The Ballast Effect in the Indian Ocean

Abstract. In this study, data obtained from a sediment trap experiments off South Java are analyzed and compared to satellite-derived information on primary production and data collected by deep-moored sediment traps in the Arabian Sea and the Bay of Bengal. The aim was to study the relative importance of primary production and the ballast effect on the organic carbon export and the CO₂ uptake of the biological carbon pumps. Therefore, data obtained from sediment trap experiments carried out in other ocean basins were also integrated into the data analysis and a four-box model was developed. Our data showed that the organic carbon flux in the highly-productive upwelling system in the Arabian Sea was similar to those in the low productive system off South Java. Off South Java as in other river-influenced regions, lithogenic matter supplied from land mainly controls the organic carbon flux via its ballast effect in sinking particles, whereas carbonate produced by marine organisms appears to be the main ballast material in the high productive regions. Since the carbonate flux tends to increase with an increasing export production, it is difficult to quantify the relative importance of productivity and the ballast effect on the organic carbon flux into the deep sea. However, the export of organic matter into the deep sea represents a loss of nutrients for the pelagic ecosystems, which needs to be balanced by mode water nutrient supply into the seasonal thermocline to sustain the productivity of the pelagic system. The amount of preformed nutrients utilized during the formation of the exported organic matter strongly influences the impact of the ballast effect on the CO₂ uptake of the organic carbon pump. Accordingly, this is stronger at higher latitudes where preformed nutrients are formed than at lower latitudes where the euphotic zone is nutrient depleted. Nevertheless, the ballast effect enhances the export of organic matter into the deep sea and favors the sedimentation of organic matter in river-influenced regions. Since globally > 80 % of organic carbon burial occurs in river-dominated systems, the lithogenic ballast is assumed to play an important role in the Earth’s climate system on geological time scales.