Environmental Engineering Reference
In-Depth Information
composition of oceanic combined nitrogen, N
2
-fixation and denitrification have
diametrically opposed effects and the occurrence of both processes within
the same water column creates unique challenges in interpreting the isotopic
systematics of the basin. Clearly, this may also complicate the interpretation
of the sedimentary δ
15
N record, which has typically been viewed in terms of
denitrification intensity alone.
The isotopic contrast between sinking particles at 100 and 500 m depth
provides another example of the complex interplay between denitrification and
N
2
-fixation in this basin. While denitrification sets the overall baseline isotopic
composition of the basin, N
2
-fixation can act locally to lower the δ
15
Nof
both suspended and sinking particles. Because our cruise coincided with a
major bloom of
Trichodesmium
, our data likely represent one extreme in the
spectrum of interaction between denitrification and N
2
-fixation in the Arabian
Sea and field studies at other seasons will likely reveal a simpler propagation
of the high δ
15
N associated with denitrification through the water column.
Finally, our data also provide strong support for the view that the δ
15
Nof
sedimentary organic nitrogen acts as an integrative proxy for nitrogen cycle
processes on a regional scale [e.g., 4]. In particular, we found elevated δ
15
N
values in suspended particles well south of the OMZ and in a region where
denitrification doesn't occur. Further studies of the role of the surface circulation
in redistributing the isotopic signature of denitrification throughout the basin
are clearly warranted.
Acknowledgements
We thank our colleagues on cruise ME32/3 as well as the officers and crew
of the F/S Meteor for their assistance at sea. Financial support for this project
was provided by the German Ministry for Education, Science and Technology
“Der Stickstoffkreislauf im Arabischen Meer als Antrieb f ur neue Produktion
und Exportfluß von organischem Kohlenstoff” F orderkennzeichen 03F0241D
and NSF grants OCE9530187, OCE9977528, and OCE0425583 to JPM.
References
[1]
Altabet M.A. A time-series study of the vertical structure of nitrogen and particle dynamics
in the Sargasso Sea. Limnol Oceanogr 1989; 34(7):1185-1201.
[2]
Altabet M.A., Deuser W.G., Honjo S. and Stienen C. Seasonal and depth-related changes
in the source of sinking particles in the North Atlantic. Nature 1991; 354:136-39.
[3]
Altabet M.A. and François R. Sedimentary nitrogen isotopic ratio records surface ocean
nitrate utilization. Global Biogeochem Cy 1994; 8:103-16.
[4]
Altabet M.A., François R., Murray D.W. and Preil W.L. Climate-related variations in
denitrification in the Arabian Sea from sediment
15
N/
14
N ratios. Nature 1995; 373:506-09.
Altabet M.A. and McCarthy J.J. Vertical patterns in
15
N natural abundance in PON from
the surface waters of warmâ^core rings. J Mar Res 1986; 44:185-201.
[5]
