Study Area

Two series (M and N) of sediment traps were deployed along the equator in the western Pacific (Fig. 1). Trap sites Ml and N2 were located in the hemi-pelagic zone, where the Mindanao and Halmahera eddies are prominent features (Godfrey, 1996). The other trap sites in these series were located either in the WPWP or in the equatorial upwelling area. Except at site N10, the ENSO condition did not change for the entire duration of each trap's deployment, i.e., either El Nino or La Nina (Fig. 2) conditions prevailed during trap deployment. The north equatorial current (NEC), equatorial

Figure 1: Location of sediment traps in the western equatorial and southeastern Pacific Ocean. Bathymetric contours correspond to 4,000 m water depth.

Figure 1: Location of sediment traps in the western equatorial and southeastern Pacific Ocean. Bathymetric contours correspond to 4,000 m water depth.

N2 N3 N10S13

1991 1993 1995 1997 1999 Year

Figure 2: Variations in the equatorial southern oscillation index (SOI) during the sediment trap experiments and approximate duration of sampling, shown by the positions of the trap site names (see details in Table 1).

N2 N3 N10S13

1991 1993 1995 1997 1999 Year

Figure 2: Variations in the equatorial southern oscillation index (SOI) during the sediment trap experiments and approximate duration of sampling, shown by the positions of the trap site names (see details in Table 1).

counter current (ECC) and south equatorial current (SEC) are the three major surface ocean currents in this area. West-bound equatorial currents (NEC and SEC) are the major forces causing the accumulation of warm water in the WPWP (Gordon and Fine, 1996). Also important is the equatorial undercurrent (EUC), an east-bound flow having the potential to affect the settling-particle flux along the equator (Hansell et al., 1997a). Sea surface temperature (SST) is usually higher than 28 °C throughout the WPWP, but thermal and spatial variations may occur owing to changes in solar irradiation, ENSO events and global warming or cooling (Yan et al., 1992).

Another series of traps was deployed in the south-west Pacific near the Australian continent. Important hydrographical features in this region are the tropical convergence and Tasman front. The subtropical east Australian current (EAC) merges with the temperate Tasman current at about 30°S and forms the Tasman front, which diverts towards the central Pacific (Mulhearn, 1987). The merger results in a complicated pattern of eddies (Tate et al., 1989), whose deep structure extends from the sea surface to abyssal depths (Mulhearn et al., 1986). Sites S11 and S12 were located north of the tropical convergence in the Coral Sea, while site S13 was located south of the Tasman front in the Tasman Sea. The average monthly SST varies between 26.5 °C and 30.2 °C around S11 and between 24.7 °C and 29.0 °C around S12. The Tasman Sea is characterised by seasonal SST changes with larger variation (between 16.2 °C and 21.9 °C), caused by latitudinal fluctuation of the Tasman front.

Bottom-tethered sediment-trap moorings were deployed in the western equatorial Pacific and south-west Pacific Ocean. The locations of moorings

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