Precessional forcing of coccolithophore assemblages in the northern Arabian Sea: Implications for monsoonal dynamics during the last 200,000 years

Abstract

The paleoceanographic potential of coccolithophores was used to decipher the monsoon induced paleoproductivity changes in the northern Arabian Sea during the past 200,000 years. Core SO 90–94 KL was taken in the north-eastern part of the study area representing the primarily NE-monsoon-influenced domain. For comparison, core ODP117–723 A from the western side of the Arabian Sea off Oman under the dominating influence of the SW-monsoon was investigated. Samples were prepared using a quantitative dilution/filtering technique and investigated in the SEM. We focus here only on the most important contributors to the assemblages Gephyrocapsa oceanica, Gephyrocapsa ericsonii, Emiliania huxleyi, Florisphaera profunda contributing at least 63% up to 92.5% of the coccolith assemblages. Abundances of the two species E. huxleyi and G. ericsonii were summed up and depicted as a combined record (EhuxGeric). The 200,000 year record from the north-eastern Arabian Sea yielded a significant Precession derived signal in the abundances of EhuxGeric which could be related by cross spectral analysis to maximum low latitudinal insolation. In contrast, G. oceanica seemed to correspond to insolation minima. Relative abundances of F. profunda are generally rather low compared to other oceanographic regions and exhibit no distinct variations except for the thermal maxima of isotope stages II/I and V. A schematic model is presented to show the changes in surface water stratification in the Arabian Sea driven by changing monsoon intensities over periodicities of the precessional band of Milankovitch cycles. This model is based on the variations of the two taxa groups G. oceanica and EhuxGeric together with changes in the abundance of Cocco total and F. profunda. Probably the Arabian Sea remained eutrophic at least during one season of the year despite drastic changes in the summer monsoon strength. Lower productivities during the summer monsoon were probably compensated by higher phytoplankton growth due to stronger winter monsoons.