Results and Discussion

The first set of sediment trap data obtained from the Arabian Sea, covering the years 1986 and 1987, showed a strong monsoon-driven seasonality with enhanced fluxes during the NE and SW monsoon (Nair et al., 1989). The 10 years' records on deep ocean fluxes generated subsequently have further confirmed the strong monsoon-driven seasonality (Fig. 2a). In order to produce an average record covering the annual cycle, the time-series data have been converted into bi-weekly means (Fig. 2b). It may be noted that the d15N data are available only for the period between 1986 and 1992. The extended time series reveals that fluxes during the peak of the NE monsoon (January/ February) can be as low as during the spring inter-monsoon in May (Fig. 2). In January/February, the mixed layer reaches its maximum depth and exceeds the depth of the euphotic zone (Dickey et al., 1998). Accordingly, deep mixing of photoautotrophic organisms initiating light-limiting conditions have been suggested as possible reason for the low organic carbon fluxes during the peak of the NE monsoon (Prahl et al., 2000; Rixen et al., 2002). The relatively high organic carbon fluxes during the early NE monsoon (November/December) correspond with the initial mixed-layer deepening.

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-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-r—I-1-1-1-1-1-1-1-1

1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998

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1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998

Jan Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec.

Figure 2: (a) Particulate organic carbon (POC) fluxes measured at 3,000-m water depth in the western Arabian Sea from 1986 to 1998. (b) Same POC fluxes plotted vs. the period of the year during which the sampling took place (black circle). The line indicates the mean bi-weekly POC fluxes during the year.

Jan Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec.

Figure 2: (a) Particulate organic carbon (POC) fluxes measured at 3,000-m water depth in the western Arabian Sea from 1986 to 1998. (b) Same POC fluxes plotted vs. the period of the year during which the sampling took place (black circle). The line indicates the mean bi-weekly POC fluxes during the year.

A second peak of organic carbon fluxes occurs in March/April (late NE monsoon) when the mixed layer shoals and approaches the depth of the euphotic zone (Rixen et al., 2002). Such an interplay between the depth of the mixed layer and the euphotic zone is known from parts of the north Atlantic where the shoaling of the deep winter mixed layer is associated with the development of the spring bloom and the deepening of the shallow summer mixed layer leading to the autumn bloom (Lalli and Parsons, 1993). In the western Arabian Sea, the organic carbon fluxes are higher during spring (late NE monsoon) than during the autumn (early NE monsoon; Fig. 2).

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