-"Thorium balance in a single volume of seawater is 

 described by equation: 



The flux from the surt"ace water layer at depth H is 

 determined as follows: 



^A';- 



where 





(7) 



Njj^ — -"Th atoms concentration in sea medium; 



A'^ — -'*U atoms concentration in sea medium; 



X^ — -w-j-j^ radioactive decay constant; 



A.j^, — -'''U radioactive decay constant; 



k — factor of -"Th accumulation in suspended organic 



substance; 

 P — rate of biosedimentation removal of 



suspended organic substance from single 



water volume. 

 The general solution of differential equation ( 1) in relation 



to A'^ is as follows: 



^n.,n = ^v.,0, • \' 



h 



-{X„+lfPi-i 



•P)J^ 



^n,ore 



(X„,+k''Pj-\ \ -{\,,+ k- P}y 



-(X„ ^ If PI- 1 



(2) 



where N^.„„ and Nyh,,,,, are -'■U and ''■'Th concentrations 

 respectively, at times zero. 



Transforming equation (2) with regard for small value \ 

 and N^, = constant to the results in: 





(3) 



For steady state conditions (dNj^dt = 0). the 

 biosedimentation rate is determined by equation ( 1 ): 



P = 



k • /V,„ 



(4) 



Introducing specific activities of -'"U and -"Th in seawater, 

 C^: = N,,'\, and C„, = A'„«Ar„ (4a) 



and specific activity of Th in the particulate organic matter: 



Cn'' = Nr„'\,.'k (4b) 



we obtain the equation to calculate the biosedimentation rate: 



p _(Q, - C„,) "A,,, ,c, 



•^ ^ »- ' 



The residence time of -'"'Th, r„, in the upper layers of water 

 is inversely proportional to the rate of the biosedimental 

 removal of this isotope (Coale & Bruland, 1987): 



Cr, 



(C, -C„,) • \„ 



(6) 



During the Third US-USSR Joint Research Expedition in 

 J uly-August 1 988 experiments were carried out at 8 stations in 

 the Bering Sea and at 3 stations in the Chukchi Sea. These 

 measurements included the vertical profiles of -"*U and -"Th 

 and the concentration -"Th in particulate organic matter 

 (POM). These data were then used to calculate biosedimentation 

 parameters in the region. 



The "^U concentrations in seawater were calculated based 

 on the close correlation of -'-U with salinity (Turekian & Chan, 

 1971;Ku£'f(;/., 1977): 



Ci, = 0.07081 'S 



(8) 



where Q = specific activity of -"U (dpm l-^y,S = salinity ("/qo). 



-"Thorium concentrations were determined separately in 

 filtered seawater and in POM. The POM samples were 

 collected at 3-4 depths within the upper 100-m layer. The 

 water (about 1 m') was forced by a vibrating immersion pump 

 through a "Midiya" filtration unit. The filters were 

 FPP-15-1.5, which retained suspended particles larger than 

 \i 0.5 m (Vakulovsky. 1986). The following procedure was 

 used to determine -"Th concentration in the filtered water. In 

 plastic tanks cosedimentation of Th(4+) with Fe(3+)-hydroxide 

 was performed on 100 1 of water. The precipitate was isolated 

 by paper filtration, dried, and redissolved in 

 50-100 ml 8N HCl. This solution was then brought to a volume 

 of 150 ml with 8N HCl. Thorium was isolated by passing the 

 solution through glass columns packed with cation-exchange 

 resin, Dowex in (H*) form. The resin was ashed at 450-500°C. 

 The ash residue was placed in a scintillation vial, dissolved in 

 about 1 ml of 0.5N HCl, and then 5-10 ml of scintillation 

 cocktail was added. The p-activity of the sample was measured 

 by liquid scintillation counting in a "Rack P-121, Wallac 

 LKB." The radiochemical purity of the -"Th was checked by 

 double measurement of activity , once immediately after isolation 

 and then again 24 days later. 



The filters with the POM were dried at 60°C. The dry 

 weight of the particulate matter and its concentration in water 

 were determined by the difference in weight before and after 

 filtration. The filters were then ashed in a muffle furnace at 

 450-500°C and the weight of the ash residue (with correction 

 for ashing of the filter itself) was determined. The ash residue 

 was dissolved in a HCl and HNO, mixture (3:1 ratio) then 

 evaporated in a sand bath. The resulting dry residue was re- 

 dissolved in 8M HCl. Any insoluble residue (silicates) was 

 isolated by means of centrifugation. The supernatant was 

 collected. Then 7 ml of 8M HCl was added to the remaining 

 precipitate, mixed, and centrifuged again. The supernatant was 

 again collected and added to the first collection. The procedure 

 was repeated three times. The combined supernatant liquid 

 was brought to a volume of 1 50 ml with 8M HCl. The -"Th was 

 then determined as described above for the seawater samples. 



225 



