Particulate barium fluxes and their relationships to biological productivity

To understand better the processes that control the transport of particulate barium through the water column and its preservation in marine sediments, we measured particulate barium fluxes along an equatorial transect at 140°W using moored sediment traps. The fluxes of barium correlate strongly with the fluxes of organic carbon; however, this relationship is non-linear—higher carbon fluxes have proportionately less associated barium. As a result we observe spatial and temporal variations of roughly a factor of three in the barium-to-organic carbon ratio. Understanding this variability may help to define the processes that determine the geochemical behavior of Ba in the oceans. Several hypotheses that could influence the flux of Ba and its relationship to organic carbon flux have been proposed: barite formation in barium- and sulfate-enriched microenvironments formed during particle settling; lateral advection of carbon and barium from continental margins; the influence of seawater barium concentration; and Ba scavenging by aluminosilicates. Our study reveals temporal variability in the Ba/Corg values that occurs over timescales of less than one month. Also, depth profiles of carbon and Ba fluxes show that the variability originates at depths less than 1200 m and is conveyed throughout the water column. Both the rapid changes and the upper water column origin of the signals point to upper-ocean biological processes as the predominant cause of the variability in the barium-to-organic carbon ratios. We also observe, however, a 25% increase in Ba flux below 1200 m. The deep sources of Ba could result from barite formation linked to continued organic carbon degradation or from lateral sources of particulate barium. Because the spatial and temporal changes in Ba/Corg values correlated to changes in particulate opal and organic carbon fluxes, ocean ecology appears to have an important influence on barium fluxes. A better understanding of the processes that contribute to the particulate barium flux is needed before the accumulation of barium in marine sediments can be used as a quantitative proxy for ocean productivity.