First evidence of denitrification vis-à-vis monsoon in the Arabian Sea since Late Miocene

Oxygen minimum zones (OMZ) is the regions of oxygen deficient (O2 < 20 μM) water located in the tropical oceans, which have been proposed to expand in the present scenario of global warming. OMZs play a significant role in producing N2O (a powerful greenhouse gas through the process of denitrification), when the dissolved O2 levels fall below 1 µΜ. A perennial OMZ develops between 150 and 1000 m water depth in the Arabian Sea due to various natural factors such as high surface water productivity and reduced ventilation. The anoxic centers of these OMZs occupy only ~0.8% of the world ocean but are responsible for the highest production of N2 through denitrification (~35% of the global production) out of which the Arabian Sea contributes the largest proportion (~17% of global N2 production).

This study give a synoptic view of long-term evolution of OMZ spanning the past several million years, from the Eastern Arabian Sea. The squeeze cake samples from Site U1456 (sedimentation rate ~10 cm kyr -1) in the Eastern Arabian Sea, were analyzed during the IODP Expedition 355.

The long-term OMZ variability and its coupling with surface water productivity were established by analyzing multiple isotopic and geochemical proxies viz. δ 15N, δ 13C, TOC, TN, and C/N ratio of sedimentary organic matter (SOM).

Our record from Site U1456 spans ~10.2 to 0.03 Ma, but includes several hiatuses dated to ~8.2–9.2 Ma, ~3.7–5.4 Ma, and ~1.6–2.2 Ma. Nevertheless, we interpret that surface water productivity in the Eastern Arabian Sea was low from 10.15 Ma to 3.2 Ma as evident from uniformly low values of TOC and TN.

During this period, the δ 15N did not reach the threshold value (~6‰) indicative of denitrification. This implies that neither the surface water productivity (TOC, TN) nor the OMZ intensity supports any major intensification in SAM strength from ~10 to ~3.2 Ma, which is also documented in the different regions (the South China Sea, the Northern Arabian Sea and the Bay of Bengal). We find that the SAM was weak at ~10 Ma indicating that East Asian Monsoon (EAM) and SAM varied in consonance, without any apparent time lag, on tectonic timescale. At around 8 Ma, δ 15N values vary between 3.7‰ to 5.8‰, i.e., the OMZ was not intense enough to cause denitrification and the surface water productivity was diminished, which implies that SAM did not intensify at ~8 Ma. During the study period, for the first time, the OMZ intensified to the level that denitrification takes place was at ~3.2–2.8 Ma. Recently investigated that the role of the Tibet Plateau in affecting SAM, and found that it simply acts as a physical barrier for northerly cool, dry winds. Its role as an elevated heat source is of secondary importance in affecting the SAM. EAM dynamics is also affected by the Tibet Plateau, which is located in the path of subtropical jet streams. The increase in both the EAM and SAM during ~3.6–2.6 Ma could have resulted in the increased weathering and organic carbon burial, as evident by higher TOC leading to atmospheric CO2 drawdown that would have possibly contributed to Northern Hemisphere Glaciation (NHG) at 2.7 Ma. Thereafter, from 2.8 Ma to ~1.0 Ma, δ 15N values as well as the surface water productivity declined in parallel, indicating relatively weaker SAM. Previous studies also reported the weakened EAM and SAM after ~2.6 Ma, confirming our results, which coincides with the onset of NHG. Finally, the OMZ reached its modern strength, i.e., denitrification became a permanent feature, at about ~1.0 Ma closely following the enhanced surface water productivity. It implies that SAM intensified from ~1.0 Ma as reported in earlier studies viz. the enhanced sedimentation rate in the Indus Fan, the increased chemical weathering from the Bengal Fan and the South China Sea, the rise of magnetic susceptibility and mean sediment flux from the Indian Ocean.

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