Holocene strengthening of the Oxygen Minimum Zone in the northwestern Arabian Sea linked to changes in intermediate water circulation or Indian monsoon intensity?

A high resolution 14,000 year old record of benthic foraminifera from ODP Hole 723A was produced to understand changing intensity of the NW Arabian Sea Oxygen Minimum Zone (OMZ) and its links with intermediate water circulation and Indian Monsoon Intensity. During the latest Pleistocene, the OMZ was very weak marked by the abundance of oxic species. From early to mid Holocene period (up to 6500 years BP), intense SW monsoon-related high surface productivity led to the development of an OMZ in the Arabian Sea; however, presence of oxic and suboxic species indicate that OMZ intensification was minimized by cyclic incursions of oxygen-rich intermediate waters from the South. The middle Holocene was a transitional period to intense OMZ conditions. The intensification of the OMZ could be due to replacement of oxygen-rich southern source Sub-Antarctic Mode and Antarctic Intermediate Waters (SAMW-AAIW), with oxygen poor Red Sea Water (RSW) from the north during the middle Holocene. As the intensity of the summer monsoon weakened during the late Holocene (~ 4200 years BP onwards) the Arabian Sea OMZ intensified, suggesting total cut off from the oxygenated SAMW-AAIW. Spectral analysis of Information Function (H), benthic foraminifer Bolivina spp. and oxic species of benthic foraminifera indicate that the intensity of the OMZ and deep water conditions respond closely to solar cycles.