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| Titel |
Isotope fractionation and isotope decoupling during nitrate reduction in marine sediments |
| VerfasserIn |
Kirstin Dähnke, Bo Thamdrup |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250104233
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| Publikation (Nr.) |
 EGU/EGU2015-5425.pdf |
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| Zusammenfassung |
| In summer 2010, we sampled marine sediments in the Skagerrak, covering a gradient of
reactivity, oxygen consumption, and manganese concentration in the sediment. Along this
gradient, we aimed to evaluate links between nitrogen cycling and sediment properties. The
focus of the study was the interplay of nitrate and nitrite reduction rates and concomitant
nitrate and nitrite isotope changes in sediment incubations.
As expected, nitrate reduction was fastest in sediments with highest sediment reactivity
and oxygen consumption. At the shallower sampling sites, denitrification was the main
removal pathway of nitrate and nitrite, but acetylene inhibition experiments pointed towards
significant importance of anammox at the deepest site in the Skagerrak.
The N-isotope of denitrification effect varied with depth, with stronger N-isotope
fractionation at deeper, and less reactive, sites, and ranged from -12 to -16o. At the deepest
site in the Skagerrak, anammox was the dominant N2 production pathway. For this site, we
calculated the intrinsic isotope effect of anammox in marine sediments, and found that it is
~-15o, which is in accordance with recent culture studies.
The isotope effect of oxygen, however, was not consistent pattern along the gradient of
sediment reactivity. The oxygen isotope effect of nitrate reduction was entirely decoupled
from the nitrogen isotope effect. Surprisingly, this variability in oxygen isotope fractionation
was not linked to the occurrence of anammox, but rather to intermediate nitrite accumulation
in the anoxic incubations. Consequently, the ratio of 18É / 15É was highly variable in all
sediments we investigated.
We presume that such decoupling of oxygen and nitrogen isotopes is due to anoxic nitrite
oxidation, which rises in turn with nitrite accumulation in the sediment incubations.
These findings suggest that the ratio of 18É / 15É in marine environments is highly
flexible, and might, especially in regions with considerable nitrite accumulation,
not be a reliable indicator of nitrogen turnover processes in anoxic environments. |
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