Introduction

The mid-latitude central Pacific is sensitive to climatic change, because its location corresponds to the position of the Northern Hemisphere westerly wind system, in a transition zone between subtropical and subarctic waters (e.g., Thompson, 1981; Thunell and Mortyn, 1995). The central North Pacific is characterized by two major water masses (the subarctic and subtropical water masses) and a transition zone (the Kuroshio Current and Kuroshio Extension; Tchernia, 1980). The subarctic region extends from the Bering Sea to approximately 40°N, where it borders the Kuroshio Current and Kuroshio Extension along a complex oceanographic front. The subarctic water mass is characterized by the presence of a permanent halocline at 100-200 m. The subtropical region is located between 31°N and 10°N. The northern boundary of this region is determined by the position of the Kuroshio Current and Kuroshio Extension, which together form a narrow transition region with a biological composition distinct from that found to the north or south. The subtropical region is characterized by a permanent thermocline, in contrast to the permanent halocline of the subarctic region (Tchernia, 1980). The thermal contrast between the subarctic and subtropical water masses is so large that a latitudinal shift of the Kuroshio Current and Kuroshio Extension can affect the climate in the Japanese islands and North America (Kennett and Ingram, 1995; Behl and Kennett, 1996; Ujiie and Ujiie, 1999) (Fig. 1).

The differing physical parameters of the subarctic and subtropical water masses result in differences in primary productivity and biological components. The subarctic water mass shows high primary productivity, which may reduce the partial pressure of carbon dioxide (pCO2) in surface water during the late spring (Kawahata et al., 1998a). In contrast, the subtropical water mass is characterized by oligotrophic conditions, with low primary productivity due to the surface water's permanent thermocline. A study of pigments and picoplankton groups in the central North Pacific along a transection at 175°E showed definite gradients in the meridional distribution of

120"E 130" 14Q" 15Q" 160" 170" 180" 170"W

120"E 130" 14Q" 15Q" 160" 170" 180" 170"W

West Pacific Subtropical High

Figure 1: Locations of the four sediment-trap mooring sites for this study. Surface hydrography of the central North Pacific is also presented. The subtropical and subarctic fronts showed seasonal latitudinal shift. Sites 6, 5, 7, and 8 were located at 30°00.1/N, 174°59.70E; 34°25.3/N, 177°44.20E; 37°24.20N, 174°56.70E and 46°07.20N, 175°01.90E, respectively.

Figure 1: Locations of the four sediment-trap mooring sites for this study. Surface hydrography of the central North Pacific is also presented. The subtropical and subarctic fronts showed seasonal latitudinal shift. Sites 6, 5, 7, and 8 were located at 30°00.1/N, 174°59.70E; 34°25.3/N, 177°44.20E; 37°24.20N, 174°56.70E and 46°07.20N, 175°01.90E, respectively.

divinyl chlorophyll-a and autotrophic picoplankton, in terms of carbon biomass reflecting water masses (Ishizaka et al., 1994; Suzuki et al., 1995). In spite of the significance of biological effects on the ocean carbon cycle, the downward flux and settling particle composition has been studied in the mid-latitude central North Atlantic, but not in the central North Pacific (Honjo and Manganini, 1993).

In order to understand the chemical properties of settling particles and carbon cycles in the central North Pacific, four sediment trap moorings were deployed from 1993 to 1994 (Table 1). The studied sites were located in the subtropical water mass (Site 6), the transition zone (Sites 5 and 7), and the subarctic water mass (Site 8). This paper documents carbonate, organic matter (OM), biogenic opal, and lithogenic fluxes of settling particles in order to characterize biological pump activity at various locations in the central North Pacific. Iron-addition experiments carried out in recent years showed substantial increases in chlorophyll-a and primary production and a reduction in pCO2 in the North-Pacific subarctic gyre (Tsuda et al., 2003; Boyd et al., 2004). Moreover, the mid-latitude central Pacific is potentially a receiving area for eolian dust from the Asian continent (Duce et al., 1991). Consequently, the contribution of eolian dust to primary production and lithogenic flux is also discussed in this paper.

Table 1: Locations of the mooring sites, deployment times, trap, and water depths as well as the durations of the sampling intervals

Trap name

Position

Seafloor

Trap depth

Sampling

depth (m)

(m)

duration

Station 6

30°00.rN

174°59.7'E

5,390

3,873

16-Jun-93 to 1-Jun-94

Station 5,

34°25.3'N

177°44.2'E

3,365

1,342

16-Jun-93 to

shallow

16-Apr-94

Station 5,

JJ

»

2,848

16-Jun-93 to

deep

1-Jun-94

Station 7,

37°24.2'N

174°56.7'E

5,105

1,482

1-Jun-93 to 9-

shallow

Apr-94

Station 7,

JJ

»

4,588

1-Jun-93 to 9-

deep

Apr-94

Station 8

46°07.2'N

175°01.9'E

5,435

1,412

16-Jun-93 to 16-Apr-94

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