Minor Element Models in Coastal Waters - ACS Symposium Series

4 Minor Element Models in Coastal Waters P E T E R G . B R E W E R and D E R E K W . S P E N C E R

Downloaded by UNIV OF CINCINNATI on May 25, 2016 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0018.ch004

Woods Hole Oceanographic Institution, Woods Hole, Mass. 02543

It is now almost twenty years since Richards (1) discussed the state of our knowledge of trace elements in the ocean. In a brief, and somewhat pungent paper he pointed out that by 1956 there had been no valid analyses of deep ocean water for Sb, Ba, Cd, Cs, Ce, Cr, Co, Ga, Ge, La, Pb, Hg, Mo, Ni, Sc, Se, Ag, T1, Th, W, Sn, V, Y and Zr; that the assumption of a single oceanic concentration level for these elements was doubtful, and that computed residence times for these elements were of limited use in assessing their oceanic chemistry. Since then a great deal of work has been carried out. Recently, one of us was obliged to review (2) what is now known of minor element concentrations in sea water. The task set was simply to review the observational data base; discussions of analytical methods, theoretical models, or calculations of chemical speciation were to be kept to a minimum. The result was disturbing, for it was soon apparent that we are still far from having an adequate set of analytical data on which to base extensive theoretical models of minor element chemical processes in the ocean (3, 4, 5). In this paper, we briefly examine what is known of some minor element concentrations in sea water. We ask what chemical processes may be controlling their distribution, and inquire as to under what circumstances these processes may be revealed through direct observation. Analytical Data It is now true that, of the list of elements given above, some reasonable estimate of the deep water concentrations may be given. A listing of these data, and the calculated residence times are given in Table I.How accurate are these data, and what may be inferred from the residence time calculation? An inspection of the recent literature shows that, of the elements listed by Richards 0), we may now be sure of the deep water abundance of Ba, Cs, Mo, Sc (6, 7, 8, 9). Of these elements, Mo and Cs exhibit essentially conservative behavior, 80 Church; Marine Chemistry in the Coastal Environment ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

Minor Element Models

4. BREWER AND SPENCER

81

though the c o n c e n t r a t i o n o f Mo may be perturbed i n anoxic systems; Sc shows a s m a l l , but s t a t i s t i c a l l y s i g n i f i c a n t , i n crease w i t h depth from 1.42 x l O " M/l above 2000m. to 2.04 x 1 0 " M/l below 2000m; 8a e x h i b i t s a w e l l documented range o f values from c a . 3 x 1 0 " M/l t o c a . 22 x 1 0 " M / l . The covariance o f Ba w i t h Si i s s t r o n g and a good deal may be s a i d o f i t s oceanic d i s t r i b u t i o n and chemistry. For the r e s t , data on Sb, Cd, Ce, C r , Co, Ga, Ge, L a , Pb, Hg, N i , S e , A g , T l , Th, W, S n , V, Y and Zr are p o s s i b l y c o r r e c t to an order o f magnitude. Sampling problems are severe f o r Sb, C d , C r , Co, Pb, Hg, N i , A g , Sn, V, and sample contamination i s probably a l i m i t i n g f a c t o r at the present t i m e . Problems o f d e t e c t i o n l i m i t , and p a r t i c u l a r l y the p r e c i s i o n o f the d e t e r m i n a t i o n , are troublesome f o r Ce, Ga, Ge, L a , Se, T l , Th, W, Y and Z r . The goal o f a t t a i n i n g at l e a s t one, a c c u r a t e , deep water value i s e x t r a o r d i n a r i l y l i m i t e d . A more productive e f f o r t would be the determination o f a d e t a i l e d v e r t i c a l p r o f i l e i n a major ocean b a s i n . From t h i s , the c o r r e l a t i o n of the element w i t h w e l l known oceanic v a r i a b l e s (T, S % , 0 , N 0 , POi,, S i ( 0 H K , z C 0 , a l k a l i n i t y ) can be examined. This leads to estimates o f b i o l o g i c a l c y c l i n g , and important i n f o r m a t i o n on s o u r c e s , s i n k s and t r a n s p o r t mechanisms. There i s a general f e e l i n g t h a t t r a c e metal data are "gett i n g l o w e r a l l the time" as sampling and a n a l y t i c a l methods i m prove. Has t h i s i n f a c t occurred? Figure 1 shows data taken from references 2 and 10, f o r s e v e r a l t r a c e elements, p l o t t e d a g a i n s t y e a r o f p u b l i c a t i o n f o r the l a s t 40 o r so y e a r s . It i s by no means c l e a r t h a t a d e f i n i t e trend towards lower concent r a t i o n i s observed. Rather, the data show e r r a t i c v a r i a t i o n s , o f t e n w i t h a f a i r l y wide ( i . e . order o f magnitude) range o f values f o r any one author. This may be taken to mean: ( i ) t h a t the ocean contains i n h e r e n t t r a c e metal v a r i a b i l i t y over s h o r t s c a l e s o f d i s t a n c e o r t i m e , ( i i ) t h a t a n a l y t i c a l problems e x i s t such t h a t the data presented are e s s e n t i a l l y n o i s e , o r ( i i i ) t h a t sampling problems e x i s t to the extent t h a t even the most c a r e f u l a n a l y s i s w i l l see o n l y a s i g n a l dominated by the l a r g e v a r i a t i o n s o f the metal ions coming from, o r t o , the i n d i v i d u a l c o n t a i n e r surfaces. Hypotheses ( i i ) and ( i i i ) must c o n t r i b u t e g r e a t l y to the s c a t t e r apparent i n the d a t a . Not a l l samples have been s t o r e d i n the a c i d i f i e d c o n d i t i o n to minimize a d s o r b t i o n (11) and sample contamination has been shown to e x i s t i n a number o f cases (12, 1 3 ) . F i n a l l y , recent data on c a r e f u l l y taken oceanic samples (14, 15) do show a markedly lower abundance f o r Cu ( 1 - 3 x 10" M/l) andPFor Mn (5 x 1 0 " M/kg) than the mean c o n c e n t r a t i o n reported over the l a s t 40 y e a r s . These d a t a , made p o s s i b l e by the remarkable s e n s i t i v i t y and s p e c i f i c i t y o f g r a p h i t e f u r nace atomic a b s o r p t i o n spectroscopy, provide the most c o n v i n c i n g evidence so f a r of a h i s t o r y of a n a l y t i c a l o r sampling problems over the l a s t few decades. 1 1

1 1


8

0

9

2

8

3

z

1 0

Church; Marine Chemistry in the Coastal Environment ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

MARINE CHEMISTRY

82


MANGANESE IN SEA WATER

_ \

v

1930

1940

1

t1950

1960 TIME

1970 1980

CHROMIUM IN SEA WATER

48

is

I

B

20

I

5?

10 •J 1930

1940 1950

1960

TIME

1970

1980

Ok VANADIUM IN SEA WATER

1

C

|,00 i \ i \

I S 50

1930

1940

1950

1960 TIME

1970 1980

Figure 1. Estimates of the concentration of three trace elements in seawater with time. Data taken principally from references 2 and 10. No clear trend is apparent.



4.

BREWER

A N D SPENCER


In s p i t e o f these caveats a s u b s t a n t i a l body o f evidence may be advanced i n f a v o r o f hypothesis ( i ) . I f t h i s i s c o r r e c t then the concept o f a residence t i m e , as given i n Table I, i s not p a r t i c u l a r l y h e l p f u l . A residence time o f 10,000 years corresponds to approximat!ey 10 global oceanic s t i r s , s u f f i c i e n t to smear out a l l observable c o n c e n t r a t i o n g r a d i e n t s . An element w i t h a h a l f - l i f e o f t h i s time s c a l e would be q u a s i - u n i f o r m l y d i s t r i b u t e d . However, small s c a l e r e g i o n a l v a r i a t i o n s have been shown to e x i s t (11) and to be c o r r e l a t e d w i t h hydrographic f e a t u r e s , o r l o c a l b i o l o g i c a l p e r t u r b a t i o n (16) f o r Cu, Zn, Fe and Mn. This demonstrates t h a t chemical o r B i o l o g i c a l c o n t r o l s o f some kind may be o p e r a t i n g on a time s c a l e two o r three orders of magnitude s h o r t e r than the conventional residence time would suggest. MODEL CALCULATIONS How might we, even c r u d e l y , assess t h i s ? In the case o f b i o l o g i c a l e f f e c t s , we can o b t a i n average values f o r primary p r o d u c t i v i t y , and estimates o f the elemental composition o f marine phytoplankton are a v a i l a b l e Q 7 ) . Assuming t h a t primary p r o d u c t i v i t y i s ^ 100 g C/m /yr., the depth o f the euphotic zone i s 100 m, then the growth r a t e i s approximately 2.5 x 10*3 g dry weight of phytoplankton/l/yr. In Table II,we show estimates o f removal times o f some t r a c e elements i n s u r f a c e waters w i t h respect to removal by phytoplankton, assuming e i t h e r complete uptake and removal by s i n k i n g , o r 90% r e c y c l i n g o f the element i n a manner analogous to phosphate. The true r e s u l t probably l i e s somewhere between. The values range from < 1 to ^ 1 0 y e a r s . Some o f the data are m i s l e a d i n g ; f o r i n s t a n c e , the low values f o r Al and Ti are not due to pronounced metabolic a c t i v i t y . They probably r e f l e c t the i n c o r p o r a t i o n o f c l a y p a r t i c l e s i n t o the phytoplankton sample. Let us c o n s i d e r the s i m p l e s t p o s s i b l e c a s e . I t i s i m p l i c i t i n the t r a d i t i o n a l residence time concept t h a t the continents represent a constant s o u r c e , and the oceans a uniform s i n k . We i n c o r p o r a t e a c o a s t a l zone i n t e r f a c e connecting the two v i a h o r i z o n t a l d i f f u s i o n . Given what we know o f c o n c e n t r a t i o n gradients and uptake rates (Table I l h w o u l d non-conservative e f f e c t s be d i s t i n g u i s h a b l e from purely c o n s e r v a t i v e t r a n s p o r t by d i f f u s i o n ? I f we c o n s i d e r a one dimensional model o f the s u r f a c e ocean w i t h d i s t a n c e from the c o a s t , a simple case could be given by 2

5

dc dt

_ "

„ K

(1)

where c i s the c o n c e n t r a t i o n o f a c o n s t i t u e n t , x i s d i s t a n c e from the c o a s t , t i s t i m e , x i s a f i r s t order removal r a t e c o n s t a n t , and K i s the h o r i z o n t a l eddy d i f f u s i v i t y .


MARINE CHEMISTRY

TABLE I The Concentration and Residence Times o f Some Minor Elements i n Sea Water


Element

Concentration (M/l)

*Residence Time (Years)

Be

6 x 10"

1 1

? Short

Al

7 x 10"

8

1 x 10

Sc

2 x 10"

1 1

4 x 10*

Ti

2 x 10"

8

1 x 10*

V

5 x 10"

8

8 x 10*

Cr

6 x 10"

9

6 x 10

3

Mn

5 x lO"

1 0

2 x 10

3

Fe

3 x 10"

8

2 x 10

2

Co

8 x 10"

1 0

3 x 10*

Ni

3 x 10"

Cu

8 x 10"

Zn Ga

2

5 x 10*

8

2 x 10*

9

8 x 10"

8

2 x 10*

4 x 10"

1 0

1 x 10*

Ge

7 x 10"

1 0

As

5 x 10"

8

Se

3 x 10"

9

Rb

1.4 x 1 0 "

Y

1 x 10

Zr

3 x 10"

1 0

1 0

- l u

Nb

1 x 10"

Mo

1 x lO"

Ag

4 x 10"

Cd

1 x 10

Sn

8 x 10"

Sb

2 x 10"

I

5 x 10

7

1 0

5 x 10* 2 x 10* 6

4 x 10

6

? ? ? 2 x 10

5

4 x 10

H

?

- 9

1 1

9

- 7

?

? 7 x 10

3

4 x 10

5




TABLE I (Continued)


Element

Concentration (M/1) *Residence Time (Years)

Cs

3 x 10"

Ba

1-3 x 10~

La

2 x 10"

Ce

1 x 10"

1 0

?

Pr

4 x 10"

1 0

?

ND

1.5 x 1 0 "

Sm

3 x 10"

1 2

Eu

6 x 10"

1 3

?

Gd

4 x 10"

1 2

?

Tb

9 x 10"

1 3

?

Dy

6 x 10"

1 3

?

Ho

1 x 10"

1 2

?

Er

4 x 10"

1 2

1

Tm

8 x 10'

1 3

?

Yb

3 x 10"

1 2

1

6 x 10

9

4 x 10*

7

6 x 10

u

1 1

2

? ?

?

Lu

5

?

Hf

4 x 10"

Ta

1 x 10"

1 1

W

5 x 10"

1 0

Re

4 x 10"

1 1

Au

2 x 10-

1 0

Hg

1.5 x 1 0 "

Tl

5 x 10"

Pb

1 x 10"

1 0

Bi

1 x 10"

1 0

?

Ra

3 x 10"

1 6

?

Th

4 x 10"

U

1 x 10"

?

n

1 0

1 x 10

5

2 x 10

5

8 x 10*

1 1

u

8

4 x 10

2

2 x 10

2

3 x 10

6

* Residence Time ( ) i s defined as T

T

"

A W3i


86

MARINE CHEMISTRY

where A i s the t o t a l amount o f the element d i s s o l v e d i n the ocean, and dA/dt i s the amount introduced t o , o r removed from, the ocean each y e a r . A steady s t a t e i s assumed.


TABLE

II

Concentration o f Some Minor Elements i n Phytoplankton and Estimated Residence Times i n Surface Waters With Respect to B i o l o g i c a l Removal Element

Sr Ba Al Fe Mn TiCr Cu Ni Zn Ag Cd Pb Hg

*Concentration tResidence Time (Years) i n Phytoplankton (ug/g dry A (complete removal) B (90% r e c y c l i n g ) weight) 100 _ 700 20 300 10 400 200 1500 5 15 0 30 5 1 15 1 10 10 100 0.1 1 1 5 0 - 10 0 0.2

-

4 x 10 13 1 0.8 26 t 13 -v 40 27 40 40 4 15 » 40 > 40 3

-

3 x 10 200 40 6 80

-

800 400 400 40 80

4

4 x 10 130 10 8 260 130 400 270 400 400 40 150 > 400 40

4

*Data taken from references 2 , 18 tPrimary p r o d u c t i v i t y = 100 g c/nr/yr. Depth o f euphotic zone = 100 m ^ Growth r a t e = 1 x 10"3 g c/l/yr 2.5 x 1 0 " g dry 6

-

3 x 10 2000 400 60 800

-

8000 4000 4000 400 800

-

5

weight/l/yr.




87

Assuming a steady s t a t e c o n d i t i o n i s achieved then dc and a p a r t i c u l a r s o l u t i o n o f the equation ( 1 ) i s dt given by r


C

_ ~ =

C

o

-Mx e

, +

e^-e ^ MXm -MXm e -e

/ (

C

m

~ " °

r m

C

g

=

-MXnu

6

}

where M j r . Co i s a f i x e d c o n c e n t r a t i o n a t X = 0 and Cm i s a f i x e d c o n c e n t r a t i o n a t X = Xm. Figure 2 shows s e v e r a l s o l u t i o n s o f t h i s model where we assume the c o n c e n t r a t i o n a t the c o a s t to be f i x e d at 1 0 u n i t s by r i v e r i n p u t and the c o n c e n t r a t i o n i n the open ocean, a t 2 0 0 km from the s h o r e , f i x e d a t a lower l e v e l o f 0 . 1 u n i t s maintained by a d i f f u s i v e f l u x from below. The f i g u r e shows s o l u t i o n s f o r eddy d i f f u s i v i t i e s (K) o f 1 0 and 1 0 cm s e c ' l and removal r a t e constants g i v i n g residence times o f 0 . 5 , 1 and 5 y e a r s . With the p r e c i s i o n t h a t i s normally reported f o r t r a c e element d e t e r m i n a t i o n s . i t i s apparent t h a t a t eddy d i f f u s i v i t i e s o f 1 0 cm sec"' a residence time o f even 0 . 5 y e a r could hardly be d i s t i n g u i s h e d from the s t r a i g h t d i f f u s i o n model. With a eddy d i f f u s i v i t y o f 1 0 cm sec"' a residence time of one y e a r should be d e t e c t a b l e but i t i s doubtful t h a t a f i v e y e a r residence time could be seen. A somewhat more s o p h i s t i c a t e d model could c o n s i d e r the case o f a c o a s t a l zone i n w h i c h , i n a d d i t i o n to a f i x e d boundary conc e n t r a t i o n maintained by r i v e r i n p u t , there occurs an a d d i t i o n a l i n p u t e i t h e r from an atmospheric f l u x o r f l u x from the bottom sediments. Such a case could be modelled by 6

7

K ^ 4

=

2

2

6

^

7

2

- XC + a = 0

(2)

dx^

a t

where X i s a f i r s t order removal r a t e constant and a i s an i n p u t f u n c t i o n which decreases w i t h d i s t a n c e from the coast according to a =a o

e

-ux

so t h a t the maximum i n p u t i s i n the near c o a s t a l zone. A s o l u t i o n to equation ( 2 ) i s given by o ' , .2 a

C = C

G

e"Jl

e

-yX

.

e

- J X

where, a t i n f i n i t e X, C approaches z e r o . Such a model may be a simple approximation f o r Pb, and o t h e r heavy metals t h a t have a s i g n i f i c a n t atmospheric f l u x , however, we know o f no s t a b l e t r a c e element data s u i t a b l e to t e s t t h i s model. 228 A p o s s i b l e a p p l i c a t i o n i s to the d i s t r i b u t i o n o f Ra Moore and S a c k e t t ( J 8 ) and Moore Q 9 ) have suggested t h a t the


88

MARINE

CHEMISTRY

R 2 2 8 distribution in the surface ocean is strongly influenced by input from the continental shelves. This conclusion has been adopted by Kaufman et a l . ( 2 0 ) who give substantial amounts of surface data for R a , including a horizontal traverse from the near shore of Long Island into the Sargasso Sea. These authors modelled the data of this traverse using the simple f i r s t order decay - diffusive model given by equation ( 1 ) to suggest an eddy diffusion coefficient o f about 2 x 1 0 cnr s e c " . However, of the 3 8 0 km represented by the traverse, about 2 0 0 km is on the continental shelf and one would expect that equation ( 2 ) would be a more appropriate model. Figure 3 shows the R a data from Kaufman et a l . ( 2 0 ) together with three solutions of equation ( 2 ) . The f i r s t solution allows no input from the continental shelf ( i . e . sets a =0 ) . This would model the case of a boundary condition maintained by river input and a reasonable f i t to the observed data can be obtained with this boundary concentration set at 2 5 dpm/100 kg. The second solution a r b i t r a r i l y sets a = 1 0 0 dpm/100 kg/ y r . and u = 0 . 0 5 . The boundary concentration, Co, necessary for a reasonable f i t to the observed data is about 1 8 dpm/ 1 0 0 kg. The third solution sets the boundary concentration, Co, to zero and, in this case, a reasonable f i t to the data could be obtained with a = 1 3 0 0 e " ° - dpm/100 kg/yr. If Moore ( 2 1 ) i s correct in his statement that the river concentration of R a is extremely low 2 dpm/100 kg) then the third solution is the most appropriate. However, a l l three models give an agreeable f i t to the data and, i f the last is most appropriate, then the flux of R a from the bottom cannot be maintained over the whole shelf but must be restricted to the immeJiate 2 0 - 3 0 mile near shore area. In the shelf region off Long Island the existence of a stable "winter water" cold layer in the outer shelf region at about 5 0 meters depth may establish a sufficiently stable water column that vertical diffusion of the Ra input from the sediments may be essentially restricted to the well mixed near shore zone. It should be pointed out that the f i t to the data given by the three models described above was obtained through the use of a radioactive decay constant appropriate to a h a l f - l i f e of 5 . 7 5 years. This is the value gi ven in recent compilations of isotopic data (e.g. reference ( 2 2 ) ) , but i t differs by about 20% from the older value ( 2 3 ) of 677"years quoted in many papers on the use of R a as a geochemical tracer ( 1 8 , 1 9 , 2 0 , 2 1 ) . It is interesting to see how stable the models are to perturbations of this kind, since i t is certain that trace metal removal rates, as for instance those calculated in Table II, are subject to large errors. A f i t of the third model to the data, using a decay constant of 0 . 1 0 3 (t«j/ = 6 . 7 years) gave a = 8 5 0 - 0 . i x dpm/100 kg/yr., with K = 1 x 1 0 cm /sec. The result is numerically different, requiring a diffusion coefficient of about half the previous value, but the conclusions are substana

2 2 8

6

1


2 2 8

0

0

l x

2 2 8

2 2 8

8

2 2 8

2

6

2

e



BREWER AND SPENCER


Co=10

X Figure 2. Simple model of diffusive transport of a nonconservative tracer from a coastal source to an oceanic sink. X is distance from coast. The removal rate is first order, and the concentration gradient Co/Cm = 100. At a diffusion coefficient of 10 cm /sec, a 1 year half-life would produce significant perturbation; at JO env/sec > 5 years half-life could be distinguished. 7

2

6

20

to 10 o I

0

100

200

300

X Figure 3. Application of Equation 2 to the Ra data of Kaufman et al. (20). X is distance from coast. Three solutions fit to the data: Q = Ra data in dpm/100 kg; oa = 0,Co =25 dpm/100 kg; X a = 100 dpm/100 H/yr-> — 0.05, Co = 18; + a = 1300 e-° dpm/100 kg/yr., Co — 0. 228

228

o

0

lx


90

MARINE CHEMISTRY

t i a l l y the same. I f r i v e r i n p u t i s s e t to z e r o , then the f l u x from the sediments i s s i g n i f i c a n t only i n the 2 0 - 3 0 mile near shore a r e a .


Removal Processes o f S h o r t e r Time Scale Superimposed upon processes such as a c t i v e b i o l o g i c a l upt a k e , and processes o f a time s c a l e o f months to y e a r s , are many s h o r t term e f f e c t s . P r e c i p i t a t i o n and adsorbtion f a l l i n t o t h i s category. I t i s commonly assumed t h a t these p r o cesses must take p l a c e i n the ocean and probably operate on a time s c a l e o f minutes to days. Under what circumstances could these processes by i n v e s t i g a t e d by d i r e c t observation? We must again c o n s i d e r the r a t e o f the process r e l a t i v e to the r a t e o f d i f f u s i o n . Equations ( 1 ) and ( 2 ) d e a l t w i t h a simple case i n which the d i f f u s i o n c o e f f i c i e n t was h e l d to be c o n s t a n t . T h i s i s v a l i d over d i s t a n c e s o f the o r d e r o f s e v e r a l tens o f k i l o m e t e r s . However, h o r i z o n t a l d i f f u s i o n i n the ocean i s n o n - F i c k i a n ( 2 4 , 2 5 ) and over s h o r t e r d i s t a n c e s t h i s must be taken i n t o account. For F i c k i a n d i f f u s i o n the variance ( c r ) o f the c o n c e n t r a t i o n around a p o i n t source i s p r o p o r t i o n a l to time ( t ) , p e r m i t t i n g the d e f i n i t i o n o f a constant d i f f u s i o n c o efficient. In the ocean the variance i s observed to i n c r e a s e f a s t e r than t ' , the p h y s i c a l reason b e i n g e s s e n t i a l l y t h a t l a r g e eddies c o n t a i n p r o p o r t i o n a t e l y more energy than small ones. Thus, the r a t e o f d i s p e r s i o n increases w i t h t i m e . Okubo ( 2 4 ) has prepared oceanic d i f f u s i o n diagrams, reducing the r e s u l t s o f many experiments, and one o f these i s reproduced i n Figure 4 . The s l o p e o f a l l the p o i n t s i s given by a

2

= 0.018 t 2

3 4

3 Theory would p r e d i c t a dependence on t . The r e l a t i o n s h i p between the apparent d i f f u s i v i t y (Ka) and s c a l e length ( 1 ) , defined as 3 a , i s Ka

= 0.0103

l

1

'

1

5

A g a i n , theory would p r e d i c t a dependence on l ^ . Okubo has shown t h a t the 4 / 3 power law may h o l d l o c a l l y . These data have been obtained from dye experiments, i n which a q u a n t i t y o f Rhodamine B dye i s r e l e a s e d , so as to r e semble as c l o s e l y as p o s s i b l e an instantaneous p o i n t source. The i n i t i a l v a r i a n c e , a , may be represented as 3

Q

2

where P i s a d i f f u s i o n v e l o c i t y (^ 1 cm sec"^) and % i s a c h a r a c t e r i s t i c growth time from a p o i n t source to s i z e o ^ .


BREWER AND

SPENCER


10m


1000

100

10000

/(cm) Deep Sea Research

Figure 4. Oceanic diffusion diagram showing the relationship between apparent diffusivity (Ka) and scale length (I), with the fit of the 4/3 power law locally



92

MARINE CHEMISTRY

R e l i a b l e data on t u r b u l e n t d i f f u s i o n may only be obtained at some c r i t i c a l time > to. A time o f 10 to i s o f t e n chosen. In d e s c r i b i n g dye d i f f u s i o n experiments o f t h i s k i n d i t i s f r e q u e n t l y s t a t e d t h a t they are o f p r a c t i c a l importance i n connection w i t h the d i s p e r s a l o f poi1utants o r chemical t r a c e r s . I f the r a t e o f removal o f a t r a c e r i s f i r s t order ( r a d i o a c t i v e decay, a d s o r b t i o n , p r e c i p i t a t i o n ) then a s o l u t i o n to the d i f f u s i o n equation can be found and, i n p r i n c i p l e , knowing the d i f f u s i o n c o e f f i c i e n t and measuring the d i s p e n s i o n , a removal r a t e may be c a l c u l a t e d from d i r e c t o b s e r v a t i o n . So f a r as we know, t h i s has not been attempted. We can enquire whether i t would be p o s s i b l e t o c o n s t r u c t an experiment i n which the cont r o l l e d r e l e a s e o f a t r a c e m e t a l , or o t h e r t r a c e r s u b s t i t u t e f o r a dye, i n the ocean would provide i n f o r m a t i o n on the l o c a l removal processes o f s h o r t time s c a l e . This would be analogous to the problems posed by the d i s p e r s a l o f metals from waste water d i s c h a r g e s , o r a c a t a s t r o p h i c p o l l u t i o n event. The problem does not appear to be s i m p l e . F i r s t l y , the inherent s c a t t e r i n the oceanic d i f f u s i o n diagrams i s such t h a t i t would be i m p o s s i b l e to p r e d i c t the l o c a l d i f f u s i o n c o e f f i c i e n t to much b e t t e r than an order o f magnitude. T h i s probably means t h a t measurements o f some c o n s e r v a t i v e t r a c e r , r e l e a s e d s i m u l taneously w i t h the element of i n t e r e s t , would have to be made i n order to provide estimates o f l o c a l t u r b u l e n t d i f f u s i o n . (In f a c t , the measured variance o f a dye patch i s independent o f any f i r s t o r d e r decay, the measure o f removal being given by the change i n peak c o n c e n t r a t i o n and mass b a l a n c e . However, f o r a t r a c e r w i t h a r a p i d removal r a t e , the d i s p e r s i o n would provide an i n f e r i o r estimate o f t u r b u l e n t d i f f u s i o n . ) An obvious candidate f o r such a c o n s e r v a t i v e t r a c e r would be Rhodamine B dye. I t has been w i d e l y used, i s harmless, and may be measured w i t h great s e n s i t i v i t y . U n f o r t u n a t e l y , i t i s not c l e a r whether i t s behavior i s t r u l y c o n s e r v a t i v e . Mass balances o f 50-70% are o f t e n reported ( 2 4 ) , the l o s s being a t t r i b u t e d to photochemical decomposition o r a d s o r b t i o n on to p a r t i c l e s . If the l a t t e r i s a s i g n i f i c a n t f a c t o r then the dye would compete f o r a d s o r b t i o n s i t e s w i t h the element o f i n t e r e s t . Secondly, f o r most mi nor elements i n seawater, there are problems of d e t e c t i o n 1 i m i t and severe problems i n generating a s i g n i f i c a n t c o n c e n t r a t i o n g r a d i e n t . The d e t e c t i o n l i m i t f o r Rhodamine B dye i s approximately 1 0 " g/kg. For convenience i t i s customary to use a v i s u a l l i m i t o f about 10~ g/kg f o r observing the spread o f the dye patch by eye and p l a n n i n g sampling s t r a t e g y . For d i f f u s i o n experiments o f the o r d e r o f one week i n time s c a l e perhaps 50 kg o f dye would be used. In t h i s manner c o n c e n t r a t i o n gradients o f 10-100 from the center o f the dye patch to the edge can be maintained. For many chemical elements t h i s would be i m p o s s i b l e . I t i s true t h a t the c o n c e n t r a t i o n o f mi nor elements i n seawater i s not maintained by 8

5


4.

BREWER

A N D SPENCER


93

e q u i l i b r i u m w i t h t h e i r l e a s t s o l u b l e phase, but the n a t u r a l l e v e l s may o f t e n not be exceeded g r o s s l y before s u p e r s a t u r a t i o n does o c c u r . In Table I I I , we show a comparison o f the n a t u r a l l e v e l o f some t r a c e metals i n seawater w i t h t h e i r l i m i t i n g c o n c e n t r a t i o n w i t h r e s p e c t t o the formation o f a s o l i d phase ( 2 6 ) . The c a l c u l a t e d values f o r Co and Ni would suggest t h a t l o c a l c o n c e n t r a t i o n s c o u l d be r a i s e d d r a s t i c a l l y , but t h a t Cu, Zn and Ba occur n a t u r a l l y w i t h i n a f a c t o r o f 10 o f t h e i r maximum p e r m i s s i b l e c o n c e n t r a t i o n . K i n e t i c f a c t o r s may, o f c o u r s e , i n h i b i t p r e c i p i t a t i o n , as i n the case o f Mn where o x i d a t i o n and p r e c i p i t a t i o n as Mn0 i s slow below pH c a . 8 . 5 Were p r e c i p i t a t i o n t o o c c u r then our d i f f u s i o n experiment would simply reveal the r a t e o f d i s p e r s i o n o f the p a r t i c l e s . For many problems o f p r a c t i c a l importance t h i s would be most v a l u a b l e , and s t u d i e s o f s o l i d phase formation and t r a n s p o r t d u r i n g s e t t l i n g are much needed. Reference t o the oceanic d i f f u s i o n diagram (Figure 3) shows t h a t i f p r e c i p i t a t i o n are s e t t l ing and complete w i t h i n 24 hours, then the v a r i a n c e o f the t r a c e r d i s t r i b u t i o n i n the sediments around the source would be o f the o r d e r o f one k i l o m e t e r . Local advection would elongate the d i s t r i b u t i o n p a t t e r n . The a c t u a l r a t e o f formation o f p a r t i c l e s must take p l a c e on a much s h o r t e r time s c a l e , and may be i m p o s s i b l e t o observe adequately i n s i t u . I f the r a t e o f removal by a d s o r b t i o n i s o f i n t e r e s t then we are l i m i t e d by the q u a n t i t y o f the s o l i d phase n a t u r a l l y p r e s e n t , and our ignorance o f i t s s u r f a c e chemical c h a r a c t e r i s t i c s . The q u a n t i t y o f p a r t i c u l a t e matter i n c o a s t a l waters i s h i g h l y v a r i a b l e ; i t can range from > 1 mg/kg t o c a . 100 ug/kg. The e f f e c t i v e s u r f a c e area o f t h i s m a t e r i a l i s unknown; a value o f 100 m /gram may be an approximate value f o r a c l a y mineral such as i l l i t e ( 2 6 ) . L e t us assume a value o f 50 m /gram f o r marine suspended matter. At a c o n c e n t r a t i o n o f 1 mg/kg we thus have 0.05 m /kg a v a i l a b l e f o r a d s o r b t i o n . The a d s o r b t i o n d e n s i t y o f v a r i o u s ions i n seawater on to marine p a r t i c u l a t e matter i s unknown. Taking Co as an example o f an elemental t r a c e r , then at a c o n c e n t r a t i o n o f 10" M/l i n seawater the a d s o r b t i o n dens i t y on i l l i t e would be 1 0 " moles/m , and on 6-Mn0 5 x 10" moles/m ( 2 6 ) . Using a h y p o t h e t i c a l value o f 1 0 " moles/m f o r the a d s o r b t i o n d e n s i t y on marine p a r t i c u l a t e matter then we have 5 x 1 0 " moles Co adsorbed per kg o f seawater. I f we were t o add 1 kg (17 moles) o f Co t o seawater t o i n i t i a t e a t r a c e r experiment, then under the c o n d i t i o n s d e s c r i b e d above, a patch 1 m deep and approximately 10 km radius would form before complete removal by a d s o r b t i o n c o u l d o c c u r . The l i m i t i n g f a c t o r being the presence o f adsorbing s i t e s . A patch o f t h i s s i z e would form over the time s c a l e o f about one week and would be represented by an e f f e c t i v e d i f f u s i o n c o e f f i c i e n t o f * 1 0 cm s e c " . Reference t o a diagram such as Figure 1, w i t h a p p r o p r i a t e s c a l e f a c t o r s , reveals t h a t w i t h an a n a l y t i c a l 2


2

2

2

2

7

1 0

2

2

7

2

8

2

1 0

5

2

1


94

MARINE CHEMISTRY


TABLE I I I

Natural and L i m i t i n g Minor Element Concentration i n Seawater w i t h Respect t o the Formation o f a S o l i d Phase ( 2 6 ) . Element

N a t u r a l Level (M/l)

Limi t i n g Concentration (M/l)

S o l i d Phase

x 10"

6

CaC0

8

1.1 x 1 0 "

2

Ni(OH)

8 x 10"

9

3.4 x 1 0

Zn

8 x 10"

8

4.4

x 10"

7

ZnC0

3

Ba

3 x 10"

7

3.5 x 1 0 "

7

BaS0

4

Co

8 x 10"

Ni

3 x 10"

Cu

1 0

8.5

- 7

3

CuO


2



95

p r e c i s i o n of 1% the removal r a t e would be s c a r c e l y d i s t i n g u i s h able from s t r a i g h t d i f f u s i v e t r a n s p o r t o f a conservative t r a c e r .


Conclusions In w r i t i n g a simple review paper o f t h i s k i n d , we are f o r c i b l y reminded o f the acute lack o f accurate o b s e r v a t i o n a l data on the ocean, and i n p a r t i c u l a r o f the c o a s t a l zone. The admittedly crude estimates o f uptake rates given here would tend t o suggest t h a t non-conservative e f f e c t s i n t r a c e element chemistry may w e l l be hard t o d i s t i n g u i s h through d i r e c t observat i o n . This should i n no way d e t e r good experimental work. It i s c l e a r t h a t f u r t h e r advances w i l l not come from casual o b s e r v a t i o n , but from c a r e f u l l y planned e x p e d i t i o n s i n a defined oceanographic s e t t i n g w i t h f u l l r e a l i z a t i o n o f what the a n a l y s t may have to reveal through h i s d a t a . Some o f the problems discussed h e r e , the c o a s t a l - t o - o c e a n i c elemental gradients and the a d s o r b t i v e p r o p e r t i e s o f marine p a r t i c u l a t e matter, appear to be o f b a s i c importance and should y i e l d to experimental work. I f t h i s paper can s t i m u l a t e some work i n these f i e l d s , we w i l l count ourselves f o r t u n a t e . Acknowledgements This work was supported by the U. S. Atomic Energy Commission under Contract No. AT ( l l - l ) - 3 5 6 6 , C o n t r i b u t i o n Number 3539 from the Woods Hole Oceanographic I n s t i t u t i o n . Abstract The concept o f a t r a c e element residence time i n the ocean leads to estimates o f c a . 1 0 y e a r s . Were t h i s the only f a c t o r then t r a c e element g r a d i e n t s would be e f f e c t i v e l y e l i m i n a t e d . However, some d a t a , and simple uptake c a l c u l a t i o n s , show t h a t gradients i n s u r f a c e waters do o c c u r . Attempts to evaluate t h i s by a simple d i f f u s i v e model o f t r a n s p o r t o f a non-conservative t r a c e r from a c o n t i n e n t a l source to an oceanic s i n k through a c o a s t a l zone, as i n 4

2

show t h a t a " h a l f - l i f e " o f >> 5 years can probably not be d i s t i n g u i s h e d from t r u l y c o n s e r v a t i v e behavior. The model may be modified t o i n c l u d e an e x p o n e n t i a l l y v a r y i n g i n p u t f u n c t i o n , as from inshore sediments. This model i s a p p l i e d to p u b l i s h e d data on R a ( t 1/2 = 5.75 y e a r s ) as an example. Estimates o f removal processes o f s h o r t e r time s c a l e are g i v e n . We conclude t h a t b a s i c data on c o a s t a l to oceanic elemental g r a d i e n t s , and 2 2 8


96

MARINE CHEMISTRY

the adsorptive properties of marine particulate matter, are sadly lacking.

Literature Cited Richards, F. A. Geochim. Cosmochim. Acta (1956) 10, 241. Brewer, P. G. In "Chemical Oceanography" 2nd ed., Academic Press, London, 1975. (3) Morel, F., Morgan, J. J. Environ. Sci. Techno1. (1972) 6, 58. (4) Craig, H. Earth Planet. Sci. Lett. (1974) 23, 149. (5) Millero, F. J. In "The Sea", Vol. 6, Interscience. In press. (6) Bacon, M. P., Edmond, J. M. Earth Planet. Sci. Lett. (1972) 16, 66. (7) Spencer, D. W., Robertson, D. E . , Turekian, K. K. and Folsom, T. R. J. Geophys. Res. (1970) 75, 7688. (8) Morris, A. W. Deep Sea Res. (1975) 22, 49. (9) Brewer, P. G., Spencer, D. W., Robertson, D. E. Earth Planet. Sci. Lett. (1972) 16, 111. (10) Høgdahl, O. T. "The Trace Elements in the Ocean. A Bibliographic Compilation", Central Institute for Industrial Research, Norway, 1963. (11) Spencer, D. W., Brewer, P. G. Geochim. Cosmochim. Acta (1969) 33, 325. (12) Schutz, D. F., Turekian, K. K. Geochim. Cosmochim. Acta (1965) 29, 259. (13) Robertson, D. E. Geochim. Cosmochim. Acta (1970) 34, 553. (14) Boyle, E . , Edmond, J. M. Nature (1975) 253, 107. (15) Bender, M. Unpublished work. (16) Morris, A. W. Nature (1971) 233, 427. (17) Martin, J. H . , Knauer, G. A. Geochim. Cosmochim. Acta (1973) 37, 1639. (18) Moore, W. S., Sackett, W. M. J. Geophys. Res. (1964) 69, 5401. (19) Moore, W. S. Earth Planet. Sci. Lett. (1969) 6, 437. (20) Kaufman, A . , Trier, R. M., Broecker, W. S., Feely, H. W. J. Geophys. Res. (1973) 78, 8827. (21) Moore, W. S. J. Geophys. Res. (1969) 74, 694. (22) Heath, R. L. In "Handbook of Chemistry and Physics", 53rd ed., The Chemical Rubber Co., Ohio, 1972-1973. (23) Curie, M., Debierne, A . , Eve, A. S., Geiger, H., Hahn, O., Lind, S. C., Meyer, St., Rutherford, E . , Schweidler, E. Revs. Mod. Phys. (1931) 3, 427. (24) Okubo, A. Deep Sea Res. (1971) 18, 789. (25) Csanady, G. T. Turbulent diffusion in the environment, Reidel, 1973. (26) Murray, J. W., Brewer, P. G. In "Marine Manganese Deposits", Elsevier, in press. Downloaded by UNIV OF CINCINNATI on May 25, 2016 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0018.ch004

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Minor Element Models in Coastal Waters - ACS Symposium Series

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