Sea surface temperatures during the SW and NE monsoon seasons in the western Arabian Sea over the past 20,000 years

The western Arabian Sea is influenced by the seasonal southwest and northeast monsoon wind systems. In the modern day, SW monsoon induced upwelling of cold water leads to lowest SSTs in summer, while in glacial periods, weakened upwelling and increased cooling by NE monsoon winds may have resulted in lowest SST in winter. In order to reconstruct the western Arabian Sea seasonal sea surface temperature (SST) evolution during the monsoons over the past 20,000 years, a novel proxy is used that captures the contrast in seasonal SSTs. The presented proxy concerns the difference in δ18O between two shallow dwelling species of planktonic foraminifera; Globigerinoides ruber, which calcifies throughout the year, and Globigerina bulloides, which mainly calcifies during SW monsoon driven upwelling. We convert this δ18O difference to the difference in calcification temperature (ΔTcrub− bul), and use it in combination with Mg/Ca-derived temperatures to reconstruct seasonal SSTs. We present the monsoon evolution record of Core NIOP929, located NW of the island of Socotra. Based on our results we distinguish three distinct modes. I) A glacial monsoon mode (20–13 ka BP) characterised by weakly negative ΔTcrub− bul values, and SW monsoon SSTs (TSW) that are ∼ 2 °C higher than NE monsoon SSTs (TNE). This pattern is indicative for a weak SW monsoon and stronger cooling by glacial NE monsoon winds compared to the modern situation. II) A transitional mode (13–8 ka BP) characterised by ΔTcrub−bul values around 0, and thus a reduced difference between TSW and TNE. III) The modern monsoon mode, which started at 8 ka BP with the main shift toward low summer SSTs and high winter SSTs. The modern mode is characterised by ΔTcrub− bul values around + 4 °C, indicative of a strong SW monsoon associated with strong upwelling, and weak influence of the NE monsoon on SST. We propose that measuring Mg/Ca on both species will improve the accuracy of the SST estimates.