Sedimentary sources and processes in the eastern Arabian Sea: Insights from environmental magnetism, geochemistry and clay mineralogy

• Deep water sediments contain high proportion of magnetic minerals.
• Magnetic minerals in the SSD and SP size range.
• Magnetic properties significantly correlated with Al, indicating terrigenous source.
• OMZ characterized by high OC content.
• CaCO3 values high in shallow sediments when compared with deep-sea sediments.

The spatial distribution patterns of surficial sediment samples from different sedimentary domains (shallow to deep-sea regions) of the eastern Arabian Sea were studied using sediment proxies viz. environmental magnetism, geochemistry, particle size and clay mineralogy. Higher concentrations of magnetic minerals (high χlf) were recorded in the deep-water sediments when compared with the shallow water sediments. The magnetic mineralogy of one of the shallow water samples is influenced by the presence of bacterial magnetite as evidenced from the χARM/χlfvs. χARM/χfd biplot. However, the other samples are catchment-derived. The high correlation documented for χlf, anhysteretic remanent magnetisation (χARM) and isothermal remanent magnetisation (IRM) with Al indicates that the deep-sea surficial sediments are influenced by terrigenous fluxes which have been probably derived from the southern Indian rivers, the Sindhu (the Indus) and the Narmada-Tapti rivers. A lower Mn concentration is recorded in the upper slope sediments from the oxygen minimum zone (OMZ) but a higher Mn/Al ratio is documented in the lower slope and deep-sea sediments. Clay minerals such as illite (24–48.5%), chlorite (14.1–34.9%), smectite (10.6–28.7%) and kaolinite (11.9–27.5%) dominate the sediments of shallow and deep-sea regions and may have been derived from different sources and transported by fluvial and aeolian agents. Organic carbon (OC) data indicate a low concentration in the shallow/shelf region (well oxygenated water conditions) and deeper basins (increased bottom-water oxygen concentration and low sedimentation rate). High OC concentrations were documented in the OMZ (very low bottom-water oxygen concentration with high sedimentation rate). The calcium carbonate concentration of the surface sediments from the continental shelf and slope regions (<1800 m) up to the Chagos-Laccadive Ridge show higher concentrations (average = 58%) when compared to deep basin sediments (average = 44%). Our study demonstrates that particle size as well as magnetic grain size, magnetic minerals and elemental variations are good indicators to distinguish terrigenous from biogenic sediments and to identify sediment provenance.

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