464
Holmesetal.
Summary and Conclusions
The concentration of nitrate in surface water and
the degree to which it is utilized are the predominant controls on 8 1S N in organic matter produced
in the Angola and Cape Basins. This is seen in the
correlation between 8 1S N values in sinking particles and SST and particle flux rates. Surface
sediments corroborate this view in that surface
sedimentary 8 1S N was negatively correlated with
surface water nitrate concentrations. Bacterial
remineralization appears to have altered the sedimentary 8 1S N signal to some extent, but euphotic
zone conditions are clearly reflected in sedimentary 8 1S N. It appears that neither denitrification nor
the presence of terrestrial material play important
roles in determining the nitrogen isotopic composition of surface sediments in this area ( except in the
Zaire estuary). Likewise, denitrification does not
appear to have occurred to a significant extent
during the past 300 kyr in this region and changes
in terrestrial input were undetectable on the basis
of ISN/14N ratios. Nutrient utilization in surface
water over the past 300 kyr in the Angola Basin
has responded primarily to changes in boreal summer insolation and trade wind intensity. Time series analyses revealed the presence of 23 kyr cycles in the 8 1S N record, suggesting that nitrate availability in this region is controlled by the intensity of
trade wind-driven upwelling. Glacial8 ,s N values
were significantly lower than the8 1S N ofinterglacial sediments, indicating less intense depletion of
the surface nitrate pool during glacials due to elevated nitrate concentrations.
Our results provide encouraging evidence for
the successful hindcasting of nitrate utilization in
surface water from 8 1S N records in Late Quaternary sediments. There are, however, uncertainties
regarding the interpretation of nitrogen isotope
records that point out the need for future research.
First, a wide range ofterrestrial8 1s N values have
been reported and, unlike 8l3C, 8 1S N in terrestriallyderived material does not seem to be consistently
lighter or heavier than marine-derived material,
complicating its use as a measure of the amount
of terri genous input to a sedimentary system. Fortunately, the question of terrestrial contribution
to sedimentary 8 1S N records can usually be avoided
in paleoceanographic studies by the choice of core
locations which are remote from continental
influences. Unavoidable, however, for paleoceanographers is the problem of organic matter decomposition. More work is needed to clarity the extent
of, and mechanisms behind, changes in ISN/14N
ratios in sinking particles and sediments, especially
if we hope to quantitatively reconstruct nutrient
utilization and concentrations from sedimentary
8 1S N. Also to be considered in the interpretation of
nitrogen isotope records is the hydrographical regime of the site under investigation. Another important question around which some debate centers
involves glacial-interglacial changes in the extent
of denitrification and how these may have affected
global NO]· concentrations and 8 1S N-NO J ·• Considering the relevance of nutrient drawdown and
denitrification to global ocean chemistry, 8 'SN
promises to be an important proxy in the study of
paleoceanography.
Acknowledgments
The authors wish to thank P. MUller, R. Schneider
and M. HUttel for thought-provoking discussions,
M. Segl, B. Meyer-Schack and I. Pesza for
technical assistance and M. Voil and D. Sigman
for their constructive comments on the manuscript.
This work was funded by the Deutsche
Forschungsgemeinschaft (Sonderforschungsbereich 261 at Bremen University, Contribution no. 202) and by the Bundesminister
ftir Bildung, Wissenschaft, Forschung and
Technologie (BMBF). Data are available under
www.pangaea.de/Projects/SFB261.
References
Altabet M (1988) Variations in nitrogen isotopic composition between sinking and suspended particles:
implications for nitrogen cycling and particle transformation in the open ocean. Deep-Sea Res 35:
535-554
Altabet M (1996) Nitrogen and carbon isotopic tracers
of the source and transformation of particles in the
deep sea. In: Ittekkot V, Schafer P, Honjo S (eds)
Particle Flux in the Ocean. J Wiley & Sons, New York,
pp 155-184
Holmesetal.
Summary and Conclusions
The concentration of nitrate in surface water and
the degree to which it is utilized are the predominant controls on 8 1S N in organic matter produced
in the Angola and Cape Basins. This is seen in the
correlation between 8 1S N values in sinking particles and SST and particle flux rates. Surface
sediments corroborate this view in that surface
sedimentary 8 1S N was negatively correlated with
surface water nitrate concentrations. Bacterial
remineralization appears to have altered the sedimentary 8 1S N signal to some extent, but euphotic
zone conditions are clearly reflected in sedimentary 8 1S N. It appears that neither denitrification nor
the presence of terrestrial material play important
roles in determining the nitrogen isotopic composition of surface sediments in this area ( except in the
Zaire estuary). Likewise, denitrification does not
appear to have occurred to a significant extent
during the past 300 kyr in this region and changes
in terrestrial input were undetectable on the basis
of ISN/14N ratios. Nutrient utilization in surface
water over the past 300 kyr in the Angola Basin
has responded primarily to changes in boreal summer insolation and trade wind intensity. Time series analyses revealed the presence of 23 kyr cycles in the 8 1S N record, suggesting that nitrate availability in this region is controlled by the intensity of
trade wind-driven upwelling. Glacial8 ,s N values
were significantly lower than the8 1S N ofinterglacial sediments, indicating less intense depletion of
the surface nitrate pool during glacials due to elevated nitrate concentrations.
Our results provide encouraging evidence for
the successful hindcasting of nitrate utilization in
surface water from 8 1S N records in Late Quaternary sediments. There are, however, uncertainties
regarding the interpretation of nitrogen isotope
records that point out the need for future research.
First, a wide range ofterrestrial8 1s N values have
been reported and, unlike 8l3C, 8 1S N in terrestriallyderived material does not seem to be consistently
lighter or heavier than marine-derived material,
complicating its use as a measure of the amount
of terri genous input to a sedimentary system. Fortunately, the question of terrestrial contribution
to sedimentary 8 1S N records can usually be avoided
in paleoceanographic studies by the choice of core
locations which are remote from continental
influences. Unavoidable, however, for paleoceanographers is the problem of organic matter decomposition. More work is needed to clarity the extent
of, and mechanisms behind, changes in ISN/14N
ratios in sinking particles and sediments, especially
if we hope to quantitatively reconstruct nutrient
utilization and concentrations from sedimentary
8 1S N. Also to be considered in the interpretation of
nitrogen isotope records is the hydrographical regime of the site under investigation. Another important question around which some debate centers
involves glacial-interglacial changes in the extent
of denitrification and how these may have affected
global NO]· concentrations and 8 1S N-NO J ·• Considering the relevance of nutrient drawdown and
denitrification to global ocean chemistry, 8 'SN
promises to be an important proxy in the study of
paleoceanography.
Acknowledgments
The authors wish to thank P. MUller, R. Schneider
and M. HUttel for thought-provoking discussions,
M. Segl, B. Meyer-Schack and I. Pesza for
technical assistance and M. Voil and D. Sigman
for their constructive comments on the manuscript.
This work was funded by the Deutsche
Forschungsgemeinschaft (Sonderforschungsbereich 261 at Bremen University, Contribution no. 202) and by the Bundesminister
ftir Bildung, Wissenschaft, Forschung and
Technologie (BMBF). Data are available under
www.pangaea.de/Projects/SFB261.
References
Altabet M (1988) Variations in nitrogen isotopic composition between sinking and suspended particles:
implications for nitrogen cycling and particle transformation in the open ocean. Deep-Sea Res 35:
535-554
Altabet M (1996) Nitrogen and carbon isotopic tracers
of the source and transformation of particles in the
deep sea. In: Ittekkot V, Schafer P, Honjo S (eds)
Particle Flux in the Ocean. J Wiley & Sons, New York,
pp 155-184