and C. Slickers for
laboratory work for total, opal and organic carbon
determinations. This research was funded by
the Deutsche Forschungsgemeinschaft (Sonderforschungsbereich 261 at Bremen University, Contribution no. 190), and by the Marine Life Research
Group and the Geosciences Research Division of
the Scripps Institution of Oceanography. Data
are available under www.pangaea.de/Projects/
SFB26 1.