A high resolution, multi-proxy approach to reconstructing Arabian Sea Oxygen Minimum Zone dynamics

Abstract

A new deep-sea core from the base of the Arabian Sea Oxygen Minimum Zone (OMZ) spanning the last glacial (75-10 ka) has been analyzed for changes in OMZ intensity. Processes maintaining the Arabian Sea OMZ are surface productivity, stratification of the water mass and advection of oxygen poor deep waters. Productivity is controlled by the strength of the Atlantic Meridional Overturning Circulation (AMOC) and the SW and NE summer and winter monsoons. Minimal OMZ conditions are reconstructed from pteropod and Globorotalia abundance records which reflect the CO32- saturation state of the bottom waters and the depth of convectional overturning of the surface waters, respectively. The three periods of minimal OMZ intensity which are found in the record lag precession maxima by ~7,000 years. A similar lag has been found for minimum AMOC strength. Thus, we conclude Arabian Sea OMZ variability is mainly driven by changes in the AMOC rather than the monsoons. Comparing the PASOM 3 bromine and Corg record with similar records from Murray Ridge stations spanning water depths ranging from 565-2387 m reveals that individual minima and maxima can be correlated in great detail. Shallow stations show sharp minima and maxima while records from deeper stations are much smoother. The PASOM 3 core shows a transition between these two types of records, reflecting the close proximity of the core site to the lower OMZ boundary. Due to the absence of benthic foraminiferal data we do not know whether the OMZ boundary dropped to below the core site at 1172 m depth during the last glacial. A high resolution study has been conducted for the period around Heinrich event 4 (~46-38 ka). Micropaleontological and geochemical proxies are used to reconstruct OMZ variability in this interval. Results show two periods of well oxygenated deep waters (~44-43 & ~40.5-39 ka), occurring during stadial 11 and Heinrich event 4. The composition of the benthic assemblage shows that the base of the OMZ never dropped below the core site at 1172 m during the studied period. Deep overturning of the surface waters, reduced productivity and advection of oxygen-rich Indian Central Water related to the weakening of the AMOC are induced to explain the weakening of the OMZ during H4.