Abstract. Data obtained from long-term sediment trap experiments in the Indian Ocean in
conjunction with satellite observations illustrate the influence of primary
production and the ballast effect on organic carbon flux into the deep sea.
They suggest that primary production is the main control on the spatial
variability of organic carbon fluxes at most of our study sites in the Indian
Ocean, except at sites influenced by river discharges. At these sites the
spatial variability of organic carbon flux is influenced by lithogenic matter
content. To quantify the impact of lithogenic matter on the organic carbon
flux, the densities of the main ballast minerals, their flux rates and
seawater properties were used to calculate sinking speeds of material
intercepted by sediment traps. Sinking speeds in combination with
satellite-derived export production rates allowed us to compute organic
carbon fluxes. Flux calculations imply that lithogenic matter ballast
increases organic carbon fluxes at all sampling sites in the Indian Ocean by
enhancing sinking speeds and reducing the time of organic matter respiration
in the water column. We calculated that lithogenic matter content in
aggregates and pellets enhances organic carbon flux rates on average by
45 % and by up to 62 % at trap locations in the river-influenced
regions of the Indian Ocean. Such a strong lithogenic matter ballast effect
explains the fact that organic carbon fluxes are higher in the low-productive
southern Java Sea compared to the high-productive western Arabian Sea. It
also implies that land use changes and the associated enhanced transport of
lithogenic matter from land into the ocean may significantly affect the
CO2 uptake of the organic carbon pump in the receiving ocean areas.