- Email: [email protected]
226Ra222Rn210Pb systematics in seawater near the bottom of the ocean
Earth and Planetary Science Letters, 80 (1986) 36-40 Elsevier Science Publishers B.V.. Amsterdam - Printed in The Netherlands
36
226 Ra_
222
Rr~-~l’Pb systematics
in seawater near the bottom of the ocean Y oshiyuki
Received
Nozaki
March 24. 1986: revised version rcceivcd July 11. 1986
Four vertical ““Pb profiles obtained within 250 m above bottom in the deep western North Pacific showed no significant variation with depth. irrespective of marked increase of excess 27’Rn relative to ‘IhRa near the seafloor. Consequently. the apparent box-model residence time baaed on ““‘Pb/ lzrRn activity ratio sharply decreases from - 100 years in ocean interior to < 10 years near the bottom. However, the ‘I” Pb data can also be explained using a vertical mixing and first-order scavenging model without any enhanced scavenging near the sediment-water interface. This model calculation indicates that bottom excess 22’Rn can hardly influence ““Pb in the water and the short residence time derived using box-model is an artifact and does not reflect chemical reactivity of Pb. This also implies that the deficiency of ““Pb as a result of removal by scavenging is best estimated based on ““Ra and not on zzrRn. The ““Pb scavenging at the sediment-water interface that has been inferred from the oceanic distributions of ““Pb appears difficult to be detected from bottom 2”‘Pb profile measurements.
1. Introduction
based on (*“Pb -226 Ra) can not exceed -45 dpm/lOO kg on the basis of the highest 226Ra concentration in the North Pacific [12]. In the earlier GEOSECS papers [3,6], however, the effect of bottom excess 222Rn was implicitly ignored in the calculation of *l”Pb deficiency. Apparently, the above discrepancy needs to be clarified for our understanding of the boundary scavenging. Here I report four bottom *“Pb profiles in the western North Pacific for which 222Rn data are available. Implications of *“Pb- ***Rn- 226Ra disequilibria regarding ‘l”Pb scavenging at the sediment-water interface are described in this paper.
Recent extensive studies on *“Pb distribution in the GEOSECS program [l-6] have envisaged the importance of oceanic boundaries as a potential sink of reactive metals in seawater. The “‘Pb deficiency relative to its ancestor 226Ra generally increases from mid-ocean interior towards the continental edges and bottom seafloor where it is more intensively scavenged. The Pb removal processes near the boundaries probably include not only particulate scavenging within the water column but also scavenging at the sediment-water interface as first suggested by Bacon et al. [l]. Although particulate scavenging has been investigated using sediment trap [7,8] and by filtering water [1,5,9-111, it seems difficult to estimate the scavenging rate at the sediment-water boundary from direct measurements. Within about 100 m above seafloor, there exists a large amount of excess 222Rn relative to 22hRa due to its supply from sediments through pore water diffusion. This 222Rn decays to *“Pb, hence its effect on the distribution of *“Pb in the bottom water needs to be understood. For example, the deficiency of ““Pb relative to 222Rn is generally more negative than - 100 dpm/ 100 kg in the waters just above bottom, whereas the deficiency 0012-821X/86/$03.50
G 1986 Elsevier
Science Publishers
2. Sampling and methods Water samples were collected using 23-liter Niskin bottles during the KH-80-2 R.V. “Hakuho-Maru” cruise (Cygnus Expedition) from April to June, 1980 [13]. The distance between the sampling bottles and the seabed was determined using a pinger record. The sampling locations are shown in Fig. 1. For *“Pb analysis, - 3-liter aliquot of unfiltered seawater was acidified with 10 ml of concentrated HCl aboardship and returned to the land-based laboratory. The analytical procedure for “‘Pb is described by Thomson and Turekian B.V.
3-i
160'
140' E
Fig. 1. Map showing Pacific.
the Cygnus
stations
160’ w
in the western
3.2. Near-bottom “OPb The data of *“Pb in bottom waters are given in Table 1 together with the 210Pb/222Rn activity ratio and the apparent box-model residence time. Although 222Rn varies considerably in the bottom water, *i”Pb is nearly constant with depth. It is also noted that the variation depending on location is small for 210Pb (- 10%) as compared to that of 222Rn (more than a factor of two). This suggests that the bottom excess 222Rn may have little effect on the variation of *“Pb. The apparent box-model residence time based North TABLE
[14] and Nozaki et al. [3]. ***Rn was measured aboardship immediately after sampling. The 222Rn data and their implications are given by Gamo and Horibe [15]. 3. Results and discussion 3.1. Comparison with the GEOSECS data During the GEOSECS program, three laboratoand ries participated in the *l’Pb measurements considerable efforts were devoted to intercomparison of their data sets [16]. Since Cygnus station 11 is a reoccupation of GEOSECS station 226, the results can be compared with the earlier GEOSECS data [3]. As shown in Fig. 2, there is no systematic difference between the two data sets. 20 _ I”
’
3o dpm/lON! I
4. I
Distance above bottom(m)
E
-
““Pb
and
“‘Pb/*“Rn
z’OPb a (dpm/lOO
kg)
ratio
2’0Pb/222 Rn activity ratio
Stu~orl CY-IO: 29O25’N, 176O5O’W; 5431 m 0.670 23.6 * 1.3 195 0.742 23.6 t 0.9 117 0.530 23.011.2 6X 0.170 25.1* 1.1 3X 0.160 23.4+ 1.2 19 0.16X 25.9 I 1.3 9 Stutiou CY-II: 14x 74 40 20 10 5
30°34’N. 170”36’E; 5504 m 0.783 26.4 & 1.2 0.648 22.1 i0.X 0.694 22.7 & 0.9 0.600 22.5 k 1.0 0.146 25.7* 1.3 0.121 23.6+ 1.0
Stutrofl CY-13: 30”01’N,
159”59’E:
in the western
Box-model residence time h (yr)
66 93 36 6.6 6.2 6.5
117 60 73 4x 5.5 4.4
5730 m 0.753 0.288 0.188 0.136 0.113 0.134
99 13 7.5 5.1 4.1 5.0
Sj~tro~ CY-16: 30”03’N. 146”55’E; 625X m 0.678 21.7& 1.2 243 0.546 22.1 t 1.5 145 0.411 21.3 k 1.2 77 0.427 22.5 &0.X 4X 0.345 22.1 f 1.6 2x 0.283 20.2& 1.3 14 0.220 20.4 + 1.2 4
67 38 22 24 17 13 9.0
191 113 64 34 15 5 x
1
Near-bottom North Pacific
24.7k1.5 21.3 F 1.6 25.1 & 1.5 24.0 + 1.3 21.2 * 1.2 23.Ok1.4
“‘Pb A
GEOSECS
226
Fig. 2. Comparison of 210Pb profiles at GEOSECS station 226 [3] and Cygnus station 11. The 226Ra data are based on Chung and Craig [12].
’ Errors are one sigma counting statistics. ’ Based on the residence time, 7 = [R/(1 - R)](l/h) where R is the activity ratio of 2”‘Pb/222Rn and X is the decay constant of ““Pb.
38
on 210Pb/ 222Rn activity ratio sharply decreases from - 100 years at a distance of > 150 m above bottom to < 10 years near the seafloor (Table 1). From these calculations, one may arrive at a conis greatly enclusion that the “‘Pb scavenging hanced near the bottom boundary. The rapid 210Pb scavenging near bottom is likely to be caused by resuspension of sediment particles and/or direct contact with surface sediments at the bottom interface. However, this interpretation is misleading because the bottom water is not a closed system, and on a time scale of several years, the water which contains excess 222Rn at the time of measurement must have spent most of the time away from the boundary without having any excess 222Rn in it. Therefore, the steady state assumption for the bottom excess 2’2Rn is no longer valid.
The bottom 222Rn profiles from the western North Pacific were grouped into three types of distribution [15]. Here, I assume, as a simple case, an exponential formula written by: A Kn =Al7a + (AOR”-AKa)
exp(-/$zj
(2)
where Ain is the activity of 12’Rn at z = 0 and A,, is the activity of 226Ra assumed to be constant with depth. Then the solution of equation (1) with the boundary conditions. A,, = A!,, at z = 0 and A,, =X,,A,,/(X,, + k) at z + c/j is given by:
3.3. A firsr-order scavenging model The above consideration leads us to use a more realistic model to describe the 210Pb- 222Rn- “‘Ra system near the bottom of the ocean. Particularly, mixing and scavenging are the necessary components to be taken into account. One of the models with those parameters is described as: . -yKK-aA,,
at
aAm az2
-exp[
+ h~tJ~n-(X,,+k)A,,=O
(1) where A is the activity of the nuclide, X is the decay constant, z is the distance above bottom, K is the vertical eddy diffusion coefficient, k is the in-situ scavenging rate constant of 2’0Pb, and the subscripts Pb and Rn represent ‘i”Pb and 222Rn, respectively. In the equation, “‘Pb scavenging is formulated to be a first-order irreversible process a treated by Craig et al. 1171 and others [l-6]. However, Nozaki et al. [18,19] and Bacon and Anderson [20] have shown that scavenging may best be described by a reversible process for Th isotopes and may be so for Pb as well. In addition, lateral transport of 210Pb by eddy diffusion from ocean interior to boundary sinks appears to be also important for the 210Pb removal. Therefore, k is only a crude measure reflecting all removal processes of 2’0Pb and may not adequately represent the real in-situ scavenging process. Nevertheless the model is simple and useful to assess the importance of bottom excess 222Rn on the “‘Pb distribution.
- @zl]
(3)
Now it is possible to compare the model curves and the data. An example for Cygnus 16 is shown in Fig. 3. In the calculation, K= 69 cm2/s based on the 222Rn profile [15] and k = 0.01 yr- ’ based on the 2’oPb/226Ra activity ratio at intermediate depths are used. The two model curves for “‘Pb are based on the bottom boundary values, A& = 20.5 dpm/lOO kg obtained by real measurement and A “p,,= 0 assumed for an extreme case of complete uptake at the sediment-water interface. The 210Pb data fit well to the model curve with a finite Agb value (Fig. 3). Thus it is possible to interpret the 210Pb profile without any enhancement of in-situ scavenging rate near the bottom interface. Looking at equation (3), the third term of the right handside represents the effect of 222Rn/ 226Ra disequilibrium and it disappears when Ai,, = A,,. For CY-16, equation (3) can be rewritten as, A Pb = A:,, exp( - 4.34
X
10-6z)
+ 0.758A,,
x [ 1 - exp( - 4.34 X 10-hz)] +0.00047(
A;,, -A..)
39 300 Cygnus
16
F E 2 200 2 E B m
‘,,O
8 s 100
“‘Pb ‘.
E n
‘26Ra
“S
222R”
;.... ‘.
“Q -:
-‘-a- . ... .. ...__n 0
90’ Activity
(dp~lOOkg)
Fig. 3. Comparison of vertical first-order scavenging model curves with 2”‘Pb data at Cygnus 16 (see text). The calculated model profiles using any finite A& values between 0 and 20.5 dpm/lOO kg show their vertical gradients in between the two curves indicated in the figure and do not fit the data. The “‘Rn data are based on Gamo and Horibe [15].
does not fit the data at all. The nearly constant *“Pb profile suggests that scavenging reaction at the seafloor is so slow compared to vertical mixing and therefore most of the 210Pb transported by diffusion from the ocean interior must be reflected at the boundary. This is analogous to consumption of oxygen at the seafloor which is hardly identified from its bottom profile. Spencer et al. [4] have estimated the “‘Pb scavenging rate of 0.04 dpm cm-’ yr- ’ at the deep-sea bottom boundary of the Atlantic. Dividing the value by the vertical eddy diffusion coefficient (e.g. K = 69 cm2/s for CY-16), the vertical 2’0Pb concentration gradient which is necessary to yield the flux can be calculated to be - 2 x lop4 (dpm/lOO kg)/m. This small gradient would not be detectable by direct bottom *“Pb profile measurements. Acknowledgements
x [exp( -4.34 -exp(
x lo-6z)
- 1.74 X 10p4z)]
14)
Since the last term is a decreasing function with respect to z, the maximum effect of bottom excess 222Rn can be evaluated by 0.00047( Ai,, - AR,)/ A,,. This value is 0.0014 for CY-16 implying that the excess 222Rn can contribute at most only 0.14% to the observed *‘OPb concentration. The highest bottom 222Rn concentration that has been observed so far in the North Pacific is - 1000 dpm/ 100 kg [15,21]. If this value is used, the 222Rn contribution becomes - 2% as a maximum, which is still negligibly small. Clearly, these calculations indicate that bottom excess 222Rn can hardly inwater. Therefore. fluence 210Pb in the bottom ignorance of excess 222Rn from the 226Ra- 222Rn*“Pb system in the GEOSECS papers [3,6] can be justified.
I would like to thank Drs. Y. Horibe and T. Gamo for collaboration in sampling and Ms. K. Hasegawa for typing the manuscript. This work was partially supported by the Ministry of Education, Science and Culture, Japan under grant No. 65117009 to the University of Tokyo. References 1 M.P. Bacon, D.W. Spencer and P.G. Brewer, 2’oPb/‘2hRa
2
3
4
5
3.4. The bottom boundary value of ‘lOPb Since the above “‘Pb model calculations are very sensitive to the bottom boundary value, it seems necessary to discuss the processes maintainFig. 3 clearly indiing the 2’0Pb concentration. cates that complete uptake of “‘Pb by surface sediments is not taking place at the bottom boundary, since the model curve with A&, = 0
6
7
8
and ‘r’Po/ “‘Pb disequilibria in seawater and suspended particulate matter, Earth Planet, Sci. Lett. 32, 277, 1976. Y. Nozaki, J. Thomson and K.K. Turekian, The distribution of 2’oPb and 2’oPo in the surface waters of the Pacific Ocean, Earth Planet. Sci. Lett. 32, 304, 1976. Y. Nozaki, K.K. Turekian and K. Von Damm, “‘Pb in GEOSECS water profiles from the North Pacific, Earth Planet. Sci. Lett. 49, 393, 1980. D.W. Spencer. M.P. Bacon and P.G. Brewer, Models of the distribution of 2’0Pb in a section across the North Equatorial Atlantic Ocean. J. Mar. Res. 39, 119, 1981. Y. Chung and H. Craig, 21”Pb in the Pacific: the GEOSECS measurements of particulate and dissolved concentrations, Earth Planet. Sci. Lett. 65, 406, 1983. J.K. Cochran, M.P. Bacon, S. Krishnaswami and K.K. in the central and Turekian, “‘PO and “‘Pb distributions eastern Indian Ocean, Earth Planet. Sci. Lett. 65, 433, 1983. D.W. Spencer, P.G. Brewer, A. Fleer, S. Honjo, S. Krishnaswami and Y. Nozaki, Chemical fluxes from a sediment trap experiment in the deep Sargasso Sea, J. Mar. Res. 36. 493, 1978. P.G. Brewer, Y. Nozaki, D.W. Spencer and A.P. Fleer.
40
9
10
11
12 13
14
15
Sediment trap experiments in the deep North Atlantic: isotopic and elemental fluxes, J. Mar. Res. 38. 703, 1980. S. Krishnaswami, D. Lal, B.L.K. Somayajulu, R.F. Weiss and H. Craig, Large volume in-situ filtration of deep Pacific waters: mineralogical and radioisotope studies, Earth Planet. Sci. Lett. 32, 420, 1976. S. Krishnaswami, M.M. Sarin and B.L.K. Somayajulu, Chemical and radiochemical investigations of surface and deep particles of the Indian Ocean, Earth Planet. Sci. Lett. 54, 81, 1981. B.L.K. Somayajulu and H. Craig, Particulate and soluble *“Pb activities in the deep sea, Earth Planet. Sci. Lett. 32, 268, 1976. Y. Chung and H. Craig, 226Ra in the Pacific Ocean, Earth Planet. Sci. Lett. 49, 267, 1980. Y. Horibe, Preliminary Report of the Hakuho Maru cruise KH-80-2 (CYGNUS Expedition) and the Hakuho Maru cruise KH-82-1 (CEPHEUS Expedition), 78 pp., Ocean Research Institute, University of Tokyo, 1983. J. Thomson and K.K. Turekian, ‘t”Po and *“Pb distributions in ocean water profiles from the eastern south Pacific, Earth Planet. Sci. Lett. 32. 297, 1976. T. Gamo and Y. Horibe, Excess bottom ***Rn profiles and
16
17
18
19
20
21
their implications in the northwestern Pacific Ocean, Earth Planet. Sci. Lett. 71, 215. 1984. Y. Chung, R. Finkel, M.P. Bacon, J.K. Cochran and S. Krishnaswami, Intercomparison of *“Pb measurements at GEOSECS station 500 in the northeast Pacific, Earth Planet. Sci. Lett. 65, 393, 1983. H. Craig, S. Krishnaswami and B.L.K. Somayajulu, *“Pb226Ra: radioactive disequilibrium in the deep sea, Earth Planet. Sci. Lett. 17, 295, 1973. Y. Nozaki, Y. Horibe and H. Tsubota, The water column distributions of thorium isotopes in the western North Pacific, Earth Planet. Sci. Lett. 54, 203, 1981. Y. Nozaki and Y. Horibe, Alpha-emitting thorium isotopes in northwest Pacific deep waters, Earth Planet. Sci. Lett. 65, 39, 1983. M.P. Bacon and R.F. Anderson, Distribution of thorium isotopes between dissolved and particulate forms in the deep sea. J. Geophys. Res. 87, 2045, 1982. J.L. Sarmiento, H.W. Feely, W.S. Moore, A.E. Bainbridge and W.S. Broecker, The relationship between vertical eddy diffusion and buoyancy gradient in the deep sea, Earth Planet. Sci. Lett. 32, 357, 1976.
36
226 Ra_
222
Rr~-~l’Pb systematics
in seawater near the bottom of the ocean Y oshiyuki
Received
Nozaki
March 24. 1986: revised version rcceivcd July 11. 1986
Four vertical ““Pb profiles obtained within 250 m above bottom in the deep western North Pacific showed no significant variation with depth. irrespective of marked increase of excess 27’Rn relative to ‘IhRa near the seafloor. Consequently. the apparent box-model residence time baaed on ““‘Pb/ lzrRn activity ratio sharply decreases from - 100 years in ocean interior to < 10 years near the bottom. However, the ‘I” Pb data can also be explained using a vertical mixing and first-order scavenging model without any enhanced scavenging near the sediment-water interface. This model calculation indicates that bottom excess 22’Rn can hardly influence ““Pb in the water and the short residence time derived using box-model is an artifact and does not reflect chemical reactivity of Pb. This also implies that the deficiency of ““Pb as a result of removal by scavenging is best estimated based on ““Ra and not on zzrRn. The ““Pb scavenging at the sediment-water interface that has been inferred from the oceanic distributions of ““Pb appears difficult to be detected from bottom 2”‘Pb profile measurements.
1. Introduction
based on (*“Pb -226 Ra) can not exceed -45 dpm/lOO kg on the basis of the highest 226Ra concentration in the North Pacific [12]. In the earlier GEOSECS papers [3,6], however, the effect of bottom excess 222Rn was implicitly ignored in the calculation of *l”Pb deficiency. Apparently, the above discrepancy needs to be clarified for our understanding of the boundary scavenging. Here I report four bottom *“Pb profiles in the western North Pacific for which 222Rn data are available. Implications of *“Pb- ***Rn- 226Ra disequilibria regarding ‘l”Pb scavenging at the sediment-water interface are described in this paper.
Recent extensive studies on *“Pb distribution in the GEOSECS program [l-6] have envisaged the importance of oceanic boundaries as a potential sink of reactive metals in seawater. The “‘Pb deficiency relative to its ancestor 226Ra generally increases from mid-ocean interior towards the continental edges and bottom seafloor where it is more intensively scavenged. The Pb removal processes near the boundaries probably include not only particulate scavenging within the water column but also scavenging at the sediment-water interface as first suggested by Bacon et al. [l]. Although particulate scavenging has been investigated using sediment trap [7,8] and by filtering water [1,5,9-111, it seems difficult to estimate the scavenging rate at the sediment-water boundary from direct measurements. Within about 100 m above seafloor, there exists a large amount of excess 222Rn relative to 22hRa due to its supply from sediments through pore water diffusion. This 222Rn decays to *“Pb, hence its effect on the distribution of *“Pb in the bottom water needs to be understood. For example, the deficiency of ““Pb relative to 222Rn is generally more negative than - 100 dpm/ 100 kg in the waters just above bottom, whereas the deficiency 0012-821X/86/$03.50
G 1986 Elsevier
Science Publishers
2. Sampling and methods Water samples were collected using 23-liter Niskin bottles during the KH-80-2 R.V. “Hakuho-Maru” cruise (Cygnus Expedition) from April to June, 1980 [13]. The distance between the sampling bottles and the seabed was determined using a pinger record. The sampling locations are shown in Fig. 1. For *“Pb analysis, - 3-liter aliquot of unfiltered seawater was acidified with 10 ml of concentrated HCl aboardship and returned to the land-based laboratory. The analytical procedure for “‘Pb is described by Thomson and Turekian B.V.
3-i
160'
140' E
Fig. 1. Map showing Pacific.
the Cygnus
stations
160’ w
in the western
3.2. Near-bottom “OPb The data of *“Pb in bottom waters are given in Table 1 together with the 210Pb/222Rn activity ratio and the apparent box-model residence time. Although 222Rn varies considerably in the bottom water, *i”Pb is nearly constant with depth. It is also noted that the variation depending on location is small for 210Pb (- 10%) as compared to that of 222Rn (more than a factor of two). This suggests that the bottom excess 222Rn may have little effect on the variation of *“Pb. The apparent box-model residence time based North TABLE
[14] and Nozaki et al. [3]. ***Rn was measured aboardship immediately after sampling. The 222Rn data and their implications are given by Gamo and Horibe [15]. 3. Results and discussion 3.1. Comparison with the GEOSECS data During the GEOSECS program, three laboratoand ries participated in the *l’Pb measurements considerable efforts were devoted to intercomparison of their data sets [16]. Since Cygnus station 11 is a reoccupation of GEOSECS station 226, the results can be compared with the earlier GEOSECS data [3]. As shown in Fig. 2, there is no systematic difference between the two data sets. 20 _ I”
’
3o dpm/lON! I
4. I
Distance above bottom(m)
E
-
““Pb
and
“‘Pb/*“Rn
z’OPb a (dpm/lOO
kg)
ratio
2’0Pb/222 Rn activity ratio
Stu~orl CY-IO: 29O25’N, 176O5O’W; 5431 m 0.670 23.6 * 1.3 195 0.742 23.6 t 0.9 117 0.530 23.011.2 6X 0.170 25.1* 1.1 3X 0.160 23.4+ 1.2 19 0.16X 25.9 I 1.3 9 Stutiou CY-II: 14x 74 40 20 10 5
30°34’N. 170”36’E; 5504 m 0.783 26.4 & 1.2 0.648 22.1 i0.X 0.694 22.7 & 0.9 0.600 22.5 k 1.0 0.146 25.7* 1.3 0.121 23.6+ 1.0
Stutrofl CY-13: 30”01’N,
159”59’E:
in the western
Box-model residence time h (yr)
66 93 36 6.6 6.2 6.5
117 60 73 4x 5.5 4.4
5730 m 0.753 0.288 0.188 0.136 0.113 0.134
99 13 7.5 5.1 4.1 5.0
Sj~tro~ CY-16: 30”03’N. 146”55’E; 625X m 0.678 21.7& 1.2 243 0.546 22.1 t 1.5 145 0.411 21.3 k 1.2 77 0.427 22.5 &0.X 4X 0.345 22.1 f 1.6 2x 0.283 20.2& 1.3 14 0.220 20.4 + 1.2 4
67 38 22 24 17 13 9.0
191 113 64 34 15 5 x
1
Near-bottom North Pacific
24.7k1.5 21.3 F 1.6 25.1 & 1.5 24.0 + 1.3 21.2 * 1.2 23.Ok1.4
“‘Pb A
GEOSECS
226
Fig. 2. Comparison of 210Pb profiles at GEOSECS station 226 [3] and Cygnus station 11. The 226Ra data are based on Chung and Craig [12].
’ Errors are one sigma counting statistics. ’ Based on the residence time, 7 = [R/(1 - R)](l/h) where R is the activity ratio of 2”‘Pb/222Rn and X is the decay constant of ““Pb.
38
on 210Pb/ 222Rn activity ratio sharply decreases from - 100 years at a distance of > 150 m above bottom to < 10 years near the seafloor (Table 1). From these calculations, one may arrive at a conis greatly enclusion that the “‘Pb scavenging hanced near the bottom boundary. The rapid 210Pb scavenging near bottom is likely to be caused by resuspension of sediment particles and/or direct contact with surface sediments at the bottom interface. However, this interpretation is misleading because the bottom water is not a closed system, and on a time scale of several years, the water which contains excess 222Rn at the time of measurement must have spent most of the time away from the boundary without having any excess 222Rn in it. Therefore, the steady state assumption for the bottom excess 2’2Rn is no longer valid.
The bottom 222Rn profiles from the western North Pacific were grouped into three types of distribution [15]. Here, I assume, as a simple case, an exponential formula written by: A Kn =Al7a + (AOR”-AKa)
exp(-/$zj
(2)
where Ain is the activity of 12’Rn at z = 0 and A,, is the activity of 226Ra assumed to be constant with depth. Then the solution of equation (1) with the boundary conditions. A,, = A!,, at z = 0 and A,, =X,,A,,/(X,, + k) at z + c/j is given by:
3.3. A firsr-order scavenging model The above consideration leads us to use a more realistic model to describe the 210Pb- 222Rn- “‘Ra system near the bottom of the ocean. Particularly, mixing and scavenging are the necessary components to be taken into account. One of the models with those parameters is described as: . -yKK-aA,,
at
aAm az2
-exp[
+ h~tJ~n-(X,,+k)A,,=O
(1) where A is the activity of the nuclide, X is the decay constant, z is the distance above bottom, K is the vertical eddy diffusion coefficient, k is the in-situ scavenging rate constant of 2’0Pb, and the subscripts Pb and Rn represent ‘i”Pb and 222Rn, respectively. In the equation, “‘Pb scavenging is formulated to be a first-order irreversible process a treated by Craig et al. 1171 and others [l-6]. However, Nozaki et al. [18,19] and Bacon and Anderson [20] have shown that scavenging may best be described by a reversible process for Th isotopes and may be so for Pb as well. In addition, lateral transport of 210Pb by eddy diffusion from ocean interior to boundary sinks appears to be also important for the 210Pb removal. Therefore, k is only a crude measure reflecting all removal processes of 2’0Pb and may not adequately represent the real in-situ scavenging process. Nevertheless the model is simple and useful to assess the importance of bottom excess 222Rn on the “‘Pb distribution.
- @zl]
(3)
Now it is possible to compare the model curves and the data. An example for Cygnus 16 is shown in Fig. 3. In the calculation, K= 69 cm2/s based on the 222Rn profile [15] and k = 0.01 yr- ’ based on the 2’oPb/226Ra activity ratio at intermediate depths are used. The two model curves for “‘Pb are based on the bottom boundary values, A& = 20.5 dpm/lOO kg obtained by real measurement and A “p,,= 0 assumed for an extreme case of complete uptake at the sediment-water interface. The 210Pb data fit well to the model curve with a finite Agb value (Fig. 3). Thus it is possible to interpret the 210Pb profile without any enhancement of in-situ scavenging rate near the bottom interface. Looking at equation (3), the third term of the right handside represents the effect of 222Rn/ 226Ra disequilibrium and it disappears when Ai,, = A,,. For CY-16, equation (3) can be rewritten as, A Pb = A:,, exp( - 4.34
X
10-6z)
+ 0.758A,,
x [ 1 - exp( - 4.34 X 10-hz)] +0.00047(
A;,, -A..)
39 300 Cygnus
16
F E 2 200 2 E B m
‘,,O
8 s 100
“‘Pb ‘.
E n
‘26Ra
“S
222R”
;.... ‘.
“Q -:
-‘-a- . ... .. ...__n 0
90’ Activity
(dp~lOOkg)
Fig. 3. Comparison of vertical first-order scavenging model curves with 2”‘Pb data at Cygnus 16 (see text). The calculated model profiles using any finite A& values between 0 and 20.5 dpm/lOO kg show their vertical gradients in between the two curves indicated in the figure and do not fit the data. The “‘Rn data are based on Gamo and Horibe [15].
does not fit the data at all. The nearly constant *“Pb profile suggests that scavenging reaction at the seafloor is so slow compared to vertical mixing and therefore most of the 210Pb transported by diffusion from the ocean interior must be reflected at the boundary. This is analogous to consumption of oxygen at the seafloor which is hardly identified from its bottom profile. Spencer et al. [4] have estimated the “‘Pb scavenging rate of 0.04 dpm cm-’ yr- ’ at the deep-sea bottom boundary of the Atlantic. Dividing the value by the vertical eddy diffusion coefficient (e.g. K = 69 cm2/s for CY-16), the vertical 2’0Pb concentration gradient which is necessary to yield the flux can be calculated to be - 2 x lop4 (dpm/lOO kg)/m. This small gradient would not be detectable by direct bottom *“Pb profile measurements. Acknowledgements
x [exp( -4.34 -exp(
x lo-6z)
- 1.74 X 10p4z)]
14)
Since the last term is a decreasing function with respect to z, the maximum effect of bottom excess 222Rn can be evaluated by 0.00047( Ai,, - AR,)/ A,,. This value is 0.0014 for CY-16 implying that the excess 222Rn can contribute at most only 0.14% to the observed *‘OPb concentration. The highest bottom 222Rn concentration that has been observed so far in the North Pacific is - 1000 dpm/ 100 kg [15,21]. If this value is used, the 222Rn contribution becomes - 2% as a maximum, which is still negligibly small. Clearly, these calculations indicate that bottom excess 222Rn can hardly inwater. Therefore. fluence 210Pb in the bottom ignorance of excess 222Rn from the 226Ra- 222Rn*“Pb system in the GEOSECS papers [3,6] can be justified.
I would like to thank Drs. Y. Horibe and T. Gamo for collaboration in sampling and Ms. K. Hasegawa for typing the manuscript. This work was partially supported by the Ministry of Education, Science and Culture, Japan under grant No. 65117009 to the University of Tokyo. References 1 M.P. Bacon, D.W. Spencer and P.G. Brewer, 2’oPb/‘2hRa
2
3
4
5
3.4. The bottom boundary value of ‘lOPb Since the above “‘Pb model calculations are very sensitive to the bottom boundary value, it seems necessary to discuss the processes maintainFig. 3 clearly indiing the 2’0Pb concentration. cates that complete uptake of “‘Pb by surface sediments is not taking place at the bottom boundary, since the model curve with A&, = 0
6
7
8
and ‘r’Po/ “‘Pb disequilibria in seawater and suspended particulate matter, Earth Planet, Sci. Lett. 32, 277, 1976. Y. Nozaki, J. Thomson and K.K. Turekian, The distribution of 2’oPb and 2’oPo in the surface waters of the Pacific Ocean, Earth Planet. Sci. Lett. 32, 304, 1976. Y. Nozaki, K.K. Turekian and K. Von Damm, “‘Pb in GEOSECS water profiles from the North Pacific, Earth Planet. Sci. Lett. 49, 393, 1980. D.W. Spencer. M.P. Bacon and P.G. Brewer, Models of the distribution of 2’0Pb in a section across the North Equatorial Atlantic Ocean. J. Mar. Res. 39, 119, 1981. Y. Chung and H. Craig, 21”Pb in the Pacific: the GEOSECS measurements of particulate and dissolved concentrations, Earth Planet. Sci. Lett. 65, 406, 1983. J.K. Cochran, M.P. Bacon, S. Krishnaswami and K.K. in the central and Turekian, “‘PO and “‘Pb distributions eastern Indian Ocean, Earth Planet. Sci. Lett. 65, 433, 1983. D.W. Spencer, P.G. Brewer, A. Fleer, S. Honjo, S. Krishnaswami and Y. Nozaki, Chemical fluxes from a sediment trap experiment in the deep Sargasso Sea, J. Mar. Res. 36. 493, 1978. P.G. Brewer, Y. Nozaki, D.W. Spencer and A.P. Fleer.
40
9
10
11
12 13
14
15
Sediment trap experiments in the deep North Atlantic: isotopic and elemental fluxes, J. Mar. Res. 38. 703, 1980. S. Krishnaswami, D. Lal, B.L.K. Somayajulu, R.F. Weiss and H. Craig, Large volume in-situ filtration of deep Pacific waters: mineralogical and radioisotope studies, Earth Planet. Sci. Lett. 32, 420, 1976. S. Krishnaswami, M.M. Sarin and B.L.K. Somayajulu, Chemical and radiochemical investigations of surface and deep particles of the Indian Ocean, Earth Planet. Sci. Lett. 54, 81, 1981. B.L.K. Somayajulu and H. Craig, Particulate and soluble *“Pb activities in the deep sea, Earth Planet. Sci. Lett. 32, 268, 1976. Y. Chung and H. Craig, 226Ra in the Pacific Ocean, Earth Planet. Sci. Lett. 49, 267, 1980. Y. Horibe, Preliminary Report of the Hakuho Maru cruise KH-80-2 (CYGNUS Expedition) and the Hakuho Maru cruise KH-82-1 (CEPHEUS Expedition), 78 pp., Ocean Research Institute, University of Tokyo, 1983. J. Thomson and K.K. Turekian, ‘t”Po and *“Pb distributions in ocean water profiles from the eastern south Pacific, Earth Planet. Sci. Lett. 32. 297, 1976. T. Gamo and Y. Horibe, Excess bottom ***Rn profiles and
16
17
18
19
20
21
their implications in the northwestern Pacific Ocean, Earth Planet. Sci. Lett. 71, 215. 1984. Y. Chung, R. Finkel, M.P. Bacon, J.K. Cochran and S. Krishnaswami, Intercomparison of *“Pb measurements at GEOSECS station 500 in the northeast Pacific, Earth Planet. Sci. Lett. 65, 393, 1983. H. Craig, S. Krishnaswami and B.L.K. Somayajulu, *“Pb226Ra: radioactive disequilibrium in the deep sea, Earth Planet. Sci. Lett. 17, 295, 1973. Y. Nozaki, Y. Horibe and H. Tsubota, The water column distributions of thorium isotopes in the western North Pacific, Earth Planet. Sci. Lett. 54, 203, 1981. Y. Nozaki and Y. Horibe, Alpha-emitting thorium isotopes in northwest Pacific deep waters, Earth Planet. Sci. Lett. 65, 39, 1983. M.P. Bacon and R.F. Anderson, Distribution of thorium isotopes between dissolved and particulate forms in the deep sea. J. Geophys. Res. 87, 2045, 1982. J.L. Sarmiento, H.W. Feely, W.S. Moore, A.E. Bainbridge and W.S. Broecker, The relationship between vertical eddy diffusion and buoyancy gradient in the deep sea, Earth Planet. Sci. Lett. 32, 357, 1976.