Fluxes of 234Th, 210Po and 210Pb Determined by Sediment Trap Experiments in Pelagic Oceans*

Transcription

1 Journal of the Oceanographical Society of Japan Vol. 42, pp.192 to 200, 1986 Fluxes of 234Th, 210Po and 210Pb Determined by Sediment Trap Experiments in Pelagic Oceans* Koh Haradat and Shizuo Tsunogait Abstract: Sediment trap experiments were carried out in two oceans, the eastern Pacific Ocean and the Antarctic Ocean, which have very different biological productivities. The natural radionuclides, 234Th, 210Po and 210Pb were used as tracers of reactive metals. Larger particulate fluxes of these radionuclides were found in the seas where total mass fluxes were larger, although the concentrations of these radionuclides in the settling particles were some what smaller. The concentrations of 234Th in the settling particles varied widely and irregularly with depth, whereas the concentrations of 210Po and 210Pb in the settling particles steadily increased with increasing water depth. The ratios of 210po/210Pb in the settling particulates were larger than unity which the ratio of 234Th/excess 210Po as larger than 234Th/210Po in the deep water. These results suggest that, when the particles sink through the water column, these radionuclides are being absorbed by settling particles in the order 234Th> 210 Po> 210pb. The observed particulate fluxes of 210Pb are about one eighth of those calculated from the disequilibria between 226Ra and 210Pb at the stations in the subtropical eastern Pacific, although the observed fluxes are the same as the calculated ones in the northern North Pacific and the Antarctic Ocean. Thus, there must be a horizontal flow carrying these reactive metals from the oligotorophic ocean to the biologically productive ocean where the metals are removed by settling particles even in deep water. 1. Introduction Recent sediment trap experiments have revealed that settling particles carrying chemical substances from the surface to the deep sea are intimately related to biological activity in the surface water (Honjo et al., 1982; Tsunogai et al., 1982). For example, the temporal variation of total mass fluxes in the deep sea corresponds to the seasonal variation of primary productivity in the surface water (Honjo, 1982; Deucer et al., 1981) and the geographical pattern of total mass fluxes is similar to that of the biological productivity in the world ocean (Noriki et al., 1985; Noriki and Tsunogai, 1986a). On the other hand, naturally occurring radioisotopes generated by decay of their parents throughout the water column have been used to study the removal mechanisms of reactive elements from seawater by falling particles. Many scientists have discussed the behavior of reactive radionuclides such as 234Th (Bhat et al., 1969, Matsumoto, 1975), 210Pb and 210Po (Rama et al., 1961; Craig et al., 1973; Krishnaswami et al., 1975; Nozaki et al., 1976; Nozaki and Tsunogai, 1976; Tsunogai and Harada, 1980 etc.) in seawater based on concentrations in seawater. Some efforts have been made to collect large amounts of particles from seawater to analyze the radiochemical composition of particulate matter, for example, by the filtration of large volumes of seawater (Krishnaswami and Sarin, 1976, Bishop et al., 1977). Another technique is the sediment trap (Spencer et al., 1978; Brewer et al., 1980; Moore et al., 1981; Bacon et al., 1985). The later has the advantage of permitting direct determination of the setting The purpose of this study is to clarify how biological productivity in the surface water controls the removal of reactive metals from seawater. For this purpose, sediment trap experiments were carried out in the subtropical eastern Pacific Ocean and the Antarctic Ocean where biological productivities differ greatly. The natural radionuclides 234Th, nopo and 210Pb were used as tracers of the reactive elements in sea-

2 Fluxes of 234Th, 210Po and 210Pb Determined the bottom of the cylinder were collected on a 2. Sampling and analysis Settling particle samples were collected with sediment traps. The sediment traps were deployed at four stations in the eastern Pacific (KH 82-5 cruise of the R/V Hakuho-Maru) and at one station in the Antarctic Ocean (KH 83-4 cruise). Locations of sampling sites are shown in Fig. 1. A NH type sediment trap was used in this investigation and consisted of six P.V.C. cylinders with a mouth area of 490 cm2. Five NH type sediment traps were usually deployed on an array at each station. Further details of the sediment traps and their method of deployment were described elsewhere (Noriki and Tsunogai, 1986a, b). 234Th, 210ph and 210Po activities were determined by using one cylinder sample (1-35 mg) among the six cylinders of each NH trap for the eastern Pacific samples and one fourth of the one cylinder sample for the Antarctic Ocean samples. Radiochemical separation and the counting of 234Th activities were carried out on board ship as follows. Upon recovery, the settling particles which Nuclepore filter (0.6 pm of pore size) and subjected to freeze-drying. These samples were completely dissolved in a mixture of HNO3, HF and HClO4 in a Teflon vessel after the addition of 230Th, 208Po and common lead to determine chemical yields. The radionuclides, 234Th, 210ph and 210Po in the solution were analyzed by the method of Harada and Tsunogai (1985) and Matsumoto (1975) with some modification. The procedure is briefly described as follows. The sample solution was evaporated to dryness in a Teflon beaker and then redissolved with 0.6 M HCl. By floating a silver disc on the solution, polonium in the solution was spontaneously electroplated onto the silver disc. The remaining solution was evaporated to dryness, redissolved with 7.5 M HNO3 and loaded on an anion-exchange column which retained thorium. Lead was eluted with 7.5 M HNO3. Thorium on the column was eluted by washing with 6M HC1 and electroplated onto a silver disc in a solution containing ethanol. 210Pb in the effluent was electrodeposited as Pb02 onto a platinum anode together with the common lead carrier. This lead oxide was dissolved in a mixture of H2O2 and HNO3 and was again precipitated as PbCrO4. This precipitate was stored for one or two months while 210Bi was radiochemically equilibrated with 210Pb. For some other samples collected at Stations 5 and 27 in the eastern Pacific, 210Pb was determined by analyzing 210Po that had grown in a solution stored for about one year. The a-activities of 2ospo, 21opo and 230Th were counted by using a pulse -height analyzer (512 or 1,024 channels) coupled with a surface barrier silicon detector (ORTEC R ). The Ĉ-activities of 234Th (strictly speaking, 234Pa) and 210ph (210Bi) were counted by using a low background 27r gas flow counter (Aloka LBC-451). The results of 234Th and 210Po presented in Table 1 are corrected to the time of collection. Decay corrections were made using the following relations: Fig.1. Map for sampling stations.

3 Harada and Tsunogai Table 1. Concentrations of 234Th, nopb ani210and210po in the settling particles. where A0 is the activity of the nuclide (dpmg-1) in a freshly depositing particles, Am is its measured activity, ti is the time interval for which the trap was deployed, t2 is the time elapsed between the recovery of the sample to the counting of the activity of the nuclide, and is the decay constant of the nuclide. The subscripts of Th, Po and Pb mean 234Th, nopo and 210Pb, respectively. 3. Results The results for total mass flux and nuclide concentrations are shown in Table 1 and plotted in Figures 2-4. Mean total mass fluxes in the deep sea ranged from 7 gm-2 yr-1 at stations in the eastern Pacific, except Station 5, to 360 gm-2 yr-1 at Station in the Antarctic Ocean, and showed a wide variation of two orders of magnitude from place to place. The concentrations of 234Th in the settling particles (Fig. 2) obtained at the stations in the eastern North Pacific range from 3 to 30 ~103 dpm g-1, whereas in the Antarctic Ocean, they ranged from 3 to 9 ~102 dpm g-1, i.e., one order of magnitude lower than in the eastern and northern Pacific. The 234Th concentrations showed no systematic trend with depth at these stations. The concentrations of 210Pb in the settling particles ranged from 20 to 280 dpm g-1 at stations 5, 7 and 11 in the eastern Pacific near the American Continent, from 70 to 840 dpm g-1 at Station 27 near the Hawaiian Island and from

4 Fluxes of 234Th, 210Po and 210Pb Determined Fig.2. Vertical profiles of concentration of 234Th in the settling particles. Fig.3. Vertical profiles of concentration of 210Pb in the settling particles. 10 to 23 dpm g-1 in the Antarctic Ocean. These concentrations increased with increasing water depth (Fig. 3). The concentrations of 210Po in the settling particles ranged from 29 to 610 dpm g-1 at Stations 5, 7 and 11, from 390 to 2,000 dpm g-1 at Station 27 and from 34 to 62 dpm g-1 in the Antarctic Ocean, and increased with increasing water depth like 21"Pb (Fig. 4). 4. Discussion 4.1. Relationship between 234Th, 210 pb and 210po in settling particles and total mass flux The vertical flux of the settling particulate matter depends upon the biological productivity in the surface water. The total mass fluxes in highly productive oceans such as the Antarctic Ocean and the northern North Pacific are large, but those in oligotrophic oceans such as the subtropical eastern Pacific are extremely small

5 Harada and Tsunogai Fig.4. Vertical profiles of concentration of 210Po in the settling particles. Fig.5. Relationship between concentrations of 210Pb in the settling particles and the total mass fluxes. Open circles, solid circles, open triangles, solid triangles, open squares, solid squares refer Stations 5, 7, 11, 27 of KH 82-5, Station 3B of KH 83-4 and Station 3 of KH 78-3, respectively. (Table 1). We studied the relationship between the total mass fluxes and the concentrations and the fluxes of these nuclides. Figure 5 shows a plot of the concentrations of 210Pb versus the total mass fluxes. The 210Pb concentrations in the settling particles decreased when the total mass fluxes become large and the smallest value was found in the Antarctic Ocean. The settling particulate fluxes of 210Pb however, increased with increasing total mass flux (Fig. 6). Es- Fig. 6. Relationship between fluxes of 210Pb in the settling particles and the total mass fluxes. sentially the same relationships were observed for 234Th and210po. This tell us that the effect of the increase in total mass flux caused by biological activity is stronger than the dilution effect due to biogenic material when these radionuclides are removed from seawater. It is therefore concluded that these radionuclides are preferentially removed in some biologically productive oceans where the total mass fluxes are one or two orders of magnitude larger than in the oligotrophic subtropical oceans Vertical distributions of 234Th, 210Pb and 210Po in the settling particles The concentrations of 234Th, in the settling particles (Fig. 2) show wide and non-systematic

6 Fluxes of 234Th, 210Po and 210Pb Determined variation with depth at all the observation stations. If the short-lived nuclides such as 234Th were incorporated into the particles only during biological assimilation processes in the surface layer, the concentration of 234Th in the settling particles should decrease with increasing depth. For example, when the settling velocity of particles is 20 m day-1 (Brewer et al., 1980), almost all the 234Th atoms in the settling particles would decay out before reaching deep water. Otherwise, when the settling velocity is extremely rapid, the concentration of 234Th in the settling particles obtained by sediment traps deployed for 24 days would not vary so widely with depth at the same station. It, therefore, is concluded that the settling particles are picking up 234Th atoms or rapidly exchanging thorium atoms during their sinking through deep water. The concentrations of 234Th in the suspended particles were observed to be 2.2 ~10 dpm and 5.4 ~103 dpm g-1 for samples collected by (pore size 0.6 pm) at a depth of 100-m at Stations 27 and 11, respectively. These values are not significantly different from those of the settling particles. The attachment of 234Th to particulate matter in deep water is considered to be a surface reaction because thorium is thermodynamically unstable in seawater. According to Stokes' law which states that the settling velocity is proportional to the square of the particle size, the suspended particles should be smaller than the settling particles in the sediment traps. Since the smaller particles have a Table2. Ratios of radionuclides in the settling particles.

7 Harada and Tsunogai larger specific surface area, the concentration of 234Th in the smaller suspended particles is expected to be greater than in the larger settling particles. The resulting similarity between concentrations of 234Th in the settling and sus. pended particles which were obtained, only at 100 m water depth, suggests that there must be a reversible exchange of 234Th between the suspended particles and the settling particles bearing 234Th in deep water. The concentration of 234Th in the settling particles records only the recent history of the settling particles because of its shorter life time, and the 234Th concentrations in the particles do not show systematic trends with depth unlike the concentrations of 210Pb and 2"Po which increase with depth as shown below. Both the concentrations of 210Pb and 210po increase with increasing depth. The ratios of those nuclides to Al also increase with increasing depth (Table 2), despite the fact that the concentration of Al increases with increasing depth (Brewer et al., 1980; Tsunogai et al., 1982). This indicates that the settling particles pick up 210Pb and 210Po as well as 234Th in seawater when they sink through the water column. The ratios of 210Po (the grand daughter of 210Pb) to 210Pb are greater than unity at all stations (Table 2). This suggests that 210Po is more intimately associated with the settling particles than 210Pb. This is consistent with previous studies (Shannon et al., 1970; Bacon et al., 1976). The ratios of 234Th to excess 210Po relative to 210Pb in the settling particles are also listed in Table 2. The 234Th/210Po ratios in deep seawater are found to range from 10 to 15. If the settling particles are picking up 234Th and 210Po with the same ratio as in deep seawater, the ratio of 234Th and 210Po caught by the settling particles in the deep sea should be smaller than the 234Th./210Po ratio in deep seawater because of the shorter half-life of 234Th. If excess 210Po relative to 210Pb in the settling particles can be regarded as newly attached 210Po from deep water, we can compare the 234Th/excess 210Po in the settling particles and the 234Th/210Po ratio in deep seawater. Table 2 shows greater 234Th/ excess 210Po ratios in the settling particles than the 234Th/210Po ratio in seawater. This suggests that 234Th is apt to be caught by the large settling particles preferentially to 210Po Comparison of directly observed 210Pb tration of 210Pb in seawater The settling flux of 210Pb from seawater can also be estimated from the vertical profiles of the concentrations of 210Pb and its parent, 226Ra, in seawater (Craig et al., 1973; Nozaki et al., 1976; Tsunogai et al., 1980). We tried to compare the observed settling fluxes with those calculated at the stations in the northern North Pacific, the eastern Pacific and the Antarctic Ocean. The data on the concentrations of 226Ra and 210Pb used in this calculation were obtained at Stations 216, 344 and 282 of the GEOSECS expedition in the northern North Pacific, the eastern Pacific and the Antarctic Ocean, respectively (Ku et al., 1976; Chung and Craig, 1980, 1983). The deposition rates of 210Pb from the atmosphere are assumed to be 5,000, 4,000 and 1,600 dpm M-2 yr-1 in the northern North Pacific, the eastern North Pacific and the Antarctic Ocean, respectively, based on deposition rates estimated by Turekian et al. (1977). As shown in Fig. 7, the observed fluxes of 210Pb in the northern North Pacific and the Antarctic Ocean are nearly equal to or somewhat larger than the fluxes calculated from the inventories of 226Ra and 210Pb in the seawater. On the other hand, at Station 11 in the eastern Pacific, the observed flux at 3,600 m, 1.2 ~103 dpm M-2 yr-1, is much smaller than the calcu- Fig.7. Comparison of the 210Pb fluxes observed with the sediment traps and the fluxes calculated from the inventories of 228Ra and 210Pb in the water column. The solid line and dashed line refer the observed fluxes and the calculated fluxes, respectively.

8 Fluxes of 234Th, 210Po and 210Pb Determined lated flux at the same depth, 8 ~103 dpm m-2 yr-i. This suggests that most of 210Pb atoms supplied to the water column are not removed in this area of smaller total fluxes. As shown in Table 1, a considerable part of the world ocean is comprised of the area of smaller total fluxes, such as the subtropical Pacific and Atlantic Oceans. Therefore, we can conclude that a large proportion of metals such as Pb and Th supplied to the oligotrophic subtropical oceans is not removed there but transported to the eutrophic, subboreal or coastal seas by a horizontal flow when they are then removed by settling biogenic particles. It was noted by Bacon et al. (1976) that the 210Pb/226Ra ratio in sea water increases with increasing distance from lateral boundaries of continents, and they suggested that 210Pb was scavenged effectively by the sediment-water interface at the margins of the ocean. Our interpretation although different from theirs is comparable with their results because the margin of the ocean is generally a biologically productive zone. Acknowledgements We would like to thank Profs A. Hattori and T. Nemoto, scientists, officers and crew aboard the R/V Hakuho Maru. We also wish to thank to Dr. Noriki and the staff of Laboratory of Analytical Chemistry, Faculty of Fisheries, Hokkaido University for carrying out the sediment trap experiments. We are indebted to the reviewers of this paper for their valuable suggestions. References Bacon, M. P., D.W. Spencer and P.G. Brewer (1976): 210Pb/226Ra and 210Po/210Pb disequilibria in sea - water and suspended particulate matter. Earth Planet. Sci. Lett., 32, Bacon, M. P., C.- A. Huh, A. P. Fleer and W. G. Deuser (1985): Seasonality in the flux of natural radionuclides and plutonium in the deep Sargasso Sea, Deep-Sea Res., 32, Bhat, S.G., S. Krishnaswami, D. Lal, Rama and W. S. Moore (1969): Th-234/ U-238 ratios in the ocean, Earth Planet. Sci. Lett., 5, Bishop, J. K. B., J.M. Edmond, D. R. Ketten, M. P. Bacon and W. B. Silker (1977): The chemistry, biology and vertical flux of particulate matter from the upper 400 m of the equatorial Atlantic Ocean, Deep-Sea Res., 24, Brewer, P. G., Y. Nozaki, D. W. Spencer and A. P. Fleer (1980): Sediment trap experiments in the deep North Atlantic: isotopic and elemental Chung, Y. and H. Craig (1980): 226Ra in the Pacific Ocean. Earth Planet. Sci. Lett., 49, Chung, Y. and H. Craig (1983): 210Pb in the Pacific: the Geosecs measurements of particulate and dissolved concentrations. Earth Planet. Sci. Lett., 65, Craig, H., S. Krishnaswami and B. L. K. Somayajulu (1973): 210Pb-226Ra: Radioactive disequilibrium in the deep sea. Earth Planet. Sci. Lett., 17, Deucer, W. G., E. H. Ross and R. F. Anderson (1981): Seasonality in the supply of sediment to deep Sargasso Sea and implications for the rapid transfer of matter to the deep ocean. Deep-Sea Res., 28, Harada, K. and S. Tsunogai (1985): A practical method for the simultaneous determinations of 234Th, 226Ra, 210pb and 210Po in seawater. J. Oceanogr. Soc. Japan, 41, Honjo, S., S. J. Manganini and J. J. Cole (1982): Sedimentation of biogenic matter in the deep ocean. Deep-Sea Res., 29, Honjo, S.(1982): Seasonality and interaction of biogenic and lithogenic particulate flux at the Panama Basin. Science, 218, Krishnaswami, S., B. L. K. Somayajulu and Y. Chung (1975):210Pb/226Ra disequilibrium in the Santa Barbara basin. Earth Planet. Sci. Lett., 27, Krishnaswami, S. and M. M. Sarin (1976): Atlantic surface particulates: Composition, settling rates and dissolution in the deep sea. Earth Planet. Sci. Lett., 32, Ku, T-L. and M-C. Lin (1976): 226Ra distribution in the Antarctic Ocean. Earth Planet. Sci. Lett., 32, Matsumoto, E. (1975): 234Th-238U radioactive disequilibrium in the surface layer of the ocean. Geochim. Cosmochim. Acta, 39, Moore, W. S., K. W. Bruland and J. Michel (1981): Fluxes of uranium and Thorium series isotopes in the Santa Barbara Basin, Earth Planet. Sci. Lett., 53, Noriki, S., K. Harada and S. Tsunogai (1985): Sediment trap experiment in the Antarctic Ocean. p In: Marine and Esturine Geochemistry, ed. by A. C. Sigleo and A. Hattori, Lewis Pub., Chelsea. Noriki, S. and S. Tsunogai (1986a): Particulate fluxes and major components of settling particles from sediment trap experiment in the Pacific Ocean.

9 Harada and. Tstnogai Deep-Sea Res., in press. Noriki, S. and S. Tsunogai (1986b): Sediment trap comparison experiments: Existence of light particles collected in the narrow sediment traps. J. Oceanogr. Soc. Japan, 42, (in Japanese). Nozaki, Y. and S. Tsunogai (1976): 226Ra, 210Pb and 210Po disequilibria in the western North Pacific. Earth Planet. Sci. Lett., 32, Nozaki, Y., J. Thomson and K. K. Turekian (1976): The distribution of 210Pb and 210Po in the surface waters of the Pacific Ocean. Earth Planet. Sci. Lett., 32, Rama, M. Koide and E. D. Goldberg (1961): 210Pb in natural waters. Science, 134, Shannon, L. V., R. D. Cherry and M. J. Orren (1970): Polonium-210 and lead-210 in the marine environment, Geochim. Cosmochim. Acta, 34, Spencer, D. W., P. G. Brewer, A. Fleer, S. Honjo, S. Krishnaswami and Y. Nozaki (1978): Chemical deep Sargasso Sea. J. Mar. Res., 36, Tsunogai, S. and K. Harada (1980): 226Ra and 210Pb, in the western North Pacific. p In: Isotope Marine Chemistry, ed. by E. D. Goldberg,. Y. Horibe and K. Saruhashi, Uchida Rokakuho, Pub., Tokyo. Tsunogai, S., M. Uematsu, S. Noriki, N. Tanaka and M. Yamada (1982): Sediment trap experiment. in the northern North Pacific: Undulation of settling particles. Geochem. J., 16, Turekian, K. K., Y. Nozaki and L. K. Benninger (1977): Geochemistry of atmospheric radon and radon products. Ann. Rev. Earth Planet. Sci., 5,