A Late Quaternary Foraminiferal Record of Deep-Water Ventilation Changes below OMZ in the Northwest Arabian Sea

ABSTRACT This study is primarily based on the census data of late Quaternary foraminifera from Ocean Drilling Programme (ODP) hole 722B, which is situated beneath the Oxygen Minimum Zone (OMZ) at 2028 m of water depth in the northwest Arabian Sea near Owen Ridge. The purpose of this investigation is to improve our understanding of the history of deep-water ventilation in the northwest Arabian Sea under the combined influence of surface productivity produced by monsoonal upwelling and the northward outflow of deep water originating in the south. The prominent occurrence of Bulimina alazanensis-Globocassidulina subglobosa assemblage along with low organic matter influx at the seafloor during interglacial MIS 5 (including 5.5 and 5.1) and Holocene indicate better ventilated bottom water due to enhanced northward incursion of oxygen-rich North Atlantic Deep Water (NADW) up to the northwest Arabian Sea. On the other hand, enhanced upwelling caused by an intense southwest monsoon yields more productive surface water, as demonstrated by higher relative abundance of Globigerinita glutinata . Thus, despite the strong SW monsoon circulation and increased surface productivity, the higher organic matter remineralization under better deep-sea ventilation reduced the amount of available organic matter on the sea floor. The prevalence of Bulimina aculeata assemblage, increased abundance of dysoxic taxa and higher percentage of organic matter during latter part of Last Glacial Period (~55 to 15 ka BP) indicate decline in the deepwater ventilation and better trophic condition at the bottom possibly due to reduced incursion of NADW into the northwest Arabian Sea. In the deep-sea during glacial MIS 4 and cold stadial MIS 5.2, the prevalence of Bulimina aculeata and Uvigerina proboscidea assemblages along with higher abundance of shallow infaunal taxa shows a minor decline in the dissolved oxygen or nearly unchanged deep-water ventilation with moderately higher trophic levels.