Particulate Flux and Cd/P Ratio of Particulate Material in the Pacific Ocean

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1 Journal of Oceanography, Vol. 55, pp. 693 to Particulate Flux and Cd/P Ratio of Particulate Material in the Pacific Ocean SHINICHIRO NORIKI 1, KAZUHIRO HAMAHARA 2 and KOH HARADA 3 1 Laboratory of Marine and Atmospheric Geochemistry, Graduate School of Environmental Earth Science, Hokkaido University, Sapporo , Japan 2 Hokkaido Institute of Environmental Sciences, Sapporo , Japan 3 National Institute for Resources and Environment, Agency of Industrial Science and Technology, Onogawa, Tsukuba , Japan (Received 24 July 1998; in revised form 13 July 1999; accepted 13 July 1999) Time-series Mark 7 sediment traps were deployed at three stations at 0 N, 13 N and 48 N along 175 E to investigate seasonal and spatial variations of particulate material flux. Chemical analysis of particulate material was performed for four major chemical components, viz. opal, CaCO 3, organic material and clay minerals. Cd and P in the particulate material were also determined. We discuss the characteristics of particulate material at each site and the transportation of Cd and P to deep water by the particulate material. The total mass fluxes and variation of fluxes at each site reflect oceanographic conditions, such as biological productivity and kind of major planktonic organisms. At the northern site, large mass fluxes with a spring bloom and high ratios of opal are characteristic. Relatively small mass fluxes with high ratios of CaCO 3 are distinct, and dissolution of CaCO 3 due to sinking is recognized in the middle latitude and 0 N sites. The larger flux at the lower trap than the upper trap at the equatorial site suggests influence by lateral transport in the deep water. Distinctive decreasing Cd/P ratio and CaCO 3 concentrations in the particulate material with increasing depth suggests that the change of Cd/P ratio in the intermediate and deep water occurs through the dissolution of CaCO 3. The dissolved Cd/P ratios in the deep water are proportional to the age of the deep water in the Atlantic but not in the Pacific. This is explained by the difference of kinds of particulate material transporting Cd and P in the deep water between the oceans. That is, the major planktonic organisms are planktons of CaCO 3 tests in the Atlantic Ocean and diatoms of opal tests in the North Pacific Ocean. Keywords: Cd, P, settling particle, CaCO 3, opal, sediment trap, Pacific Ocean. 1. Introduction The vertical distributions of some trace metals in the ocean are of a nutrient type, which implies that they are involved in a water column cycle that includes a particle formation process at the surface, followed by particle sinking and then demineralization. Several studies have found a correlation between dissolved Cd and phosphate. The dissolved Cd/P ratio was about 0.2 nm/µm in the deep North Atlantic water (Bruland and Franks, 1983; Danielsson et al., 1985), but about nm/µm in the deep water of the Southern Ocean (Bordin et al., 1987; Nolting et al., 1991). In the Atlantic deep water, the dissolved Cd/P ratio seems to increase with the age of the deep water. The dissolved Cd/P ratio was 0.3 nm/µm in the North Pacific (Boyle et al., 1976; Bruland, 1980). The dissolved Cd/P ratio in seawater should vary due to the biological activity in seawater and the mixing of water masses having different dissolved Cd/P ratios (e.g., Kudo et al., 1996). The factor controlling the dissolved Cd/P ratio in the deep water is not yet definitively understood. Boyle (1981) indicated that the Cd/Ca ratio of foraminiferal shells was correlated with that of seawater and was a useful paleoceanographic tracer. The Cd/Ca ratio of benthic foraminifera has been used to reconstruct the past distributions of nutrients in ambient seawater (Hester and Boyle, 1982; Boyle, 1988; Delaney, 1989). The dissolved Cd/P ratio in the present deep water, however, varies among the oceans (Nolting et al., 1991; De Baar et al., 1994). It is necessary to understand the mechanism controlling the dissolved Cd/P ratio in the deep water. Copyright The Oceanographic Society of Japan. 693

2 Particulate matter produced by biological activity in the surface water carries Cd and P to the deep water. The vertical fluxes and chemical compositions of biogenic materials vary very greatly in the ocean, both spatially and seasonally (Honjo et al., 1982, 1995; Noriki and Tsunogai, 1986; Honda et al., 1997). In this study, sediment traps were deployed at the equatorial, mid-latitude and high-latitude regions in the North Pacific Ocean, where the fluxes were expected to be moderate, small and large, respectively. Total fluxes and concentrations of major components were measured to describe the seasonal and spatial variation of sinking particulate material. Moreover, the P and Cd concentrations of particulate material were determined. We discuss the characteristics of particulate material at each site and the transport of P and Cd to the deep water by the particulate material. 2.1 Time-series sediment trap Time-series Mark 7 sediment traps (Honjo and Doherty, 1988) were deployed at three stations at 0 N, 13 N and 48 N along 175 E (Fig. 1) in 1993 and The sediment trap mooring depths and the sampling periods are listed in Table 1. The settling particles were obtained from N, E (Stn. WNP in Fig. 1) with a time-series HX-10 sediment trap to measure elemental concentrations in the biogenic particles using a sequential method for chemical separation (Noriki et al., 1995, 1997). Each receiving cup was filled with a solution of 10% neutral formaldehyde/filtered seawater to prevent degradation of organic matter. Samples were kept in the dark at 4 C until analysis. Swimmers were hand picked from the sample solution in each receiving cup. The particulate materials in each cup were collected onto an 0.6 µm Nuclepore filter, then dried in vacuum at 60 C and weighed. 2.2 Analysis The particulate material was digested in a sealed Teflon vessel with a mixture of perchloric, nitric and hydrofluoric acids at 150 C for 10 h, and concentrations 2. Sampling and Chemical Analysis Fig. 1. Sampling stations. Table 1. Location and summary of the sediment trap experiments. Station Water depth Trap depth Sampling Duration Location (m) (m) Start End (days) Stn. 1 (NH94-48N) /08/17 94/04/28 13 or N E Stn. 2 (NH94-13N) , /09/25 94/04/ N E Stn. 3 (NH94-Eq) , /09/20 94/08/ N E Stn. WNP /06/30 90/01/ N E 694 S. Noriki et al.

3 Table 2. Total mass fluxes and chemical constituents of particulate materials. Stn. 1 (48 N, 175 E) of chemical elements were measured (Noriki et al., 1980). Aluminum and Cd were measured by flameless atomic absorption spectrophotometry. Phosphate was determined by molybdenum blue colorimetry. Silicate in the digestion solution was determined by molybdenum yellow colorimetry. Biogenic opal was estimated from the difference in concentrations between total Si and lithogenic Si, by assuming that lithogenic aluminosilicate contained 8.1% Al and 28% Si (Taylor, 1964). Clay content was obtained by assuming that clay contained 8.1% Al. Phosphoric acid was added to the sample and the generated CO 2 was measured with a coulometer, UIC coulometrics Model The CaCO 3 content was calculated from the CO 2 content. The content of organic carbon was measured with a CHN elemental analyzer after removing calcium carbonate. Concentrations of Cd and P in three separate fractions of particulate materials from Stn. WNP were determined by the method of Noriki et al. (1997). In the method, the fraction extractable by 2% acetic acid solution is considered to be carbonate, the fraction extractable by 30% hydrogen peroxide is organic material, and the fraction that dissolves in 0.5% sodium carbonate solution is opaline silica. All the methods for chemical analysis used in this study were calibrated using Pond Sediment certified by the National Institute for Environmental Studies, Japan (Okamoto, 1982). Each element was measured with a relative error less than ±5%. The results of the chemical analyses of particulate materials are listed in Table Results and Discussion 3.1 Total mass flux and major components of settling particle The settling particles consist of four major components; opal, CaCO 3, organic material and clay. Total mass fluxes and concentrations of major components were calculated based on chemical analysis (Table 2 and Fig. 2). Stn. 1 A large flux and a large seasonal variation of total particulate flux characterize this site (Fig. 2(A)). The average flux during the whole sampling time is 231 mg/ (m 2 day) and a large peak of 829 mg/(m 2 day), probably related to spring bloom, started in late February and continued to late April. The total mass fluxes were lower than 100 mg/(m 2 day) in January to February, but were higher than 100 mg/(m 2 day) in the other seasons. There were large mass fluxes in the last 2 periods of The opal contents of these particles were about 80%, higher than Particulate Flux and Cd/P Ratio of Particulate Material in the Pacific Ocean 695

4 Table 2. (continued). Stn. 2 (13 N, 175 E) 696 S. Noriki et al.

5 Table 2. (continued). Stn. 3 (0 N, 175 E) nd: Not determined. Particulate Flux and Cd/P Ratio of Particulate Material in the Pacific Ocean 697

6 Fig. 2. Temporal variations of total mass fluxes at the stations along 175 E. (A) Stn. 1 (48 N, 175 E); (B) Stn. 2 (13 N, 175 E); (C) Stn. 3 (0 N, 175 E). : Opal, : CaCO 3, : Organic matter, : Clay. those in other seasons. The high opal content was also observed in August of The relationship between the total mass flux and each component was investigated. The best relationship was observed between the total mass flux and the opal flux at Stn. 1. The total mass fluxes vs. the opal fluxes are plotted in Fig. 3. The relation is expressed as: [Opal flux, mg/(m 2 day)] = 0.84[Total mass flux, mg/(m 2 day)] S. Noriki et al.

7 Fig. 4. Plots of CaCO 3 flux versus total mass flux at Stn. 2; gn l ei pa Fe s i. t ws h3x o ta su t me nl bd of n i l a ta x ap u lo l e f Rt a flux = [flux at (n + 3)-th period at 5.1 km] [flux at n-th n t l. S) sl a1 x sa t u ap o l mot f ( = period 0 at 1.5 km]. CaCO 3 = 3.8( Total flux) 1.3. The correlation coefficient is calculated to be A large diatom bloom in spring and a small diatom bloom in autumn were observed at high latitudes in the western North Pacific (Taniguchi and Kawamura, 1972). The slope of 0.84 means that diatom with opaline skeleton contributes largely to total mass fluxes and their variation in the northern North Pacific. Stn. 2 The average of total mass flux was 18 mg/(m 2 day) at 1.5 km depth. Large fluxes (about twice the average) were observed in late October and December of 1993 and March of 1994 (Fig. 2(B)). At the deep depth, an average of total mass flux was 12 mg/(m 2 day), which was significantly smaller than that at the 1.5 km depth. A large mass flux in late October was also observed. However, two definite peaks in December and March were not found at 5.1 km depth. The calcium carbonate contents of settling particles were over 50% and were dominant at both depths in all seasons. The sinking velocity of planktonic siliceous skeleton was found to be m/day at Stn. PAPA in Alaska Bay (Takahashi and Honjo, 1983). If a calcium carbonate shell has a sinking velocity of the same order as the diatom, a particle at 1.5 km depth will reach 5.1 km depth after about one month. As the sampling period was 10 days at this station, the time lag between the two depths should be about three periods. Figure 4 shows a plot of differences in CaCO 3 fluxes between period n at 1.5 km depth (S) and period (n + 3) at 5.1 km depth (D), f [CaCO 3 ] n+3,d f [CaCO 3 ] n,s, against differences in total mass fluxes between period n at 1.5 km depth (S) and period (n + 3) at 5.1 km depth (D), f [Total mass] n+3,d f [Total mass] n,s. The correlation coefficient is The correlation coefficients of the same plots for opal, clay and organic matter are 0.76, 0.20 and 0.96, respectively. It seems that there is a positive relation, mainly between the decrease in the total mass flux with depth and that of CaCO 3 in the water column. Stn. 3 There were large total mass fluxes during February to April at 2.2 km depth, and also at 4.3 km depth (Fig. 2(C)). Average fluxes during the period from 94/1/30 to 94/5/9 were 53 mg/(m 2 day) at 2.2 km depth and 70 mg/ (m 2 day) at 4.3 km depth. Total mass flux seems to be increased with increasing depth at this station. The dominant constituent was CaCO 3 at both depths in all seasons. Using the same method described at Stn. 2, the differences of total mass fluxes were plotted against the differences of fluxes of four major constituents between the two depths (Fig. 5). The time lag is two sampling periods, because the sampling period was 16.5 days at this station. At Stn. 3, fluxes of all four major constituents increased with depth. The correlation coefficients were calculated to be 0.73, 0.95, 0.67 and 0.89 for opal, CaCO 3, clay and organic matter, respectively. Many studies have suggested that total mass fluxes increase with depth due to horizontal transport of particulate materials from coastal regions to the open ocean (Biscaye et al., 1988; Tsunogai et al., 1990; Saito et al., 1992; Ramaswamy et al., 1997). Although the origin of the CaCO 3 is still unknown, the increase of total mass flux may be mainly related to an increase of the CaCO 3 flux with depth. 3.2 Cd/P ratios of particulate material and in the deep water The contents of Cd and P, and Cd/P ratio in each sample are listed in Table 2. Cd contents of particulate materials were 0.1 ppm or less at the northern site. The average Cd contents of sam- Particulate Flux and Cd/P Ratio of Particulate Material in the Pacific Ocean 699

8 Fig. 5. Relationship between total mass flux and material flux at Stn. 3; flux = [flux at (n + 2)-th period at 4.3 km] [flux at n-th period at 2.2 km]. : CaCO 3, : Opal, : Organic matter, : Clay. Fig. 6. Plots of PO 4 -P contents versus Cd concentration in particles. +: Stn. 1, : 1.5 km at Stn. 2, : 5.2 km at Stn. 2, : 2.2 km at Stn. 3, : 4.3 km at Stn. 3. Table 3. Averages of Cd/P ratio and contents of CaCO 3 and organic matter at Stns. 2 and 3. Cd/P ratio CaCO 3 Organic matter (µmol/mmol) (%) (%) Stn km ± ± ± km ± ± ± 2.2 Stn km ± ± ± km ± ± ± 2.7 ples are calculated to be ppm, ppm and ppm at Stns. 1, 2 and 3, respectively. Cd contents decreased with increase depth at both Stns. 2 and 3. The average P contents of particulate materials (% as PO 4 -P) are and at shallow depths, and and at deep depths, at Stns. 2 and 3, respectively. Vertical change of P contents of particulate materials were small as compared with Cd. Consequently, the Cd/P ratios of samples decrease with increase depth at Stns. 2 and 3 (Table 3). The plots of Cd and P contents of settling particles shows good correlation at each depth of each station (Fig. 6). The Cd/P ratios in the deep depths are lower than those of the shallow depths at Stns. 2 and 3. The concentrations of Cd and CaCO 3 in particles are shown in Fig. 7. Cd concentrations in particles of deep depth are lower than those in the shallow depth at Stns. 2 and 3. The Cd/ P ratios and CaCO 3 contents in particles are plotted in Fig. 8. Fig. 7. Relationship between CaCO 3 content and Cd concentration in particle. +: Stn. 1, : 1.5 km at Stn. 2, : 5.2 km at Stn. 2, : 2.2 km at Stn. 3, : 4.3 km at Stn. 3. Fig. 8. Plot of CaCO 3 contents versus Cd/P ratio in particle. +: Stn. 1, : 1.5 km at Stn. 2, : 5.2 km at Stn. 2, : 2.2 km at Stn. 3, : 4.3 km at Stn S. Noriki et al.

9 Table 4. Cd and P and Cd/P ratio in biogenic fractions of particulate materials from the deep water at Stn. WNP. Depth Cd (%) P (%) Cd/P (µmol/mmol) (km) CaCO 3 Organic matter Opal CaCO 3 Organic matter Opal CaCO 3 Organic matter ± ± 6 ND 31 ± ± ± (n = 8) ± 7 11 ± 4 ND 15 ± 3 64 ± ± (n = 13) ND: Not detected. At Stn. 2, total mass flux decreases with increase depth and the CaCO 3 flux also decreases related to decrease of total mass flux, as shown in Fig. 4. The decrease of Cd/P ratio with increase in depth occurs with the decrease of the CaCO 3 contents. At Stn. 3, the fluxes of the total mass, organic matter, and the CaCO 3 in the lower depth were higher than those in the shallow depth, but the Cd/P ratio decreased with depth, the same as at Stn. 2. The lower Cd/P ratio of settling particles at the deep depth may be mainly caused by the dissolution of CaCO 3 with higher Cd/P ratio than that in the organic matter of settling particles (Table 4 and Fig. 9), though one cannot conclude that those low Cd/P ratio particles are aggregated with settling particles during the sinking process between the depths of trap sampling as at Stn. 3. In the North Atlantic Deep Water, the dissolved Cd/ P ratio was about 0.2 nm/µm (Bruland and Franks, 1983; Danielsson et al., 1985). The dissolved Cd/P ratio in the Atlantic deep water increased toward the south, becoming 0.27 nm/µm in the Southern Ocean deep water (Nolting et al., 1991; Saager et al., 1992). It seems that the dissolved Cd/P ratio increases with the southward advection of North Atlantic Deep Water (Nolting et al., 1991; Saager et al., 1992) due to the dissolution of CaCO 3 (Takahashi et al., 1980). Noriki and Tsunogai (1992) found that the particulate Cd flux decreased steeply with depth in the water column shallower than 1000 m. The flux of organic material also decreased. We can show that the labile organic particles regenerate and Cd in this fraction dissolves in the surface water. As a result, refractory organic materials and hard parts of plankton settle into the deep water. The Cd/P ratios in organic tissues and hard parts of biogenic materials in deep water should be the most important factor controlling the dissolved Cd/P ratio in the deep water. The dissolved Cd/P ratios in the deep water were less than 1 nm/µm (e.g., Nolting et al., 1991). The dissolved Cd/P ratio increases with the dissolution of CaCO 3 in the deep water (Takahashi et al., 1980; Saager et al., 1992), so the dissolved Cd/P ratio in the mid and deep waters is higher than that in the surface water in the Atlantic Ocean. Fig. 9. Plot of organic matter contents versus Cd/P ratio in particle. +: Stn. 1, : 1.5 km at Stn. 2, : 5.2 km at Stn. 2, : 2.2 km at Stn. 3, : 4.3 km at Stn. 3. The dominant constituent of particulate material is opal in the northern North Pacific (Table 2 and Noriki and Tsunogai, 1986). Thus, the particulate flux of Cd to the northern North Pacific deep water should be small compared to the southern Pacific deep water. Plots of dissolved Cd versus phosphate in the two stations (32 41 N, W; N, W) of the eastern Pacific reported by Bruland (1980) are shown in Fig. 10. Bruland (1980) concluded that dissolved Cd is related to phosphate by the equation; [Cd] = [0.347 ± 0.007][PO 4 ] [0.068 ± 0.017] (R 2 = 0.99), where Cd is in nmol/kg and PO 4 is in µmol/kg (line A in Fig. 10). The data from the upper of the oxygen minimum layers in the two stations show that the relation is expressed by the equation; [Cd] = [0.38][PO 4 ] [0.11] (R 2 = 0.99) (line B in Fig. 10). The slope for the data in the upper the oxygen minimum layer is larger than that for the data as a whole. This means that the particulate material that is poor in Cd relative to phosphate should dissolve in the deep water of the eastern North Pacific. On the other hand, the dissolved Cd/P ratio in the surface water is higher than Particulate Flux and Cd/P Ratio of Particulate Material in the Pacific Ocean 701

10 crew of R/V Hakurei Maru of Metal Mining Agency of Japan for their assistance in sediment trap experiments. Thanks are extended to the captain and crew of T/S Hokusei Maru of Hokkaido University for their assistance in sediment trap deployment and recovery. We would like to thank Dr. P. G. Wangersky and Dr. Akira Nishimura for their helpful advice and critical reading of the manuscript. Special thanks are extended to anonymous reviewers for the critical reading of the manuscript and valuable comments. Fig. 10. Relationship between phosphate and dissolved Cd in the eastern Pacific (after Bruland, 1980). : Data from the upper of the oxygen minimum layers. that in the deep water in the Atlantic Ocean, where CaCO 3 is dominant in particulate material (e.g., Honjo et al., 1982). These findings can consistently explain the fact that the dissolved Cd/P ratio was not proportional to the age of the deep water in the Pacific Ocean (Saager et al., 1992). 4. Summary and Conclusion The results can be summarized as follows: The total mass flux observed with sediment traps varied seasonally, spatially and vertically, in relation to biological productivity. At the northern site, large mass fluxes with a spring bloom and high ratios of opal are characteristic. Relatively small mass fluxes with high ratios of CaCO 3 are distinct and dissolution of CaCO 3 through sinking is recognized in the middle latitude and 0 N sites. The larger flux at the lower trap than the upper trap at the equatorial site suggests influence by lateral transport in the deep water. The Cd/P ratio and the CaCO 3 contents of settling particles decreased with depth; however, the organic matter contents of settling particles did not vary with depth. These results suggest that the dissolved Cd/P ratio increases with the southward direction of flow of the deep water in the Atlantic Ocean due to the dissolution of CaCO 3, while the ratio is not proportional to the age of water in the northern Pacific Ocean where the particulate flux of opal is much larger than that of CaCO 3. Acknowledgements We would like to thank Dr. S. Tsunogai and the staff of the Laboratory of Marine and Atmospheric Geochemistry, Graduate School of Environmental Earth Science, Hokkaido University for their valuable comments and suggestions. We wish to thank the captain and References Biscaye, P. E., R. F. Anderson and B. L. Deck (1988): Flux of particles and constituents to the eastern United States continental slope and rise: SEEP-I. Cont. Shelf Res., 8, Bordin, G., P. Appriou and P. Treguer (1987): Repartitions horizontale et verticale du cuivre, du manganese et du cadmium dans le secteur indien de l Ocean Antarctique. Oceanol. Acta, 10, Boyle, E. A. (1981): Cadmium, zinc, copper and barium in foraminifera tests. Earth Planet. Sci. Lett., 53, Boyle, E. A. (1988): Cadmium: chemical tracer of deep water paleoceanography. Paleoceanography, 3, Boyle, E. A., F. R. Sclater and J. M. Edmond (1976): On the marine geochemistry of cadmium. Nature, 263, Brewer, P. G., Y. Nozaki, D. W. Spencer and A. P. Fleer (1980): Sediment trap experiments in the deep North Atlantic: isotopic and elemental fluxes. J. Mar. Res., 38, Bruland, K. W. (1980): Oceanographic distributions of cadmium, zinc, nickel, and copper in north Pacific. Earth Planet. Sci. Lett., 47, Bruland, K. W. and R. P. Franks (1983): Manganese, nickel, copper, zinc and cadmium on the western North Atlantic. p In Trace Metals in Seawater, 4 (9), ed. by C. S. Wong, NATO Conference Series, Plenum Press, New York. Danielsson, L.-G., B. Magnusson and S. Westerlund (1985): Cadmium, copper, iron, nickel and zinc in the north-east Atlantic Ocean. Mar. Chem., 17, De Baar, H. J. W., P. M. Saager, R. F. Nolting and J. van der Meer (1994): Cadmium versus phosphate in the world ocean. Mar. Chem., 46, Delaney, M. L. (1989): Uptake of cadmium into shells by planktonic foraminifera. Chem. Geol., 78, Hester, K. and E. A. Boyle (1982): Water chemistry control of the Cd content of benthic foraminifera. Nature, 298, Honda, M. C., M. Kusakabe, S. Nakabayashi, S. J. Manganini and S. Honjo (1997): Change in pco 2 through biological activity in the marginal seas of the western North Pacific: The efficiency of the biological pump estimated by a sediment trap experiment. J. Oceanogr., 53, Honjo, S. and K. W. Doherty (1988): Large aperture time-series sediment traps, design objectives, construction and application. Deep-Sea Res., 35, Honjo, S., S. J. Manganini and J. J. Cole (1982) Sedimentation of biogenic matter in the deep ocean. Deep-Sea Res., 29, S. Noriki et al.

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