Mineralogical (X-ray diffraction, FTIR spectroscopy analysis), magnetic mineral and Sr-Nd isotopic analyses have been carried out on the carbonate-free fraction of three gravity cores from the Andaman Sea and Bay of Bengal. The purpose is to set up proxies indicative of the intensity of chemical weathering and erosion of the Himalaya and to relate this erosional pattern to climatic parameters such as monsoon intensity. Thesources of detrital material are the Irrawaddy/Salween rivers for two of the cores and the Ganges/Brahmaputra rivers for the third one. The time scale is based on d18O of planktonic and benthic foraminifera and ASM 14C ages. These three cores record two climatic cycles. The clay fraction < 2mm contains chlorite, illite, mixed layer, smectite and kaolinite. When compared to interglacial periods in which smectite and mixed layer clays dominate, glacial ones show enrichment in illite, chlorite and quartz.
The magnetic mineralogy was determined by the measurement of various magnetic properties of the sediments (hysteresis parameters, K, NRM, ARM, IRM, SIRM, S ratio). Titanomagnetite, grain size varies depending on climatic regime. Interglacial periods are characterised by small magnetic grains sizes and glacial periods by coarser grains. These variations in grain size may be modulated by processes of transport and/or by chemical alteration inducing a magnetic grain dissolution. In two cores from the Andaman Sea, eNd implies that both glacial and interglacial sediments share a common crustal source. In contrast, a strong climatic signal is recorded by 87Sr/86Sr ratios: interglacial periods yield values between 0.714 and 0.717, whereas glacial periods show higher values between 0.718 and 0.719. When plotted versus 87Rb/86Sr, two pseudo-isochrons give apparent ages of 153±39 Ma (87Sr/86Sr × 0.7082±0.0019) and 30±5 Ma (87Sr/86Sr × 0.71654 ±0.00036) for interglacial and glacial periods, respectively. The apparent age of 30±5 Ma is close to an Himalayan collision age and is interpreted as buffered by primary minerals. The age of 153±39 Ma is without any geological meaning and is interpreted as a mixing line due to the increased chemical weathering during warm periods on the continent, weathering which releases preferentially alkaline elements such as Rb and associated radiogenic 87Sr.
When compared with the time scale and d18O pattern, it appears that the characteristics of the detritic material are affected both by moonsoon and global climate. Each proxy gives, however, a different response to the various forcings.