In the Arabian Sea, South Asian monsoon (SAM)-induced high surface water productivity coupled with poor ventilation of intermediate water results in strong denitrification within the oxygen minimum zone (OMZ). Despite the significance of denitrification in the Arabian Sea, we have no long-term record of its evolution spanning the past several million years. Here, we present the first record of denitrification evolution since Late Miocene (~10.2 Ma) in the Eastern Arabian Sea, where the SAM generates moderate surface water productivity, based on the samples retrieved during the International Ocean Discovery Program (IODP) Expedition 355. We find that (i) the SAM was persistently weaker from ~10.2 to 3.1 Ma; it did not intensify at ~8 Ma in contrast to a few previous studies, (ii) on tectonic timescale, both the SAM and the East Asian Monsoon (EAM) varied synchronously, (iii) the first evidence of denitrification and productivity/SAM intensification was at ~3.2–2.8 Ma that coincided with Mid-Pliocene Warm Period (MPWP), and (iv) the modern strength of the OMZ where denitrification is a permanent feature was attained at ~1.0 Ma.

Denitrification in the Arabian Sea is closely related to the monsoon-induced upwelling and subsequent phytoplankton production in the surface water. The δ15N values of bulk sediments collected at Site U1456 of the International Ocean Discovery Program (IODP) Expedition 355 reveal the orbital-scale denitrification history in response to the Indian Monsoon. Age reconstruction based on the correlation of planktonic foraminifera (Globigerinoides ruber) δ18O values with the LR04 stack together with the shipboard biostratigraphic and paleomagnetic data assigns the study interval to be 1.2 Ma. Comparison of δ15N values during the last 800 kyrs between Site U1456 (Eastern Arabian Sea) and Site 722B (Western Arabian Sea) showed that δ15N values were high during interglacial periods, indicating intensified denitrification, while the opposite was observed during glacial periods. Taking 6‰ as the empirical threshold of denitrification, the Eastern Arabian Sea has experienced a persistent oxygen minimum zone (OMZ) to maintain strong denitrification whereas the Western Arabian Sea has undergone OMZ breakdown during some glacial periods. The results of this study also suggests that five principal oceanographic conditions were changed in response to the Indian Monsoon following the interglacial and glacial cycles, which controls the degree of denitrification in the Arabian Sea.

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The Mi-1 glaciation (ca. 23 Ma), which marks the Oligocene-Miocene boundary, was an aberrant cooling event that led to a build-up of the Antarctic ice sheet, which reached the near-modern volume (or greater) from its ephemeral or partial existence. An increase of ~1‰ in the d 18 O of benthic foraminifera during this interval has been attributed to the development of Antarctic ice sheets and deep-water cooling. Without definitive evidence, Northern Hemisphere (NH) glaciation has not been a material consideration for the d 18 O increase. Here we investigate the interhemispheric temperature contrast during Mi-1, with the movement of the Intertropical Convergence Zone (ITCZ) at a site (10°31′N) in the East Pacific (Integrated Ocean Drilling Program Site U1333), to understand NH cooling and the possibility of NH glaciation. The measured 143 Nd/ 144 Nd, 87 Sr/ 86 Sr, and clay mineral compositions of eolian dust fractions indicate unequivocally the deposition of Asian dust during Mi-1, and of Central American and South American dust before and after Mi-1. This is attributed to the southward displacement of the ITCZ over Site U1333 during Mi-1. The ITCZ shifts toward the warmer hemisphere. Thus our results suggest that the cooling during Mi-1 was more significant in the NH than in the Southern Hemisphere, which underwent a sudden expansion of continental ice sheets. Our data call for a forcing mechanism to drive significant NH cooling during this episode. Based on the available data, we propose that the widespread growth of NH ice sheets and/or changes in the production of North Atlantic-origin deep water could be possible causes of the NH cooling at that time.

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