Environmental Engineering Reference
In-Depth Information
NITROGEN CYCLING IN SUBOXIC WATERS:
ISOTOPIC SIGNATURES OF NITROGEN
TRANSFORMATION IN THE ARABIAN SEA
OXYGEN MINIMUM ZONE
Joseph P. Montoya
1
and Maren Voss
2
1
School of Biology, Georgia Institute of Technology, 311 Ferst Dr., Atlanta, GA 30332, USA
2
Baltic Sea Research Institute Warnemunde, Seestrasse 15, 18119 Rostock, Germany
Abstract
The natural abundance of the stable isotopes of nitrogen provides a powerful in
situ tracer for N cycle processes in and around suboxic zones. Denitrification
in the suboxic zone of the Arabian Sea raises the δ
15
N
of the residual nitrate,
which is mixed into the surface layer during periods of deep convection and
upwelling. Complete assimilation of available nitrate during primary production
transfers this isotopic signature into surface particulate matter, which then con-
tributes to the flux of sinking organic matter. A small fraction of the nitrogen is
ultimately deposited on the seafloor with an isotopic signature well above the
δ
15
N
of average oceanic NO
3
−
. We illustrate the patterns in isotopic variation
of organic matter in and near one of the major oceanic suboxic zones with data
from the German JGOFS cruise in the Arabian Sea in May-June 1995. On this
cruise, we collected samples of particulate organic matter and nutrients along a
transect on 65
◦
E between 21
◦
N and the equator. Interestingly, we also encoun-
tered a massive bloom of N
2
-fixing cyanobacteria during this cruise. This bloom
introduced isotopically depleted N into upper water column particles as well as
sinking organic matter. Nonetheless, we found enriched nitrogen isotope values
in suspended particles throughout the upper 1500m of the water column as far
south as the equator. This lateral spread of the isotopic signature of denitrification
is of obvious importance in paleoceanographic studies of the nitrogen cycle, as
is the interaction between N
2
-fixation and denitrification in setting the isotopic
composition of organic matter in the water column.
Keywords:
Denitrification, marine nitrogen cycle, stable isotope biogeochemistry, N
2
-fixation
1. INTRODUCTION AND BACKGROUND
The major Oxygen Minimum Zones (OMZs) of the world ocean are char-
acterized by partial to complete consumption of midwater NO
3
−
, leading to
significant removal of combined nitrogen from the ocean. On long time scales,
the interplay between denitrification in OMZs and N
2
-fixation elsewhere in the
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