Introduction

Marine organisms influence the exchange of CO2 between the ocean and the atmosphere via photosynthesis and precipitation of carbonates. These processes affect the fugacity of CO2 (fCO2) in the surface water by changing the concentration of total dissolved inorganic carbon (DIC) and the total alkalinity (TA) (Zeebe and Wolf-Gladrow, 2001). A decrease in surface water fCO2 favouring the CO2 uptake from the atmosphere is caused by a reduced DIC concentration and an enhanced TA. During the precipitation of carbonate, DIC and TA are consumed resulting in a net increase of fCO2 in surface water. Accordingly, the precipitation of carbonates is referred to as the 'carbonate counter pump' (Heinze et al., 1991). In contrast, the photo-synthetic production of organic matter lowers the fCO2 by the direct consumption of DIC and nutrients and their subsequent export into the deep sea. The net effect of the so-called organic carbon pump and the carbonate counter pump on the fCO2 comprises the biological pump (Volk and Hoffert, 1985). Owing to counteracting effects of the two pumps on the fCO2, the ratio between particulate organic carbon and particulate inorganic carbon in the material exported into the deep sea is suggested as an indicator for the CO2 uptake efficiency of the biological pump (Berger and Keir, 1984). Model studies have shown that an increase of the global mean POC/PIC (rain) ratio by a factor of 2 and can reduce the atmospheric CO2 concentration by 28.5 and 70ppm, respectively (Heinze et al., 1991; Archer et al., 2000). The 70-ppm decrease was achieved by assuming that the export production is driven by siliceous diatoms growing at the expense of carbonate-producing coccolithophorids. Since diatoms depend on the availability of silica, changes of their relevance in the plankton community are believed to result from reorganisation of the marine silica cycle or enhanced silica inputs from terrestrial sources (Froelich et al., 1992; Harrison, 2000; Conley, 2002; Matsumoto and Sarmiento, 2002).

In the framework of a joint Indo/German project and the Joint Global Ocean Flux Studies (JGOFS), PIC and POC exported from the surface ocean have been collected continuously by deep-moored sediment traps deployed at several sites in the northern Indian Ocean (Nair et al., 1989; Haake et al., 1993; Rixen et al., 1996; Lee et al., 1998; Honjo et al., 1999; Rixen et al., 2002; Unger et al., 2003; Fig. 1). The western Arabian Sea (WAST) was one of the main JGOFS study sites where in addition to sediment trap studies comprehensive field experiments were performed in 1994 and 1995. This

20.0 33.6 34.2 34.8 35.4 36.0 36.6 37.2 50,0 Annual Mean Salinity (0-50m)

Figure 1: Map of the northern Indian Ocean showing the mean annual salinity averaged for the upper 50 m of the water column (Data: World Ocean Atlas, 1998, http://www.cdc.noaa.gov/cdc/data.woa98.html). Black circles show the long-term sediment trap site in the western Arabian Sea (WAST). White circles and black squares indicate the other joint Indo/ German and the US JGOFS sediment trap sites, respectively. US JGOFS water sampling sites are shown by the red circles (For colour version, see Colour Plate Section).

20.0 33.6 34.2 34.8 35.4 36.0 36.6 37.2 50,0 Annual Mean Salinity (0-50m)

Figure 1: Map of the northern Indian Ocean showing the mean annual salinity averaged for the upper 50 m of the water column (Data: World Ocean Atlas, 1998, http://www.cdc.noaa.gov/cdc/data.woa98.html). Black circles show the long-term sediment trap site in the western Arabian Sea (WAST). White circles and black squares indicate the other joint Indo/ German and the US JGOFS sediment trap sites, respectively. US JGOFS water sampling sites are shown by the red circles (For colour version, see Colour Plate Section).

work combines results obtained by the Indo/German sediment trap experiment program and JGOFS and aims at improving our understanding of processes controlling the POC/PIC ratios in settling particles.

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