Discrete suspended particles of barite and the barium cycle in the open ocean

Abstract

Barite particles are a universal component of suspended matter in the Atlantic and Pacific Oceans. This is demonstrated by scanning electron microscope and electron microprobe analyses of samples collected during the GEOSECS program. These discrete particles, about 1 μm in diameter, account for by far the greatest part of the total particulate barium of most of the filters collected at different depths. Total particulate barium (mean value: 20 ng/kg seawater) was measured on the same filters by instrumental neutron activation analysis. Several observations indicate that biochemical, rather than purely chemical, processes are involved in the formation of the BaSO 4 mineral in the water column. Sr/Ba molar ratios among the individual barite grains, particularly from surface waters are extremely variable, which would not be anticipated for purely chemical interactions. Barite crystals occurring within fecal debris have been observed throughout the water column. Within such debris decomposition of the abundant organic matter may provide the micro-environment predicted as necessary for the precipitation of BaSO 4. Finally, a strong correlation between nutrient content and particulate barium is found in the upper 1000 m of the water column, which also suggests a control of barite formation by biota. Some of the barite dissolves at depth in the water column. Dissolution rates were calculable for two GEOSECS stations, from which a dissolved barium flux of 0.4 μg/cm 2 yr was deduced. This figure is of the same order as the dissolved barium flux calculable from the barium content and known dissolution rates of calcareous and siliceous tests: approximately 0.5 μg/cm 2 yr. These fluxes represent the largest source of dissolved barium in the water column, the other being river input (0.6 μg/cm 2 yr). This supports the contention that the barium in the water column is mostly recycled. The residual flux of barite-Ba reaching the sea floor is of about equal importance as the flux of barium associated with fast-settling fecal material. These two sources together are almost sufficient to account for the total sedimentation rate of barium.