Calcareous peloids in the north-western Arabian Sea: implications of late Quaternary marine sedimentation and paleoclimate

Yellowish to brownish grey, medium to fine sand-sized predominantly ellipsoidal shaped grains, here termed peloids, are studied from the core samples of ODP site 723A (latitude 18° 03.079′ N and longitude 57° 36.561′ E; water depth 808m), north-western Arabian Sea for their origin and correlation to the intensity of transporting media and paleoclimatic changes during the past 150kyrs. The total carbonate, organic matter, acid insoluble residue, magnetic susceptibility and oxygen isotope ratio data are also utilized to corroborate the above objectives. The ellipsoidal grains appeared as calcareous peloid dominantly composed of coccoliths with few micro-foraminifers tests set in micritic matrix. Dolomite rhombs are present almost in every peloids studied. Mineralogically they are dominantly made up of calcite with very small amounts of dolomite, gypsum and quartz. The predominance of coccolith plates in these peloidal grains indicates that they are probably formed from faecal matter of small herbivorous planktonic organisms that feed selectively on coccolithophorids. Alternatively, these peloids may be floccules formed by coagulation of grains during settling process. They were deposited on the sea floor below swell wave base, predominantly during the interglacial stages, when the sea water level was high. These grains were later eroded and churned by waves and tidal processes, subsequently transported (as suspension-rolling sediments) to the present site by low density turbidity current activated due to strong monsoon. The strong wave energy further caused more fragmentation of particles during the same stages. The silt to clay-sized quartz and gypsum in them indicate detrital transport by north-westerly wind originated from the adjoining Arabian Peninsula. Dolomite is authigenic origin. Although wind becomes an important agent for detrital sediment incorporation into the north-western Arabian Sea during glacial stages and higher sedimentation rate, peloidal grain contribution appears to be less affected by it. Moreover, relatively lower peloid contribution during glacial stages also indicates gradual decline of sea level and lesser intensity of monsoon generated waves and tidal processes. The millennial-scale climatic events have insignificant role in peloid contribution, its mean grain size, sorting and roundness. The peloid-based parameters show cycles of dominant climatic periodicities at Milankovitch (Precession) and sub-Milankovitch time-scales. These