Quaternary radiolarian faunal changes in the tropical Indian Ocean: Inferences to paleomonsoonal oscillation of the 10 degrees S hydrographic front

Abstract

The northern Indian Ocean is characterized by three distinct surface water masses, i.e. (1) highly saline (greater than 34.5 ppt) Arabian Sea, (2) low saline (less than 34.5 ppt) Bay of Bengal and (3) a moderate salinity Indian Ocean watermass south of 10 degrees S hydrographic front. Hence, changes in the surface salinity in this region are index for the paleomonsoon. Radiolaria, the oceanic microplanktons, are extremely sensitive to the monsoonal salinity, temperature and productivity in the tropical Indian Ocean. We refined the earlier radiolarian factor assemblage model by relinquishing the rare groups (less than 2% abundance) that caused noise and suppressed the environmental signals. Refined analysis resulted in an additional factor characterizing the 10 degrees S hydrographic front. The 4-factor based radiolarian model exhibited better relationship with the surface hydrographic characteristics of the overlying watermasses. Changes in downcore factor values in a sediment core SK/1, which has an excellent geological record for the last 0.2-1.4 million years. Down-core factorial fluctuations corroborate to changes in the paleomonsoon. Conspicuous variation in the values of factor-4 (0.2-0.6) characterized the oscillation of 10 degrees S hydrographic front due to changes in the regional oceanic precipitation over the core-site coupled with variations in the monsoonal fresh water debauched by the Indian rivers into the Bay of Bengal. The frontal oscillation indicated an apparent cyclicity of approx 120,000 years (plus or minus 20,000), which corresponds to the approx 100,000 years Earth's orbital eccentricity cycle during the Quaternary.