Distribution of bioactive trace metals (Fe, Co, Ni, Cu, Zn and Cd) in the Sulu Sea and its adjacent seas

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Deep-Sea Research II 54 (2007) 14–37 www.elsevier.com/locate/dsr2

Distribution of bioactive trace metals (Fe, Co, Ni, Cu, Zn and Cd) in the Sulu Sea and its adjacent seas K. Norisuye, M. Ezoe, S. Nakatsuka, S. Umetani, Y. Sohrin Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan Received 18 June 2005; received in revised form 6 March 2006; accepted 28 April 2006 Available online 23 January 2007

Abstract Here we present the first vertical distribution of dissolved (D) and acid-dissolvable (AD) bioactive trace metals (Fe, Co, Ni, Cu, Zn and Cd) in the Sulu Sea and the South China Sea, together with those observed previously by the same method in the western North Pacific. At a depth of 50–75 m, concentration minima of some trace metals were observed in the Sulu Sea (D-Zn, Cu and Fe) and the South China Sea (D-Zn), which coincided with the Chl a maximum, indicating scavenging of these metals including biological uptake. Above 150 m in the South China Sea, elevated concentrations of D-Co and Cu were observed, which could result from fluvial/coastal waters and/or shelf-sedimentary sources. In the surface mixed layer in the Sulu Sea, D-Fe concentrations were high (2 nM), presumably due to the flowing of waters across the Philippine Archipelago. Except for Fe, concentrations of D and AD species at each corresponding depth were not significantly different. Concentrations of AD-Fe (6.8–17 nM), below 500 m in the Sulu Sea were much higher than those of D-Fe (0.2–1.7 nM), indicating that particulate Fe is abundant in the basin, probably coming from nearby sedimentary sources. The vertical distributions of D-Ni, Cu, Zn and Cd in the South China Sea were nutrient-like and deviated slightly from those in the western North Pacific. This is caused by a rapid flushing of waters from the Philippine Sea coupled with vertical mixing within the South China Sea. The distribution of these metals in the Sulu Sea was more uniform below 500 m. The average concentrations of D-Ni, Cu, Zn and Cd in the deep water of the Sulu Sea were 6.1, 1.5, 3.3 and 0.5 nM, significantly lower than those of the western North Pacific (9.6, 2.8, 10.1 and 1.0 nM, respectively). We also found lower DCd/PO4, Ni/PO4 and Zn/Si(OH)4 ratios in the Sulu Sea than those of the Pacific. The depletion of these concentrations and the D-Cd/PO4 and Ni/PO4 ratios in the Sulu Sea is probably because the deep water of the basin is rapidly exchanged with shallow waters but isolated from nutrient-rich deep waters derived from the North Pacific, due to the surrounding topographic barriers including the islands and the shallow sills (o420 m). The lower D-Zn/Si(OH)4 ratios in the Sulu Sea are probably due to the relatively low D-Zn/Si(OH)4 ratios in the upper water coupled with the preferential supply of Si(OH)4 relative to Zn at depth. While subsurface waters were flowing along density surfaces (sigma-T values of
26) from the South China Sea to the Sulu Sea, the concentrations of macronutrients, D-Ni, Cu, Zn and Cd increased due to the regeneration from biogenic

Corresponding author. Tel.: +81 774 38 3095; fax: +81 774 38 3099.

E-mail address: [email protected] (K. Norisuye). 0967-0645/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2006.04.019

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particles. The increases in the concentrations of bioactive elements relative to that of PO4 were N : P : Si : Ni : Cu : Zn : Cd ¼ 15 : 1 : 40 : 2.4  103 : 0.6  103 : 2.7  103 : 0.03  103 in a molar unit. r 2006 Elsevier Ltd. All rights reserved. Keywords: Trace metals; Macronutrients; Semi-enclosed basin; Sulu Sea; South China Sea; North Pacific

1. Introduction Bioactive trace metals, such as Fe, Co, Ni, Cu, Zn and Cd, in seawater have been well studied (Sclater et al., 1976; Boyle et al., 1977; Bruland, 1980; Knauer et al., 1982; Martin et al., 1989; Bruland et al., 1994; Yeats et al., 1995; Saager et al., 1997; Fujishima et al., 2001; Ezoe et al., 2004) because of their critical role on the growth of phytoplankton in the euphotic zone of the ocean (Bruland et al., 1991; Morel and Price, 2003). Fe, Co, Ni, Cu and Zn are required by phytoplankton for various metabolic functions. The biological requirement for Cd by organisms also has been documented (Price and Morel, 1990; Cullen et al., 1999), and recent studies provided evidence for Cd playing an important role as a Cd-specific carbonic anahydrase in certain diatoms (Lane and Morel, 2000; Lane et al., 2005). Over the past three decades, developments in sampling and analytical techniques have made it possible to reveal reliable vertical profiles of these bioactive trace metals and mechanisms that control their oceanic distributions (Bruland and Franks, 1983; Chester, 2000). Profiles for dissolved Fe, Ni, Zn and Cd in the open ocean generally show surface depletion and deep-water enrichment, which is a result of uptake by biota in surface water, the sinking of biogenic particles, and regeneration at depth. The distributions of Ni, Cu, Zn and Cd show a large inter-ocean fractionation: the highest concentrations are found in North Pacific deep waters due to the regenerative input through a long journey of the thermohaline circulation (Chester, 2000). Concentrations of some bioactive trace metals and algal macronutrients in the ocean show a good correlation, and their relationships and the metal/ macronutrient ratios are studied to have a better understanding of biogeochemical cycling in the ocean. Cd and PO4 in oceanic waters have been intensively studied (de Baar et al., 1994; Lo¨scher et al., 1998; Elderfield and Rickaby, 2000, references therein), because datasets of Cd and PO4 in oceanic waters can be used to reconstruct past PO4 distributions using Cd/Ca ratios in fossil foraminiferal tests (Boyle, 1988).

The Sulu Sea is one of the subtropical Southeast Asian basins and distinctively different from neighboring basins. The Sulu Sea is a semi-enclosed basin with a maximum depth greater than 5000 m and surrounded with the shallow sills (o420 m) of the Mindoro, Sibutu and other straits, which prevent the deep water from mixing with that of other basins. The average water temperature throughout the deep water column is approximately 10 1C, which is much higher than that of the subtropical western North Pacific (o4 1C) (Quadfasel et al., 1990; Nozaki et al., 1999). On the other hand, the Pacific deep waters rapidly flush the South China Sea (Broecker et al., 1986), since the Luzon Strait (with sill depths 2000–2500 m) between the South China Sea and the western North Pacific is deep. Therefore, the biogeochemical cycling of trace metals in the Sulu Sea is expected to be unique and quite different from that in the South China Sea and the North Pacific. However, no dataset of bioactive trace metals has been reported in the subtropical Southeast Asian basins. We had an opportunity to investigate the Sulu Sea and the South China Sea during the KH-02-4 cruise of R/V Hakuho-Maru (Nishida and Gamo, 2004). In this paper, we describe the distribution of dissolved (D) and acid-dissolvable (AD) bioactive trace metals, such as Fe, Co, Ni, Cu, Zn and Cd, and the metal/macronutrient ratios (D-Cd/PO4, Ni/ PO4 and Zn/Si(OH)4) in these basins and compare the results with those previously obtained by using the same method in the western North Pacific (Ezoe et al., 2004), and discuss the mechanisms that control the inter-basin fractionation of these metal concentrations and ratios. 2. Methods 2.1. Sampling Sampling locations are shown in Fig. 1. Stn 10 (81500 N 1211480 E, Sulu Sea) and Stn 16 (131300 N 1191300 E, South China Sea) were occupied during the KH-02-4 cruise (November–December 2002) of R/V Hakuho-Maru (The University of Tokyo), and

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A

B

40 °N

western North Pacific

30 °N

Stn16

20 °N

Luzon Strait

Mindoro Strait

BO 07

South China Sea

Philippine Sea

10 °N

10 °N

Stn10

Izu-BoninMariana Ridge Sulu Sea

EQ

Sulu Sea 6 °N Sibutu Strait

10 °S 120 °E

130 °E

140 °E

150 °E

120 °E

124 °E

Fig. 1. Sampling locations, Stn 10 (the Sulu Sea) and Stn 16 (the South China Sea) during the KH-02-4 cruise and BO 07 (the western North Pacific) during the KH-00-3 cruise of R/V Hakuho-Maru. Maps show the southeast Asian basins (A) and the Sulu Sea enlarged in (B).

BO 07 (211590 N 1501590 E) during the KH-00-3 cruise (June–July 2000) of the same vessel (Ezoe et al., 2004). Water samples were collected with 12-L X type or lever-action type Niskin bottles (General Oceanics, FL, USA) mounted on an epoxy-coated aluminum frame of a CTD-carousel system (SeaBird Electronics, Washington, USA) suspended on a titanium-armored cable (8 mm). The interiors of the Niskin bottles were coated with Teflon and thoroughly cleaned with detergent and HCl. Seawater was transferred from the Niskin bottle to an acid-cleaned polyethylene bottle (Nalge, LDPE) on board. Immediately, a portion of the sample was filtered through an acid-cleaned polycarbonate Nuclepore membrane 0.2-mm filter by N2 gas pressure in a clean room (class 100) on the vessel and acidified to pH 2.2 with 420 mL of 6 M ultrahigh purity HCl (TAMAPURE AA-10, Tama Chemicals) for dissolved (D) species. Likewise, a portion of seawater was acidified without filtration for AD species. Details of the sampling, filtration and acidification are described in a previous paper (Ezoe et al., 2004). 2.2. Analysis The preconcentration and determination of dissolved (D) and AD trace metals were carried out in

our laboratory using the established analytical method (Sohrin et al., 2001; Ezoe et al., 2004). Briefly, the seawater sample was preserved for 1 yr at temperatures varying seasonally from
0 1C in winter to 38 1C in summer. Some 250 mL of the seawater sample, the pH of which was adjusted to 4.5 with ultra-high purity NH4OAc buffer (103 M), were passed through a column of fluorinated metal alkoxide glass immobilized 8-hydroxyquinoline (MAF-8HQ). Sea salt remaining in the column was washed away with 50 mL of the NH4OAc buffer (103 M). Back flushing with 25 mL of ultrahigh purity HNO3–H2O2 (0.5–103 M) eluted the metals. The concentrations of trace metals were determined with an inductively coupled plasmamass spectrometer (ICP-MS DRC II, Perkin-Elmer, Wellesley, USA) using standard solutions containing HNO3–H2O2. All the procedures were carried out within a clean bench to avoid any contamination from airborne particles. Table 1 shows results of the determination of trace metals in a seawater reference material for trace metals, NASS-5 (National Council Canada, Ottawa, Canada). Although concentrations of most of the trace metals were accurately determined, the measured concentrations of Co and Cd were 7075% and 7575% of the certified values, respectively, under the precise control of pH

ARTICLE IN PRESS K. Norisuye et al. / Deep-Sea Research II 54 (2007) 14–37 Table 1 Analytical results of seawater reference material NASS-5 Element

Fe Fe Co Ni Ni Cu Cu Zn Zn Cd Cd

Measured isotope

54 56 59 60 62 63 65 66 68 111 114

n

3 3 3 3 3 3 3 3 3 3 3

Concentration (nM) Determined

Certified

3.6970.18 3.7270.38 0.1370.01 3.9970.01 3.9970.02 4.4470.04 4.2970.03 1.2570.15 1.2370.16 0.1570.01 0.1670.01

3.7170.63 3.7170.63 0.1970.05 4.3170.48 4.3170.48 4.6770.72 4.6770.72 1.5670.60 1.5670.60 0.2070.03 0.2070.03

(70.02). The low recovery for Cd is attributable to the relatively low affinity of Cd to 8HQ in the presence of Cl. Recovery for Cd was further examined by the analysis of seawater to which known amounts of Cd were added; the resulting recovery was 72% (n ¼ 6). The reason for the low recovery for Co is not well known but it could involve oxidation of Co(II) to inert Co(III) on the column with the latter species being slowly eluted. Further investigation was carried out by the addition of known amounts of Co to seawater. The recovery for Co was 60%, being lower than those calculated from the certified value (70%). Considering these results, we corrected the measured concentrations of Co and Cd in seawater samples against the certified values, as was done in a previous study (Ezoe et al., 2004). Precisions were better than 10%. Analytical blank was examined for the entire step of the preconcentration including acidification, buffering, sample loading, washing and eluting for each MAF-8HQ column, and deionized water purified with a Milli-Q system was substitutively used for seawater. The blank tests were carried out before and after a suite of analysis for the filtered or unfiltered samples from each station. The blank values were 0.03, 0.0003, 0.02, 0.01, 0.05 and 0.0003 nM for Fe, Co, Ni, Cu, Zn and Cd, respectively. Although blank values for the filtration were not determined, they would be sufficiently low because the present study was subjected to the same protocol for the filtration and washing as for the previous study analyzing open-ocean samples (Ezoe et al., 2004).

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3. Results All the data for temperature, salinity, sigma-T, dissolved oxygen, macronutrients, Chl a and trace metals at Stn 10 and Stn 16 are presented in Table 2. Except for sigma-T and trace metals, the data were taken from the preliminary report of the KH-02-4 cruise (Nishida and Gamo, 2004). The sigma-T was calculated from the salinity and temperature according to the way of Millero (2000). As for trace metals, although only the D-species were determined at Stn 16, both D- and AD-species were determined at Stn 10 and BO 07. All the data for BO 07 were quoted from Ezoe et al. (2004). The data of samples that were obviously contaminated or which could not be determined were removed. The former includes the concentrations of D-Fe at depths of 5 and 30 m at Stn 10, which were significantly higher than those of AD-Fe. 3.1. Hydrography Figs. 2(A) and (B) show vertical profiles of temperature and salinity in the upper water column (o500 m) at Stn 10, Stn 16 and BO 07. The values of salinity in the surface-mixed layer at Stn 16 were
33.2, indicating the influence of fluvial/coastal waters. The average salinity in the surface mixed layer at Stn 10 was
34.2, taking a medium value between that at Stn 16 and BO 07. Salinity and temperature in a depth range of 200–500 m at Stn 10 were close to those at Stn 16. Thus, it seems plausible that the water at this depth range in the Sulu Sea comes from the South China Sea through the Mindoro Strait with a sill depth of 420 m (Quadfasel et al., 1990; Nozaki et al., 1999), and near-surface water comes from the western North Pacific during the winter monsoon (Amakawa et al., 2000). As shown in Fig. 2(D), temperature below 500 m at Stn 10 was approximately 10 1C, which was much higher than that at Stn 16 and BO 07 (1.5–3.3 1C). At Stn 10, the values of salinity in the deep water were also close to that at a depth of 500 m (Figs. 2(B) and (E)). Thus, the deep water of the Sulu Sea probably derives from the neighboring South China Sea, flowing just above the Mindoro Strait. The renewal time of the deep water of the Sulu Sea is estimated to be 3007150 yr (Nozaki et al., 1999). This rapid exchange of the deep water is due to vertical mixing within the basin. As a result, the shallow and deep

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Depth (m)

Temperature (1C)

Salinity

Sigma-T

NO2 (mM)

NO3 (mM)

PO4 (mM)

Si(OH)4 (mM)

204

ND

0.63

0.05

1.27

0.13

201 206 198 191 186 108 87 85 77 73 80 81 74 67 67 59 55 52 51 51 51 50 51

0.02 ND ND 0.23 0.38 0.03 0.01 ND ND

0.74 0.47 0.39 0.80 1.62 15.0 20.0 22.7 24.9 27.6 28.9 29.9 30.7 31.1 31.0 31.5 31.4 31.3 31.2 31.1 31.0 31.0 31.0

0.05 0.04 0.07 0.11 0.15 0.96 1.28 1.47 1.64 1.92 1.99 2.06 2.14 2.18 2.19 2.24 2.20 2.21 2.26 2.24 2.23 2.24 2.22

1.21 1.24 1.38 2.01 2.75 18.3 24.1 30.0 36.8 47.2 52.1 58.6 64.4 70.5 73.2 81.2 87.0 91.9 93.6 94.3 94.3 94.3 94.1

0.16 0.20 0.22 0.64 0.45 0.05 0.01 0.01 0.01

ND ND ND 0.06

ND ND ND ND

0.63 0.59 0.65 1.45

0.13 0.13 0.16 0.31

Stn 16 (131300 N, 1191300 E; December 9, 10, 2002) 11 28.643 33.231 20.857 201 20 28.648 33.233 20.857 201 30 28.577 33.291 20.923 202 49 27.339 33.811 21.716 192

ND ND ND ND

Chl a (mg L1)

D-Fe (nM)

1.5 2.3 0.7 1.0 3.5 1.8 1.1 1.0 3.3 1.0 1.7 1.4 0.9 1.0 1.0 0.7 1.3 0.6 0.5 0.4 0.2 0.2

0.54 1.21 0.74 0.25

AD-Fe (nM)

D-Co (pM)

AD-Co (pM)

D-Ni (nM)

AD-Ni (nM)

D-Cu (nM)

AD-Cu (nM)

D-Zn (nM)

AD-Zn (nM)

D-Cd (nM)

1.6 2.1 3.0 1.4 5.0 6.0 9.9 10.4 10.2 8.6 13.1 16.8 8.8 6.8 7.8 9.0 12.3 10.0 9.9 9.9 9.5 9.8 9.1 9.6

32 31 32 7 8 8 10 9 8 26 28 22 24 22 20 13 12 11 9 6 7 ND ND 1

51 54 54 48 50 50 46 40 34 36 31 30 32 22 15 11 8 11 15 13 13 ND ND 4

2.28 2.35 2.41 2.43 2.45 2.41 3.30 3.66 3.97 4.52 5.01 5.20 5.50 5.78 5.95 6.02 6.28 6.61 6.63 5.91 5.70

2.51 2.59 2.56 2.43 2.55 2.55 3.42 3.65 4.03 4.45 4.94 5.29 5.10 5.45 5.40 5.59 5.26 6.13 6.51 6.52 6.46 6.42 6.22 6.36

1.08 0.97 0.91 0.83 0.72 0.80 1.22 1.15 1.14 1.20 1.26 1.20 1.16 1.23 1.25 1.28 1.33 1.66 1.76 1.61 1.55 1.54 2.00 1.65

1.62 1.17 1.18 0.96 0.89 0.94 1.41 1.37 1.40 1.38 1.44 1.38 1.34 1.36 1.38 1.43 1.54 1.92 1.82 1.98 1.98 2.15 1.93 2.09

4.30 0.50 0.55 1.90 0.49 0.26 1.24 2.23 2.91 2.46 3.22 4.12 2.45 3.50 3.15 3.24

4.84 0.49 0.40 2.11 0.41 0.47 1.86 1.62 4.32 2.65 2.97 4.71 3.07 2.86 2.92

ND ND ND ND ND ND 0.21 0.28 0.16 0.45 0.43 0.28 0.57 0.49 0.53 0.56 0.45 0.64 0.55 0.44 0.45

52 50 51 40

2.70 2.61 2.60 2.36

1.59 1.42 1.42 1.28

3.43 3.81 3.09 3.01

1.38 0.64 0.37 0.66

3.81 3.62 3.20 3.86 3.18 4.27

ND 0.023 0.024 ND

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Stn 10 (81500 N, 1211480 E; December 3, 2002) 0 28.900 34.169 21.476 5 10 28.702 34.169 21.542 20 28.624 34.201 21.592 30 28.383 34.266 21.719 50 27.940 34.319 21.904 75 27.701 34.345 22.002 100 23.326 34.391 23.379 125 18.815 34.434 24.637 150 15.983 34.497 25.366 199 14.081 34.498 25.782 299 12.459 34.469 26.088 497 10.940 34.461 26.367 745 10.259 34.448 26.477 992 10.113 34.531 26.567 1239 10.098 34.449 26.505 1486 10.105 34.455 26.509 1979 10.148 34.464 26.508 2471 10.211 34.469 26.502 2962 10.285 34.474 26.492 3453 10.364 34.509 26.507 3940 10.444 34.475 26.466 4427 10.527 34.475 26.451 4845 10.601 34.474 26.438 4941 10.618 34.474 26.435

D-O2a (mM)

K. Norisuye et al. / Deep-Sea Research II 54 (2007) 14–37

Table 2 Data for hydrography, dissolved oxygen, macronutrients, Chl a, and dissolved (D) and acid-dissolvable (AD) trace metals

34.230 34.385 34.573 34.583 34.559 34.475 34.430 34.435 34.477 34.541 34.574 34.596 34.622 34.628 34.630 34.628 34.629 34.626 34.627 34.627 34.619

22.963 24.127 25.125 25.383 25.738 26.234 26.602 26.767 27.127 27.397 27.510 27.573 27.630 27.644 27.645 27.640 27.638 27.632 27.627 27.626 27.620

140 113 114 117 117 110 95 83 79 87 92 95 103 107 111 110 113 114 115 115 115

0.11 0.02 ND 0.01 ND

6.03 13.4 16.4 17.3 19.3 24.1 29.4 32.4 36.3 38.5 39.3 39.9 39.7 39.2 39.2 39.3 39.3 39.1 38.9 39.0 39.0

0.43 0.85 1.08 1.15 1.32 1.66 2.07 2.32 2.64 2.81 2.88 2.90 2.87 2.78 2.77 2.80 2.79 2.79 2.75 2.78 2.80

6.40 10.5 15.4 17.7 23.1 37.5 57.0 69.7 100 129 142 151 154 153 152 152 151 148 150 148 149

0.28 0.05 ND ND ND

0.96 1.00 0.63 0.55 0.77 0.74 0.98 1.01 0.86 1.03 0.95 0.82 0.60 1.42 0.62 0.83 1.14 1.05 0.92 0.82

ND, not detected. a Original data in a unit mL L1 were multiplied by 44.6 to obtain D-O2 concentrations in a unit mM.

42 36 28 26 25 27 28 23 16 16 15 21 11 9 9 11 16 28 30 30 3

2.62 2.86 2.98 3.07 3.55 4.55 5.67 6.04 6.97 8.06 8.42 9.06 8.96 9.00 8.87 9.09 9.36 9.44 9.64 9.46 8.76

1.08 1.06 0.91 0.81 0.94 1.18 1.27 1.34 1.63 1.96 2.35 2.53 2.65 2.88 2.87 2.92 2.96 3.04 3.06 3.10 2.99

0.43 0.70 1.18 1.14 1.68 2.51 3.80 4.64 5.39 7.68 7.64 8.15 8.97 8.76 9.46 8.73 9.15 8.65 8.77 8.90

ND ND 0.27 0.27 0.35 0.49 0.59 0.65 0.73 0.76 0.83 0.89 0.80 0.80 0.88 0.80 0.75 0.92 0.85 0.84 0.87

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24.329 20.631 17.261 16.194 14.515 11.722 9.434 8.417 6.096 4.221 3.375 2.883 2.469 2.365 2.369 2.406 2.445 2.491 2.553 2.561 2.569

K. Norisuye et al. / Deep-Sea Research II 54 (2007) 14–37

74 99 124 148 197 296 397 497 745 992 1239 1487 1980 2470 2961 3449 3937 4428 4914 4975 5030

19

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A

B

Temperature (°C) 0

10

20

30

33

30

33

C

Salinity 34

35

36

35

36

20

Sigma-T 24

28

0

Depth (m)

100 200 300 400 500

D

E

Temperature (°C) 0

10

20

F

Salinity 34

20

Sigma-T 24

28

0

1000

Depth (m)

2000

3000

4000

5000

6000 Fig. 2. Vertical profiles of temperature, salinity and sigma-T at Stn 10 (circle), Stn 16 (diamond) and BO 07 (square). Sigma-T was calculated according to the way of Millero (2000).

waters of the Sulu Sea are heavier and lighter, respectively, than waters at the same depths of the western North Pacific (Figs. 2(C) and (F)). 3.2. Chl a and bioactive trace metals in the upper water column at Stn 10 and Stn 16 Fig. 3 shows vertical profiles of trace metals and Chl a in the upper water column at Stn 10. The concentration of D-Ni and Cd was low in the surface mixed layer and increased with depth. Profiles of D-Zn, Cu and Fe showed a minimum at a depth range of 50–75 m, which coincided with the enhanced maximum in Chl a (
0.65 mg L1 at a depth of 50 m). The concentrations of D-Co were also low at a depth range of 30–150 m.

The average AD-metal/D-metal ratios in the upper 500 m at Stn 10 were 4.273.1, 3.872.3, 1.070.1, 1.270.1, 1.170.3 and 1.270.7 for Fe, Co, Ni, Cu, Zn and Cd, respectively. The concentration of AD-Fe was
1.6 nM in the surface water and then rapidly increased to 10 nM at a depth of 100 m. On the other hand, the concentrations of D-Fe ranged from 0.7 to 3.5 nM with minima of 0.7 and 1 nM at a depth of 50 and 200 m, respectively. Fig. 4 shows vertical profiles of D-trace metals and Chl a in the upper water column at Stn 16. The Chl a maximum was observed at a depth of 50–75 m (
0.3 mg L1), which coincided with a minimum DZn concentration (
75 m). Concentrations of D-Co and Cu above 150 m at Stn 16 were high (Fig. 4) and this trend was not

ARTICLE IN PRESS K. Norisuye et al. / Deep-Sea Research II 54 (2007) 14–37

A

B

Fe (nM) 0

10

20

0

Co (pM) 30

60

C

Ni (nM)

0

4

21

8

D

Cu (nM)

0

1

2

0

Depth (m)

100 200 300 400 500

E

Zn (nM) 0

3

F 6 0

Cd (nM) 0.4

G 0.8 0

Chl a (µg L-1) 0.4

H 0.8 0

Fe (nM) 2

4

0

Depth (m)

100 200 300 400 500 Fig. 3. Vertical profiles of dissolved (open circle) and acid-dissolvable (closed circle) trace metals in the upper water column (o 500 m) at Stn 10 (A–F) and a profile of Chl a (G). The D-Fe profile also is given in a small concentration range (H).

prominent at Stn 10 (Fig. 3). In contrast, concentrations of D-Fe and Zn in the surface layer at Stn 10 (
2 and 4.3 nM, respectively) were high (Fig. 3), which was not prominent at Stn 16 (Fig. 4). Profiles of D-Ni and Cd did not show a large difference between the two stations.

3.3. Trace metals in the deep water column at Stn 10, Stn 16 and BO 07 3.3.1. D- and AD-trace metals at Stn 10 Fig. 5 shows vertical profiles of D- and AD-trace metals in the deep-water column at Stn 10. The average AD-metal/D-metal ratios at a depth range of 1000–4000 m were 1275, 1.370.6, 1.070.1, 1.170.1, 1.070.1 and 1.370.1 for Fe, Co, Ni, Cu, Zn and Cd, respectively. The distribution of DFe exhibited a scavenging-type profile with a gradual decrease in concentration from 1.7 nM at a depth of 750 m to 0.2 nM near the bottom (Fig. 6(A), open circles). The AD-Fe concentrations were very high (6.8–17 nM) throughout the water column and showed a sharp maximum (17 nM) at a depth of 500 m (Fig. 5(A)). At this depth, AD-Zn also exhibited a distinct maximum (4.7 nM).

3.3.2. Comparison of D-trace metals between the three deep water columns Fig. 6 shows vertical profiles of D-trace metals at Stn 10, Stn 16 and BO 07. The profiles for Stn 10 are shown again to make an inter-basin comparison. The D-Fe and the D-Co profiles showed considerably large geographic variations. The D-Fe profile at BO 07 was nutrient-like, while that at Stn 10 scavenging-type. The concentrations of D-Fe at Stn 10 and Stn 16 were higher throughout the water column than those at each corresponding depth at BO 07. Below 4000 m, the concentration of D-Fe decreased with depth to a bottom value of 0.2 nM at Stn 10, while it was elevated to 1 nM at Stn 16 and relatively constant (0.5 nM) at BO 07. The distribution of D-Co at all stations showed a gradual decrease with depth below 1000 m, while the concentration of D-Co was elevated near the bottom at Stn 16 (
30 pM) and depleted at Stn 10. Profiles of D-Ni, Cu, Zn and Cd at Stn 16 and BO 07 were close to each other, although there were differences in a deep water maximum (Fig. 6): the profile of D-Zn at Stn 16 did not show a deep water maximum that was observed at a depth of 1500 m at BO 07. As for D-Ni and D-Cd, a deep-water maximum observed at a depth between 1000 and

ARTICLE IN PRESS K. Norisuye et al. / Deep-Sea Research II 54 (2007) 14–37

22

A

D-Fe (nM) 0

1

2

B

D-Co (pM)

0

30

F

D-Cd (nM)

0

0.4

60

C

D-Ni (nM)

0

4

D

D-Cu (nM)

0

1

8

2

0

Depth (m)

100 200 300 400 500

E

D-Zn (nM) 0

3

6

G 0.8 0

Chl a (µg L-1) 0.4

0.8

0

Depth (m)

100 200 300 400 500 Fig. 4. Vertical profiles of trace metals in the upper water column (o500 m) at Stn 16 (A–F) and a profile of Chl a (G).

1500 m at Stn 16 was slightly broader than that at BO 07. The distribution of D-Ni, Cu, Zn and Cd at Stn 10 was more uniform below 500 m compared to that at BO 07 (Fig. 6). The concentrations of D-Ni, Cu, Zn and Cd below 500 m were considerably lower at Stn 10 than at the other stations. The average concentrations of D-Ni, Cu, Zn and Cd in a depth range of 1000–4000 m were 6.1, 1.5, 3.3 and 0.5 nM at Stn 10, and 9.6, 2.8, 10.1 and 1.0 nM at BO 07, respectively. 4. Discussion 4.1. Trace metals in the upper water column at Stn 10 and Stn 16 For Ni, Cu, Zn and Cd, the average AD-metal/Dmetal ratios at Stn 10 were 1.0–1.2, indicating a predominance of dissolved species. We observed that the minima of some D-trace metals coincided with the Chl a maximum at Stn 10 and Stn 16 (Figs. 3 and 4). This can be attributed to scavenging of these metals including uptake by phytoplankton in the euphotic zone. Concentrations of D-Co and Cu above 150 m were higher at Stn 16 than Stn 10 (Figs. 3 and 4).

The high concentrations seemed to be associated with low salinity (Figs. 2 and 4) as seen in the following correlations between the concentration of D-metals and salinity in the upper water column: DCo ¼ 617  17  Salinity DCu ¼ 14:8  0:40  Salinity

ðr2 ¼ 0:90; n ¼ 10Þ, ðr2 ¼ 0:87; n ¼ 10Þ.

Saito et al. (2004) reported dissolved Co did not correlate with salinity but more correlated with macronutrients in the Peru upwelling system, and invoked reduced sediments as the important source of Co in the upwelled and surface waters in this system. Knauer et al. (1982) measured dissolved Co in the San Francisco Bay and off the bay, and reported that the high levels of Co in nearshore waters probably resulted from continental weathering input processes as suggested by a remarkable Co-salinity correlation. Also, dissolved Co correlated strongly with salinity (r2 ¼ 0.93) in a surface transect from the Sargasso Sea to coastal zone off New England (Saito and Moffett, 2002). The relationship indicates a Co concentration of 785 pM at zero salinity, which is comparable to the D-Co concentration at zero salinity in this

ARTICLE IN PRESS K. Norisuye et al. / Deep-Sea Research II 54 (2007) 14–37

A

B

Fe (nM) 0

10

20

0

4

0

Co (pM)

23

C

Ni (nM) 5

30

60

0

Zn (nM) 6

12

0

10

0

1000

Depth (m)

2000

3000

4000

5000

6000

D 0

Cu (nM) 2

E

F

Cd (nM) 0.6

1.2

0

1000

Depth (m)

2000

3000

4000

5000

6000 Fig. 5. Vertical profiles of dissolved (open circle) and acid-dissolvable (closed circle) trace metals over the entire depth range at Stn 10.

study. Although the value for Co at zero salinity in this study (617 pM) is not a representative fresh water-endmember due to the scavenging of the metal (Saito and Moffett, 2002), the clear metalsalinity relationships in the above equations indicate Co and Cu derive from coastal zone or as wet deposition. We discuss below the relative importance of these sources for Co and Cu. Although an annual precipitation in the South China Sea is large (
2000 mm), our observation was conducted during the dry season. Further, Nozaki

et al. (1998) have published data that showed very high concentrations of estuarine and/or shelf-sediment derived 228Ra and low concentrations of atmospherically derived 210Pb at four stations in the South China Sea. Thus we consider that coastal sources are more important than wet deposition for Co and Cu in the present study. Several large rivers flow into the South China Sea, such as the Mekong River, with an annual discharge of suspended sediments and water as high as 160 million tons (Chester, 2000) and 450 billion m3 (Kite, 2001),

ARTICLE IN PRESS K. Norisuye et al. / Deep-Sea Research II 54 (2007) 14–37

24

A

B

D-Fe (nM) 0

2

6

0

C

D-Co (pM) 30

60

D-Ni (nM)

0

5

F

D-Cd (nM)

10

0

1000

Depth (m)

2000

3000

4000

5000

6000

D

D-Cu (nM) 0

2

4

E

D-Zn (nM)

0

6

12 0

0.6

1.2

0

1000

Depth (m)

2000

3000

4000

5000

6000 Fig. 6. Comparison of vertical profiles of dissolved (D) trace metals at Stn 10 (circle), Stn 16 (diamond) and BO 07 (square).

respectively, and the Chao Phraya and Pearl rivers. Cu from fluvial sources is often reported to behave conservatively within the estuarine mixing zone (Boyle et al., 1982; Danielsson et al., 1983; Edmond et al., 1985), while Co may be scavenged. Alternatively, Co and Cu may derive from reduced sediments (Saito et al., 2004) and from mildly reduced sediments (Boyle et al., 1981), respectively. The data of Nozaki et al. (1998) show that salinity in surface water decreases monotonically from 34.12 at S 53 (191550 N 1191270 E) to 32.48 at S 51 (31310 N

1081320 E), indicating mixing of waters in the northern basin with less saline waters deriving from the southwestern basin where vast area is shallow shelf-zone. In the case of the present study, the shallow sediments of the vast area as well as nearby sedimentary sources may influence the concentrations of Co and Cu. Thus, the high concentrations of Co and Cu above 150 m at Stn 16 could result from fluvial/coastal waters and/or shelf sediments, though the relative importance of these sources cannot be elucidated at present.

ARTICLE IN PRESS K. Norisuye et al. / Deep-Sea Research II 54 (2007) 14–37

Alibo and Nozaki (2000) found that there were excess REEs in the upper 150 m of the water column at PA 11 (151220 N 1151170 E), and noted that surface waters of the East China Sea and the West Philippine Sea as well as coastal/fluvial and shelf waters also contribute to the surface water at PA 11. Surface water circulation in the South China Sea is strongly influenced by monsoonal forcing with cyclonic flow during the northeast monsoon in winter (Wu et al., 2003). Compared to PA 11, the upper waters at Stn 16, which is located near the eastern rim of the basin, would be more strongly affected by this anticlockwise circulation, and thus would be less influenced by the surface waters of the East China Sea and the West Philippine Sea. We observed higher D-Fe and Zn concentrations in near-surface water at Stn 10 than Stn 16 (Figs. 3 and 4). The dissolved Zn concentrations in surface water of the eastern North Pacific (Bruland, 1980) and the subantarctic region east of New Zealand (Ellwood, 2004) did not show such a surface maximum. Since our samples of surface water were collected near the vessel, it might be a result of contamination. However, Ezoe et al. (2004) reported a regional difference in D-Zn concentrations in surface waters observed by the same vessel, R/V Hakuho-Maru, with high concentrations of D-Zn (3–7 nM) at depths of 0–30 m in the oligotrophic western North Pacific compared to undetectable levels (o0.4 nM) in the subarctic North Pacific (Fujishima et al., 2001). Thus, the high concentrations observed in the Sulu Sea and the South China Sea cannot be thoroughly ascribed to contamination from the vessel. The Zn enrichment in surface waters of the Sulu Sea is a contrast to the lack of such a feature in the eastern North Pacific Central Gyre (Bruland, 1980), though such a difference could be produced by the differences in region and analytical procedures, including sample storage. It is unknown whether such high levels of Fe and Zn are sporadically brought in via aeolian input to the Sulu Sea. Wyrtki (1961) described that the surface water of the Sulu Sea during the winter monsoon comes from the western North Pacific. The water across the Philippine Archipelago is presumed to have been enriched with metals derived from weathering of young oceanic islands as inferred from a highly radiogenic eNd value of 1.3 (Amakawa et al., 2000). Thus, the high concentrations of Fe in the surface water at Stn 10 are presumably caused by influx of such surface water. More investigation is required to identify

25

such a source for Zn, since sufficient data is not yet available. 4.2. Comparison of D- and AD-Fe in the deep water column at Stn 10 The average AD-metal/D-metal ratios were 1.0–1.3 for Co, Ni, Cu, Zn and Cd, indicating most of species are dissolved form. In contrast, the average AD-Fe/D-Fe ratio was high with an average of 1275. We obtained a scavenging-type profile of D-Fe showing a gradual decrease in concentration with depth from 1.7 nM at a depth of 750 m to 0.2 nM near the bottom. Further, we found the AD-Fe concentrations were considerably elevated throughout the deep-water column (6.8–17 nM), with a sharp maximum at a depth of 500 m. Neither a scavenging-type D-Fe profile nor high AD-Fe concentrations is generally found in the open ocean. Although the distribution of Fe is controlled by biological activity in surface water in the ocean, it is also influenced strongly by local sources, such as eolian input, hydrothermal input, and resuspension of marine sediments. Ezoe et al. (2004) reported that the concentrations of AD-Fe (0.7–4.7 nM) were significantly higher than those of D-Fe (0.4–1.4 nM) at each corresponding depth over the subarctic and subtropical North Pacific stations (BO 01–07), and discussed that the difference between AD- and D-Fe represents labile particulate Fe, which includes iron oxyhydoxides and species adsorbed on clay minerals and incorporated in organisms. They further argued that the general increase in the AD-Fe concentrations toward the bottom arises from resuspension of sediments as previously reported for particulate Al (Bruland et al., 1994). Shelf sediments have been invoked to be the important source of Fe in coastal and upwelling systems in the absence of large river discharge (Croot and Hunter, 1998; Chase et al., 2002, 2005). Johnson et al. (1999) reported dissolvable Fe concentrations in the California Current System during strong upwelling conditions and attributed the source of Fe to resuspended sediments. They reported light transmission was markedly lower in waters with high Fe (60% decrease in transmission corresponding to an increase in
100 nM Fe concentration). The high AD-Fe concentrations at Stn 10 indicate that particulate Fe is abundant in the Sulu Sea. We observed a sharp maximum in the AD-Fe and Zn concentrations (17 and 4.7 nM, respectively)

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K. Norisuye et al. / Deep-Sea Research II 54 (2007) 14–37

at a depth of 500 m, which may be related to the maximum of dissolved oxygen at 500–600 m depth shown by Gamo et al. (2007). It is likely that the maximum of particulate Fe probably reflects the advection of water over the sill of the Mindoro Strait. The water is enriched with particulate Fe derived from the sediments of sill bottom and slopes, subjected to rapid vertical mixing, resulting in abundant particulate Fe in deep water. The light transmission less strongly responded to the AD-Fe variations in the present study than those of Johnson et al. (1999). This is probably because the levels of labile particulate Fe in the Sulu Sea are at best one-tenth those in the California Current System (Johnson et al., 1999). The reason of a gradual decrease in the concentration of D-Fe with depth is not clear. The D-Fe concentrations near the bottom at Stn 10 were
0.2 nM, being lower than those in the global deep waters (Parekh et al., 2004). Thus, it may be that scavenging is stronger in the Sulu Sea than in the open ocean. Alternatively, if D-Fe in the Sulu Sea consists primarly of colloidal fraction whose main source is resuspension from the sill bottom, apparent decrease in D-Fe with depth may be due to the coagulation of colloidal matter forming particulate matter. 4.3. Comparison of D-trace metal profiles in the deep water column between Stn 10, Stn 16 and BO 07 4.3.1. Comparison between Stn 16 and BO 07 Profiles of D-Fe and D-Co were variable and different between the two basins. The D-Fe concentrations at Stn 16 were higher than those at each corresponding depth at BO 07 throughout the water column. Below 4000 m, the elevated D-Fe concentrations of 1 nM were found at Stn 16. The D-Co profiles at the two stations showed a gradual decrease with depth from 1000 to 4000 m, while the elevated concentrations were found below 4000 m at Stn 16 (
30 pM). The oceanic residence times of Fe and Co are estimated to be
150 and
50 yr (Knauer et al., 1982; Bruland et al., 1994; Johnson et al., 1997), respectively. For surface waters, the residence times are considerably shorter (Saito and Moffett, 2002; Croot et al., 2004). Therefore, local input and removal of these metals are expected to reflect on the inter-basin fractionation. The high concentrations of D-Fe above 1000 m at Stn 16 can be attributed to resuspension of sediments from the

shelf of the South China Sea and the East China Sea. The elevated concentrations of D-Fe and Co below 4000 m probably derive from resuspended bottom sediments. The average concentrations of D-Ni, Cu, Zn and Cd at a depth range of 1000–4000 m at Stn 16 were 8.9, 2.6, 8.5 and 0.8 nM, which were comparable to, but slightly lower than, those at BO 07 (9.6, 2.8, 10.1, 1.0 nM, respectively). The average ratios of concentrations in the deep-water column at Stn 16 to those at BO 07 were calculated to be 0.92, 0.95, 0.84 and 0.81, for Ni, Cu, Zn and Cd, respectively. A profile of D-Zn at Stn 16 showed no deep water maximum such as observed at a depth of 1500 m at BO 07. As for D-Ni and D-Cd, a deep maximum at Stn 16 was slightly broader than that at BO 07. On the other hand, macronutrients showed a broader maximum and slightly higher concentrations below
3000 m at Stn 16 than those at BO 07 (Fig. 7). The average ratios of the concentrations in the deep column at Stn 16 to those at BO 07 were 1.00, 1.02 and 1.02 for NO3, PO4 and Si(OH)4, respectively. The deep water of the South China Sea is thought to derive from the neighboring Philippine Sea through the Luzon Strait. Broecker et al. (1986) estimated the flushing time of the deep water of the South China Sea is shorter than 100 yr based on the almost identical D14C value (–200%) at a depth range of 1300–4000 m between the South China Sea and the Philippine Sea. Although raw values of D14C in the South China Sea were slightly higher than those in the western North Pacific due to the sample leakage, correction of the values resulted in the comparable D14C values between the two regions (Broecker et al., 1986). Thus, the average concentrations of macronutrients at the depth range of 1000–4000 m are not significantly different between Stn 16 and BO 07. In contrast, Gamo (1993) argued that chemical characteristics of the Philippine Sea bottom waters are significantly deviated from the trends of the main northwestern Pacific bottom water based on diagrams of potential temperature vs. salinity, Si(OH)4 and D14C. It is probable that the topographic barrier effect of IzuBonin-Mariana Ridge sequence coupled with vertical mixing in the Philippine Sea broadens the intermediate O2 minimum, deep silica maximum, and deep D14C maximum in the Philippine Sea relative to those in the northwestern Pacific (Gamo, 1993). Thus, broad maxima in D-Ni and Cd, and disappearance of a D-Zn maximum at Stn 16 are also caused by a rapid flush of waters from the

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A

NO3 (µM) 0

25

50

B

PO4 (µM)

0

1.8

C 3.6 0

27

Si(OH)4 (µM) 80

160

0

1000

Depth (m)

2000

3000

4000

5000

6000 Fig. 7. Comparison of vertical profiles of macronutrients at Stn 10 (circle), Stn 16 (diamond) and BO 07 (square).

Philippine Sea, of which deviations in the chemical properties reflect the barrier effect coupled with mixing within the Philippine Sea. Also, relatively low concentrations of D-Ni, Cu, Zn and Cd are probably due to scavenging and vertical mixing within the South China Sea. 4.3.2. Comparison between Stn 10 and BO 07 Profiles of D-Fe and D-Co showed considerably large geographic variations. The D-Fe profile was nutrient-like at BO 07, while scavenging-type at Stn 10. Mostly, the D-Fe concentrations at Stn 10 were higher than those at each corresponding depth at BO 07 above 3000 m. The higher concentrations of D-Fe probably result from surrounding islands or shelves. This supports the Sulu Sea is particle-rich environment as seen in the higher AD-Fe concentrations (Fig. 5(A)). The D-Co profiles showed a gradual decrease with depth below 1000 m at both stations, while near-bottom concentrations at Stn 10 were more depleted. The shape of vertical profiles of D-Co is highly variable between various oceanic regimes: D-Co shows nutrient-like profiles in the upper water column of the Costa Rica upwelling system (Saito et al., 2005) and in the mid-latitude Northeast Pacific (Martin and Gordon, 1988). Water-column profiles of D-Co from the oligotrophic North Pacific (Ezoe et al., 2004) appear to be a combination of nutrient-type and scavenging-type, and the

vertical profile of this metal may fall best in the ‘hybrid type’ as expressed by Bruland and Lohan (2003). However, the D-Co profile at Stn 10 was a scavenging-type profile as observed inshore and in the subarctic North Pacific (Knauer et al., 1982; Fujishima et al., 2001; Ezoe et al., 2004). Saito et al. (2004) reported cobalt speciation measurements showing labile Co species occupied a significant fraction of the total dissolved Co in the Peru upwelling system while strongly organic binding species predominantly existed in South Pacific surface waters. Strongly complexed D-Co is probably stabilized in seawater and protected from particulate scavenging (Saito et al., 2005). We hypothesize that labile Co is a dominant species at least in the deep water of the Sulu Sea and is more easily removed by particulate matters. The best candidate of the scavenger is Mn-oxide as the distribution of particulate Co in deep waters of the Atlantic mimics that of particulate Mn (Scherrell and Boyle, 1992) and Mn nodules in the global marine sediments are enriched with Co. Thus, dissolved labile Co is scavenged by Mn-oxides, producing a scavenging type profile at Stn 10. Although profiles of D-Ni, Cu, Zn and Cd at BO 07 were nutrient-like, the concentrations of these metals at Stn 10 were more uniform below 500 m. The concentrations of D-Ni, Cu, Zn and Cd below 500 m were considerably lower at Stn 10 than BO 07. The average concentrations of D-Ni, Cu, Zn and

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K. Norisuye et al. / Deep-Sea Research II 54 (2007) 14–37

Cd in a depth range of 1000–4000 m were 6.1, 1.5, 3.3 and 0.5 nM at Stn 10 compared to those at BO 07 (9.6, 2.8, 10.1 and 1.0 nM, respectively) and at depths of 400 and 500 m at Stn 16 (5.9, 1.3, 4.2 and 0.6 nM, respectively). Further, the distribution of macronutrients also showed lower concentrations at Stn 10 than at BO 07 (Fig. 7). The concentrations of REEs except for Ce were also low in the Sulu Sea (Nozaki et al., 1999). The ratios of the concentrations in the deep water of Stn 10 to those of BO 07 were calculated to be 0.80, 0.80, 0.56, 0.64, 0.53, 0.33, 0.51, 0.62, 0.49, 0.76 and 0.66 for NO3, PO4, Si(OH)4, Ni, Cu, Zn, Cd, Y, La, Gd and Er, respectively (data for Y and REEs were taken from Table 2 in Nozaki et al., 1999). The age of the deep water of the Sulu Sea has been estimated to be several hundred years (Quadfasel et al., 1990; Broecker et al., 1986; Nozaki et al., 1999), which is a consequence of rapid exchange of the deep water with water from the South China Sea. Concentrations of nutrient-type metals (Ni, Zn and Cd) and Cu are known to be higher in Pacific old deep waters than in young deep waters due to the regeneration coupled with thermohaline circulation (Chester, 2000). Thus, the lowest concentrations of macronutrients and nutrient-type trace metals at Stn 10 can be explained as follows: since the Sulu Sea is a semi-enclosed basin with a maximum sill depth of 420 m, the deep water of the Sulu Sea is isolated from Pacific old deep waters enriched with regenerated elements. Instead, a supply of macronutrients and nutrient-type trace metals to the deep water of the Sulu Sea depends on vertical mixing and biological recycling. Also, the rapid renewal of the deep water prevents regenerated elements from residing in the basin for a long time. 4.4. Comparison of metal/macronutrient ratios in the deep-water column at Stn 10, Stn 16 and BO 07 In order to have a better understanding of biogeochemical cycling, we describe the D-metal/ macronutrient ratios (in a unit nM/mM) and discuss the factors controlling the inter-basin fractionations of these ratios. 4.4.1. D-Cd/PO4 ratios The D-Cd/PO4 ratios in the upper waters at Stn 10, Stn 16 and BO 07 are compared in Fig. 8(A). The ratios in the surface mixed layer were very low at Stn 10 and Stn 16 due to the undetectable levels

of D-Cd, while the ratios at BO 07 were not available because the levels of PO4 were below the detection limits. Below the surface mixed layer, the D-Cd/PO4 ratios increased (Figs. 8(A) and (D)). The average D-Cd/PO4 ratios in a depth range of 200–500 m were 0.2170.07 (n ¼ 3), 0.2870.01 (n ¼ 4) and 0.1970.09 (n ¼ 4) for Stn 10, Stn 16 and BO 07, respectively. The average D-Cd/PO4 ratios in a depth range of 1000–4000 m were 0.237 0.04 (n ¼ 8), 0.2970.02 (n ¼ 8) and 0.3670.03 (n ¼ 7) for Stn 10, Stn 16 and BO 07, respectively. The average ratio in this depth range at BO 07 was close to the value of 0.33, a typical ratio observed in North Pacific deep waters (de Baar et al., 1994). The average ratio in the deep water (0.36) is significantly higher than that in the upper water (0.19) at BO 07. This is probably because Cd is preferentially taken up by phytoplankton relative to PO4 from oceanic surface waters and regenerated at depth from biogenic particles (Elderfield and Rickaby, 2000). This trend is consistent with Cd/PO4 profiles obtained from other locations in the North Pacific (de Baar et al., 1994; Elderfield and Rickaby, 2000). The average D-Cd/PO4 ratio in the deep water at Stn 10 (0.23), close to that of 0.21 in the upper water at the station, is significantly lower than that at BO 07 (0.36). This depletion of the D-Cd/ PO4 ratio at depth at Stn 10 might be due to the preferential biological uptake of PO4 relative to Cd in surface waters under the favorable growth conditions of diatoms (Cullen et al., 2003; references therein). However, the D-Cd vs. PO4-plot at Stn 10 (Fig. 9) showing a concave rather than convex shape does not support this. Rather, we consider the lower D-Cd/PO4 ratios in the Sulu Sea is probably because the deep water of the Sulu Sea is rapidly exchanged with shallow waters but isolated from deep waters derived from the North Pacific with a high Cd/PO4 ratio of 0.33 (de Baar et al., 1994). The average ratio in the depth range of 1000– 4000 m at Stn 16 takes a medium value (0.29) of the other two average ratios at Stn 10 and BO 07 and it is similar to the upper ratio of 0.28 at Stn 16. A combination of a supply of waters with high D-Cd/ PO4 ratios from the Philippine Sea and vertical mixing within the South China Sea plays a role on the medium value. 4.4.2. D-Ni/PO4 ratios Figs. 8(B) and (E) show the D-Ni/PO4 ratios at Stn 10, Stn 16 and BO 07. The ratios in the surface mixed layer were very high (410) due to the

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A

D-Cd/PO4 (nM/µM) 0

0.2

0.4

B

D-Ni/PO4 (nM/µM)

0

10

E

D-Ni/PO4 (nM/µM)

0

3

20

29

C

D-Zn/Si(OH)4 (nM/µM)

0

0.3

F

D-Zn/Si(OH)4 (nM/µM)

0

0.06

0.6

0

Depth (m)

100

200

300

400

500

D

D-Cd/PO4 (nM/µM) 0

0.2

0.4

6

0.12

0

1000

Depth (m)

2000

3000

4000

5000

6000 Fig. 8. Comparison of vertical profiles of the D-Cd/PO4, Ni/PO4 and Zn/Si(OH)4 ratios at Stn 10 (circle), Stn 16 (diamond) and BO 07 (square). The Ni/PO4 and Zn/Si(OH)4 ratios for the three deepest data at Stn 10 were calculated from the AD-Ni and AD-Zn concentrations, respectively. Note different scales between upper and lower figures for Ni/PO4 and Zn/Si(OH)4 ratios.

depletion of PO4. Below the surface-mixed layer, the ratios took a minimum value (2.6–2.7) in a depth range of 300–750 m at all stations. Below the minimum layer the ratios showed a gradual increase with depth for BO 07, were relatively constant for Stn 10, or showed an intermediate trend for Stn 16. The average D-Ni/PO4 ratios at a depth range of 200–500 m were 2.770.1 (n ¼ 3–4) for Stn 10 and Stn 16, and 6.872.7 (n ¼ 4) for BO 07, respectively. The average D-Ni/PO4 ratios at a depth range of 1000–4000 m were 2.870.2 (n ¼ 8), 3.170.2 (n ¼ 8) and 3.570.3 (n ¼ 7) for Stn 10, Stn

16 and BO 07, respectively. The ratio at BO 07 was close to the average ratios observed in the North Pacific (3.5–4.0, Sclater et al., 1976; Bruland, 1980). The formation of the ratio-minimum layer can be explained by a combination of depletion of PO4 in the euphotic zone and regeneration of Ni at deeper layer than PO4. The latter is consistent with the fact that the concentration of D-Ni can be best correlated with a combination of those of softtissue-derived PO4 and hard-parts-derived Si(OH)4 (Sclater et al., 1976; Bruland, 1980). Based on these

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30 1.2 1.0

D-Cd (nM)

0.8 0.6 0.4 0.2 0.0 0.0

1.0

2.0 PO4 (µM)

3.0

4.0

Fig. 9. D-Cd vs. PO4 at Stn 10 (circle), Stn 16 (diamond) and BO 07 (square). The regression curve for Stn 10 represents [D-Cd] ¼ 0.0957[PO4]2.18, r2 ¼ 0.96.

results, we infer that D-Ni/PO4 ratios in global deep waters could increase with increasing the age of the waters. In spite of a large deviation of D-Ni/PO4 ratios in the upper waters, the average D-Ni/PO4 ratios in a depth range of 200–500 m at Stn 10 and Stn 16 show good agreement (a value of 2.7), and these average ratios are also close to the average value of 2.8 in the depth range of 1000–4000 m at Stn 10. These results also support that the deep water of the Sulu Sea probably comes from the neighboring South China Sea flowing through the Mindoro Strait. The average ratio in the deep water at Stn 10 is lower than that at BO 07. This depletion of the D-Ni/PO4 ratio at Stn 10 is probably due to the combination of vertical mixing within the basin coupled with the isolation of the deep water of the Sulu Sea from that of the western North Pacific. The average ratio in the depth range of 1000–4000 m at Stn 16 takes a medium value (3.1) between the two average ratios at depth at Stn 10 and BO 07, and is similar to the average ratio in the depth range of 200–500 m at Stn 16 (2.7). A combination of a supply of waters from the Philippine Sea and vertical mixing within the South China Sea plays a role on the uniform distribution with the medium value of the D-Ni/PO4 ratio. 4.4.3. Zn/Si(OH)4 ratios Figs. 8(C) and (F) show the D-Zn/Si(OH)4 ratios at Stn 10, Stn 16 and BO 07. The D-Zn/Si(OH)4

ratios in the surface mixed layer at all stations were very high due to the depletion of Si(OH)4. Below this depth range, the D-Zn/Si(OH)4 ratios decreased gradually to the depth of 1000 m. Below 1000 m, the ratios showed a gradual decrease with depth (Stn 10), a slight increase with depth (Stn 16), or a constant trend (BO 07). The average D-Zn/ Si(OH)4 ratios at a depth range of 200–500 m were 0.07170.007 (n ¼ 3), 0.06870.003 (n ¼ 4) and 0.5270.35 (n ¼ 4), for Stn 10, Stn 16 and BO 07, respectively. The average D-Zn/Si(OH)4 ratios at a depth range of 1000–4000 m were 0.0417 0.008 (n ¼ 8), 0.05870.003 (n ¼ 7) and 0.0707 0.006 (n ¼ 7) for Stn 10, Stn 16 and BO 07, respectively. The latter two ratios fell into a range of the average ratios obtained from 16 stations in a vast area of the subtropical and subarctic North Pacific (0.053–0.076, Bruland, 1980; Martin et al., 1989; Fujishima et al., 2001; Ezoe et al., 2004). As seen in the D-Ni/PO4 ratios, the average DZn/Si(OH)4 ratios in the 200–500 m column are close to each other between Stn 10 and Stn 16, suggesting the upper water of the Sulu Sea comes from the South China Sea at this depth range. However, the D-Zn/Si(OH)4 ratio decreases with depth (Fig. 8(F)) and the average D-Zn/Si(OH)4 ratio in the depth range of 1000–4000 m at Stn 10 is significantly lower than the average value in the depth range of 200–500 m at Stn 10. This suggests that Si(OH)4 may be preferentially supplied at depth from suspended particles and sediments relative to Zn. This is supported by the distribution of Si(OH)4, which shows a concomitant increase with depth (Fig. 7(C)). In contrast, Zn removed biologically from the euphotic zone appears to be rapidly regenerated within the shallow layer, showing the uniform vertical profile in the deep water column as like NO3 and PO4 (Figs. 5(E) and 7). The observed decoupling of Zn and Si(OH)4 in the vertical profile is consistent with the previous studies which show that Zn taken up by phytoplankton is mainly associated with organic tissue rather than diatom opal (Collier and Edmond, 1984; Ellwood and Hunter, 1999). The D-Zn/Si(OH)4 ratios in the depth range of 1000–4000 m are lower at Stn 10 than at BO 07. This is probably due to the relatively low D-Zn/Si(OH)4 ratios in the upper water (the source of the deep water) at Stn 10 coupled with the preferential supply of Si(OH)4 relative to Zn at depth.

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4.5. Regeneration of bioactive elements 4.5.1. Regeneration of bioactive elements in the isopycnal waters flowing from the South China Sea to the Sulu Sea As described above, the values of temperature and salinity at the depth range of 200–500 m at Stn 10 are almost the same as those at the similar depth range at Stn 16. Since the sill depth of the Mindoro Strait is 420 m, the waters at the depth range of
200–420 m will flow into the Sulu Sea. The flowing waters have sigma-T values of
25.7–26.6 (Table 2). In this situation, we can directly estimate how the concentrations of bioactive elements are modified by the regeneration in the isopycnal waters flowing from the South China Sea to the Sulu Sea. In Fig. 10, the concentrations of bioactive elements at Stn 10 and Stn 16 are plotted against the sigma-T values. Three lines in each figure represent isopycnal waters with sigma-T values of 25.738, 26.234 and 26.602, which can be found at depths of 197, 296 and 397 m, respectively, at Stn 16. The concentrations of nutrient-type elements at sigma-T values of
26.6 were not largely different between the two stations, suggesting that sizable regeneration does not occur during the flowing of the waters with these sigma-T values. On the other hand, the concentrations of macronutrients, D-Fe, Ni, Cu and Zn at sigma-T values between
25.7 and 26.6 were higher at Stn 10 than Stn 16 due to the regeneration of the elements or other inputs. We calculated the increment in the elemental concentrations at two sigma-T values of 25.738 and 26.234 assuming the elemental concentrations at these sigma-T values at Stn 10 result from the linear interpolation of neighboring data points. The results are given as an average for each element in Table 3. In Table 3, the increases in the elemental concentrations normalized to that of the PO4 concentration also are shown to be directly compared with the elemental compositions reported for plankton samples (Martin and Knauer, 1973; Martin et al., 1976; Collier and Edmond, 1984) or those calculated from the drawdown of the dissolved bioactive elements during the mesoscale Fe fertilization in the subarctic western North Pacific (Kinugasa et al., 2005). We also calculated the changes in the element/PO4 ratios in the isopycnal waters (Table 4). The increases in the concentrations of macronutrients and trace metals showed positive values (Table 3). The P-normalized increase in the con-

31

centration of NO3 was 15, which is close to the Redfield ratio (N/P ¼ 16), suggesting the effect of denitrification is small. Since the magnitude of changes in the D-Ni/PO4, Cu/PO4 and Zn/Si(OH)4 ratios are negligibly small (o2%, Table 4), D-Ni, Cu, Zn and perhaps Cd cycle together with macronutrients. The regenerated elemental composition was calculated to be N:P:Si:Ni:Cu:Zn:Cd ¼ 15:1:40:2.4  103:0.6  103:2.7  103:0.03  103. The P-normalized increase in the D-Zn concentration (2.7) is comparable with the composition of bulk plankton samples in the Central Equatorial Pacific (3.0) but considerably higher than other reported values (Table 3). The P-normalized increase in the DCd concentration, 0.03 nM mM1, seems to be lower than any reported values but the differences are insignificant due to the large uncertainty in our data. The P-normalized increases in the D-Ni and Cu concentrations are higher than any reported values. These differences between our data and the reported values for Zn, Ni and Cu may be due to the differences in the physiological status of organisms, community structure, and biological processes as one previous study provides the composition of phytoplankton while our data may contain the regeneration of elements from other biogenic sources, such as zooplankton, fecal pellets and organic aggregates. Oxygen consumption relative to PO4 regeneration in the isopycnal waters is –112 (Table 3). The value significantly deviates from the Redfield ratios of –138 in the classical definition or –150 of a new estimate (Li et al., 2006), but falls within the range of the DO2/DP ratios observed in the upper waters in the Arabian Sea (shallow waters above the DO2/ DP maximum-layer in Fig. 4 in Rixen and Ittekkot, 2005). The modest consumption of D-O2 relative to the regeneration of PO4 or NO3 in our data is not ascribed to diapycnal mixing as surrounding waters have low D-O2/PO4 ratios (o100 mmol D-O2/mmol PO4 below 150 m at Stn 10 and Stn 16), but will be due to a more rapid cycle of phosphorus and nitrogen than carbon (Tyrrell, 2001). We suggest that it is important to establish a consumption or regeneration stoichiometry of C:N:P:O2 in upper waters of the various oceanic regimes. Given this kind of stoichiometry, the analysis of the regeneration of bioactive trace metals in such waters can provide us valuable insights for the biogeochemical cycling of trace metals in the ocean. For example, if the flux of PO4 is given, other elemental fluxes also can be estimated using the composition of regenerated elements.

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The P-normalized increase in the D-Fe concentration (5.1) agrees well with the top three reported values of 4.6–5.2 in Table 3. However, the increase in the D-Fe/PO4 ratio was 137% (Table 4), suggesting this metal is supplied not as regeneration from biogenic particles but from other sources, such as shelf and sill sediments. The increase in

the D-Co/PO4 ratio was –16%, which implies that the scavenging of this metal cancels out the regeneration. 4.5.2. Regeneration of silica within the Sulu Sea The downward fluxes of bioactive elements are important since they are profoundly involved with

45

3.6

PO4 (µM)

NO3 (µM)

25.738 30

15

2.4

1.2 26.234 26.602 0.0

0 20

24

20

28

12

180

8

120

Si(OH)4 (µM)

D-Zn (nM)

24

28

Sigma-T

Sigma-T

4

60

0

0 20

24

20

28

24

28

Sigma-T

Sigma-T 4.5

8

3.0

D-Ni (nM)

D-Cu (nM)

12

1.5

4

0.0

0 20

24 Sigma-T

28

20

24

28

Sigma-T

Fig. 10. Plots of the concentrations of bioactive elements vs. sigma-T at Stn 10 (circle) and Stn 16 (diamond). Three lines represent waters with sigma-T values of 25.738, 26.234 and 26.602.

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2.4

AD-Fe (nM)

D-Fe (nM)

3.6

1.2

0.0

10

0 20

24

28

20

Sigma-T

24

28

Sigma-T

60

1.2

40

0.8 D-Cd (nM)

D-Co (pM)

33

20

0.4

0.0

0 20

24

20

28

Sigma-T

24

28

Sigma-T

D-O2 (µM)

220

110

0 20

24

28

Sigma-T

Fig. 10. (Continued)

the global carbon cycles. Since the Sulu Sea is a semi-enclosed basin where horizontal fluxes at depth from neighboring basins are negligible, we can directly estimate the vertical regeneration flux of bioactive elements when applying the two-box model to the present study. Ni, Zn, Cd, NO3, PO4

and Si(OH)4 generally show significant vertical concentration gradients in the open ocean. However, as shown in Figs. 5 and 7, the former five elements show small vertical gradients in the Sulu Sea. In contrast, the vertical gradient of Si(OH)4 concentrations in the Sulu Sea is large. This

34

Table 3 Changes in elemental concentrations in the isopycnal waters flowing from the South China Sea to the Sulu Sea

Increase in conc.a Percent increase (%)b P-normalized increasec Ratios in phytoplanktond Ratios in assemblages of planktone Ratios in assemblages of planktonf Decrease in conc.g

NO3

PO4

Si(OH)4

D-O2

D-Fe

D-Co

D-Ni

D-Cu

D-Zn

D-Cd

5.170.4 2476 1574

0.3570.07 2371 1.070.0 1.0 1.0 1.0 1.0

1471 48712 4075

3872 3370 112728

2.072.4 2667323 5.175.9 5.2 5.0 4.6 1.2

0.572.2 278 0.976.3

0.8070.15 2077 2.470.9 0.21 0.35 0.98 0.61

0.1870.10 18713 0.670.4 0.18 0.38 0.54 0.38

0.9570.17 4675 2.770.0 0.84 1.8 3.0 1.2

0.0070.10 3725 0.0370.30 0.07 0.46 0.61 0.05

16

27

23

Increase in ratiosa Percent increase (%)b

D-Fe/PO4 (nM/mM)

D-Co/PO4 (pM/mM)

D-Ni/PO4 (nM/mM)

D-Cu/PO4 (nM/mM)

D-Zn/Si(OH)4 (nM/mM)

D-Cd/PO4 (nM/mM)

0.6270.77 1377176

2.972.0 1679

0.0370.13 175

0.0170.05 277

0.0070.00 174

0.0470.04 12715

Each data was given as an average71s calculated by the same way as Table 3. a Increase in the metal/macronutrients in the subsurface waters from Stn 16 to Stn 10. b Percentage of the increase in the ratios relative to the ratios in the subsurface water.

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Table 4 Changes in metal/macronutrient ratios in the isopycnal waters flowing from the South China Sea to the Sulu Sea

K. Norisuye et al. / Deep-Sea Research II 54 (2007) 14–37

Each data in this study was given as an average71 s calculated from the values at the two isopycnal waters with sigma-T of 25.738 and 26.234. The values at sigma-T of 25.738 and 26.234 for Stn 10 were obtained assuming elemental concentrations changed linearly with sigma-T. a Increase in the concentrations in the subsurface waters from Stn 16 to Stn 10 with a unit mM for macronutrients and D-O2, nM for D-metals except for D-Co, and pM for D-Co. b Percentage of the increase. c Increase in the concentrations of elements (mM, nM or pM) divided by that in the concentration of PO4 (mM). d Monterey Bay [Table 2 in Bruland et al. (1991) (original data from Martin and Knauer (1973))]. e Sta 54–88 in the subtropical eastern Pacific [Table 2 in Bruland et al. (1991) (original data from Martin et al. (1976))]. f MANOP C site in the Central Equatorial Pacific [calculated from Table 1 in Bruland et al. (1991) (original data from Collier and Edmond (1984))]. g Decrease in dissolved elemental concentrations observed during the iron fertilization SEEDS at the subarctic western Pacific (Kinugasa et al., 2005).

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indicates the regeneration of Si(OH)4 from silica occurs at greater depth than the other elements with the latter regenerated primarily via microbial and oxidative destruction of biogenic particles within the upper waters in the Sulu Sea. The regeneration of Si from biogenic particles within the deep-water column is discussed here. The regeneration flux (Freg) of Si from biogenic particles can be described by the two-box model: F reg þ F b ¼ z=T  ðC deep  C upper Þ, where Fb, z, T, Cdeep and Cupper are the benthic flux of Si, the thickness of the deep water column, the residence time of the deep water, the average concentrations of Si(OH)4 in the deep and upper water column, respectively. When the upper box is the water column above 100 m, z is
4800 m. We used a value of
0.15 mol m–2 yr–1 for the benthic flux of Si measured in the Sulu Sea (Minami et al., 2005). Using 82 mM of Cdeep, 6 mM of Cupper and 300 yr of T (Nozaki et al., 1999), the Freg results in 1.1 mol m–2 yr–1. This value is higher than the global export flux of 0.3–0.4 mol m–2 yr–1, which is comprised of regeneration and burial fluxes, 97% and 3%, respectively (Nelson et al., 1995). The higher Si flux is suggestive of the higher primary productivity of diatoms within the Sulu Sea than the global average productivity, since the flux of Si in highly productive coastal upwelling areas is an order of magnitude higher than the global average (Nelson et al., 1995). Alternatively, the effect of temperature also may be important. Since the rate constant of silica dissolution increases with increasing temperature (Rickert et al., 2002), the higher temperature of the deep water in the Sulu Sea (
10 1C) also may cause the higher flux of Si in this system.

35

cal removal and external input, although there are differences between the two basins. Results of inter-basin comparisons between the deep-water columns are summarized as follows: compared to the western North Pacific, the biogeochemical cycling of trace metals in the Sulu Sea and the South China Sea is more strongly influenced by (1) topography, (2) supply from the coastal/fluvial waters, shelf, sill bottom and benthic sediments, and oceanic islands, and (3) water circulation within or through basins. Further, the Sulu Sea and the South China Sea are affected by these factors in a different way. As a result, the deep water of the Sulu Sea is enriched with particulate Fe and depleted with nutrient-type elements, and exhibits the distinctly low D-Cd/PO4, Ni/PO4 and Zn/Si(OH)4 ratios. The South China Sea, where Pacific deep waters rapidly enter, has comparable levels of nutrient-type elements to those in the western North Pacific, but shows medium values of the metal/macronutrient ratios between the western North Pacific and the Sulu Sea. Our results suggest that metal/macronutrient ratios in various deep regimes characterized by unique topography will vary from those of the open ocean and thus investigation of the metal/ macronutrient ratios in deep waters of such systems will be important for the reconstruction of the past oceanic distributions of elements based on the application of the ratios. Finally, the present observation field provides a unique opportunity for exploring the regeneration of bioactive elements in the isopycnal waters and within the semi-enclosed basin. The application of such analyses to other fields will be useful in estimating regeneration fluxes of bioactive elements. Acknowledgements

5. Conclusions The first vertical datasets of bioactive trace metals (Fe, Co, Ni, Cu, Zn and Cd) in the Sulu Sea and the South China Sea were obtained. Because the identical analytical technique was applied to the Sulu Sea, the South China Sea and the western North Pacific, a reliable comparison was made as for metal concentrations and metal/macronutrient ratios to show the unique inter-basin fractionations as for the deep-water column. In the upper water column in the Sulu Sea and the South China Sea, the distributions of bioactive trace metals are controlled by water circulation, biologi-

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