The Ballast Effect of Lithogenic Matter and its Influences on the Carbon Fluxes in the Indian Ocean

Abstract. Data obtained from long-term sediment trap experiments in the Indian Ocean were analysed in conjunction with satellite-derived observations and results obtained from a box model to study the influence of primary production and the ballast effect on organic carbon flux into the deep sea. The results are used to better understand the associated impacts on the CO2 uptake of the organic carbon pump. In line with other findings derived from global data sets, our results suggest that a preferential export of organic matter in slower-sinking particles reduces the transfer efficiency of exported organic matter in high-productive systems compared with low-productive regions. The resulting enhanced respiration of organic matter maintains the high nutrient availability in the surface ocean and thus the high productivity during the summer and winter bloom in the Arabian Sea. In turn, mineral ballast is essential for the transport of organic matter and nutrients into the deep sea. The additional lithogenic ballast effect can increase organic carbon fluxes by 60 %. Our model results indicate that lithogenic ballast enhances the CO2 uptake of the organic carbon pump by increasing the amount of nutrients utilised by the organic carbon pump to bind CO2. By enhancing the export of organic matter into the deep sea, the ballast effect increases the residence time of these nutrients in the ocean. They lose the attached CO2 if they are introduced into the surface ocean at higher latitude, where the lack of light prevents photosynthesis in winter. Considering the impact of the lithogenic ballast effect on the organic carbon export into the deep sea and the enhanced mobilisation of lithogenic matter due to land-use changes, it is assumed that humans influence the CO2 uptake of the organic carbon pump, which might hold relevance for the discussion about the anthropogenic CO2 uptake of the ocean.