Controls of contrasting provenance and fractionation on the sediment magnetic records from the Bay of Bengal

Abstract

We present the results of rock magnetic, granulometric, microscopy and mineralogical analyses of shelf sediments off Krishna, Godavari, and Mahanadi Rivers, Bay of Bengal. The aim of the study was to assess the influence of contrasting lithologies and hydrodynamic controlled grain size fractionation processes on the magnetic properties of shelf sediments. Sediment magnetic mineralogy mainly consist of ferrimagnetic titanomagnetite grains of detrital origin with variable concentrations and grain sizes. A clear trend of alongshore (SE - NW directed) decrease in magnetite concentration followed by fining of magnetic grain size can be attributed to the enhanced change in magnetic contribution from Deccan basaltic (mafic) to granitic (felsic) gneissic dominated sediment province coupled with differential sediment fluxes at each site. Bivariate plot of magnetic and grain size parameters clearly records such shift in sediment provenance and differential transport mechanism of magnetic particles at the studied sites. Magnetogranulometric ratio (ARM/IRM) positively correlates with the magnetite content in majority of the studied samples. Higher magnetic susceptibilities were found associated with coarser magnetic particles. Granulometry of the magnetic fraction showed that magnetic grain size is independent of physical grain size above >40 ?m. The non-monotonous relationship between the magnetic and physical grain size can be explained by the fact that few individual finer particles occurred as magnetic inclusions protected within host (coarser silt and sand) fractions. Rockmagnetic and granulometry based approach presented in this study bears high potential for efficient tracking of sediment provenance and fractionation mechanism of magnetic particles in the shelf system, where contrasting sediment sources and complex transport dynamics create a large variability in magnetite content and grain size