Variations in Denitrification and Ventilation Within the Arabian Sea Oxygen Minimum Zone During the Holocene

The continental slope of India is exposed to an intense perennial oxygen minimum zone (OMZ) supporting pelagic denitrification. Sediments that are presently in contact with the lower boundary of the denitrification zone indicate marked changes in the intermediate and bottom waters ventilation of OMZ...

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Published in:Geochemistry, geophysics, geosystems : G3 geophysics, geosystems : G3, 2018-07, Vol.19 (7), p.2179-2193
Main Authors: Kessarkar, Pratima M., Naqvi, S. W. A., Thamban, M., Fernandes, Lina L., Siebert, Christopher, Rao, V. Purnachandra, Kawahata, H., Ittekkot, V., Frank, Martin
Format: Article
Language:English
Subjects:
Arabian Sea
Continental slope
Denitrification
Deoxygenation
Holocene
Ice cores
Isotope composition
Isotopes
Mo isotopes
Nitrous oxide
Oxygen
oxygen minimum zone
Sediment
Sediments
Ventilation
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Summary:The continental slope of India is exposed to an intense perennial oxygen minimum zone (OMZ) supporting pelagic denitrification. Sediments that are presently in contact with the lower boundary of the denitrification zone indicate marked changes in the intermediate and bottom waters ventilation of OMZ during the past 9,500 years. The δ15N of sediment suggests that the OMZ waters were less ventilated during the early Holocene (between 9.5 and 8.5 ka BP) resulting in intensified denitrifying conditions with an average δ15N value of 7.8‰, while at the same time stable Mo isotope composition (average δ98Mo of −0.02‰) indicates that the bottom waters that were in contact with the sediments were better oxygenated. By the mid‐Holocene OMZ became more oxygenated suppressing denitrification (average δ15N of 6.2‰), while bottom waters gradually became less oxygenated (average δ98Mo of 1.7‰). The mid‐Holocene reduction in denitrification coincided with a global decrease in atmospheric N2O as inferred from ice core records, which is consistent with a decreased contribution from the Arabian Sea. Since ~5.5 ka BP OMZ waters have again been undergoing progressive deoxygenation accompanied by increasing denitrification. Key Points Sedimentary delta15N and delta98Mo suggest less ventilated intermediate waters and more oxygenated bottom waters during early Holocene Reorganized subsurface circulation during mid‐Holocene let to more oxygenated intermediate waters and anoxic bottom waters Weaker denitrification was associated with stronger summer monsoon and lower N2O levels in the atmosphere
ISSN:1525-2027
1525-2027
DOI:10.1029/2017GC007286