Particulate fluxes and major components of settling particles from sediment trap experiments in the Pacific Ocean

Deep-Sea Research, Vol. 33, No. 7. pp. 903-912

0198-0149/86 $3.0(I + I).00 Pergamon Journals Ltd.

Printcd in Great Britain.

Particulate fluxes and major components of settling particles from sediment trap experiments in the Pacific Ocean SHINICHIRO NORIKI* a n d SHIZUO TSUNOGAI*

(Received 23 July 1985; in revised Jorm 12 December 1985; accepted 27 January 1986) Abstract--Total particulate fluxes in the deep water column, as measured with sediment traps, were 6-82 mg m 2 day ~ in the subtropical central Pacific of Hawaii and the eastern Pacific off Cafifornia, 300-420 mg m 2 day t in the western Pacific off Japan, and 790-1200 mg m 2 day ~ in the Antarctic Ocean. The fluxes of CaCO~ particles observed at the five stations did not vary widely from station to station. The opal contents, on the other hand, increased with the total particulate fluxes, with the highest observed opal content of the settling particles being 80% in the Antarctic Ocean. Assuming that the clay fraction in the settling particles is refractory, our results show that the regeneration of biogenic particles occurs mainly in the bottom water and at the sediment surface.

INTRODUCTION

SETI'LING particles have a role in the vertical transport of chemical substances from the surface to the deep ocean. Recently settling particles have been observed directly with sediment traps (e.g. WIEBE et al., 1976; HONJO, 1978; BREWERet al., 1980; TSUNOGAIet al., 1982; MARTIN and KNAUER, 1983; JICKELLSet al., 1984; BACONet al., 1985). HONJO et al. (1982a) have reported the total particulate fluxes of 4--30 g m -2 y-~ in the subtropical or tropical Atlantic and the Pacific oceans, as measured with cone-type sediment traps, showing that an about 50% fraction of the settling particles consists of CaCO3. In the northern North Pacific, TSUNOGAI et al. (1982) have observed the total fluxes one order of magnitude larger than those in the Atlantic (HoNJo et al., 1982a), and found that the opal contents in the settling particles is over 50%. HONJO (1982) and THUNELLet al. (1983) have shown a seasonal variation of particulate fluxes in the deep, the result of variations in surface primary production; BETZER et al. (1984) and SUESS (1980) reached similar conclusions. If total particulate flux depends strongly on primary productivity at the surface, the removal rates of biologically related chemical elements will show similar temporal and spatial variations. In this paper we report on the results of the sediment trap experiments from various stations in the subtropical ocean (low biological productivity) and in the subboreai and Antarctic oceans (high productivity) (Fig. 1), The spatial variation of the chemical composition of major components in the settling particles will be related to the total particulate fluxes. * Department of Chemistry, Faculty of Fisheries, Hokkaido University, Hakodate, 041 Japan. 903

904

S. NORIKIand S. TSUNOGAI

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20°

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METHODS

Each sediment trap (Fig. 2) consisted of six cylinders of polyvinyl chloride. Each cylinder was 25 cm in diameter and 57 cm high. The cross-shaped baffle of 10 cm deep was placed in the top of the cylinder. Each cylinder was set radially to the stainless steel frame. The hinged lids were closed before the retrievement operation by the messenger system. A bactericide, sodium azide, was added to two of the receiving cups of the six cylinders of each trap. The samples contained in each cylinder were filtered through a preweighed Nuclepore filter (0.6 lam). Dry weights were determined by the method of UEMATSUet al. (1978), and chemical components by the methods of NORIKI et al. (1980). The dried samples were ignited at 450°C for about 24 h. We have assumed that the ignition loss is equal to the organic matter content. The content of biogenic silica, that is opal, was obtained from the difference in concentrations between total Si and aluminosilicate-Si in the sample. The concentration of aluminosilicate-Si was calculated by multiplying the AI concentration of the sample by the ratio Si : A1 --- 28 : 8 which is the average value of crustal material. The clay content was obtained by assuming that clay contained 8% of AI.

905

Particulate fluxes from sediment trap experiments

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Sediment trap used in this work; a, nylon rope dp 12 mm; b, messenger; c, trigger; d, gut: e, baffle, f, cylinder; g, frame; h, spring; i, lid; i receiving cup.

Table 1. Summary o f the sediment trap experiments in the open ocean Station code Location Term Duration (days) Trap depth (km)

Water depth (km)

AO

WP

61.5°S 150.5°E 12/83-1/84 24 [/.52 0.77 1.20 2.26 3.11 3.58

41.5°N 146.5°E 8/83-9/83 19 1.33 1.65 3.25

5.16

EP5 37.0°N 127.6°W 12/82-12/82 7 11.51 0.72 1.25 3.37 4.22 4.75

EP7 31.7°W 124.6°W 12/82-1/83 40 11.511 0.72 1.25 3.38 3.80 4.20

EPII

CP27

17.5°N 26.8°N 1 1 7 . 0 ° W 146.8°W 12/82-1/83 2/83-2/83 24 2 11.47 0.46 11.69 th67 1.22 1.21 3.34 3.33 3.66 4.28 3.87 5.15

R E S U L T S AND D I S C U S S I O N

Total particulate flux and major components of settling particle Mean fluxes calculated from the measured fluxes for 5 or 6 cylinders of each sediment trap are listed in Table 2. The total particulate fluxes observed in the presence of sodium azide in the cylinders were not significantly different from those deployed for 24 days without the bactericide (NORIKI et al., 1985a). The total particulate flux at 'Site AO' in the Antarctic Ocean was the largest among those observed at the other five stations; average flux for the five depths was about 1000 mg m-2 day -I, which is about two times that measured in the Drake Passage (WEFER et al., 1982). The flux at 'Site AO' is also 20-70 times those at 'Site EP5', 'Site EP7', 'Site EPll' and 'Site CP27' in the subtropical and tropical Pacific.

906

S. NORIKI and S. TSUNOGAI

Table 2.

The major Jour components of the settling particles and their particulate fluxes Organic matter

Clay Depth (km)

Total flux*

AO

0,52 11).77 1,20 2,26 3,11

1123 1101 970 795 951

WP

1.33 1.65 3.25

381 419 301

NP3+

1.04 2.16 4.38

208 195 148

EP5

(}.51 111.72 1.25 3,37 4.22

EP7

Station

%

CaCO~

%

Flux*

%

~,

Flux*

15.4 16.0 14.7 15.1 14.5

173 176 143 12(I 138

77.9 811.6 8111.1 82.7 83.7

875 887 777 657 796

2.3 2.111 2.1 2.1 2.2

26 22 211! 17 21

45.7 52.8 55,9

26. l 20.11 13.9

99 84 46

49.9 49.0 49.2

187 21(15 148

8.9 8.8 8.9

34 37 27

3.3 4.4 8.111

6,9 8.6 12

18.5 16.2 12.8

38.5 31,6 18,9

510.1 69.2 53.2

1104 135 78

26.9 6.2 24.8

56.0 12 36.7

81.1 81.9 69.9 54.0 46.3

3.5 19./0 3.9 2.5 12.3

2.8 15.6 2.7 1.4 5.7

37.0 38.1 37.4 34.(} 29.(I

3111,0 31,2 26.0 18.4 13.4

37.9 9.16 30.0 22.4 41(I. 1

30.9 31.9 36.() 26.4 43.7

25.1 26.1 25.1 14.3 20.2

0.51(I (t.72 1,25 3.38 3.80

18.9 15.2 14.8 16.8 16,2

6.10 8.3 15.9 15.0 21.3

1.1 1.3 2.4 2.5 3.5

43.8 32.2 21.6 23.8 16.9

8.28 4.89 3.211 4.0(I 2.74

1(}.4 18.8 23.3 18.3 19.9

1.9 2.8 3.45 3.07

EPI I

0.47 0.69 1.22 3.34 3.66

25,5 27.3 22.3 24.11/ 18.2

6.5 6.1 12.1 12.4 12.4

1.7 1.7 2.7 3.3 2.3

36.9 36.9 33.1 25.t0 22.6

9.45 1111.I 7.38 6.011} 4. ll

22.3 18.11 21.6 2t0.111 25.t0

CP27~:

O.46 11.67 1.21 3.33 4.28

34.8 2111.4 14.111 11.0 6.49

0.075 0.54 0.93 11.124 11).121 12.(I 12.6 18.6

Flux*

Opal

0.84 10.59 11.911) /t.99 1.15

Flux . . . .

30.7 7.5 20.9 12,1 18,6

3.22

34.8 3(1.5 45.(I 3(1.3 48.5

6.58 5.55 6.66 6.1()9 7.86

5.71 4.91 4.82 4.97 4.55

36.8 38.1 35.9 37.3 34.3

9.42 1111.4 8.01 8.911) 6.24

*mgm2day i. ~- TStINO(;AI et al, (1982). :~: The chemical components were not determined.

The mean particulate fluxes in the open ocean from various previous studies are summarized in Table 3. The sediment traps used by HoNJO et al. (1982a) were a cone type (HoNJO et al., 1980). Although the trapping efficiency may vary with the design of the sediment trap (GARDNER, 1980), the chemical composition of particles does not seem to vary with the shape of the trap in the open ocean (NomKJ and TSUNOGAI, 1986). Although total particulate flux varies widely fron station to station (Table 3) there seems to be some relation between the flux and the geographical position of the sampling station. Particle fluxes are higher in the Antarctic Ocean (Site AO), the western Pacific (Site WP), the northern North Pacific (Site NP3) and the Panama Basin (Site PB), where primary productive rates also are higher (KoBLENTz-MIsnKE, 1970). This obviously

907

Particulate fluxes from sediment trap experiments Table 3.

Mean total particulate fluxes and the fractions of major components of the settling particles observed in the open ocean

Station

(Code)

Depth range with the number of traps deployed (km) (n*)

Pacific Eastern Pacific North Atlantic Eastern Pacific Tropical Atlantic Eastern Pacific Panama Basin N. North Pacific Western Pacific Antarctic Ocean

(P)t (EP7) (S)t (EPI 1) (E)? (EP5) (PB)t (NP3)$ (WP) (AO)

0,30--5.58 0.50-3.80 0.98-5.21 0.47-3.66 0.39-5.1/7 11.51-4.22 0.67-3.79 1.112~1.38 1.33-3.25 11.52-3.11

Total Organic flux matter (rag m 2 day 11 (%)

(5) (5) (3) (5) (4) (5) (6) (3) (3) (5)

13 16 17 23 53 67 144 184 367 990

Opal (%)

22.8 22.7 22.5 30.9 14.5 35,1 17.6 15.8 20.0 15.1

CaCO~ Clay (%) (%)

9.1 18.1 5.0 21.5 9.7 27.9 19.5 57.5 54.9 81.0

61.7 40.2 45.5 36.4 55.7 33.8 32.5 19.3 8.9 2.1

11t.3 13.3 13.0 9.9 19.9 8.2 30.0 5.2 16.2 0. I

* Number of traps deployed. -I- HONJO et al. (1982). :~:TSUNOGAIet al. 119821.

reflects the fact that the total particulate flux in the deep water is related primarily to the biological productivity in the surface water (SuEss, 1980). The fractions of organic matter, opal, CaCO3 and clay of the settling particles (Table 2) show that over 80% of the settling particles at any station consists of biogenic material. The average total particulate fluxes are plotted against the contents of biogenic materials in Fig. 3. The CACO3 fluxes are 5-30 mg m-e day -~. WEFER et al. (1982) reported the Mean flux, g Im2day

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CP27

Fig. 3.

Mean total flux and bio~nic components of settling particles. ~ o p a l ; maner; ~ CaCO3. Data are provided in Table 3.

~

organic

908

S. NORIKIand S. TSUNOGAI

CaCO 3 flUX of 15 mg m-2 day-I in the Drake Passage. MARTIN and KNAUER (1983) observed the CaCO3 flux of 89 mg m -2 day-I at 50 m depth and 18-53 mg m -2 day t at the deeper depths off California (VERTEX I). It seems that the CaCO3 flux does not vary spatially in the open ocean. The opal flux, on the other hand, varies spatially by two orders of magnitude in the open ocean (Table 2). The fact that the larger the total particulate flux is related to higher content of opal suggests that the total particulate flux depends strongly on the production of opaline silica particles in the surface water. This conclusion coincides with that in a coastal area described in a previous paper (NORIKI et al., 1985b). Regeneration of biogenic material from settling particles in the water column The clay flux and the concentration of clay generally increase with depth at each site (Table 2). TSUNOGAI et al. (1982) have found that the concentration of refractory components such as Al and Fe increased with depth down to the bottom sediments and have discussed that one possible explanation of increase of the particulate flux with depth is vertical advective motion of deep water. HONJOet al. (1982b) also found that the flux of lithogenic particles increases linearly with depth. They suggested that the continental slope sediments are transported horizontally. Although the smectite flux increased with depth, the ratio of [smectite]/[quartz + feldspar] of the settling particles decreased with depth at the sites of Panama Basin and Damerara Abyssal Plain (HONJO el al.. 1982b). This indicates that the increase of the flux of the lithogenic particles collected by sediment traps is not caused only by the horizontal transport of the slope sediment. BREWERet al. (1980) and ALLER and DEMASTER (1984) have reported that the horizontal movement of water does not contribute largely to the downward flux of clay. Although the lithogenic materials may be transported horizontally (HONJO el al., 1982b) or be caught in vertical circulation cells with undulation (TsvNOGAI et al., 198(}, 1982), the decreasing ratio of biogenic material to clay with depth (Table 4) indicates the chemical degradation during horizontal and/or vertical transport. The ratios of the biogenic materials to clay decrease most abruptly in the upper 1000 m layer. We, however, choose a depth near 1000 m as a reference depth in the following calculation of regenerated fractions because we unfortunately have no data for depths shallower than 1000 m at some stations. The depths chosen as the reference depth are shown in Table 4 with asterisks. The fractions of biogenic materials degradating in the deep water and in the bottom water between the deepest trap depth and the surface sediment, and the fraction accumulating in the sediment are given as follows: For the regeneration in the deep water ( 1 - [B/Clay]D/[B/ClaYIR) x 100, % For the regeneration in the bottom water (([B/Clay]D- [B/Clay]s)/[B/Clay]R)) x 100, % For the accumulation in the sediment ([B/Clay]s/[B/Clay]R) x 100, % where B and Clay are the fractions of a biogenic component in question and clay,

Particulate fluxes from sediment trap experiments

Table 4.

Station AO

909

Ratios of biogenic material to clay calculated for settling particles and sediments Trap depth (km) 0.52 0.77 1.20" 2.26 3.11 Sediment

,

Org. :Clay

Opal: Clay

CaCO3 :Clay

205 297 154 122 120 1.78

1039 1493 861 667 692 13.3

30.7 37,0 22,6 16.9 18,2 0,492

WP

1.33" 1.65 3.25 Sediment

2.18 1.59 0.747 0.107

4.08 3.89 2.65 0.510

0.742 0.698 0.478 I).0562

NP3t

1.04" 2.16 4.38 Sediment

5.61 3.68 1.60 0.0663

15.2 15.7 6.65 0.196

8.15 1.41 3.10 0.0398

EPll

0.47 0.69* 1.22 3.34 3.66 Sediment

5.68 6.05 2.74 2.02 1.82 0.0624

3.43 2.95 1.79 1.67 2.02 0.125

5.66 6.25 2.97 2.99 2.77 0.0412

* Reference depth: see text. t TSUNOGAIet al. (1982).

respectively, and the subscripts, R, D and S refer to the reference depth, the depth of the deepest trap, and the sediment surface, respectively. This calculation has been made for only the four stations where the chemical composition of the surface sediment was determined (Table 5). The results show that at 'Site WP' in the western Pacific only small portions (5% for organic matter, 13% for opal and 8% for CaCO3) of the settling particles at the reference depth are retained in the sediment surface. At the other three sites, the regenerated fractions of the three biogenic components are 95-99%. At any site in the Pacific, the dissolution of opal occurs chiefly in the 'bottom water' (including the sediment surface), and to a lesser extent in the 'deep water' (Table 5). The regeneration fluxes of opal are calculated to be about 270 g m -2 y-l, 60 g m-2 y-t, 40 g m -2 y-t, and 2 g m -2 y-t at 'Site AO' in the Antarctic Ocean, 'Site WP' in the western Pacific, 'Site NP3' in the northern North Pacific and 'Site EPll' in the eastern Pacific, respectively. TAKAHASHI and HONJO (1983) have shown that large phaeodarian skeletons quickly sink to the abyssal depth. Only small pieces are dissolved during their descent in the water column. This conclusion agrees with our result in Table 5. The regenerated fractions of opal and CaCO3 in the bottom water (including the surface sediment) are larger than that of organic matter. This is evidence that the regeneration of biogenic silica in the hard parts of organisms is slower (that is occurs in the deeper depths) than the regeneration of phosphate or nitrogen in the soft tissue of

1

24 75

AO

1

20 79

Opal

2

20 78

CACO~

5

66 29

Org

13

35 52

Opal

WP

8

35 57

CaCO~

3

71 26

Org

3

56 41

Opal

NP3*

Fate o f major components in the settling particles

* TSUNOGAI et al. 11982). t In the water column between the reference depth and the deepest trap. $ Below the deepest trap.

Fraction accumulating in Sediment (%)

Fraction dissolving in Deep watcrt (%) Bottom water3: (%)

Org

Table 5.

1

62 37

CaCO

1

711 29

Org

4

32 64

1

56 43

EPII Op~d CaCO~

Particulate fluxes from sediment trap experiments

911

organisms. This conclusion explains why in the open ocean the depth of maximum phosphate or nitrate concentration is generally shallower than that of silicate.

Acknowledgements--We wish to thank K. Harada, N. Ishimori, T. Suzuki, K. Taguchi and the staff of the Laboratory of Analytical Chemistry, Faculty of Fisheries, Hokkaido University for their cooperation in the arrangements of sediment trap experiments and the chemical analysis of samples. We are also grateful to A. Hattori and T. Nemoto and the scientist, officers and crew aboard R.V. Hakuho Maru of the University of Tokyo for assistance in the sediment trap experiment.

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