Koh Harada

Particulate removal of230Th and231Pa in the biologically productive northern North Pacific

Settling particles were collected by sediment traps at four stations in the high latitudes of the North Pacific and analyzed for234Th,232Th,230Th,228Th and231Pa. In the northern North Pacific, including the Bering Sea, the average activity ratio of unsupported230Th to231Pa in settling particles was 5.5, whereas that in the Gulf of Alaska in the eastern North Pacific was about 14. The northern North Pacific may be an efficient sink for231Pa relative to230Th because the average ratio in the world ocean is equal to the production ratio of 10.8 in seawater. The activity ratios of230Th/231Pa of the settling particles agree well with those of the surface sediments at the respective stations. This indicates that both nuclides in the surface sediments originated from settling particles and their dissolution or uptake during early diagenesis are not effective. n nThe ratio of the particulate nuclide flux (F) to the overlying water column production rate (P) did not correlate well with the total mass flux for230Th. However, that for231Pa increased almost linearly with increase in total mass flux and reached unity when the total mass flux was about 110 mg/m2 d. This suggests that231Pa transported from oligotrophic regions is being actively removed in the biologically productive northern North Pacific.

Removal of trace metals from seawater during a phytoplankton bloom as studied with sediment traps in Funka Bay, Japan

Abstract To study biological effects on the particulate removal of chemical elements from seawater, sediment trap experiments were carried out successively ten times throughout the spring phytoplankton bloom in Funka Bay. Sediment traps were deployed every one to two weeks at 1, 40 and 80 m depths. The settling particles obtained were analyzed for trace metals, phosphate and silicate. The propagation of diatoms in spring results in larger particulate fluxes than that of dinoflagellates. The biogenic silicate concentration is higher in the earlier period, when diatoms are predominant, than in the subsequent period, when dinoflagellates are predominant. The concentrations of aluminum, iron, manganese and cobalt in the settling particles comprising largely biogenic particles are lower during phytoplankton bloom. The concentration of copper is not reduced by the addition of biogenic particles, and its vertical flux is approximately proportional to the total flux, indicating that its concentration in the biogenic particles is nearly equal to that in the non-biogenic particles. The results for nickel and lead show the same tendency as for copper. Cadmium is more concentrated in biogenic particles than in non-biogenic particles, and the concentration of cadmium in the settling particles decreases with depth, similarly to phosphate and organic matter. Thus, metals in seawater are segregated by biological affinities, and the degree of incorporation into biogenic particles is in the order Cd > Pb, Ni, Cu > Co > Mn, Fe, Al. Biogenic particles are the most important agent controlling the vertical distribution of metals in the ocean. They remove the metals from the surface water, transport them through the water column, and regenerate them in the deep.

226Ra in the Japan Sea and the residence time of the Japan Sea water

Abstract The surface water of the Japan Sea contained 226 Ra of 70 ± 4dpm m −3 which was nearly equal to that of the surface water in the North Pacific. The concentration of 226 Ra in the Japan Sea deep water below 500 m was 151 ± 8dpm m −3 , showing a vertically and regionally small variation. This concentration of 226 Ra in the deep water is unexpectedly high, because the Japan Sea deep water has a higher Δ 14 C value by about 50‰ than the Atlantic deep water containing the same 226 Ra. One of the causes to be considered is larger contribution of 226 Ra from biogenic particles dissolving in the Japan Sea deep water, but the Japan Sea is not so fertile in comparison to the Bering Sea. The other more plausible cause is the internal ventilation of the Japan Sea water, which means that the residence time of the Japan Sea Proper water is considerably long although the water is vertically mixed fairly well especially in winter. The ventilation may supply some amounts of radiocarbon and oxygen but does not change the inventory of 226 Ra. The residence times of the Japan Sea deep water and of water within the Japan Sea are calculated by solving simultaneous equations for 226 Ra and 14 C with a three-box model to be 300–400 years and 700–1000 years, respectively.

Large but variable particulate flux in the antarctic ocean and its significance for the chemistry of antarctic water

Settling particles were collected at 1,460 m and 3,760 m depth in the Antarctic Ocean with sediment traps of time series type. The total deployment period of 40 days was divided into four terms of 10 days each. Seawater samples were collected both at deployment and retrieval of the traps at each site. During the 42 days the concentration of silicate in the surface water decreased by 32%, whereas those of nitrate and phosphate decreased by only 4–5%. The total particulate flux in the Antarctic Ocean is the largest among those hitherto observed in the world ocean. The time variation of the particulate flux at 1,460 m depth almost coincided with that at 3,760 m. The settling particles were comprised roughly of 80% biogenic silica, 15% organic matter and 5% other substances including sea salt. The clay fraction was only 0.05% at 1,460 m depth. The settling flux of biogenic silica agrees fairly well with the calculated rate of change in the concentration of silicate in the surface 100 m. Thus it is concluded that preferential propagation of diatoms reduces the concentration of silicate prior to other nutrients in the Antarctic Ocean.

Fluxes of234Th,210Po and210Pb determined by sediment trap experiments in pelagic oceans

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 and210Pb 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 somewhat smaller. The concentrations of234Th in the settling particles varied widely and irregularly with depth, whereas the concentrations of210Po and210Pb in the settling particles steadily increased with increasing water depth. The ratios of210Po/210Pb in the settling particulates were larger than unity which the ratio of234Th/excess210Po as larger than234Th/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 order234Th>210Po>210Pb. The observed particulate fluxes of210Pb are about one eighth of those calculated from the disequilibria between226Ra and210Pb 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.

A practical method for the simultaneous determination of234Th,226Ra,210Pb and210Po in seawater

A practical method has been developed for the simultaneous determination of226Ra,234Th,210Pb and210Po in seawater. In the method, the samples are spiked with228Ra,230Th,208Po and common lead to determine chemical yield. These nuclides are coprecipitated with calcium carbonate and ferric hydroxide from 20 to 50 l of seawater and separated from one another by using coprecipitation and ion exchange techniques. Counting sources of Ra and the other nuclides are prepared by electrodeposition onto silver discs. Their radioactivities are counted with anα-spectrometer and a low backgroundβ-counter. This method gives a standard deviation of about 5% for replicate determination of226Ra and the other nuclides.

Seasonal variation in the difference between observed and calculated particulate fluxes of Th-234 in Funka Bay, Japan

Particulate fluxes were determined by two methods to elucidate the behavior of settling particles in seawater. One method involves direct observation of fluxes with sediment traps, while in the other method flux is indirectly calculated from the radioactive disequilibrium between U-238 and Th-234 in seawater, which gives net flux. Observations were carried out several times throughout a year in Funka Bay. When linearly extrapolated, the observed gross fluxes of Th-234 did not converge to zero at the surface. In the subsurface water the difference between the observed and calculated fluxes showed a seasonal variation. The observed fluxes roughly coincided with the calculated net fluxes in the summer stratified water but the observedfluxes were much larger than the calculated ones in the convective winter water. Conversely the observed fluxes were smaller than the calculated ones in spring when the water was exchanging. These results suggest that we can apply this two approach method to get information not only on the behavior of settling particles in seawater but also on the physical stability of water.

Anomalous Large Scavenging of 230Th and 231Pa Controlled by Particle Composition in the Northwestern North Pacific

We report the role of particle composition and lateral particle movement that influences the oceanic distribution of 231Pa and 230Th. Settling particles were collected during sediment trap experiments. These and surface sediments were obtained from five stations along 38 to 44°N in the northwestern North Pacific. The high total mass flux and seasonal variations in the marginal area of the western North Pacific are controlled by the supply of lithogenic materials and primary productivity. The high content of the lithogenic material in the settling particles in this area contributes to 230Thex fluxes that exceed the local rate of supply. The lithogenic materials are important as a carrier of 230Th and contribute to the fractionation between 230Th and 231Pa in the ocean, as the 231Paex/230Thex ratio in the settling particles decreases with increasing 232Th concentration. The 231Paex/230Thex ratio in the settling particles collected in the abyssal basin decreases with water depth, which indicates that lateral transport of the lithogenic particles from the marginal area and/or shallower depth plays an important role in determining the 231Paex/230Thex ratio in a population of settling particles and remineralization. This indicates that lateral redistribution of particles and sediment focusing influence the 231Paex/230Thex ratios in surface sediments. Thus, the observations reported here mean that the use of the sediment 231Paex/230Thex ratio as a paleoproductivity proxy will be problematic in the northwestern North Pacific.

Is lead soluble at the surface of sediments in biologically productive seas

Abstract Total concentrations of 210 Pb and 210 Po were observed in vertical profiles of coastal waters, 92 m in depth. In summer, in stratified water, the profiles show maxima at 30–40 m depth and minima at about 70 m. Below 70 m, the concentrations of 210 Pb increased towards the seabed. The activity ratios of 210 Po/ 210 Pb in the water column decreased from 0.6 at 30 m to 0.3 at 90 m; this indicates that the resuspension of these nuclides from the bottom is not effective, since the activity ratio of 210 Po/ 210 Pb of the bottom sediment was almost unity. On the basis of budget calculations, the net production rate of 210 Pb in the bottom water was 17 dpm m −2 day −1 (1dpm = 1.67 × 10 −2 Bq), whereas it was−1.4dpm m −2 day −1 for 210 Po. Taking into account the particulate removal of 210 Pb, its supply from the bottom sediments to the bottom waters was estimated to be 22 dpm m −2 day −1 . Supply of 210 Pb to the bottom water suggests strongly that lead is mobilized in the weakly reducing surface sediments and, at least, a part of the lead migrates into the bottom of the water column.

Sequential and rapid determination of Po-210, Bi-210 and Pb-210 in natural waters.

A sequential and rapid separation method for the determination of radon daughter nuclides, Pb-210, Bi-210 and Po-210 has been developed for application to natural waters. Rapid separation is attained by the use of the same hydrochloric acid solution. After isolation of the three radionuclides from the sample by co-precipitation with added Fe(3+), polonium isotopes are first spontaneously deposited onto a silver disc from a 0.5N hydrochloric acid solution. Next, bismuth isotopes are electrodeposited onto a platinum net cathode coupled with a platinum coil anode at 1.2 V. Finally, lead isotopes are electrodeposited onto a platinum net cathode at 1.8 V from the remaining solution by adding hydroxylamine hydrochloride as an anodic depolarizer. This method can be applied to meteorological precipitation samples where these three nuclides are separated within 10 hr after the sampling with chemical yields of more than 80% for Po-210 and Bi-210 and more than 70% for Pb-210. This method is applicable to other environmental water samples.