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

Abstract

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.

N<sub>2</sub>O changes from the Last Glacial Maximum to the preindustrial – Part 1: Quantitative reconstruction of terrestrial and marine emissions using N<sub>2</sub>O stable isotopes in ice cores

“…N 2 O production by nitrification increases with substrate availability and with decreasing O 2 concentration. The overall increase in marine N 2 O emission over the termination is as such consistent with paleoceanographic evidence showing an oxygen depletion across the deglaciation in the upper ocean (Jaccard and Galbraith, 2012;Moffitt et al, 2015), whereas changes in organic matter export as inferred from ocean sediments varied regionally (Kohfeld et al, 2005) and overall organic matter preservation was much higher during the LGM than the Holocene at low and mid latitudes (Cartapanis et al, 2016). The largest volumetric expansion of oxygen-depleted water masses is reconstructed for the onset of the BA, when oxygen concentrations decreased throughout the Indo-Pacific above 2500 m (Jaccard et al, 2014) and oxygen minimum zones expanded worldwide (Moffitt et al, 2015).…”

Reviews and syntheses: Present, past, and future of the oxygen minimum zone in the northern Indian Ocean

“…More vigorous upwelling, discernible from benthic foraminifera, may have led to a better ventilation of the basin as it is associated with an increased inflow of ICW from the south (Das et al, 2017). As discussed before, the inflow of ICW increases the phys- (Altabet et al, 2002), NIOP-905P (Ivanochko et al, 2005), SK148-55 (Kessarkar et al, 2018), and MD04-2876 (Pichevin et al, 2007) parallel with (c) sinking oxygen concentrations in biogeochemical model simulations driven by the Kiel Climate Model/PISCES model in the northern (yellow), eastern (red), western (blue), and southern Arabian Sea (dark grey). See text for definition of regions.…”

“…However, to our knowledge there is only one published sediment record from the Bay of Bengal spanning the entire Holocene (Contreras-Rosales et al, 2016), so we know nothing about the spatial variability within the basin. In contrast to the Bay of Bengal, denitrification in the Arabian Sea has prevailed during the warm interstadials of the Pleistocene and during the entire Holocene, as can be discerned from sedimentary δ 15 N values > 6 ‰, with maxima of > 11 ‰ (Agnihotri et al, 2003;Higginson et al, 2004;Kessarkar et al, 2018;Möbius et al, 2011;Pichevin et al, 2007). Productivity increased with the onset of the Holocene as the summer monsoon strengthened and monsoonal upwelling off Somalia and Oman commenced and became a permanent feature of the Holocene Arabian Sea (Böning and Bard, 2009;Gaye et al, 2018).…”

Present past and future of the OMZ in the northern Indian Ocean

“…In contrast to the Bay of Bengal denitrification has prevailed in the Arabian Sea during the warm interstadials of the Pleistocene and during the entire Holocene as can be discerned from δ 15 N >6 ‰ with maxima of >11 ‰ (Agnihotri et al, 2003;Higginson et al, 2004;Kessarkar et al, 2018;Möbius et al, 2011;Pichevin et al, 2007). Productivity increased with the onset of the Holocene as the summer monsoon strengthened and monsoonal upwelling off Somalia and Oman commenced and became a permanent feature of the Holocene Arabian Sea (Böning et al, 2009;Gaye et al, 2018).…”

“…show increasing denitrification since about 6000 -8000 years BP; data from the northern (yellow; light brown), eastern (red), western (blue) and southwestern (black) Arabian Sea. Sediment cores: SO9090-63KA (Burdanowitz et al, 2019), RC27-23 (Altabet et al, 2002), NIOP-905P (Ivanochko et al, 2005), SK148-55 (Kessarkar et al, 2018), MD04-2876 (Pichevin et al, 2007) (Table A1). The maps were produced with MATLALB.…”