Reconstruction of Surface Ocean Nitrate Utilization Using Stable
Nitrogen Isotopes in Sinking Particles and Sediments
M. E. Holmes 1 *, C. Eichner 2 , U. Struck 2 and G. Wefer 1
1 Universitat Bremen, Fachbereich Geowissenschaften, Postfach 33 04 40,
D-28334 Bremen, Germany
21nstitutjur Ostseejorschung, Seestraj3e 15, D-18119 Rostock-Warnemunde, Germany
*corresponding author (e-mail}:bholmes@uni-bremen.de
Abstract: Stable nitrogen isotope ratios in sinking particles and bulk sediments have been used to
document changes in relative nitrate utilization in surface waters of the subtropical southeast Atlantic.
Incubation experiments demonstrated clear isotopic increases in decomposing particulate matter due
to the removal of 15N in the form ofNH;. In spite of the diagenetic offset, ol5N values in sinking
particles at the Walvis Ridge and in surface sediments in the Angola and Cape Basins reflect changes
in upwelled nitrate concentrations in surface water and the relative depletion of this nutrient. Two
sediment cores from the Angola Basin provide a olIN record of nitrate utilization for the past 300 kyr
that indicates that nutrient availability and productivity in this region are primarily controlled by
changes in trade wind-driven upwelling intensity and the northward advection of cold, nutrient-rich
water. Additionally, nitrate was less depleted and productivity was higher in glacial periods than
during interglacials.
Introduction
Stable nitrogen isotope ratios in marine sediments
provide a record of changes in the biochemistry of
the water column. The oI5N(l) signal measured in
sediments is ultimately the result of the
fractionation of nitrogen isotopes during many biological reactions occurring in both the marine and
terrestrial environments. This parameter can therefore yield valuable information about the conditions
under which the organic matter in the sediments
was produced. Of particular interest in the study
of paleoceanography is the reconstruction of variations in nitrate utilization, denitrification and nitrogen fixation.
Uptake of dissolved inorganic nitrogen during
photosynthesis results in fractionation of nitrogen
isotopes between the inorganic source nitrogen
(NO]' supports most of the primary production in
the eastern subtropical Atlantic, Chapman and
Shannon 1985) and the organic matter which is produced. Because NO; containing the lighter isotope
is preferentially taken up during photosynthesis,
phytoplankton is typically depleted in 15N relative to
the substrate used for growth. The remaining NO)'
pool becomes progressively enriched in I'N according to Rayleigh fractionation kinetics as long as there
is no new input of NO; (Wada 1980; Cifuentes et
al. 1988; Waser et al. 1998). Thus, one of the most
important controls on the ol5N of organic matter is
relative nitrate depletion in the euphotic zone
(Mariotti et al. 1982). Nutrient concentration influences relative nitrate utilization, as seen by the inverse relationship between nutrient concentrations
and ol5N of suspended and sinking particles noted
by Montoya and McCarthy (1995) and Voss et al.
(1996). Wu et al. (1997) reported that as [NO]']
increased in a transect in the subarctic Pacific,
oI5N-NO)', ol5N of suspended particulate organic
matter (POM) and ol5N-zooplankton decreased,
From FISCHER G. WEFER G (eds). 1999. Use of Proxies in Paleoceanography: Examplesfrom the South Atlantic. Springer-Verlag
Berlin Heidelberg, pp 447-468
Nitrogen Isotopes in Sinking Particles and Sediments
M. E. Holmes 1 *, C. Eichner 2 , U. Struck 2 and G. Wefer 1
1 Universitat Bremen, Fachbereich Geowissenschaften, Postfach 33 04 40,
D-28334 Bremen, Germany
21nstitutjur Ostseejorschung, Seestraj3e 15, D-18119 Rostock-Warnemunde, Germany
*corresponding author (e-mail}:bholmes@uni-bremen.de
Abstract: Stable nitrogen isotope ratios in sinking particles and bulk sediments have been used to
document changes in relative nitrate utilization in surface waters of the subtropical southeast Atlantic.
Incubation experiments demonstrated clear isotopic increases in decomposing particulate matter due
to the removal of 15N in the form ofNH;. In spite of the diagenetic offset, ol5N values in sinking
particles at the Walvis Ridge and in surface sediments in the Angola and Cape Basins reflect changes
in upwelled nitrate concentrations in surface water and the relative depletion of this nutrient. Two
sediment cores from the Angola Basin provide a olIN record of nitrate utilization for the past 300 kyr
that indicates that nutrient availability and productivity in this region are primarily controlled by
changes in trade wind-driven upwelling intensity and the northward advection of cold, nutrient-rich
water. Additionally, nitrate was less depleted and productivity was higher in glacial periods than
during interglacials.
Introduction
Stable nitrogen isotope ratios in marine sediments
provide a record of changes in the biochemistry of
the water column. The oI5N(l) signal measured in
sediments is ultimately the result of the
fractionation of nitrogen isotopes during many biological reactions occurring in both the marine and
terrestrial environments. This parameter can therefore yield valuable information about the conditions
under which the organic matter in the sediments
was produced. Of particular interest in the study
of paleoceanography is the reconstruction of variations in nitrate utilization, denitrification and nitrogen fixation.
Uptake of dissolved inorganic nitrogen during
photosynthesis results in fractionation of nitrogen
isotopes between the inorganic source nitrogen
(NO]' supports most of the primary production in
the eastern subtropical Atlantic, Chapman and
Shannon 1985) and the organic matter which is produced. Because NO; containing the lighter isotope
is preferentially taken up during photosynthesis,
phytoplankton is typically depleted in 15N relative to
the substrate used for growth. The remaining NO)'
pool becomes progressively enriched in I'N according to Rayleigh fractionation kinetics as long as there
is no new input of NO; (Wada 1980; Cifuentes et
al. 1988; Waser et al. 1998). Thus, one of the most
important controls on the ol5N of organic matter is
relative nitrate depletion in the euphotic zone
(Mariotti et al. 1982). Nutrient concentration influences relative nitrate utilization, as seen by the inverse relationship between nutrient concentrations
and ol5N of suspended and sinking particles noted
by Montoya and McCarthy (1995) and Voss et al.
(1996). Wu et al. (1997) reported that as [NO]']
increased in a transect in the subarctic Pacific,
oI5N-NO)', ol5N of suspended particulate organic
matter (POM) and ol5N-zooplankton decreased,
From FISCHER G. WEFER G (eds). 1999. Use of Proxies in Paleoceanography: Examplesfrom the South Atlantic. Springer-Verlag
Berlin Heidelberg, pp 447-468