Late Quaternary Monsoonal Variations in the Western Arabian Sea Based on Cross-Spectral Analyses of Geochemical and Micropalaeontological Data (ODP Leg 117, Core 728A)

ODP Hole 728A (Leg 117), located at a semi-enclosed slope basin on the continental margin of Oman, underlies a seasonally monsoon-enhanced upwelling zone. The sediment composition reflects past environmental fluctuations which are linked to variations in the monsoon system. These include seasonal upwelling, palaeoproductivity, variation in the extent of the OMZ and aeolian input. The top-most Pleistocene (21.5 m; 524 ka) was sampled at 25-cm intervals for micropalaeontological, geochemical and stable isotope analyses. Twelve of the micropalaeontological and geochemical variables determined have been used as proxy indicators. These are assumed to be directly related to wind strength (Ti, Cr), biological productivity (CaC03, TOC, U, Ba, Br and I), intensity of the OMZ (planktonic foraminiferal tests whole-fragments ratio) and upwelling-induced productivity (δ 13C Globigerina bulloides-δ 13C Neogloboquadrina dutertrei). Examination for coherence and phase relationship with the SPECMAP stacked δ 180 record, using cross-spectral analyses, has revealed that productivity proxies vary in concert with OMZ proxies, and are coherent with the orbital cycles of the Milankovitch model. However, upwelling and aeolian intensity proxies may not be related directly to the orbital parameters, but show instead a more complicated behaviour.