Supra-generic radiolarian transfer function based late Miocene palaeomonsoonal changes in the northeastern tropical Indian Ocean

Abstract

A Q-mode factor analysis of the % data of 25 broadly defined supra-generic radiolarian groups in 42 deep-sea surface sediments produced four statistically significant and geographically distinct faunal assemblages related to (1) southwest cooler (27 degrees C), hypersaline (34.5 ppt) and highly productive (0.3 mgC/m sup(3)/d); (2) northeastern warmer (27 degrees C), hyposaline (34.5 ppt) and poor productive (0.2 mgC/m sup(3)/d); (3) transitional and (4) southern water masses during southwest monsoon in the central Indian Ocean. The southwest hypersaline and northeast hyposaline faunal assemblages are separated by a transitional assemblage that divides the Central Indian Ocean Basin diagonally along the 34.5 ppt isohaline south of Sri Lanka. The southern factor dominates the area influenced by the 10-12 degrees S hydrographic front. Resultant transfer functions have multiple correlation coeflicients (r) at 0.001 level of significance (99.9%) for the salinity, PPT and SST for the southwest monsoon only. Descending trend in the correlation coefficients for the salinity-July-August (r=0.834), PPT-SWM (r=0.785) SST-spring (May-July) (r=0.695) suggests order of the faunal sensitivity in the area and their respective reliabilities in the paleoestimates. SST and PPT changes using % data of the late Miocene radiolarian sequence from the Sawai Bay Formation, Neill Island, Andamans. Estimated SST and salinity during southwest monsoon are not only conformable with the qualitative paleoceanography in terms of warmer periods (W) of W-9 and W-11, but also have resolved W-8 and W-10 in late Miocene Neill Island radiolarian data-set. Salinity minima in warmer and maxima in colder periods reflect the influx of fresh water from the Ganges-Bramhputra rivers into the Bay of Bengal during southwest monsoon.