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nitrogen, isotopes, nitrification, Mediterranean Sea, atmospheric
input, 0412 Biogeochemical kinetics and reaction modeling, 0469 Nitrogen
cycling, 0460 Marine systems, 0454 Isotopic composition and chemistry
Abstract:
The eastern Mediterranean Sea is an unusually nutrient-poor oceanbasin
where the 15N/14N isotope ratios in many compartments of reactivenitrogen
are lower than in comparable oceanic settings. To elucidatepossible
reasons, we determined stable isotope ratios in nitrate,suspended
particulate nitrogen (PN), and total dissolved reducednitrogen for
stations across the eastern Mediterranean Sea occupiedin January
and February 2007; sinking PN was collected at one ofthe stations
in the period from February to September 2007. The δ15Nlevels of
all reactive N compartments in waters of the basin arevery low (grand
average 2.6‰) compared to other oceanic environments.Deep water nitrate
below 500 m water depth (δ15N = 2.2 \pm 0.3‰)was more depleted
in 15N than nitrate generally found in deep waternitrate pools of
other oceans (δ15N ranges from 4.7 to 5.4‰), whereas15N was enriched
in suspended particulate N (δ15N = 7.3 \pm 0.8‰)and reduced dissolved
N (δ15N = 5.7 \pm 3.8‰) compared to nitrateand sinking particulate
N intercepted in sediment traps (δ15N = 0.9\pm 0.8‰). We infer
that extensive mineralization is the causeof the isotopic makeup
of reactive N in deep water, in concert withthe lack of water column
denitrification. Nitrogen and oxygen isotoperatios in nitrate of
the mixed layer suggest an external source ofnitrate depleted in
15N, probably anthropogenic NOx rather than fixednitrogen. To explain
the observed isotope anomaly in the mixed layer,either the ammonium
formed by the breakdown of organic matter mustbe predominantly nitrified,
or atmospheric NOx characteristicallyenriched in 18O was present.
