Glacial-Interglacial changes and Holocene variations in Arabian Sea denitrification

Abstract

<p><strong>Abstract.</strong> At present the Arabian Sea has a permanent oxygen minimum zone (OMZ) at water depths between about 100 m and 1200 m. Active denitrification in this OMZ is recorded by enhanced δ<sup>15</sup>N values in the sediments. Sediment cores show a δ<sup>15</sup>N increase from early to late Holocene which is contrary to the trend in other regions of water column denitrification. We calculated composite sea surface temperature (SST) and δ<sup>15</sup>N in time slices of 1000 years of the last 25 ka to better understand the reasons for the establishment of the Arabian Sea OMZ and its response to changes in the Asian monsoon system. Pleistocene stadial δ<sup>15</sup>N values of 4–6 ‰ suggest that denitrification was inactive or weak. During interstadials (IS) and the entire Holocene, δ<sup>15</sup>N values of > 7 ‰ indicate enhanced denitrification and a stronger OMZ. This coincides with active monsoonal upwelling along the western margins of the basin as indicated by low SST. Stronger ventilation of the OMZ in the early to mid-Holocene period during the most intense southwest monsoon and vigorous upwelling is reflected in lower δ<sup>15</sup>N compared to the late Holocene. 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 last 4–5 ka. This was probably caused by (i) rising oxygen consumption due to enhanced northeast monsoon driven biological productivity, in combination with (ii) reduced ventilation due to a longer residence time of OMZ waters.</p>

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

“…This feedback mechanism might have played an important role in maintaining the hypoxic conditions within the Arabian Sea and Bay of Bengal OMZ by preventing the development of anoxic conditions. As discussed in the following chapters this also agrees with model and paleoceanographic results suggesting that variations of the physical oxygen supply rather than changes in the biological oxygen demand are drivers controlling the intensity of the OMZ in both the Arabian Sea and the Bay of Bengal (Gaye et al, 2018;McCreary Jr et al, 2013;Resplandy et al, 2012).…”

“…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). A rise of δ 15 N by at least 2 ‰ shows that onset of upwelling immediately strengthened the OMZ and led to https://doi.org/10.5194/bg-2020-82 Preprint.…”

“…Early diagenesis may raise sedimentary δ 15 N values by 2-5 ‰ and the diagenetic effect increases with water depth (Altabet, 2006;Tesdal et al, 2013). Nevertheless, the relative changes of δ 15 N in deep-sea sediments record variations in the OMZ intensity while records from the continental slopes are subjected to negligible diagenetic enrichments so that they retain the signal of the nitrogen source (Altabet et al, 1999;Gaye et al, 2018).…”

Intensification and deepening of the Arabian Sea oxygen minimum zone in response to increase in Indian monsoon wind intensity

“…This study also finds an intensification of land-sea thermal and pressure gradients that is consistent with the Bakun (1990) hypothesis. Other studies reveal that summer monsoon wind intensification has already been observed recently (Goes et al, 2005;Wang et al, 2013). Goes et al (2005) reported an increase in upwelling-favorable winds off Somalia over the period between 1997 and 2005 and have shown an associated increase in productivity over the same period.…”

“…Other studies reveal that summer monsoon wind intensification has already been observed recently (Goes et al, 2005;Wang et al, 2013). Goes et al (2005) reported an increase in upwelling-favorable winds off Somalia over the period between 1997 and 2005 and have shown an associated increase in productivity over the same period. Wang et al (2013) found a global intensification of the Northern Hemisphere summer monsoon since the late 1970s and attributed these trends to the interaction of the mega El Niño-Southern Oscillation and the Atlantic Multidecadal Oscillation, in addition to hemispherical asymmetric global warming.…”