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At present the Arabian Sea has a permanent oxygen minimum zone (OMZ) at water depths between about 100 m and 1200 m. Active denitrification in this OMZ is recorded by enhanced δ15N values in the sediments. Sediment cores show a δ15N increase from early to late Holocene which is contrary to the trend in other regions of water column denitrification. We calculated composite sea surface temperature (SST) and δ15N in time slices of 1000 years of the last 25 ka to better understand the reasons for the establishment of the Arabian Sea OMZ and its response to changes in the Asian monsoon system. Pleistocene stadial δ15N values of 4-6 ‰ suggest that denitrification was inactive or weak. During interstadials (IS) and the entire Holocene, δ15N values of > 7 ‰ indicate enhanced denitrification and a stronger OMZ. This coincides with active monsoonal upwelling along the western margins of the basin as indicated by low SST. Stronger ventilation of the OMZ in the early to mid-Holocene period during the most intense southwest monsoon and vigorous upwelling is reflected in lower δ15N compared to the late Holocene. The displacement of the core of the OMZ from the region of maximum productivity in the western Arabian Sea to its present position in the northeast was established during the last 4-5 ka. This was probably caused by (i) rising oxygen consumption due to enhanced northeast monsoon driven biological productivity, in combination with (ii) reduced ventilation due to a longer residence time of OMZ waters. [ABSTRACT FROM AUTHOR]

The Indian summer monsoon (ISM) brings most of the annual precipitation to the densely populated region in southern Asia. For the agricultural development and economic prosperity of the region, it is therefore vital to assess the variability of the monsoon system on societal relevant decadal- to centennial time scales. This might help to better understand how potential driving forces might be controlling ISM variability and how it might develop under future climate scenarios. Here we present a study of a sediment core from the northern Oman margin, revealing early- to mid Holocene ISM conditions on a near 20-year resolution. We assess multiple independent proxies indicative of sea surface temperatures (SST) during the upwelling season together with bottom water conditions. We use geochemical parameters, transfer functions of planktic foraminiferal assemblages and Mg/Ca paleothermometry and find evidence corroborating previous studies that upwelling intensity varies significantly in coherence to solar sunspot cycles. The dominant ~80-90-year Gleissberg cycle was apparently also affecting bottom water oxygen conditions. Although the interval from 8.4 to 5.8 ka B.P. is relatively short, the gradually decreasing trend of summer monsoon conditions was interrupted by short events of intensified ISM conditions. Results from both independent SST proxies are linked to phases of weaker OMZ conditions and enhanced carbonate preservation. This indicates that atmospheric forcing was intimately linked to bottom water properties and state of the OMZ on decadal time scales. [ABSTRACT FROM AUTHOR]