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Biogeosciences An interactive open-access journal of the European Geosciences Union
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Volume 15, issue 2 | Copyright

Special issue: Progress in quantifying ocean biogeochemistry – in honour...

Biogeosciences, 15, 507-527, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 26 Jan 2018

Research article | 26 Jan 2018

Glacial–interglacial changes and Holocene variations in Arabian Sea denitrification

Birgit Gaye1, Anna Böll1, Joachim Segschneider2, Nicole Burdanowitz1, Kay-Christian Emeis1,3, Venkitasubramani Ramaswamy4, Niko Lahajnar1, Andreas Lückge5, and Tim Rixen1,6 Birgit Gaye et al.
  • 1Institute for Geology, Universität Hamburg, Bundesstraße 55, 20146 Hamburg, Germany
  • 2Institute for Geosciences, Universität Kiel, Ludewig-Meyn-Straße 10, 24118 Kiel, Germany
  • 3Institute of Coastal Research, Helmholtz Center Geesthacht, Max-Planck-Straße 1, 21502 Geesthacht, Germany
  • 4National Institute of Oceanography, Dona Paula, Goa, 403004, India
  • 5Bundesanstalt für Geowissenschaften und Rohstoffe, Stilleweg 2, 30655 Hannover, Germany
  • 6Leibniz-Zentrum für Marine Tropenforschung (ZMT) GmbH, Fahrenheitstraße 6, 28359 Bremen, Germany

Abstract. At present, the Arabian Sea has a permanent oxygen minimum zone (OMZ) at water depths between about 100 and 1200m. Active denitrification in the upper part of the OMZ is recorded by enhanced δ15N values in the sediments. Sediment cores show a δ15N increase during the middle and late Holocene, which is contrary to the trend in the other two regions of water column denitrification in the eastern tropical North and South Pacific. We calculated composite sea surface temperature (SST) and δ15N ratios in time slices of 1000 years of the last 25kyr to better understand the reasons for the establishment of the Arabian Sea OMZ and its response to changes in the Asian monsoon system. Low δ15N values of 4–7‰ during the last glacial maximum (LGM) and stadials (Younger Dryas and Heinrich events) suggest that denitrification was inactive or weak during Pleistocene cold phases, while warm interstadials (ISs) had elevated δ15N. Fast changes in upwelling intensities and OMZ ventilation from the Antarctic were responsible for these strong millennial-scale variations during the glacial. During the entire Holocene δ15N values  > 6‰ indicate a relatively stable OMZ with enhanced denitrification. The OMZ develops parallel to the strengthening of the SW monsoon and monsoonal upwelling after the LGM. Despite the relatively stable climatic conditions of the Holocene, the δ15N records show regionally different trends in the Arabian Sea. In the upwelling areas in the western part of the basin, δ15N values are lower during the mid-Holocene (4.2–8.2kaBP) compared to the late Holocene ( < 4.2kaBP) due to stronger ventilation of the OMZ during the period of the most intense southwest monsoonal upwelling. In contrast, δ15N values in the northern and eastern Arabian Sea rose during the last 8kyr. The displacement of the core of the OMZ from the region of maximum productivity in the western Arabian Sea to its present position in the northeast was established during the middle and late Holocene. This was probably caused by (i) reduced ventilation due to a longer residence time of OMZ waters and (ii) augmented by rising oxygen consumption due to enhanced northeast-monsoon-driven biological productivity. This concurs with the results of the Kiel Climate Model, which show an increase in OMZ volume during the last 9kyr related to the increasing age of the OMZ water mass.

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The Arabian Sea has one of the most severe oxygen minima of the world's oceans between about 100 and 1200 m of water depth and is therefore a major oceanic nitrogen sink. Stable nitrogen isotopic ratios in sediments record changes in oxygen concentrations and were studied for the last 25 kyr. Oxygen concentrations dropped at the end of the last glacial and became further reduced during the Holocene, probably due to the increasing age of the low-oxygen water mass.
The Arabian Sea has one of the most severe oxygen minima of the world's oceans between about 100...