Seasonal variations in settling fluxes of major components in the oligotrophic Shikoku Basin, the western North Pacific: coincidence of high biogenic flux with Asian dust supply in spring

Marine Chemistry 91 (2004) 187 – 210 www.elsevier.com/locate/marchem

Seasonal variations in settling fluxes of major components in the oligotrophic Shikoku Basin, the western North Pacific: coincidence of high biogenic flux with Asian dust supply in spring Tie Lia,c, Toshiyuki Masuzawab,*, Hiroyuki Kitagawab a

Department of Hydrospheric–Atmospheric Science, Graduate School of Science, Nagoya University, Chikusa-ku, Nagoya 464-8601, Japan b Department of Hydrospheric–Atmospheric Science, Graduate School of Environmental Studies, Nagoya University, Chikusa-ku, Nagoya 464-8601, Japan c Department of Marine Chemistry, College of Chemistry and Chemical Engineering, Ocean University of China, 5 Yushan Road, Qingdao 266003, China Received 5 January 2004; received in revised form 22 June 2004; accepted 23 June 2004 Available online 11 September 2004

Abstract Time-series sediment traps were deployed at ca. 500 m below the sea surface (upper) and ca. 500 m above the sea bottom (lower) at station SHIBT (29830VN, 135815VE, ca. 4600 m water depth) near the center of oligotrophic subtropical gyre of the Shikoku Basin in the western North Pacific from February to December 1999 and from June to September 2000 to collect settling particle samples with high time resolution (2.5 days in spring 1999). Major components of settling particles, i.e., CaCO3, Fe–Mn oxyhydroxide (ERFe-Mn), organic matter (OM), opal and lithogenic aluminosilicate (clay), were measured by stepwise leaching. Strontium isotope ratio and grain size of clay fraction were also determined. Average total mass fluxes were 18.9 and 57.4 mg m 2 day 1 in the upper and lower traps from February to early December 1999, respectively, and 3.8 and 58.0 mg m 2 day 1 in the upper and lower traps from June to September 2000, respectively. Total biogenic flux (CaCO3+OM+opal) in the upper trap accounted for 93% of total mass flux in 1999. CaCO3 was the dominant component in the upper and lower traps. Seasonal variations in settling fluxes of major components were observed for almost 1 year. High biogenic fluxes with four peaks were observed in spring, i.e., four times of spring blooms occurred in the euphotic zone from February to May 1999. Estimated export flux of particulate organic carbon from the euphotic zone was 6.2 mg C m 2 day 1 on average in 1999, and 9.0 mg C m 2 day 1 on average during the high biological production period in spring. 87Sr/86Sr ratio of clay fraction fell within the range of 0.7163–0.7188 in spring, which was consistent with that of Asian loess and dust. This indicates that the clay fraction was mainly composed of Asian dust via eolian transport in spring 1999. The high biogenic flux accompanying Asian dust coincided with Asian dust supply with time lag in early spring 1999. The coincidence of high

* Corresponding author. Tel.: +81 52 789 3498; fax: +81 52 789 3449. E-mail address: [email protected] (T. Masuzawa). 0304-4203/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.marchem.2004.06.010

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biogenic flux with Asian dust supply suggests the possibility that high biological production in the oligotrophic subtropical Shikoku Basin might be affected by the Asian dust supply in spring. D 2004 Elsevier B.V. All rights reserved. Keywords: Settling particle; Major component; Biological production; Asian dust; Sr isotope ratio; Shikoku Basin

1. Introduction Major processes of marine biogeochemical cycles of materials are biological production in the euphotic zone, vertical transport of biological products to the middle and deep sea as settling particles (Honjo, 1996), remineralization through the transport processes, and partial burial in the seafloor. Sediment traps can continuously obtain settling particle samples, which carry information of occurrences of biological production and environmental changes in the surface ocean (Ittekkot, 1996). In the past decades, attention has been paid to air–sea interaction and global biogeochemical cycles especially for carbon cycle and their relation to climate change. The ocean is one of the largest reservoirs of carbon and can absorb anthropogenic CO2 via biological production to control atmospheric CO2 concentration. For this reason long-term observations have been carried out to study temporal–spatial variations in biogeochemical cycles in the world oceans, such as OSP in the northeast Pacific (e.g., Wong et al., 1999), HOT in the Hawaii Ocean (e.g., Karl et al., 1996), OFP/BATS in the Bermuda Atlantic (e.g., Conte et al., 2001), in the Bering Sea (e.g., Takahashi et al., 2000), KNOT in the western subarctic Pacific (e.g., Honda et al., 2002) and so on. Studies on settling particles in the subarctic western North Pacific observed high opal content (e.g., Tsunogai et al., 1982; Noriki and Tsunogai, 1986) and showed diatom predominance with high biological productivity, which deserved international attention (Honjo, 1997). Recent observations on settling particles in the western North Pacific were also focused on the Japan Trench (e.g., Saito et al., 1992; Saito et al., 1997), the Japan Sea (Masuzawa et al., 1989; Hong et al., 1997), the Okinawa Trough and the East China Sea (e.g., Iseki et al., 2003; Chung et al., 2003), among others. Generally, the subarctic North Pacific (in the north of 408N) is a bsilica oceanQ with high export flux

of organic carbon while the subtropical and tropical North Pacific (in the south of 408N) is a bcarbonate oceanQ with low export flux of organic carbon, which plays a different role in the global carbon cycle (Honjo, 1996, 1997). The classification of bsilica oceanQ and bcarbonate oceanQ corresponds to the classification of ecological provinces, i.e., the bPacific Subarctic Gyres ProvinceQ and bKuroshio current ProvinceQ in the North Pacific (Longhurst, 1998). Despite the importance of the extensive subtropical gyre area in the western North Pacific, there has been no long-term observation for a full year using sediment traps in the Shikoku Basin. The Shikoku Basin is located south of the Shikoku and Honshu Islands of Japanese Island Arc, between Shichito–Iojima Ridge in the east and Kyushu–Palau Ridge in the west with water depths of more than 4000 m, and the Kuroshio flows through the northern Shikoku Basin (Fig. 1). It is an oligotrophic subtropical ocean (Goes et al., 2001), where nutrients in the euphotic zone are supplied by vertical convection only in winter (Limsakul et al., 2001). The Shikoku Basin is also located east of the Asian Continent and can be affected by the westerly wind which transports Asian dust to the North Pacific mainly in spring (Prospero, 1996). It is also an area of typhoon pathways in summer and autumn. Thus, the Shikoku Basin might be an area sensitive to environmental changes and impacted by Asian dust events and fluvial inputs from Japanese Islands or typhoons, but less studied on the whole (cf. Saito et al., 1992). In order to clarify the seasonal variations in settling particle flux and processes of biological production, as well as possible effects of environmental changes such as eolian transport of Asian dust on biological production in the Shikoku Basin, sediment trap deployment was carried out for almost 1 year with a high time resolution of 2.5 days in spring 1999. As for the results, the present study reports seasonal variations in settling fluxes of major components, Asian

T. Li et al. / Marine Chemistry 91 (2004) 187–210

189

Fig. 1. Location of sediment trap deployment at SHIBT (29830VN, 135815VE, ca. 4600 m water depth) near the center of the Shikoku Basin in the western North Pacific from February 1999 to September 2000, as well as around this area: SB-1 in the Shikoku Basin and JT-01 in the Japan Trench (Saito et al., 1992), SST1 and SST2 in the Okinawa Trough (Iseki et al., 2003), and SB in Sagami Bay (Masuzawa et al., 2003).

dust detection by strontium isotope ratio of clay fraction, and the coincidence of high biological production with the Asian dust supply in spring.

2. Materials and methods 2.1. Sampling of settling particles and surface sediments A mooring system with two time-series sediment traps was successively deployed at ca. 500 m below the sea surface (upper: McLane PARFLUX Mark 7 with 21 cups) and at ca. 500 m above the sea bottom (lower: McLane PARFLUX Mark 6 with 13 cups) at station SHIBT (the Shikoku Basin Time Series;

29830VN, 135815VE, ca. 4600 m water depth) near the center of subtropical gyre of the Shikoku Basin in the western North Pacific (Fig. 1). This site is at the southernmost station of R/V Shumpu-Maru of Kobe Marine Observatory, Japan, which conducted annual observations four to five times till 2000 for around 50 years (Limsakul et al., 2001), and is located near a buoy (B21004 at 298N, 1358E from 1982 to 2000) of the Japan Meteorological Agency (Japan Meteorological Agency, 2000a). Three deployments were carried out from February 21 to April 14, 1999 (SH1: upper trap at 482 m, lower trap at 3985 m, 4510 m water depth), from April 18 to December 5, 1999 (SH2: upper trap at 752 m, lower trap at 4084 m, 4610 m water depth) and from June 17 to September 14, 2000 (SH3: upper trap at 542

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m, lower trap at 4014 m, 4540 m water depth) during the cruises of YK99-01, SE99-01, KT99-17, SE00-06 and KT00-13 of R/V Yokosuka of JAMSTEC, T/S Seisui-Maru of Mie University and R/V Tansei-Maru of University of Tokyo. A Doppler current meter with temperature and electrical conductivity sensors (AANDERAA RCM-9) was deployed 20 m above the upper trap, which observed the actual depth of the upper trap as shown above. The deeper water depth of the upper trap of SH2 was due to intertwining of mooring ropes. Results of current measurements will be reported elsewhere (Sekine and Masuzawa, 2004). Each sampling cup was filled with 5% formaldehyde buffered with sodium tetraborate prepared with filtered in situ seawater from 500 m depth (pH=8.0) to prevent biological degradation of the sampled particles. The settling particle sample recovered by each cup was split into two subsamples (3/4 and 1/4 aliquots) using a wet sample divider after picking up swimmers. The 3/4 aliquot was filtered with a 1-Am pore size nuclepore filter 47 mm in diameter, rinsed with a small volume of Milli-Q water to remove residual sea salt, and examined with an optical microscope. Each filtered sample was dried in a vacuum oven at 60 8C to a constant weight to obtain the total mass flux of each sampling period after correction for possible residual sea salt as shown later, then subjected to major component analyses. Three 1.0-cm-thick layers from 0 to 3.0 cm of a box core sample, A1 (29831.54VN, 135813.71VE, 4530 m water depth, 22.9 cm long), collected near SHIBT during the cruise of BO99-3 of T/S Bosei-Maru of Tokai University were also subjected to chemical analyses. For comparison, two Asian dust samples collected at Nagoya University (35.158N, 136.968E), Japan, during two Asian dust events were also analyzed: NU015 was collected with a high volume air sampler (HV1000F) from April 12 to 17, 2001 (Mizuno, 2002) and NU02 with a pan on April 10, 2002. 2.2. Major component analysis Calcium carbonate (CaCO3), organic matter (OM), biogenic silica (opal) and lithogenic aluminosilicate (clay) are major components of marine settling particles with possible residual sea salt and trace amounts of easily reducible Fe–Mn oxyhydroxide

(ERFe-Mn), which is composed of amorphous FeOOH and MnO2 (Chao, 1972). A stepwise leaching method modified from that of Tessier et al. (1979) combined with gravimetry was applied to determine each major component as directly as possible (Masuzawa et al., 2003). Briefly, about 30 mg of each dried sample was put into a 12-ml Teflon test tube and was leached subsequently with (1) 1 M NH4OAc (pH=5.1) at room temperature for 5 h to dissolve CaCO3 and possible residual sea salt, (2) 0.04 M NH2OHd HCl in 0.02 M HNO3 at room temperature for 30 min to extract ERFe-Mn (Chao, 1972) and (3) 30% H2O2 at 90 8C for 3 h to decompose OM. Each procedure of (1) to (3) was carried out twice. The residue was vacuum-dried at 90 8C, weighed and leached with (4) 2 M Na2CO3 at 85 8C for 5 h to extract opal. Na2CO3 leachate was sampled at 2.5 and 5 h for spectrophotometric analysis of biogenic opal as SiO2d 0.4H2O (Mortlock and Froelich, 1989), and (5) the residue (clay fraction) was filtered with a pre-weighed 25mm-diameter, 0.2-Am pore size nuclepore filter, vacuum-dried at 90 8C and weighed. The total recovered sample amounts in the upper trap were sometimes much less than 30 mg and could not be leached. Several two continuous samples were combined to enable leaching (SH1-U10 and U11, SH2-U6 and U7, SH2-U14 and U15, SH2-U18 and U19, and SH2-U20 and U21; Table 1). Chloride ion in NH4OAc leachate was determined by anion chromatography (DIONEX QIC), and residual sea salt content was calculated based on the salinity/Cl ratio of average seawater (Millero, 1996). Residual sea salt contents were 0–3.96% (b0.5% for 70% samples) for settling particle samples and 4.17–4.92% for sediment samples. Ca in NH4OAc leachate and Fe and Mn in NH2OHd HCl leachate were determined by ICP-AES (Thermo Jarrel Ash IRIS-AP). CaCO3 content was calculated from Ca content corrected for that in the residual sea salt. ERFe-Mn content was calculated as FeOOH+MnO2. OM content (except the sediment samples, see Section 2.3) was obtained from the weight loss between leaching procedures (1) and (3) by subtracting residual sea salt, CaCO3 and ERFe-Mn. Opal content was determined by gravimetry (weight loss by Na2CO3 leaching) and by spectrophotometry. Total mass flux and major components in Table 1 were corrected for observed residual sea salt.

Table 1 Total mass flux and major components of settling particles collected at SHIBT (29830VN, 135815VE, ca. 4600 m water depth) in the Shikoku Basin, the western North Pacific (1999 and 2000) Sample

Starting date Julian day

Upper trap SH1-U1 SH1-U2 SH1-U3 SH1-U4 SH1-U5 SH1-U6 SH1-U7 SH1-U8 SH1-U9 SH1-U10 SH1-U11 SH1-U12 SH1-U13 SH1-U14 SH1-U15 SH1-U16 SH1-U17 SH1-U18 SH1-U19 SH1-U20 SH1-U21

2/21/99 2/23/99 2/26/99 2/28/99 3/3/99 3/5/99 3/8/99 3/10/99 3/13/99 3/15/99 3/18/99 3/20/99 3/23/99 3/25/99 3/28/99 3/30/99 4/2/99 4/4/99 4/7/99 4/9/99 4/12/99

52 54.5 57 59.5 62 64.5 67 69.5 72 74.5 77 79.5 82 84.5 87 89.5 92 94.5 97 99.5 102

SH2-U1 SH2-U2 SH2-U3 SH2-U4 SH2-U5 SH2-U6 SH2-U7 SH2-U8 SH2-U9 SH2-U10 SH2-U11 SH2-U12 SH2-U13

4/18/99 4/29/99 5/10/99 5/21/99 6/1/99 6/12/99 6/23/99 7/4/99 7/15/99 7/26/99 8/6/99 8/17/99 8/28/99

108 119 130 141 152 163 174 185 196 207 218 229 240

2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 11 11 11 11 11 11 11 11 11 11 11 11 11

Total mass flux (mg m day 1)

% of total

% of total

2

CaCO3

ERFe-Mn

OM

Opal

Clay

101.1 103.1 117.6 88.8 57.5 37.2 55.0 61.8 43.9 25.7 23.1 6.0 7.0 9.0 21.4 14.8 13.7 8.1 5.0 6.1 1.6

51.7 58.3 58.7 62.3 58.2 71.1 56.3 57.1 65.4 65.3 65.3

0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.01 0.01 0.00 0.00

29.0 26.1 33.3 30.5 33.2 19.4 26.3 24.1 25.1 29.4 29.4

14.5 11.4 6.9 6.3 7.6 8.5 13.7 14.4 7.3 4.2 4.2

4.8 4.2 1.0 0.9 1.1 1.0 3.6 4.3 2.2 1.1 1.1

22.9 44.2 27.7 15.6 17.6 10.8 8.0 14.7 3.7 7.2 27.7 8.2 2.4

45.1 43.7 45.0 49.6 48.0 49.5 49.5 48.0

0.03 0.02 0.04 0.05 0.04 0.04 0.04 0.04

22.5 25.8 21.0 20.3 25.5 25.2 25.2 27.9

21.3 21.9 19.3 14.3 14.1 11.7 11.7 11.5

11.0 8.7 14.8 15.7 12.4 13.6 13.6 12.4

52.4 54.0 56.3

0.02 0.03 0.01

34.4 27.7 30.6

7.7 11.8 8.4

5.5 6.5 4.8

Corg flux (mg m 2 day 1)

Corg/Ntot (w/w)

Corg

Ntot

9.27 8.40 5.98 5.49 8.89 8.24 6.97 10.79 7.26 5.61 16.59

1.45 1.33 0.97 0.86 1.54 1.73 1.13 1.72 1.39 1.15 3.39

9.4 8.7 7.0 4.9 5.1 3.1 3.8 6.7 3.2 1.4 3.8

6.4 6.3 6.1 6.4 5.8 4.8 6.2 6.3 5.2 4.9 4.9

10.36 9.42

1.84 1.48

2.2 1.4

5.6 6.4

10.93 10.59 10.54 9.55 9.74 11.22 12.02 10.29 10.92 17.44 9.12 14.26

1.64 1.37 1.44 1.19 1.14 1.50 1.71 1.29 1.63 2.67 1.33 2.09

2.5 4.7 2.9 1.5 1.7 1.2 1.0 1.5

6.7 7.7 7.3 8.0 8.6 7.5 7.0 8.0 6.7 6.5 6.9 6.8

1.2 2.5 1.2

d 13Corg (x)

Opal/ CaCO3 (w/w)

Corg/Cinorg (mol/mol)

0.28 0.20 0.12 0.10 0.13 0.12 0.24 0.25 0.11 0.06 0.06

1.49 1.20 0.85 0.73 1.27 0.97 1.03 1.58 0.93 0.72 2.12

0.47 0.50 0.43 0.29 0.29 0.24 0.24 0.24

2.02 2.02 1.95 1.60 1.69 1.89 2.02 1.79

0.15 0.22 0.15

2.77 1.41 2.11

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m/d/y

Duration (days)

192

Table 1 (continued) Sample

Starting date Julian day

SH2-U14 SH2-U15 SH2-U16 SH2-U17 SH2-U18 SH2-U19 SH2-U20 SH2-U21

9/8/99 9/19/99 9/30/99 10/11/99 10/22/99 11/2/99 11/13/99 11/24/99

251 262 273 284 295 306 317 328

SH3-U1 SH3-U2 SH3-U3 SH3-U4 SH3-U5 SH3-U6 SH3-U7 SH3-U8 SH3-U9 SH3-U10 SH3-U11 SH3-U12 SH3-U13 SH3-U14 SH3-U15 SH3-U16 SH3-U17 SH3-U18 SH3-U19 SH3-U20 SH3-U21

6/17/00 6/21/00 6/26/00 6/30/00 7/5/00 7/9/00 7/14/00 7/18/00 7/23/00 7/27/00 8/1/00 8/5/00 8/10/00 8/14/00 8/19/00 8/23/00 8/28/00 9/1/00 9/6/00 9/10/00 9/15/00

169 173.5 178 182.5 187 191.5 196 200.5 205 209.5 214 218.5 223 227.5 232 236.5 241 245.5 250 254.5 259

Lower trap SH1-L1 SH1-L2 SH1-L3 SH1-L4 SH1-L5 SH1-L6 SH1-L7

2/21/99 2/25/99 3/1/99 3/5/99 3/9/99 3/13/99 3/17/99

52 56 60 64 68 72 76

11 11 11 11 11 11 11 11 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5

4 4 4 4 4 4 4

Total mass flux (mg m day 1) 6.2 11.8 17.5 13.4 12.1 12.4 10.0 9.1

% of total

% of total

2

Corg/Ntot (w/w)

CaCO3

ERFe-Mn

OM

Opal

Clay

Corg

Ntot

59.0 59.0 53.1 53.4 62.7 62.7 57.8 57.8

0.01 0.01 0.01 0.01 0.02 0.02 0.02 0.02

28.0 28.0 33.5 28.2 21.9 21.9 27.5 27.5

7.5 7.5 8.6 11.7 8.5 8.5 8.9 8.9

5.5 5.5 4.9 6.6 6.8 6.8 5.7 5.7

11.28 11.80 11.75 8.49 7.37 7.77 14.19

1.68 2.05 1.78 1.32 1.11 1.12 2.21

1.3 2.1 1.6 1.0 0.9 0.8 1.3

6.7 5.8 6.6 6.4 6.6 6.9 6.4

15.20 12.56 13.78 14.94

2.77 1.88 2.30 2.48

0.8 0.9 0.8 1.4

5.5 6.7 6.0 6.0

4.61 5.36 5.46 5.64 6.19 5.61 5.44

1.10 0.78 0.93 0.98 0.84 0.91 0.93

2.9 5.5 5.6 6.0 6.0 6.1 6.6

4.2 6.9 5.9 5.8 7.4 6.2 5.8

10.4 2.7 1.4 1.1 5.0 7.3 6.1 9.6 2.6 1.3 3.7 4.8 2.3 1.4 1.9 1.4 2.5 3.9 2.3 8.0 1.1

62.7 102.4 102.2 106.2 96.3 108.8 121.0

Corg flux (mg m 2 day 1)

53.9 50.7 49.4 51.6 47.6 41.6 40.4

0.15 0.13 0.11 0.12 0.12 0.13 0.13

12.8 12.5 15.0 11.3 14.1 16.8 17.6

14.6 16.9 17.2 18.0 19.3 19.5 20.9

18.4 19.8 18.3 19.0 18.8 21.9 21.0

d 13Corg (x)

23.6 24.4 23.6 24.4 25.2 24.4 23.7

Opal/ CaCO3 (w/w)

Corg/Cinorg (mol/mol)

0.13 0.16 0.22 0.14 0.14 0.15 0.15

1.59 1.85 1.83 1.13 0.98 1.12 2.05

0.27 0.33 0.35 0.35 0.41 0.47 0.52

0.71 0.88 0.92 0.91 1.08 1.12 1.12

T. Li et al. / Marine Chemistry 91 (2004) 187–210

m/d/y

Duration (days)

3/21/99 3/25/99 3/29/99 4/2/99 4/6/99 4/10/99

80 84 88 92 96 100

4 4 4 4 4 4

101.7 88.7 85.9 68.0 66.7 71.0

45.8 44.2 46.1 46.6 45.5 45.7

0.12 0.20 0.22 0.21 0.22 0.24

14.5 12.0 10.6 11.6 12.3 11.3

21.0 18.3 17.5 16.7 16.3 15.3

18.6 25.3 25.5 24.9 25.7 27.5

5.75 4.82 4.95 4.47 4.59 4.52

0.97 0.83 0.86 0.87 0.92 0.89

5.8 4.3 4.3 3.0 3.1 3.2

5.9 5.8 5.8 5.1 5.0 5.1

24.7 22.6 22.6 22.6 22.6 22.5

0.46 0.41 0.38 0.36 0.36 0.33

1.05 0.91 0.89 0.80 0.84 0.82

SH2-L1 SH2-L2 SH2-L3 SH2-L4 SH2-L5 SH2-L6 SH2-L7 SH2-L8 SH2-L9 SH2-L10 SH2-L11 SH2-L12 SH2-L13

4/18/99 5/5/99 5/22/99 6/8/99 6/25/99 7/12/99 7/29/99 8/15/99 9/1/99 9/18/99 10/5/99 10/22/99 11/8/99

108 125 142 159 176 193 210 227 244 261 278 295 312

17 17 17 17 17 17 17 17 17 17 17 17 17

62.0 69.9 61.2 57.0 63.1 39.9 40.5 47.5 38.4 38.9 38.8 43.4 43.6

39.9 39.8 38.8 38.3 35.4 37.6 40.6 37.1 35.2 41.5 43.3 43.9 41.5

0.23 0.23 0.21 0.23 0.24 0.25 0.25 0.26 0.27 0.29 0.28 0.29 0.29

13.4 14.2 15.2 14.7 14.7 14.7 12.3 13.6 15.7 10.1 7.4 8.3 11.2

19.3 19.4 17.1 16.3 15.3 15.0 15.0 17.0 13.1 14.0 14.1 12.6 13.8

27.1 26.4 28.8 30.5 34.4 32.4 31.8 32.1 35.8 34.1 35.0 34.9 33.2

5.51 5.10 4.78 4.54 4.63 6.54 5.26 4.62 5.10 5.04 5.11 4.41 5.11

0.83 0.73 0.70 0.85 0.71 0.99 0.88 0.73 0.76 0.71 0.72 0.70 0.72

3.4 3.6 2.9 2.6 2.9 2.6 2.1 2.2 2.0 2.0 2.0 1.9 2.2

6.6 7.0 6.8 5.3 6.5 6.6 6.0 6.3 6.7 7.1 7.1 6.3 7.1

22.6 22.8 23.0 23.1 23.1 23.4 23.5 23.2 23.3 23.1 23.4 23.1 23.6

0.49 0.49 0.44 0.42 0.43 0.40 0.37 0.46 0.37 0.34 0.33 0.29 0.33

1.15 1.07 1.03 0.99 1.09 1.45 1.08 1.04 1.21 1.01 0.98 0.84 1.02

SH3-L1 SH3-L2 SH3-L3 SH3-L4 SH3-L5 SH3-L6 SH3-L7 SH3-L8 SH3-L9 SH3-L10 SH3-L11 SH3-L12 SH3-L13

6/17/00 6/24/00 7/1/00 7/8/00 7/15/00 7/22/00 7/29/00 8/5/00 8/12/00 8/19/00 8/26/00 9/2/00 9/9/00

169 176 183 190 197 204 211 218 225 232 239 246 253

7 7 7 7 7 7 7 7 7 7 7 7 7

59.6 62.3 72.7 73.0 65.0 61.4 56.8 64.1 50.9 43.9 59.5 43.9 41.3

33.7 34.1 34.7 30.6 32.3 33.4 34.4 33.9 34.6 35.9 36.4 37.7 39.4

0.22 0.23 0.23 0.30 0.25 0.24 0.25 0.25 0.23 0.23 0.27 0.22 0.24

10.9 10.1 12.0 11.9 11.0 10.6 9.1 9.6 12.1 9.2 10.5 9.4 8.8

16.7 16.8 13.3 12.9 13.0 13.6 14.0 14.2 13.5 14.2 12.9 14.8 13.1

38.5 38.7 39.7 44.2 43.5 42.1 42.2 42.1 39.6 40.5 40.0 37.9 38.5

5.01 4.15 4.66 4.18 4.09 4.34 4.39 3.97 4.21 4.36 4.15 4.41 4.54

0.68 0.56 0.60 0.57 0.53 0.57 0.56 0.55 0.61 0.74 0.63 0.61 0.64

3.0 2.6 3.4 3.1 2.7 2.7 2.5 2.5 2.1 1.9 2.5 1.9 1.9

7.4 7.4 7.8 7.3 7.7 7.6 7.8 7.2 6.9 5.9 6.6 7.2 7.1

22.6 22.6 22.6 22.4 22.6 22.5 22.7 22.8 22.8 22.9 22.9 22.9 23.0

0.50 0.49 0.38 0.42 0.40 0.41 0.41 0.42 0.39 0.40 0.35 0.39 0.33

1.24 1.01 1.12 1.14 1.05 1.08 1.06 0.98 1.02 1.01 0.95 0.97 0.96

0.67 0.65 0.65

1.4 1.3 1.5

6.0 5.3 7.0

91.4 92.3 90.3

0.56 0.53 0.58

0.04 0.03 0.03

Sediment BO99-3-A1-0 BO99-3-A1-1 BO99-3-A1-2

0.52 0.45 0.59

12.8 16.0 17.7

11.5 11.7 11.8

T. Li et al. / Marine Chemistry 91 (2004) 187–210

SH1-L8 SH1-L9 SH1-L10 SH1-L11 SH1-L12 SH1-L13

8.96 9.85 8.19

193

194

T. Li et al. / Marine Chemistry 91 (2004) 187–210

The relationship between opal content by gravimetry and that by spectrophotometry is shown in Fig. 2a. Almost all samples are plotted along a line with a slope of 1 between 4% and 23% of opal content. The average value of the difference between gravimetric and spectrophotometric opal was 0.7F1.5% (1 S.D.), which was similar to that for settling particle samples from Sagami Bay (Masuzawa et al., 2003). This indicates that the two methods gave almost identical results, so the gravimetric opal content was used for discussion hereafter. 2.3. Organic carbon, total nitrogen and d 13C of organic carbon About 5 mg of each dried sample was analyzed for organic carbon (Corg) and total nitrogen (Ntot) with a CHN analyzer (Carlo Elba NC2500) after removing inorganic carbon (Cinorg) with HCl. d 13C of Corg was measured for about 20 mg of each dried sample with a mass spectrometer (VG Optima) relative to PDB standard after removing Cinorg with HCl only for lower trap samples because sample amounts of the upper trap were not enough. The relationship between Corg and OM contents is shown in Fig. 2b. Average OM/Corg ratio of these settling particles was 2.5F0.8 (1 S.D.) as shown with a solid line, which was comparable to 2.8 of organic

matter with Redfield type composition (Redfield et al., 1963) as shown with a dashed line (Fig. 2b). The observed OM contents of three surface sediment samples by this procedure were 10.8–11.1%, although Corg contents were 0.53–0.58% (Table 1). The sediment samples were mostly composed of clay (N90%). Clay minerals contain 5 to 14% layered water, which is removed by heating at 50–200 8C (Weaver and Pollard, 1973). Layered water in clay minerals might be extracted by H 2O 2 leaching at 90 8C and contributed significantly to the weight loss after H2O2 leaching due to the high content of clay fraction (N90%) in these surface sediments. Hence, the OM contents of the three surface sediment samples were estimated from Corg contents by multiplying the observed average OM/Corg ratio of 2.5 (Table 1). 2.4. Grain size of clay fraction The grain size of the clay fraction was measured with a laser scattering particle-size distribution analyzer (HORIBA LA-300). 2.5. Strontium isotope ratio and element analysis of clay fraction The dried clay fraction with the nuclepore filter was put into a PFA-7 ml vessel and dipped in 0.4 ml

Fig. 2. Relationships between opal contents determined by gravimetry and spectrophotometry (a), and between organic carbon (Corg) determined by CHN analyzer and organic matter (OM) by gravimetry (b) of settling particles and surface sediments collected at SHIBT.

T. Li et al. / Marine Chemistry 91 (2004) 187–210

of concentrated NH3d H2O overnight to dissolve the nuclepore filter (Sugiyama, 1996). After removing residual NH3d H2O by evaporation, the clay fraction was decomposed with 1 ml of HNO3–HClO4–HF (3:4:3) using a Teflon bomb (SAN’AI P-25) in a microwave oven, evaporated to dryness under an IR lamp in an evaporation chamber and finally dissolved in 10 ml of 1 M HNO3. This solution was split into two aliquots for element and Sr isotope ratio analyses. Rb and Sr were determined with a multi-collector ICP mass spectrometer (MC-ICP-MS; IsoProbe, Micromass, UK), which was set in a clean room of class 1000. To examine the accuracy of element analysis, reference rock samples from the Geological

195

Survey of Japan (JB-1 and JG-1) were also decomposed and analyzed. The measured Rb and Sr contents were 41.1 and 458 ppm for JB-1 and 184 and 185 ppm for JG-1, respectively, which were comparable to the recommended values of 41.3 and 444 ppm for JB1 and 182 and 184 ppm for JG-1, respectively (Imai et al., 1995). Sr was purified by cation exchange chromatography (Biorad 50W-X8; Asahara et al., 1995) and finally dissolved in 2% HNO3 to adjust Sr concentration to be 200 ppb. 87Sr/86Sr ratio was determined with IsoProbe by normalizing to 86Sr/88Sr=0.1194. The average 87Sr/86Sr ratio of NIST-SRM 987 was measured to be 0.710287F0.000084 (1r, n=10). 87 Sr/86Sr ratios of JB-1 and JG-1 were measured to

Fig. 3. Temporal variations in mass fluxes (solid line) and contents (dotted line) of total (a), CaCO3 (b), OM (c), opal (d) and clay (e) of settling particles in the upper trap deployed at ca. 500 m below the sea surface at SHIBT from February to December 1999 and from June to September 2000. Asian dust events in spring 1999 are marked with downward-pointing arrows, a typhoon event passed near SHIBT in 1999 (T9908) with a double circle, volcanic eruptions at Miyake Island in 2000 with open diamonds, and eight peaks of total mass flux in the upper trap in 1999 are labeled with PU1 to PU8.

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T. Li et al. / Marine Chemistry 91 (2004) 187–210

be 0.704198F0.000039 and 0.711206F0.000051, comparing with 0.704126 and 0.710995 (Yamamoto and Maruyama, 1996), respectively. All chemical procedures were done in a clean room of class 100.

3. Results 3.1. Settling fluxes of total and major components 3.1.1. Mass fluxes and contents of major components of settling particles Results of total mass fluxes and major components are shown in Table 1. Temporal variations in mass fluxes of the total and four major components (CaCO3, OM, opal and clay) in the upper and

lower traps are shown in Figs. 3 and 4, respectively. Peaks of total mass flux in the upper and lower traps in 1999 are numbered in Figs. 3a (PU1–8) and 4a (PL1–8). Average fluxes and contents of major components at SHIBT are summarized in Table 2. The range of total mass flux in the upper and lower traps in spring 1999 was comparable to those collected at 1.60, 2.67 and 3.75 km depths at SB-1 in the Shikoku Basin from January to March 1987 (47–141 mg m 2 day 1; 31832VN, 137805VE, 4.30 km water depth, Fig. 1; Saito et al., 1992). The order of average major components in the upper trap in 1999 was CaCO3 (54.1%)HOM (26.9%)Nopal (12.3%)Hclay (6.7%)HERFe-Mn (b0.1%), and total biogenic component (CaCO3+ OM+opal) accounted for 93.3% on average (Table 2).

Fig. 4. Temporal variations in mass fluxes (solid line) and contents (dotted line) of total (a), CaCO3 (b), OM (c), opal (d) and clay (e) of settling particles in the lower trap deployed at ca. 500 m above the sea bottom at SHIBT from February to December 1999, and from June to September 2000. Eight peaks of total mass flux in the lower trap in 1999 are labeled with PL1 to PL8.

T. Li et al. / Marine Chemistry 91 (2004) 187–210

197

Table 2 Average values of fluxes and contents of major components of settling particles collected at SHIBT (29830VN, 135815VE, ca. 4600 m water depth) in the Shikoku Basin, the western North Pacific (1999 and 2000) Total CaCO3 Feb. 21-Dec. 5, 1999 Upper trap Flux (482–752 m) (mg m 2 day % of total (range) Feb. 21–Nov. 25, 1999 Lower trap Flux (3985–4084 m) (mg m 2 day % of total (range) 1999 Lower/Upper Flux ratio Content ratio Jun. 17–Sep. 18, 2000 Upper trap (542 m) Flux (mg m 2 day Jun. 17–Sep. 16, 2000 Lower trap (4014 m) Flux (mg m 2 day % of total (range) 2000 Lower/Upper Flux ratio sediment (4530 m) % of total

18.9 10.2 1

Opal

Clay

ERFe-Mn

Corg

Ntot

5.1

2.3

1.3

0.003

1.85

0.28

6.7 (0.9–15.7)

0.02 9.79 1.51 (0.00–0.05) (5.49–17.44) (0.86–3.39)

) 100

54.1 26.9 12.3 (43.1–71.1) (19.4–34.4) (4.2–21.9)

57.4 23.8 1

OM

7.6

9.5

16.4

0.13

2.93

0.46

41.5 (35.2–53.9) 3.0 2.3 0.8

13.2 (7.4–17.6) 1.5 0.5

16.6 (12.6–21.0) 4.1 1.3

28.5 (18.3–35.8) 12.9 4.2

0.22 (0.11–0.29) 40.4 13.3

5.10 (3.97–6.54) 1.6 0.52

0.81 (0.35–1.10) 1.6 0.54

3.8 –

–

–

–

–

–

–

6.1

8.2

23.7

0.14

2.52

0.35

) 100

1

)

1

)

58.0 20.0 100

34.4 10.5 (30.6–39.4) (8.8–12.1)

15.1 100 0.5

1.4

The order of average major components in the lower trap was CaCO3 (41.5%)Nclay (28.5%)Nopal (16.6%)NOM (13.2%)HERFe-Mn (0.2%) in 1999, and total biogenic component accounted for 71.5% on average (Table 2). In the lower trap, OM content was lower than that in the upper trap, while opal content increased slightly and clay became the second abundant component (Table 2). The total mass flux in the lower trap averaged 3.0 times that in the upper trap in 1999 (Table 2). Clay and ERFe-Mn contents increased significantly from 6.7 and 0.02% in the upper trap to 28.5 and 0.2% in the lower trap on average, respectively (Table 2). This suggests that lateral transport dominated by clay contributed very much with scavenged ERFe-Mn to settling fluxes in the lower trap, consistent with the observation of Saito et al. (1992). Major components of the surface sediment samples at SHIBT were almost identical between 0 and 3 cm depth. Clay contents were predominant (N90%). CaCO3 decreased significantly to 0.4–0.6% from 41.5% (1999) or 34.4% (2000) in the lower trap. ERFe-Mn content increased to 0.7% from 0.2% (1999 and 2000) in the lower trap (Tables 1 and 2). Carbonate

14.1 40.8 0.24 4.34 (12.9–16.8) (37.9–44.2) (0.22–0.30) (5.01–3.79)

0.60 (0.53–0.74)

6.1

0.03

91.3

0.66

0.51

compensation depth (CCD) at 308N in the western North Pacific is ca. 3500 m (Berger, 1974). The difference in CaCO3 content between the lower trap and surface sediment indicates that CaCO3 in settling particles passed though CCD, and CaCO3 was dissolved out mainly on the sea bottom before burial. Results of Corg, Ntot, d 13C and Corg/Ntot (w/w), opal/CaCO3 (w/w) and Corg/Cinorg ratios are also listed in Table 1. Temporal variations in Corg flux, d 13C value, and Corg/Ntot and Corg/Cinorg ratios in the upper and lower traps are shown with solid circles in Fig. 5. High CaCO3 content (54.1% and 41.5% on average in the upper and lower traps in 1999, respectively) and low opal/CaCO3 (w/w) ratio (0.22 and 0.39 on average in the upper and lower traps in 1999, respectively; Table 1) indicate that the Shikoku Basin has the property of a bcarbonate oceanQ (Honjo, 1996, 1997). Opal/CaCO3 ratios in the upper trap were comparable to those in the North Pacific Gyre (0.27; Honjo, 1997), but Corg/Cinorg ratios (1.7 and 1.05 on average in the upper and lower traps in 1999, respectively; Table 1) were higher than those in the North Pacific Gyre (0.8; Honjo, 1997) or in the bcarbonate oceanQ (0.6; Honjo, 1997).

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Fig. 5. Temporal variations in Corg flux, d 13Corg, Corg/Ntot (w/w) and Corg/Cinorg in the upper (a) and lower (b) traps at SHIBT in 1999. Solid circles with solid lines show results observed in 1999, and open squares with dash lines those in 2000. Open circles show the estimated export Corg fluxes from the euphotic zone at 150 m depth by the equation observed at ALOHA (Karl et al., 1996).

Coccolithphores and foraminifera are possible main producers for CaCO3, and diatoms, silicoflagellates and radiolaria for opal. Based on microscopic examination, many foraminiferan shells were observed in the filtered settling particle samples both in the upper and lower traps. Contribution of coccolith was 29–53% of CaCO3 settling flux in the tropical and subtropical North Pacific (Honjo et al., 1982; Steinmetz, 1994; Kawahata et al., 2002). Hence, the high CaCO3 content in the present study is thought to be dominated by foraminifera and coccolith. Based on phytopigment analysis of phytoplankton samples (25–335 Am) at a station near SHIBT in April 2002 (318N, 1378E; Ra and Masuzawa, unpublished data), the ratio of diatoms to silicoflagellates was 4:1, and radiolarian tests were rarely found by microscopic examination of settling particles. Therefore, the opal in settling particles collected at SHIBT is thought to be produced mainly by diatoms.

3.1.2. Seasonal variations in mass flux and major components of settling particles Seasonal variations in total mass flux and major component composition in the upper trap can be described by dividing the whole observation period from February 21 to December 5, 1999 into four stages (Fig. 3). Stage 1 (early spring) was from mid-February to mid-April with three steep peaks of total mass fluxes from February 21 to 28 (PU1), from March 10 to 13 (PU2) and from March 28 to 30 (PU3), which reflected high biological production from later winter to early spring. The CaCO3 content was high (51.7–71.1%) accompanying the relatively low opal/CaCO3 ratio (0.06–0.28) in stage 1 (Table 1). Stage 2 (late spring) was a period of high settling flux with subdominant opal content and relatively high opal/ CaCO3 ratio (0.29–0.50) from mid-April to midJune (Table 1). Stage 3 (summer) was from midJune to mid-September when the total mass flux

T. Li et al. / Marine Chemistry 91 (2004) 187–210

was low with little variation in major components except an outstanding flux peak sampled from August 6 to 17 (PU7) in early to mid-August. The peak might have coincided with a typhoon passing near SHIBT on August 6 as shown in Fig. 3a (T9908; Japan Meteorological Agency, 2000b). Stage 4 (autumn) was from mid-September to early December with a slight increase in total mass flux over that in stage 3. A regular sequence in the fluxes and contents of major components was observed in the upper trap. During the first episode, flux peaks of opal and clay appeared 5 days (two sampling durations) prior to flux peaks of CaCO3 and the total (PU1). There were two OM flux peaks accompanying the earlier opal peak and the later CaCO3 peak (Fig. 3). During the second (PU2) and the fourth episodes (PU4), opal, OM and CaCO3 flux peaks appeared together with the total flux peak. High opal content was associated with the total flux peak, while CaCO3 content was slightly low at the total flux peak and increased after it (dotted lines in Fig. 3b and d). High flux and content of opal with OM followed by those of CaCO3 with OM in the upper trap in spring. Seasonal variation of flux peaks in the lower trap was very similar to that of the upper trap with 8–27 days delay over high backgrounds of total mass fluxes with high clay content (18.3–35.8%; Tables 1 and 2). The four stages for the upper trap in 1999 can also be applied to the lower trap with certain time lags (Fig. 4). The d 13C value in the lower trap in stage 1 (23.6–25.2x) was lower than those in other stages (22.5–23.6x; Table 1 and Fig. 5b), which associated with high OM flux in early spring. Based on the similar seasonal variations in the upper and lower traps, flux peaks of PU2-PL2, PU4-PL4 and PU7-PL7 are identified as corresponding peak-pairs between the upper and lower traps by their high opal contents. The other four peak-pairs separated by the three identified peakpairs are also considered to correspond between the upper and lower traps. Settling velocity of particles can be estimated according to the btemporal offsetQ of bbenchmarksQ (Honjo and Manganini, 1993; Honjo, 1996). The apparent settling velocity at SHIBT was estimated to be in a range from 120 to

199

480 m day 1 on average for these seven peak-pairs based on the depth differences (SH1, 3503 m; SH2, 3232 m) and the time lag between the upper and lower cups of each peak pair. These estimated settling velocities were not much different from those reported in other oceans: 80–330 (e.g., Honjo and Manganini, 1993; Berelson, 2002), or N 500 m day 1 (Bory and Newton, 2000). The higher the total mass flux, the higher was the apparent settling velocity. 3.2. Sr isotope ratio, element contents and grain size of clay fraction The results for Rb and Sr contents, Sr isotope ratio and mode diameter of grain size of clay fraction are shown in Table 3. Temporal variations in 87 Sr/86Sr ratio and mode diameter of clay fraction are shown in Fig. 6. 87Sr/86Sr ratio increased with increasing clay flux and vice versa in stage 1. In stage 2, 87Sr/86Sr ratio reached the highest value through the whole observation period in 1999. In stage 3, 87Sr/86Sr ratio was low when the clay flux was low. In stage 4, 87Sr/86Sr ratio increased with the increase in clay flux. Grain size of the clay fraction showed mainly mono-peak distribution ranging from 1 to 40 Am (N90%) with mode diameters of 6–28 Am in the upper trap and ranging from 1 to 30 Am (N90%) with mode diameters of 6–9 Am in the lower trap. Mode diameters of the clay fraction in the upper trap at SHIBT were almost constant in 1999 except in stage 1. In stage 1, mode diameters were slightly larger and variable (11–28 Am) than in other stages (6–11 Am), and varied inversely with the clay flux and 87 Sr/86Sr ratio, i.e., high clay fluxes corresponded to relatively small mode diameters in the upper trap (Fig. 6a). 87 Sr/86Sr ratio of clay fraction of bottom sediment ranged from 0.7123 to 0.7125, lower than those in the lower trap, and their mode diameters of grain size ranged from 9 to 11 Am (Table 3). 87 Sr/86Sr ratios of Asian dust samples collected at Nagoya University were 0.7164 (NU01-5) and 0.7153 (NU02), and the mode diameter of NU02 was 11 Am (Table 3). Mode diameters of clay fraction of Asian dust and settling particle samples observed here were consis-

200

T. Li et al. / Marine Chemistry 91 (2004) 187–210

Table 3 Rb and Sr contents, Sr isotopic ratio and mode diameter of grain size of clay fraction in settling particles collected at SHIBT (29830VN, 135815VE, ca. 4600 m water depth) in the Shikoku Basin, the western North Pacific (1999 and 2000) Rb (ppm)

Sr (ppm)

87

S.D. (1r)

87

136 125 101 129 135 129 95 97 75 51 51

118 128 124 158 172 165 124 106 89 77 77

0.718070 0.717863 0.716981 0.716981 0.716981 0.716981 0.717829 0.717818 0.716314 0.716314 0.716314

117 176 196 196 196 196 194 102 270 270 270

3.25 2.74 2.29 2.30 2.22 2.21 2.17 2.60 2.38 1.86 1.86

11 14 14 21 28 – 16 16 25 – –

123 171 141 195

106 124 123 126

0.718572 0.718046 0.718558 0.718755

100 131 124 110

3.28 3.88 3.23 4.39

209

120

0.717902

120

4.94

11 6 11 8 6 7 7 7

90 138 74

104 104 82

0.716734 0.717674 0.716347

147 76 167

2.43 3.74 2.53

11 8 –

88 88 117 121 194 194 125 125

107 107 101 103 98 98 104 104

0.715028 0.715028 0.716032 0.715924 0.716280 0.716280 0.716333 0.716333

208 208 140 109 161 161 129.8 129.8

2.32 2.32 3.27 3.33 5.58 5.58 3.40 3.40

8 8 6 6 8 8 6 6

Asian dust NU01-5 NU02

111 38

117 182

0.716443 0.715302

127 119

2.68 0.60

– 11

Lower trap SH1-L1 SH1-L2 SH1-L3

72 120 124

91 101 102

0.715571 0.715882 0.715865

94 85 89

2.22 3.37 3.45

6 6 6

Sample Upper trap SH1-U1 SH1-U2 SH1-U3 SH1-U4 SH1-U5 SH1-U6 SH1-U7 SH1-U8 SH1-U9 SH1-U10 SH1-U11 SH1-U12 SH1-U13 SH1-U14 SH1-U15 SH1-U16 SH1-U17 SH1-U18 SH1-U19 SH1-U20 SH1-U21 SH2-U1 SH2-U2 SH2-U3 SH2-U4 SH2-U5 SH2-U6 SH2-U7 SH2-U8 SH2-U9 SH2-U10 SH2-U11 SH2-U12 SH2-U13 SH2-U14 SH2-U15 SH2-U16 SH2-U17 SH2-U18 SH2-U19 SH2-U20 SH2-U21

Sr/86Sr

Rb/86Sr

Mode diameter (Am)

T. Li et al. / Marine Chemistry 91 (2004) 187–210 Sr/86Sr

201

Sample

Rb (ppm)

Sr (ppm)

87

SH1-L4 SH1-L5 SH1-L6 SH1-L7 SH1-L8 SH1-L9 SH1-L10 SH1-L11 SH1-L12 SH1-L13

64 62 82 99 96 175 45 59 47 73

109 79 107 108 106 112 90 100 95 99

0.715739 0.715895 0.715627 0.715724 0.715962 0.715548 0.715555 0.715594 0.715541 0.715445

93 93 79 84 75 68 74 69 83 74

1.66 2.23 2.16 2.60 2.55 4.41 1.41 1.68 1.39 2.10

SH2-L1 SH2-L2 SH2-L3 SH2-L4 SH2-L5 SH2-L6 SH2-L7 SH2-L8 SH2-L9 SH2-L10 SH2-L11 SH2-L12 SH2-L13

50 102 158 59 56 54 61 95 100 112 59 97 50

91 103 109 98 100 86 100 108 108 108 103 103 98

0.715931 0.715945 0.715969 0.716284 0.715848 0.715815 0.715965 0.715904 0.715696 0.715655 0.715269 0.715250 0.715435

79 84 89 84 58 89 90 90 95 89 79 99 74

1.55 2.79 4.08 1.71 1.60 1.76 1.73 2.49 2.62 2.94 1.63 2.66 1.44

5 6 7 6 7 6 5 9 5 6 5 6 7 6 6 5 6 5 6 6 5 6 6 7

SH3-L1 SH3-L2 SH3-L3 SH3-L4 SH3-L5 SH3-L6 SH3-L7 SH3-L8 SH3-L9 SH3-L10 SH3-L11 SH3-L12 SH3-L13

74 46 50 55 40 86 71 83 35 38 35 77 76

103 103 100 102 103 108 101 101 88 89 95 107 107

0.714943 0.714852 0.715033 0.714672 0.714801 0.715095 0.714871 0.714874 0.714937 0.714923 0.714939 0.715266 0.714845

79 71 116 93 104 104 88 98 107 80 75 100 99

2.03 1.25 1.41 1.52 1.10 2.26 1.98 2.31 1.12 1.21 1.05 2.01 2.01

6 6 7 6 6 6 6 6 7 7 6 7 8

Sediment BO99-3-A1-0 BO99-3-A1-1 BO99-3-A1-2

57 42 22

111 80 90

0.712359 0.712492 0.712316

66 67 56

1.45 1.47 0.70

9 10 11

S.D. (1r)

87

Rb/86Sr

Mode diameter (Am)

–: Not determined.

tent with those of Asian dust particles sampled in fallen snow layers at Murododaira of Mt. Tateyama in central Japan from November 1998 to March 1999 by the same method (6–21 Am; Osada et al., 2004), but were slightly larger than those of Asian dust observed in Japan by the aerodynamic method with Andersen samplers (3.3–4.7 Am; Ishizaka, 1991; Mori et al., 2003).

4. Discussion 4.1. Biogenic flux peaks related to spring blooms in 1999 From late February to May 1999 (stages 1 and 2), four episodes with steep peaks of total mass flux (PU1 to PU4) were observed (Fig. 3a). High contents (84.3–

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T. Li et al. / Marine Chemistry 91 (2004) 187–210

Fig. 6. Temporal variations in clay flux, strontium isotope ratio (87Sr/86Sr) and mode diameter of clay fraction in the upper (a) and lower (b) traps at SHIBT. Solid lines show results in 1999 and dash lines those in 2000. Asian dust events and a typhoon event in 1999 and volcanic eruptions at Miyake Island in 2000 are shown with downward-pointing arrows, a double circle and open diamonds, respectively.

99.1%) of biogenic component (CaCO3+OM+opal) indicate that settling particles were derived from biological products during spring blooms in the euphotic zone. Settling fluxes decreased from PU1 to PU3 in stage 1. The intervals were about 18 days from PU1 to PU2 and PU2 to PU3, and about 1 month from PU3 to PU4. Settling particles took 1–2 days to sink down from the euphotic zone to the upper trap at ca. 500 m depth when the estimated apparent settling velocity of PU1-PL1, PU2-PL2 and PU3PL3 between the upper and lower traps (250–480 m day 1) was applied. The settling period was much shorter than the peak intervals. The surface current at SHIBT, which is located near the center of the subtropical gyre, was estimated in 1999 to be low (0.1–0.3 m s 1) with random current directions and the surface physical condition was relatively stable (Sekine and Masuzawa, 2004). Thus, the upper trap sampled settling particles as a statistic funnel (Siegel and Deuser, 1997) with little disturbance in the settling processes due to the shallow depth, or with

little influence by surface physical condition at SHIBT. The three biogenic flux peaks with 18-day intervals observed with high time resolution in stage 1 were most probably due to separate spring blooms in the euphotic zone with decreasing biogenic fluxes from PU1 to PU3. 4.2. Export flux of particulate organic carbon Corg fluxes were from 0.8 to 9.4 mg C m 2 day 1 with an average value of 1.8 mg C m 2 day 1 at ca. 500 m depth (upper trap), and from 1.9 to 6.6 mg C m 2 day 1 with an average value of 2.9 mg C m 2 day 1 at ca. 4000 m depth (lower trap) in 1999 (Tables 1 and 2). Moreover, Corg/Ntot ratios of settling particles in the upper (4.8–6.4) and lower (4.2–6.9) traps were around that of Redfield type composition (5.7; w/w) in stage 1 (Table 1 and Fig. 5). These facts suggest the freshness of OM in the upper and lower traps and rapid descent of settling particles to the depth of lower trap during the high biological production period.

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Corg flux decreased with depth below the euphotic zone due to decomposition (Bishop, 1989). The decrease in Corg flux can be expressed by the model of F Z =F 150(Z/150)b , where F Z is the Corg flux at depth Z (m) below the euphotic zone, F 150 the export flux of Corg from the euphotic zone, 150 (m) the thickness of the euphotic zone and b= 0.818 observed at ALOHA (22845VN, 1588W) near Hawaii in the subtropical central North Pacific (Karl et al., 1996), which was modified from that of Martin et al. (1987) adapted in the latitudinal range from 148N to 358N in the eastern North Pacific. The thickness of the euphotic zone at SHIBT was also assumed to be 150 m based on chlorophyll-a profiles observed at station SHIBT by R/V Shumpu-Maru in 1999 (Japan Meteorological Agency, 2000a). Export fluxes of Corg from the euphotic zone at SHIBT in 1999 were estimated based on this model and shown with open circles in Fig. 5a. The average estimated Corg fluxes from the euphotic zone were 8.1 and 10.0 mg C m 2 day 1 in stages 1 and 2, respectively, which were about twice those in stages 3 and 4, 4.2 and 4.8 mg C m 2 day 1, respectively. The annual average Corg flux exported from the euphotic zone at SHIBT in 1999 was estimated to be 6.2 mg C m 2 day 1, which was lower than that at ALOHA (29.0 mg C m 2 day 1; Karl et al., 1996). Corg/Cinorg ratios in the upper trap were higher than 1, averaging 1.66 from February to December 1999 (Table 1 and Fig. 5d). The average Corg/Cinorg ratio in stage 1 was 1.17 and those in other stages were between 1.51 and 2.00 in the upper trap. Estimated export fluxes of Corg from the euphotic zone were 2.6- to 3.2-fold those observed in the upper trap, so Corg/Cinorg ratio would be more than 4.3 for the particles exported from the euphotic zone. This suggests that the Shikoku Basin plays the role of a sink of CO2 in the observation period based on the Corg/Cinorg ratio of settling particles (Tsunogai and Noriki, 1991; Kawahata, 2002). It was consistent with the fact that the partial pressures of CO2 of seawater was lower than those of atmosphere at 308N along 1378E line in the western North Pacific except in July (Ishii et al., 2001). 4.3. Eolian transport of Asian dust to station SHIBT Eolian transport of Asian dust from the Loess Plateau, the Gobi Desert or the Takla Makan Desert

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is a common phenomenon in spring and the dominant pathway of lithogenic aluminosilicate to hemi-pelagic and pelagic areas of the North Pacific (Uematsu et al., 1983). Station SHIBT is located around 2100 km away from the Loess Plateau of northern China. Besides Asian dust, riverine terrigenous materials from the Japanese Islands and possibly volcanic ash might be latent sources of lithogenic aluminosilicate in settling particles and sediment. Fig. 7 shows 87Sr/86Sr ratio vs. 87Rb/86Sr ratio (Rb– Sr diagrams) in the clay fraction of settling particles in the upper and lower traps at SHIBT, as well as the enclosed areas of reported lithogenic materials, which are Asian loess (Taylor et al., 1983; Nakai et al., 1993; Gallet et al., 1996; Asahara et al., 1999; Jahn et al., 2001; Kanayama et al., 2002; Sun, 2002), igneous rocks (Shibata and Ishihara, 1979; Notsu et al., 1989) and river suspended material (Goldstein and Jacobsen, 1988) in the Japanese Islands, the volcanic rocks of the Izu–Bonin Arc (Taylor and Nesbitt, 1998), and the sediments from the Yellow Sea, the East China Sea and seas around the Japanese Islands (Asahara et al., 1995). The area of Asian loess is plotted between 0.712 and 0.720 in 87Sr/86Sr ratio and between 0.3 and 3.0 in 87 Rb/86Sr ratio (Fig. 7). It is clearly separated from others. In the upper trap, 87Sr/86Sr ratios were concentrated between 0.7150 and 0.7188, but 87Rb/86Sr ratios were variable and sometimes high (Fig. 7a). Almost all data points in stage 1 fell within the area of Asian loess (Fig. 7a). 87Sr/86Sr ratios (Fig. 7a) and mode diameters (Table 3) of clay fraction in stage 1 were comparable to those of the Asian dust samples collected at Nagoya University (NU01-5 and NU02; Table 3) during Asian dust events on April 14, 2001 and April 10, 2002, respectively. These facts indicate that the clay fraction of settling particles in the upper trap in stage 1 was consistent with the nature of Asian loess and Asian dust, that is, the clay fraction collected in stage 1 was composed mainly of Asian dust. Data points in stages 2–4 are plotted between the area of the Yellow Sea sediment and that of igneous rocks and river suspended material in the Japanese Islands; they are close to the area of sediments around Japan (Fig. 7a). Mode diameters of clay fraction in the upper trap were relatively fine (6–11 Am). It seems that the clay fraction in stages 2–4 was a mixture of these sources.

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Fig. 7. 87Sr/86Sr vs. 87Rb/86Sr (Rb–Sr diagrams) for clay fraction of settling particles in the upper (a) and lower (b) traps collected at SHIBT in 1999 and 2000. Reported lithogenic materials are shown in enclosed areas: Asian loess (Taylor et al., 1983; Nakai et al., 1993; Gallet et al., 1996; Asahara et al., 1999; Jahn et al., 2001; Kanayama et al., 2002; Sun, 2002), the East China Sea sediments, the Yellow Sea sediments and sediments around the Japanese Islands (Asahara et al., 1995), igneous rocks (Shibata and Ishihara, 1979; Notsu et al., 1989) with river suspended material (Goldstein and Jacobsen, 1988) in the Japanese Islands, the volcanic rocks of the Izu–Bonin Arc (Taylor and Nesbitt, 1998) as well as GSJ reference rock samples (JB-1 and JG-1).

Fig. 7b shows a Rb–Sr diagram of clay fraction collected by the lower trap and surface sediments.87Sr/86Sr ratios in the lower trap (0.7147– 0.7162; Fig. 6b) were slightly lower than those in the upper trap (Fig. 6a). This might suggest that the clay fraction in the lower trap was a mixture of particles from the upper water containing Asian dust in spring and laterally transported resuspended sediment from

the continental slope of the Japanese Island side near to the Shikoku Basin, where 87Sr/86Sr (0.706–0.713) and 87Rb/86Sr (0.37–3.49) ratios were variable (Asahara et al., 1995). Volcanic material is another possible source with low 87Sr/86Sr ratios. No volcanic eruption was reported in 1999, but huge volcanic eruptions took place at Miyake Island (ca. 650 km northeast of SHIBT; Fig. 1) on July 14,

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August 10 and August 29, 2000 (Japan Meteorological Agency, 2001; marked with open diamonds in Figs. 3 and 6b). Volcanic ash particles were not found by microscopic examination of these settling particle samples, and their effects on the 87Sr/86Sr ratio of clay fraction in settling particles at SHIBT in 2000 were not detected (Fig. 6b). Sediment traps are efficient facilities for long-term observation of air-borne dust flux in the ocean (Jickells et al., 1998). The settling flux of Asian dust at station SHIBT was estimated to be 52 mg m 2 from February 21 to April 15, 1999 (stage 1; 52.5 days) based on the clay flux in spring 1999. This flux was much lower than the Asian dust deposition rate observed in fallen snow layers at Murododaira of Mt. Tateyama in the central Japan from November 1998 to March 1999 (7.4 g m 2; Osada et al., 2004). 4.4. Coincidence of high biogenic flux with Asian dust supply in spring in the Shikoku Basin Asian dust events observed in eight districts from East to Southwest in Japan by visual observation at meteorological observatories (Japan Meteorological Agency, 2000b), satellite images (Kinoshita et al., 2001), lidars (Murayama et al., 1999; Asian Dust Network, 2001) and dust sampled in fallen snow layers at Murododaira of Mt. Tateyama, central Japan (Fig. 1; 36834VN, 137836VE, 2450 m a.s.l.; Osada et al., 2004) are shown in Fig. 8. Based on these observations, the Asian dust events are compiled and marked with downward-pointing arrows in Figs. 3 and 6. The first and widespread Asian dust event of the year started on January 26, in 1999. Settling particle sampling began on February 21. Settling particle fluxes of total and major components are also shown in Fig. 8. Clay particles with the quality of Asian dust associated with each settling episode were derived by an Asian dust event prior to each settling flux episode. They were removed from the sea surface by biological aggregation during the spring bloom period (Fig. 8). For example, the clay peak of the first settling episode (PU1) associated with the opal peak can be considered to be the removal of the Asian dust event from February 8 to 10 with a time lag of about 2 weeks (Fig. 8). Similarly, the clay peak of PU2 can be related to the Asian dust event from March 1 to 3 (Fig. 8). Thus, the high biogenic flux of each episode (PU1 to PU3)

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accompanying the clay peak coincided with the Asian dust supply with a time lag of 10–17 days in stage 1 (Fig. 8). Eolian dust might promote high biological production by supplying nutrients and/or minor elements such as Fe to high nutrient areas (e.g., Duce, 1986; Martin et al., 1989; Jickells, 1995). Recently, Bishop et al. (2002) observed that the carbon biomass was enhanced by an Asian dust event with a 2-week time lag in a high nutrient area near station PAPA in the subarctic North Pacific. The time lag of 10–17 days between high biogenic flux and Asian dust supply at SHIBT was comparable to that reported by Bishop et al. (2002). The coincidence of high biogenic flux with the Asian dust supply might suggest a possible enhancement of Asian dust upon high biological production in the oligotrophic subtropical Shikoku Basin in spring. For around 50 years, the strong winter mixing of seawater has been observed to pull up nutrients to the euphotic zone in the Shikoku Basin (Limsakul et al., 2001). Phosphate and dissolved inorganic nitrogen (DIN) concentrations were 0.04–0.17 and 0.2–1.1 AM, respectively, in the surface layer from 0 to 150 m depth (the winter mixing depth) at SHIBT on January 23, 1999 (Japan Meteorological Agency, 2000a), about one order of magnitude lower than in the subarctic North Pacific high nutrient area (Goes et al., 2001). The standing stocks of phosphate and DIN at SHIBT were 7.2 mmol P m 2 and 94.4 mmol N m 2, respectively, in the surface layer from 0 to 150 m depth. The estimated total export flux of Corg (81 mmol C m 2) during stages 1 and 2 in 1999 corresponded to 0.8 mmol P m 2 and 12.2 mmol N m 2 based on Redfield ratio (Redfield et al., 1963). These required amounts of phosphate and DIN were much smaller than their standing stocks. The standing stocks of nutrients in January 1999 were enough to support such export flux of Corg in stages 1 and 2. Thus, the observed high biological production subsequent to the Asian dust supply might have been triggered mainly by minor elements, such as Fe contained in and dissolved out from Asian dust (Martin et al., 1989; Zhuang et al., 1992; Bishop et al., 2002) in the oligotrophic subtropical Shikoku Basin in spring. To clarify this mechanism, further field studies on the possible effects of iron on biological production will be required.

206 T. Li et al. / Marine Chemistry 91 (2004) 187–210 Fig. 8. Asian dust events observed during late January to early April in 1999: (1) satellite images (marked with solid circles; Kinoshita et al., 2001); (2) Asian dust in fallen snow layers at Murododaira, Mt. Tateyama (sampled on March 23 with a marker on January 27; Osada et al., 2004); (3) visual meteorological observations in eight districts from East to Southwest in Japan (marked with shadows; darker gray shadow areas indicate heavy dust days; Japan Meteorological Agency, 2000a,b); and (4) Asian dust events observed with lidars (marked with b*Q; Murayama et al., 1999; Asian Dust Network, 2001). Temporal variations of settling fluxes of the total and major components in stage 1 are also shown to compare the sequence with Asian dust events.

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5. Summary Settling particle samples were collected at station SHIBT by deploying time-series sediment traps with high time resolution from February to December 1999 and from June to September 2000. Major components of settling particles, the strontium isotope ratio and grain size of the clay fraction were determined to clarify seasonal variations in settling particle fluxes and biological production, eolian transport of Asian dust and its possible impact on biological production. The main conclusions are summarized as follows: (1)

(2)

(3)

(4)

Average total mass fluxes were 18.9 and 57.4 mg m 2 day 1 in the upper and lower traps from February to early December 1999, respectively, and 3.8 and 58.0 mg m 2 day 1 in the upper and lower traps from June to September 2000, respectively. Total biogenic flux (CaCO3+OM+opal) in the upper trap accounted for 93% of total mass flux in 1999. CaCO3 was the dominant component in the upper and lower traps. Seasonal variations in settling fluxes of major components were observed for almost 1 year. High settling fluxes were observed in spring 1999 due to high biological production in the euphotic zone, i.e., four spring blooms occurred from February to May 1999. The estimated export flux of Corg from the euphotic zone averaged 6.2 mg C m 2 day 1 in 1999, and 9.0 mg C m 2 day 1 during high biological production periods in spring. This was about twice the export flux of Corg from summer to early winter (4.4 mg C m 2 day 1). 87 Sr/86Sr ratio of clay fraction fell within the range of 0.7163–0.7188 in spring 1999, which was consistent with that of Asian loess and dust. The clay fraction was mainly composed of Asian dust via eolian transport in spring 1999. The settling flux of Asian dust was estimated to be 52 mg m 2 from February 21 to April 15, 1999 (52.5 days). High biogenic flux accompanying Asian dust coincided with the Asian dust supply with a time lag in early spring 1999. The coincidence of high biogenic flux with the Asian dust supply suggests the possibility that high biological production in the oligotrophic subtropical Shi-

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koku Basin might be affected by the Asian dust supply in spring.

Acknowledgements We would like to express our gratitude to the captains, officers and crews of R/V Yokosuka of JAMSTEC, T/S Seisui-Maru of Mie University and R/V Tansei-Maru of University of Tokyo for their skillful assistance in deployments and recoveries of the mooring system of sediment traps. We are grateful to Profs. E. Tanoue and T. Siano for their facilities and encouragement in conducting the sediment trap experiment, and to Drs. M. Endo, I. Nakano and H. Fujimori of JAMSTEC and Drs. A. Tanimura and K. Taguchi of Mie University, for their support in the field observations. We acknowledge Prof. Y. Kato of Tokai University for his supply of the sediment core sample. We are indebted to Ms. C. Sukigara and Dr. K. Okamura for their assistance in sampling and sample treatment, and to Mrs. M. Yamamoto, Mrs. Y. Hibi and Ms. Y. Duan for their assistance in the chemical analyses. We also thank Dr. H. Kawahata and an anonymous reviewer for their valuable comments on the earlier version of the manuscript. This study was supported in part by a grant to TM from the Special Research Project on Dynamics of Atmosphere–Ocean Interaction of the Institute for Hydrospheric–Atmospheric Sciences, Nagoya University.

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