Seasonal variations in planktonic foraminifera at three sediment traps in the Subarctic, Transition and Subtropical zones of the central North Pacific Ocean
Introduction
Variations in seasonal standing crop variations of planktonic foraminifera in response to oceanographic changes have been investigated through sediment trap experiments by many authors. Most of these studies dealt with upwelling areas such as the Panama Basin (Thunell et al., 1983, Thunell and Reynolds, 1984), San Pedro Basin (Sautter and Thunell, 1991, Thunell and Sautter, 1992), Santa Barbara Basin (Kincaid et al., 2000), Bengal Bay (Guptha et al., 1997), Arabian Sea (Curry et al., 1992), and Somalia Basin (Conan and Brummer, 2000).
Compared with these works, study of open sea areas of the North Pacific is limited. Recently, Mohiuddin et al. (2002) reported the results of sediment trap experiments moored for approximately one and a half years from 1998 to 1999 in the northwestern Pacific Ocean at WCT-1 (24°59.6′N, 136°59.6′E) in the Subtropical zone and at WCT-2 (39°01′N, 147°00′E) in the Transition zone off northern Japan. Other important sediment trap studies include the successive works at Station PAPA (50°N, 145°W) in the Subarctic region of the northeastern Pacific Ocean by Reynolds and Thunell, 1985, Reynolds and Thunell, 1986 and by Sautter and Thunell (1989).
In this study, we extend the preliminary results (Eguchi et al., 1999) with data from three sediment traps deployed in a north–south transect along 175°E longitude under the Subarctic, Transition, and Subtropical water masses in the central North Pacific for approximately 1 year, beginning in June 1993. The revised results and their explanation are presented below.
Section snippets
Oceanographic setting
In the central North Pacific, the Subarctic water mass is separated from the Transition water mass by the Subarctic Front (Fig. 1). Between the Transition and Subtropical water masses, the so-called Kuroshio Extension has been located between the Transition and Subtropical water masses based on observed or simulated structure of sea-water temperatures in this area. The Extension experiences meandering on annual and inter-annual scales (variation breadth shown in Fig. 1, adopting Mizuno and
Materials and methods
Three time-series sediment traps (PAFLUX Mark 7G-21; opening 0.5 m2) were deployed at Sites 6–8 for about 1 year beginning in May–June 1993. Each trap cup was filled with filtered deep sea-water containing a 3% formaldehyde solution with buffered sodium borate (pH>8).
Recovered sample bottles were immediately refrigerated on board ship at approximately 2–4°C. Samples were then passed through a 1 mm opening sieve, after which the <1 mm fraction was split into aliquots with a rotary splitter.
Results with some short notes
Fluxes noted in the following text and Fig. 2, Fig. 3, Fig. 4, Fig. 5 have been subjected to a 1 month correction to minimize the time lag between production in surface water and deposition in traps moored in intermediate water depths: 1412 m at Site 8, 1482 m at Site 7, and 3873 m at Site 6, respectively.
Discussion
Planktonic foraminifera collected at Site 8 in the Subarctic zone were completely different from those from the other sites in terms of variations in seasonal flux rates and in faunal composition. Thus, warmer water did not migrate northward beyond the Subarctic Front during the period of this study and Site 8 was constantly under the influence of Subarctic water.
Sautter and Thunell (1989) showed a long-term (from September 1982 to October 1985) average seasonal thermal profile at Station PAPA
Conclusions
Three time-series sediment traps were deployed for about 1 year at sites in the Subarctic, Transition and Subtropical zones along ca. 175°E longitude in the North Pacific Ocean, beginning in June 1993. A stratified thermal structure in the surface layer developed from May to December at all three sites. The contrast between this period and the season characterized by a well mixed water column was probably due to the development of vertical convection produced by strengthened westerly surface
Acknowledgements
We are deeply thankful to James C. Ingle, Jr. of the Stanford University for his critical review of the first draft of this report. This study was supported by the program ‘Northwest Pacific Carbon Cycle Study’ consigned to the Kansai Environmental Engineering Center Co., Ltd. by the New Energy and Industrial Technology Development Organization. We also thank Douglas F. Williams of the University of South Carolina, and Richard Z. Poore of the U.S. Geological Survey, Menlo Park, CA, USA, for
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