Alkenone SST and stable nitrogen isotopes in Arabian Sea sediments

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 d15N values in the sediments. Sediment cores show a d15N 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 d15N 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 d15N values of 4-6 per mil suggest that denitrification was inactive or weak. During interstadials (IS) and the entire Holocene, d15N values of >7 per mil 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 d15N 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.