Barium accumulation in the Arabian Sea: controls on barite preservation in marine sediments

Abstract

In this study, a new sequential extraction technique is used to investigate the particulate barium (Ba) phases in sediments from the Arabian Sea to determine the processes controlling Ba accumulation in marine sediments. The total solid-phase Ba concentration in Arabian Sea surface sediments increases with water depth from ∼200 ppm at 500 meters below sea surface (mbss) to ∼1000 ppm at 3000 mbss. The sedimentary Ba composition consists of three major fractions: barite, Ba incorporated in aluminosilicates, and Ba associated with Mn/Fe oxides. Accumulation of barite, which is the most important Ba fraction in sediments located below 2000 mbss, increases gradually with water depth. The Ba/Al ratio of the terrigenous fraction varies significantly across the Arabian Basin as the result of differences in grain size and provenance of the terrigenous sediment. Ba associated with Mn oxides is a relatively minor fraction compared with bulk Ba concentrations, and it only accumulates in well-oxygenated sediments below the present-day oxygen minimum zone. The water-depth–dependent accumulation of barite in the Arabian Sea is not related to the continuous formation of barite in settling organic particles or Ba scavenging by Mn oxyhydroxides but is primarily controlled by differences in Ba preservation upon deposition. A good correlation between the barite saturation index and the barite accumulation rate in the upper 2000 m of the water column may indicate that the degree of barite saturation of the bottom water is the main environmental factor regulating the burial efficiency of barite. Organic matter degradation, bioturbation, diagenetic Mn cycling, and the crystallinity of the accumulating barite may play additional roles.