91 Journal of the Sedimentological Society of Japan ΐ῕῔ Vol.02 , No. , , p. 3+ - +*- ( ,**3 ) Fluxes and inventories of,+* Pb and +-1 Cs in lake sediments from East Asia and their relationships to sedimentary environment Yutaka Kanai* The activities, fluxes and inventories of,+* Pb and +-1 Cs in sediments collected from freshwater lakes, brackish-water lakes and seabeds in the East Asia are studied to elucidate their geochemical behaviors and characteristics concerning sedimentation environments. The surface concentrations varied widely by location. The average flux of excess,+* Pb increased from freshwater lakes to brackish-water lakes to seawater, and the average flux of+-1 Cs showed an inverse trend. The inventories of excess,+* Pb and +-1 Cs were in good correlation in samples from the same lake or adjacent areas. The inventory ratio (+-1 Cs/excess ,+* Pb) was in the order of samples from seawater, increasing in brackish-water, further increasing in freshwater, and samples from Chinese lakes showed higher values. It is inferred that the inventory ratio may become a kind of index of sedimentation environment, geological, and/or radiochemical events. Key words:,+* Pb, +-1 Cs, lake sediment, East Asia, inventory, flux ῌῌῌῌῌῌῌῌῌῌῌῌῌῌῌῌῌῌῌῌῌῌῌῌῌῌῌῌῌῌῌῌῌῌῌῌῌῌῌῌῌῌῌῌῌῌῌῌῌῌῌῌῌ +3/.). Introduction A simple,+* Pb chronology is based on the following The sedimentation rate is one of the most important assumptions. pieces of information for the analysis of sedimentary (+ ) The produced,+* Pb (unsupported ,+* Pb; excess ,+* Pb) process and environment.,+* Pb and +-1 Cs have been used is adsorbed on the aerosol, removed from the atmosphere to date recent lake sediments or sea bottom sediments within a week (Poet et al.,+31, ), and deposited with (for example; Appleby and Oldfield,+312 ; Cochran et suspended matter in the water column. al.,+332 ; Goldberg and Koide, +30, ; Koide et al., +31, ; ( , ) The residence time of excess,+* Pb in water is by far +31-; Krishnaswami et al., +31+ ). The former nuclide shorter than the sedimentation time, and excess,+* Pb is belongs to the uranium series nuclides and is produced successively incorporated into the sediment. from,,, Rn gas through short half-life nuclides. Its half- (- ) Excess,+* Pb in the sediment does not move after life is,, . - y, which is appropriate for dating within about sedimentation. The sediment column is a closed system +** years. On the other hand, the latter nuclide has been for excess,+* Pb. produced mainly from the nuclear weapon tests and the (. ),+* Pb in the sediment grain (supported ,+* Pb) is in nuclear power industries. Its detection in sediments equilibrium with,,0 Ra in the sediment grain. started in+3/. , and its release became maximum in +30- ( / ) The excess,+* Pb concentration in the surface bot- (Peirson,+31+ ). Its half-life is -* . - y, and the variation tom sediment is always constant (CIC model), or the flux of its activity is also used for marking the date (+30- or (F) of excess,+* Pb is constant (CRS model; Appleby and Oldfield,+312 ). Received: April1 , ,**3 ; Accepted: December ,/ , ,**3 The activity (Ai) of excess,+* Pb at depth z is expressed * Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST)+++ῌῌ , Tsukuba, as follows in a simple model. Ibaraki-*/ῌ 2/01 Japan Aiῒ῍ῑ῎ A* ·· expl zῌ w eq. ῏ῐῌ+ Corresponding author: Y. Kanai, [email protected] 92 Y. Kanai ,**3 where A* ,l , and w are the activity at the surface sedi- ment, the decay constant of,+* Pb, and the sedimentation rate, respectively. The flux (F) is the annual input of radioactivity into the sediment: FAwΐῐῑ῍* · eq., The inventory (I) is the depth-integrated radionuclide activity through the sediment column and is calculated as follows; IAieq.ΐrfdsz··· ῎+- ῒ ῏ ῐ ῑῌ whererfsz , , and d are the solid density (assumed as,./ . in this study), the porosity, and the depth increment in + sediment column, respectively. There is a relationship Fig. Sampling locations of studied areas. between F and I (Kanai,,*** ); The map is created by using “pscoast” (URL: woodshole. er.usgs.gov/mapit/index.html). Fΐῐῑ῍l· I eq. The chronology by use of+-1 Cs is also based on the cm were taken systematically in about, km intervals in resembled assumptions mentioned above, except (. ) and the two lakes, respectively (Kanai et al., +332 b; ,**, ). (/ ). The radionuclides are assumed to associate with the Three cores were taken from o# Niigata, the Sea of Japan aerosol or suspended materials, deposit with suspended (Kanai and Ikehara,+33/ ). Two Chinese lake cores with matters in water column, and successively incorporated0*ῌ 2/ cm lengths were taken from Lake Daihai and Lake into the sediment. Heiquan (Kanai et al.,+332 a). Two cores ( .* and 0* cm We have studied the sedimentation rates in some lakes long) in Nepal lakes were taken from Lake Begnas and in Japan, China, and Nepal (Kanai et al.,+33/ ; +331 ; Lake Rupacot (Kanai et al., ,*** ). The detailed loca- +332a; +332 b; ,*** ; ,**, ) and in seas (Kanai and Ike- tions are shown in Fig. + . hara,+33/ ). In this study, validation of the model is Radioactivity measurement was conducted as follows. checked by using these data. Especially much attention Each core sample was cut at+/ῌ cm intervals, and the was paid to the fluxes and inventories of the radionuclides divided samples were dried and ground prior to analysis. to distinguish the characteristics of the sediments such as About+/ῌ g of powdered sample was taken into the fresh/brackish environment. Nalgene centrifuge tube with a cap and sealed for more than+ month to maintain radioactive equilibrium be- tween,,0 Ra and ,+. Pb. Then the activities of ,+* Pb (peak Samples and experimental energy:.0 . / keV),,+. Pb ( -/, keV), +-1 Cs ( 00+ . 0 keV), and The lake sediments were taken from Japan, China, and.* K (+.0+ keV) were measured by gamma-ray spectrome- Nepal. In Japan, Lake Suwa, Lake Shinji, and Lake try, using well-type Ge semiconductor detectors (ORTEC Nakaumi were selected. Lake Suwa is located in the center GWL-+.*,-* -S and GWL- +,*,-* -S). The peak intensi- of Nagano Prefecture, in central Japan. It is a freshwater ties were corrected by the sample configuration. The lake, and++ cores with /*ῌ 3* cm lengths (about 1, cm on activity of excess,+* Pb is calculated by subtracting that of average) were taken from the bottom (Kanai et al.,+33/ ;,+. Pb from that of ,+* Pb, assuming the above assumption +331). Lake Shinji and Lake Nakaumi are located in (. ). The additional information is shown in the references. Shimane and Tottori Prefectures, western Japan. They The adsorption experiment was conducted as follows. are brackish-water lakes. Although the salinities in these The bentonite powder, kaolinite powder (Wako Pure Chem- lakes vary with location, water depth, and season, those ical Industries, Ltd.), and iron oxyhydroxide (FeOOH; reported are,0 .ῌῌ -+ .῕῕ , +2 . (surface water), -- . .3 . ῕ , Nacalai Tesque, Inc.) were taken in test tubes with +* +. 2ῌῌ 3῕῔῕ , 1 (bottom water) for Lake Shinji, and ++ ppm Cs solutions, respectively (solid: solution ratio is + : +.῕῕, +0 . * , /ῌῌῌ +* ῕ (surface water), +2 ,/ ῕ , ,/ -* ῕ , +** ). An aliquot of the reaction solution was taken after ,*ῌ ,/῕ (bottom water) for Lake Nakaumi (Nakamura, one month, and the Cs concentration was determined by +33-; Takayasu and Tokuoka, +33- ; Tokuoka et al., an atomic emission spectrometer (Japan Jarrel Ash Ltd., +33*). Fifteen and thirteen cores ranging from -+ to 30 AA- 2/** ). ΐῒ῍ῐ῏02 ῎ , ῑ Fluxes and inventories of,+* Pb and +-1 Cs in lake sediments from East Asia 93 and their relationships to sedimentary environment constant on average for the relatively long term under Results and discussions concern. +. Activities of surface sediments and fluxes Table+ shows the activity of the surface sediment with Most of excess,+* Pb, after disintegrations from ,,, Rn in the data on sedimentation rate, flux, inventories of excess the atmosphere, is deposited in the sediments with aerosol,+* Pb and +-1 Cs in the studied areas. Figure, a depicts the particles or suspended materials. Its concentration or flux range and the average of surface activities of excess,+* Pb. (F) depends on the,,, Rn concentration in the air, and The data obtained from the same lake vary widely. For ,,,Rn concentration depends on the amount of,,0 Ra and example, those from Lake Shinji range from**0 . to *1 . ,-2U. Although the temporal ,,, Rn concentration varies Bq/g. Those from Lake Suwa and Chinese lakes (average with time and weather conditions, showing high in theῌ*+ . Bq/g) appear to be lower than those from others. morning and low in the evening (Eisenbud and Gesell, Because there is a relationship (eq. [, ]) among flux (F; , +331; Iida et a., ,*** ), it can be assumed to be almost Bq/cm /y), surface concentration (A* ; Bq/g) and sedi- Fig. , (a): Range (straight line) and average (ῌ ) of excess,+* Pb activity at surface sediment. (b): Range (straight line) and average (ῌ ) of+-1 Cs activity at surface sediment. 94 Y. Kanai ,**3 Table + Activity of the surface sediment, sedimentation rate, flux, and inventory of excess,+* Pb and +-1 Cs in the studied areas. ῔ΐ῎ῑῐ02 ῏ , ῒ Fluxes and inventories of,+* Pb and +-1 Cs in lake sediments from East Asia 95 and their relationships to sedimentary environment mentation rate (w; g/cm, /y) in an ideal sedimentation Figure- a depicts the range and the average of excess model, the surface concentration may vary with the flux,+* Pb fluxes. Although the fluxes from Lake Nakaumi and and the sedimentation rate. Flux, sedimentation rate and the Sea of Japan are scattered, the average has an increas- surface concentration are calculated from the depth pro- ing trend in the order of Chinese lakes (* . *,0 Bq/cm, /y), file of excess,+* Pb activities using the regression line by Lake Suwa (* . *-2 Bq/cm, /y), Nepal lakes ( * . *.0 Bq/ least square method.