The monsoon, carbon fluxes, and the organic carbon pump in the northern Indian Ocean

Abstract

Time series sediment trap experiments were carried out at fifteen sites in the northern Indian Ocean between 1986 and 2007. The data on particle flux rates and composition are analyzed in combination with satellite-derived estimates of primary production and results of surface ocean studies during the Joint Global Ocean Flux Study in the Arabian Sea (JGOFS-Indik). The data highlight the influence of the monsoon on the transport of organic carbon into the deep sea and the associated functioning of the organic carbon pump. The results illustrate the well-known concept of export production, which is driven by inputs of nutrients from the aphotic zone and external reservoirs (the atmosphere and the land) into the euphotic zone. The monsoon drives the organic carbon export through its impact on the physical nutrient supply mechanisms, such as upwelling, vertical mixing, and river discharges. Eolian dust and especially riverine supply of lithogenic matter increase organic carbon fluxes by accelerating the transport of organic matter into the deep sea. Nevertheless, it is preferentially respired in the sub-thermocline and the resulting trapping of remineralized nutrients at this water-depth enforces the influence of upwelling and vertical mixing on the organic carbon fluxes which in the northern Indian Ocean are among the highest worldwide. Model experiments and measured organic carbon burial rates indicate that a weakening of the summer monsoon strength hardly affected the long-term annual average organic carbon export flux into the deep sea during the last approximately 7000 years. In addition to the summer and winter monsoon strength, which are assumed to be inversely related to each other, monsoon-driven physical impacts on the nutrient trapping efficiency seem to have kept organic carbon fluxes at a high level. A feedback mechanism caused by negative impacts of oxygen concentrations on the respiration and thus nutrient trapping efficiency apparently prevents the development of anoxia to the point where sulfate reduction occurs and sets an upper limit to organic carbon fluxes. Whether changes in the phytoplankton community structure observed in recent decades indicate that this self-regulating system is becoming unstable is open to question.