Calcareous non-skeletal grains from northwestern Arabian Sea: insights into late Quaternary paleoclimate

Abstract

Calcareous non-skeletal particles studied from the ODP Hole 724C, located in the northwestern Arabian Sea, consist of peloids, limeclasts and partially coated grains. This manuscript is based on the origin of these particles and their correlation to the late Quaternary paleoclimatic changes. The Creamy-yellow to greyish white ellipsoidal peloids (regular and irregular/broken) and limeclasts are made up of skeletal fragments of coccolith plates with occasional authigenic dolomite rhombs and spherical framboidal pyrite set in micritic matrix. The partially coated grains have gypsum as a nucleus, and the cortex is compositionally almost similar to the composition of peloids and limeclasts. Although peloids look like faecal origin, their association with the limeclasts indicates more of floccules formed by the particle coagulation process during settling from suspension. All types of grains exhibit higher abundances during interglacial stages (including interstadials) and the Holocene. Lower abundances are marked during glacial stages and the majority of the millennial-scale cold events. The above observation indicates that strong monsoon waves and tidal currents during interglacial stages predominantly churned and abraded these grains to form ellipsoidal peloids and limeclasts, and also caused the creation of numerous broken, irregularly shaped, peloidal-sized grains. All kinds of grains deposited below the swell wave base during high sea levels are subsequently transported to the deep sea by low-density turbidity currents. The gypsum precipitated at the supratidal-intertidal zones (coastal sabkhas) during the dry glacial stages, when sea levels remained low. It is later eroded by sea waves during the interglacial stages and deposited as grains below the swell wave base, where they have been coated by the identical material that made peloids and limeclasts. The subsequent transportation, particularly during the interglacial stages, caused partial removal of the coating. The depth and overpressure of the water column above the sediment during the marine transgression of the interglacial stages caused precipitation of white chalk on non-skeletal grains. The chalk is partially preserved due to abrasion during transportation of the aforesaid calcareous grains. Quartz in the bulk sample, as well as in non-skeletal grains, is of detrital origin and transported from the adjoining Arabian Peninsula and Iran-Makran regions by the north-westerly and northerly winds, respectively. The orbital and suborbital (Milankovitch) precessional periodicities in the northwestern Arabian Sea controlled glacial-interglacial sea level changes, as well as monsoon upwelling-based biogenic productivity and the abundances of calcareous non-skeletal grains.