Sensing Seasonality in the Arabian Sea: a coupled δ¹⁸O-Mg/Ca approach

Abstract. Millennial scale climate variability punctuates the record of Northern Hemisphere glacials in the form of Dansgaard–Oeschger cycles. The coldest episodes within these cycles have a marked impact on the location of the Inter-tropical Convergence Zone and thereby the intensity and extent of the Asian monsoon system. Arabian Sea oceanography is closely linked to the monsoon cycle and responds rapidly to changes therein. Seasonally varying upwelling intensity, adhesive mixing and the resulting variability on sea surface temperature and salinity, however, make it challenging to precisely reconstruct the behaviour of the Indian monsoon system through the Dansgaard–Oeschger cycles. To reconstruct impact of millennial-scale changes in the monsoon on the Arabian Sea, paired single-specimen Mg/Ca and stable oxygen isotope analyses were performed on three species of planktonic foraminifera from the northern as well as the western Arabian Sea. Mismatches between Mg/Ca- and δ¹⁸O-derived temperatures reflect changes in salinity caused by regional variability in the hydrological cycle. Comparison of Mg/Ca-derived temperatures from Globigerinoides ruber, Globigerina bulloides and Neogloboquadrina dutertrei over Heinrich Event 4 and Interstadial 8 allows for reconstructing the seasonal range in temperatures (minima and maxima) and vertical stratification of the water column during these intervals. Results show that Arabian Sea surface water temperatures were lower during the North Atlantic Heinrich Event 4, and that temperatures in the northern and western Arabian Sea differed relatively much. This implies that the north east monsoon (i.e. boreal winter) was enhanced during this interval. In contrast, the summer upwelling related species G. bulloides reveals lower temperatures during Interstadial 8 in the western Arabian Sea, showing an enhanced summer monsoon during the warmers stages of millennial-scale climate variability. Together, these results show that summer and winter monsoon intensities were thus also anti-phased on a millennial scale. An intensified NE monsoon might have contributed to enhanced mixing of surface waters in the northern Arabian Sea.