Orbital-scale denitrification changes in the Eastern Arabian Sea during the last 800 kyrs

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.