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On the estimation of minimum mechanical loss during an in situ biogenic silica dissolution experiment
Marine Micropaleontology, 7 ( 1 9 8 2 / 8 3 ) 4 4 1 - - 4 4 7
441
Elsevier Scientific P u b l i s h i n g C o m p a n y , A m s t e r d a m -- P r i n t e d in T h e N e t h e r l a n d s
ON THE ESTIMATION OF MINIMUM MECHANICAL LOSS DURING AN I N S I T U BIOGENIC SILICA DISSOLUTION EXPERIMENT
D A V I D C. H U R D .1 and K O Z O T A K A H A S H I
Department o f Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 (U.S.A.) (Revised version received M a r c h 19, 1 9 8 1 ; a p p r o v e d M a r c h 26, 1 9 8 1 )
Abstract Hurd, D.C. a n d T a k a h a s h i , K., 1 9 8 3 . On t h e e s t i m a t i o n o f m i n i m u m m e c h a n i c a l loss d u r i n g an in situ b i o g e n i c silica d i s s o l u t i o n e x p e r i m e n t . Mar. M i c r o p a l e o n t o l . , 7 : 4 4 1 - - 4 4 7 . M a x i m u m d i s s o l u t i o n rates of biogenic silica as a f u n c t i o n o f d e p t h in t h e o c e a n were calculated as a f u n c t i o n of the variables t e m p e r a t u r e , pH, dissolved silicon c o n c e n t r a t i o n , a n d t h e m e a s u r e d surface area of r a d i o l a r i a n s k e l e t o n s of a single species. These rates w h e n c o m p a r e d w i t h observed w e i g h t losses d u r i n g an in situ dissolut i o n e x p e r i m e n t suggest a m e c h a n i c a l w e i g h t loss of a b o u t 50%. T h e r e l a t i o n s h i p b e t w e e n m e c h a n i c a l w e i g h t loss a n d total w e i g h t loss is linear, suggesting the usefulness of this m a t h e m a t i c a l a p p r o a c h t o d i s s o l u t i o n exp e r i m e n t s as well as t h e need for c a u t i o n in trying to a p p l y such loss data directly t o t h e e n v i r o n m e n t .
Introduction During the past few years, several in situ dissolution experiments have been performed on biogenic minerals (Berger, 1967, 1968, 1970; Milliman, 1975, 1977; Honjo and Erez, 1978; Thunell and Honjo, 1981; Erez et al., 1982}. Usually percent weight loss is plotted as a function of depth and relative or absolute dissolution rates calculated. With these experiments one difficulty which precludes the estimation of in situ dissolution rates is the calculation of mechanical weight loss -- those particles which fall through the mesh of the sample containers as compared with the actual weight losses directly attributable to the dissolution process. In this paper, we present calculations which estimate the m i n i m u m mechanical loss relating to a single species of Radiolaria and having a siliceous skeleton. We are also in
the process of producing a more comprehensive work which will consider the general problem of morphological change accompanying siliceous skeletons dissolving in the water column, as well as approaches which will combine micropaleontological counting statistics with physical chemical laboratory studies (Hurd et al., in prep.). It is our hope that the calculations involved in such a combined approach can also be applied to a wide variety of other biogenic mineral types. Methods and materials As described in greater detail (Erez et al., 1982), a single species of phaeodarian Radiolaria ( Castanidium longispinum Haecker) was cleaned with bleach and placed in a number of all-plastic containers having a ca. 245pm mesh size. The mooring was deployed for 61 days in the central, sub-tropical North Pacific. Table I gives the sample depth, and
*~ Present address: Shell D e v e l o p m e n t C o m p a n y , P.O. Box 481, H o u s t o n , Texas 7 7 0 0 1 (U.S.A.). 0377-8398/83/0000--0000/$03.00
© 1983 Elsevier Scientific Publishing C o m p a n y
442
TABLE I Water c o l u m n p r o p e r t i e s at G E O S E C S S t a t i o n 235 Sample depth (m)
Potential temp. (°C)
Dissolved silicate (tiM)
pHT,p
378 578 778 978 1378 3978 5582
8.0 6.3 5.1 4.3 3.1 1.1 0,9
45 71 88 105 130 154 136
8.0 7.9 7.9 7.9 8.0 8.1 8.2
and the specific surface area in cm2/g for each sample. It is important to note that for these particular samples the specific surface area remained essentially constant, regardless of percent weight loss. Calculations and discussion Kamatani and Riley (1979), based on laboratory experiments involving the dissolution of chemically cleaned biogenic silica at several temperatures, have shown the following equation to fit their data:
the potential temperature, dissolved silica and calculated pH for each depth. The latter two properties were estimated from data obtained at a nearby GEOSECS Station, No. 235. After the dissolution experiment had been completed and the weight losses determined (Erez et al., 1982) the specific surface area of each sample was determined by the singlepoint BET method (Gregg and Sing, 1967), using a 31.6% N:/68.4% He gas mixture. Because of the small sample size and low specific surface areas involved, small-bore (ca. 2 mm i.d.) capillary U-tubes having a small bulb blown in the base were used to avoid problems caused by thermal diffusion of the gases (Lowell, 1979). Table II gives the actual percent weight loss at each depth
In (RF) = -- (rate const) t
where RF is the remaining weight fraction of solid biogenic silica suspended in solution at time t, and the rate constant units are in reciprocal time. Hurd and Birdwhistell (in press), using equations partly developed by Nelson (1975), has suggested that the Vdis used by Nelson, and the above rate constant of Kamatani and Riley (1979) are mathematically equivalent. Further, Vdis can be related to earlier equations used by Hurd (1973), O'Connor and Greenberg (1958) and Van Lier et al. (1960): Vdi s (h- 1) = k 2 (Csat -- Csol) Asp .MW.3600 sec/h
(2) where k: is the first order heterogeneous dis-
T A B L E II Measured and calculated p a r a m e t e r s relevant t o biogenic silica dissolution rates Sample depth (m)
C'sat, n (urn)
Calculated ~ Calculated 3 Calculated rate const.k 2 rate const.Vdi s wt. loss (cm/sec(X 109)) (hv~(× 104)) (%)
Actual Measured wt. loss 4 Asp (%) (cm2]g(× 10~ 4)
378 578 778 978 1378 3978 a 39785 5582
1050 990 980 955 945 986 986 1047
16 13 11 9.9 8.5 6.5 6.5 6.3
92 81 74 57 50 36 37 52
3.8 2.8 2.3 2.0 1.6 1.3 1.3 1.3
43 34 29 25 21 17 17 17
(1)
11.3 10.2 6.1 11.0 9.7 10.9 10.3 11.2
+ + ± ~ ± ± ± ±
0.2 0.5 0.5 0.2 0.2 0.3 0.2 0.2
' C o r r e c t e d for t e m p e r a t u r e and pressure effects; 2corrected for t e m p e r a t u r e and p H effects; 3corrected for t e m p e r a t u r e , p H and assuming Asp = 1 1 × 1 0 ' c m 2 / g ; f r o m E r e z e t a l . ( 1 9 8 2 ) .
443 WEIGHT
solution rate constant in cm/sec; Csat, the equilibrium solubility value of biogenic silica at a given temperature, pH and pressure in mole/cm; Csoi the concentration of dissolved silica at time t in mole/cm3; Asp, the specific surface area of the solid in cm2/g; and MW, the molecular weight of silica in grams, 60. In a manner similar to Hurd (1973) and Griffin and Hurd (in press)we have predicted k2 as a function of temperature and pH, and Csat as a function of temperature and pressure. Table III gives the predictive equations used. If we can predict k2 and Csat, and we T A B L E III
{0
20
t000
50
LOSS , PERCENT
40
50
/
60
-
o/
70,
80
90
ACTUAL
u
2000
/: / DEPTH,
/ /
5000
/ i/
METERS
/ /' / 4000
5000
Predictive e q u a t i o n s u s e d t o e s t i m a t e Vdi s
i b
1. E f f e c t o f t e m p e r a t u r e o n k 2 at pH 8: 6000
k 2 = 5.32 x 107 X e -~19,970/RT) c m / s e c , s e a w a t e r , cleaned skeletons 2. E f f e c t o f t e m p e r a t u r e o n Csat below pH 8.3: Csat
=
4.4~
X
10- s
X
e --(4670/RT) m o | e s / c m ~
3. E f f e c t of p r e s s u r e o n s o l u b i l i t y : Sp/'~t
atm
= 1.0
+
4 . 2 , X 10 -4
X Patm
w h e r e Patm = d e p t h in m e t e r s X 0.1
have m e a s u r e d Csol and Asp, we can then calculate V d i s by using the 61-day deployment time as our value for t. The minimum weight loss which should have occurred as a result of dissolution is then: Estimated % wt. loss = 100 X [1
--e -~vdis X
61 X 24)]
(3)
The difference between this predicted weight loss and the actual measured weight loss should be close to the minimum mechanical loss. Fig. 1 shows both the calculated and actual weight losses of the bleached samples versus depth. The general trends of greater weight loss at higher temperatures are similar but not
Fig. 1. W e i g h t loss in p e r c e n t v e r s u s d e p t h in m e t e r s . C a l c u l a t e d values (closed circles) s h o w a similar t r e n d t o a c t u a l w e i g h t losses ( o p e n circles; f r o m Erez et al., 1 9 8 2 ) b u t are n o t t r u l y parallel.
truly parallel. But if, during the dissolution process, the skeletons become susceptible to mechanical loss, the two curves would n o t necessarily be parallel. Fig. 2 shows a plot of calculated versus actual weight loss. The data fit the linear curve: (Calc. wt % loss) = --1 -+ 3 + 0.44 _+ 0.04 X (actual wt % loss)
(4)
and have a correlation coefficient of r = 0.96. The constancy of increasing mechanical loss with increasing total dissolution is striking. The actual amounts of such losses are surely species dependent, but eq. 4 strongly suggests to us that as dissolution proceeds, a great deal of fragmentation must also occur. Plate I, 1 and 2 shows scanning electron micrographs of several of the radiolarians resting on the plastic mesh of the container. Although it is obvious that this mesh size would easily allow small particles to escape,
444
3
I
PLATE I F i g u r e s 1--2 are s c a n n i n g e l e c t r o n m i c r o g r a p h s a n d figures 3--4 are t r a n s m i s s i o n e l e c t r o n m i c r o g r a p h s . All o f t h e s p e c i m e n s s h o w n h e r e were b l e a c h e d a n d r e c o v e r e d f r o m 3 9 7 8 m. 1. S p e c i m e n s o f r a d i o l a r i a n s (C. longispinum) r e s t i n g o n t h e plastic mesh o f t h e c o n t a i n e r . Scale bar: 1 m m . 2. A s p e c i m e n o f C. longispinurn o n t h e plastic m e s h . Scale bar: 5 0 0 urn. 3. A c r o s s - s e c t i o n of an i n t e r v e n i n g skeletal bar o f C. longispinum. D i s t i n c t layers e x i s t n e a r t h e s u r f a c e . T h e r e are t w o d i f f e r e n t skeletal s t r u c t u r e s : (a) a ca. relatively solid u n i t ; a n d (b) a p o r o u s u n i t w h i c h is m a d e of ca. 0 . 0 1 - ~ m size particles. Scale bar: I ~ m . 4. A c r o s s - s e c t i o n o f an i n t e r v e n i n g bar o f C. longispinum w h i c h is t a k e n f r o m a d i f f e r e n t s p e c i m e n t h a n t h a t in figure 3. T h e s t r u c t u r a l u n i t s are m a d e o f h o l l o w t u b e s h a v i n g a d i a m e t e r o f ca. 0.1 pro. Scale bar: 1 ~ m .
445
C :
- I + 0.44A r:096
;bArED WE'SHr [ 0S5, P~CE T
(
ME A
WEIGHT
LOSS , PEP, CENT
Fig. 2. Calculated weight loss in percent versus actual weight loss in percent. The linear regression equation is given on the figure. The observed trend suggests that skeletons are continuously susceptible to mechanical loss as the dissolution process proceeds.
r e c o v e r e d s k e l e t o n s o f this species is p o r o u s at several levels, w h e n relatively little diss o l u t i o n has o c c u r r e d . This is s h o w n quite well b y h i g h - r e s o l u t i o n t r a n s m i s s i o n e l e c t r o n m i c r o g r a p h s o f several p o r t i o n s o f a s k e l e t o n f r o m the 3 9 7 8 m d e p t h g r o u p i n g (Plate I, 3 and 4). Several particle m o r p h o l o g i e s exist b u t it is clear t h a t at least several p o r t i o n s o f the s k e l e t o n are p o r o u s t h r o u g h o u t t h e i r s t r u c t u r e . Analyses o f C. longispinum and several o t h e r phaeodarian species f r o m p l a n k t o n t o w s in t h e P a n a m a Basin suggest that initially phaeodarian s k e l e t o n s are generally m u c h less p o r o u s t h a n is p r e s e n t e d in the Plate b u t these s k e l e t o n s q u i c k l y bec o m e p o r o u s as d i s s o l u t i o n p r o c e e d s ( H u r d et al., in p r e p . ) . P h a e o d a r i a n dissolution m o r p h o l o g y changes are in m a r k e d c o n t r a s t to t h o s e o f p o l y c i s t i n e radiolarians w h i c h a p p e a r initially to be n o n - p o r o u s or have m a n y closed p o r e s (see H u r d et al., 1 9 8 1 , for f u r t h e r details), and t h e n relatively slowly p r o c e e d to d e v e l o p an o u t e r p o r o u s l a y e r as a result o f the d i s s o l u t i o n process. T h e m e a n d i a m e t e r of t h e s k e l e t o n s is 4 7 0 -+ 34 ~ m (n = 50) a n d t h a t o f t h e c o n n e c t ing e l e m e n t s 4 - - 5 p m . Because t h e specific sur-
face area is ca. 10 X 104 cm2/g, the s k e l e t o n s m u s t be c o m p o s e d of particles at least as small as ca. 0.1 p m . A great m a n y particles o c c u r in this size range as well as t h o s e w h i c h are s o m e w h a t larger a n d in t h e process o f b e c o m i n g smaller because o f dissolution (Plate I, 3 and 4). I t is r e m a r k a b l e to us t h a t the specific surface area o f t h e s k e l e t o n s r e m a i n s c o n s t a n t over such a wide range o f weight p e r c e n t m a t e r i a l dissolved. A l t h o u g h detailed e x p l a n a t i o n is b e y o n d t h e s c o p e o f this p a p e r , changes in u l t r a s t r u c t u r a l m o r p h o l o g y o f siliceous s k e l e t o n s during the diss o l u t i o n process should p r o v i d e an intriguing area f o r f u t u r e research. In t h e a b s e n c e o f specific surface area d a t a we c a n n o t e s t i m a t e m e c h a n i c a l w e i g h t losses f o r the Berger ( 1 9 6 8 ) e x p e r i m e n t . An
a , ,,oo
d
•tb
0.43
d,o.o,
C, 0.25
II
!
@/
Fig. 3, Schematic review of dissolution experiment from Hurd (1972). Numbers beside each letter give relative silicon release rates from radiolarian skeletons (represented by asterisks), a. Sample not encapsulated, solution stirred, b. Sample encapsulated, both sample and solution stirred, c. Sample encapsulated, only outside solution stirred, d. Sample encapsulated, neither sample nor solution stirred.
446 i m p o r t a n t f a c t o r which is relevant to b o t h the Berger ( 1 9 6 8 ) e x p e r i m e n t and t h e p r e s e n t weight loss d a t a is t h e e f f e c t of the container on the dissolution rate o f biogenic silica. Fig. 3 reviews a simple t y p e o f e x p e r i m e n t w h i c h begins to test such e f f e c t s f o r the t y p e o f c o n t a i n e r s used b y Berger ( H u r d , 1972}. H u r d ' s results suggest Berger's dissolution d a t a m a y be t o o slow b y a f a c t o r o f 3 - - 4 . T h e y e l l o w cages used b y Erez et al. ( 1 9 8 2 ) are larger, have m o r e available screen area and have larger m e s h size t h a n t h o s e used b y Berger, and as such m a y h i n d e r d i s s o l u t i o n rates o n l y b y a f a c t o r o f 2 or so. Because we d o n o t k n o w t h e t r u e dissolution r e t a r d i n g e f f e c t o f the c o n t a i n e r we m u s t t e r m o u r m e c h a n i c a l weight loss e s t i m a t e s " m i n i m u m " w e i g h t losses. An e x p e r i m e n t b y H o n j o and Erez ( 1 9 7 8 ) , using their ISWAC to s t u d y d i a t o m dissolut i o n at various d e p t h s f o r a p e r i o d o f 79 days, gave p e r c e n t losses o f 6 - - 1 2 wt% and surface area changes f r o m ca. 6.5 t o 17.5 X 104 cm2/ g. This in t u r n gives Vdi s values in the range --3.3 to 6.7 X 10 -s h -1. Given an in situ t e m p e r a t u r e of ca. 1°C, we e s t i m a t e the effective specific surface area o f their s a m p l e s to be ca. 3 to 6 X 104 cm2/g (using eq. 2), which is r e a s o n a b l y close to the actual m e a s u r e d values b u t suggests the possibility t h a t s o m e stagnat i o n m a y have o c c u r r e d in their t e s t c h a m bers. In c o n c l u s i o n , we have e s t i m a t e d t h e m i n i m u m m e c h a n i c a l loss which o c c u r r e d during a p a r t i c u l a r in situ d i s s o l u t i o n experiment. The relationship between mechanical loss and t o t a l a m o u n t o f m a t e r i a l lost during the dissolution p r o c e s s a p p e a r s regular e n o u g h t h a t f u r t h e r research using the a b o v e m a t h e m a t i c a l a p p r o a c h should be p r o m i s i n g provided t h a t the e f f e c t s o f c o n t a i n e r g e o m e t r y on dissolution rates is c o n s i d e r e d in detail as well. Acknowledgements We are grateful to Dr. J o n a t h a n Erez f o r p e r m i s s i o n to use a p o r t i o n o f his previous-
ly u n p u b l i s h e d d a t a and to Mr. V e r n o n A s p e r f o r the t r a n s m i s s i o n e l e c t r o n m i c r o g r a p h s . We also t h a n k Drs S u s u m u H o n j o , J o h n Milliman, A r t h u r Chen and Wolfgang Berger f o r their c o n s t r u c t i v e criticism o f the m a n u script. This research was s u p p o r t e d b y the Office o f Naval R e s e a r c h and N a t i o n a l Scie n c e F o u n d a t i o n G r a n t No. O C E - 7 9 2 5 4 1 2 . W o o d s H o l e O c e a n o g r a p h i c I n s t i t u t i o n Cont r i b u t i o n N o . 4 8 0 2 and Hawaii I n s t i t u t e o f Geophysics Contribution No. 1130. References Berger, W.H., 1967. Foraminiferal ooze: solution at depth. Science, 156: 383--385. Berger, W.H., 1968. Radiolarian skeletons: solution at depths. Science, 159: 1237--1238. Berger, W.H., 1970. Planktonic foraminifera: selective solution and the lysocline. Mar. Geol., 8: 111--138. Erez, J., Takahashi, K. and Honjo, S., 1982. ln-situ dissolution experiment of Radiolaria in the Central North Pacific Ocean. Earth Planet. Sci. Lett., 59: 245--254. Gregg, S.J. and Sing, K.S.W., 1967. Adsorption, Surface Area, and Porosity. Academic Press, London, 371 pp. Griffin, J.W. and Hurd, D.C., in press. The effect of pressure on the solubility of 11 silicates in 2°C, pH 8 seawater. Am. J. Sci. Honjo, S. and Erez, J., 1978. Dissolution rates of calcium carbonate in the deep ocean: an in-situ experiment in the North Atlantic. Earth Planet. Sci. Lett., 40: 287--300. Hurd, D.C., 1972. Factors affecting solution rate of biogenic opal in seawater. Earth Planet. Sci. Lett., 15: 411--417. Hurd, D.C., 1973. Interactions of biogenic opal, sediments and seawater in the Central Equatorial Pacific. Geochim. Cosmochim. Acta, 37: 2257-2282. Hurd, D.C. and Birdwhistell, S., in press. On producing a general model for the accurate description of biogenic silica dissolution in natural waters: part I, inter-relation and laboratory testing of existing equations. Am. J. Sci. Hurd, D.C., Pankratz, H.S., Asper, V., Fugate, J. and Morrow, H., 1981. Changes in the physical and chemical properties of biogenic silica from the Central Equatorial Pacific: part III, specific pore volume, mean pore size and skeletal ultrastructure of acid-cleaned samples. Am. J. Sci., 281: 833--895.
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Hurd, D.C., Takahashi, K. and Asper, V., in prep. On the preservation of siliceous skeletons: an interdisciplinary approach. Kamatani, A. and Riley, J.P., 1979. Rate of dissolution of diatom silica walls in seawater. Mar. Biol., 55: 29--35. Lowell, S., 1979. Introductioa to Powder Surface Area. Wiley-Interscience, New York, N.Y., 199 pp. Milliman, J.D., 1975. Dissolution of aragonite, Mgcalcite, and calcite in the North Atlantic Ocean. Geology, 3: 461--462. Milliman, J.D., 1977. Dissolution of calcium carbonate in the Sargasso Sea (North Atlantic). In: N.A. Andersen and A. Malahoff (Editors), The Fate of Fossil Fuel CO 2 in the Oceans. Plenum Press, New York, N.Y., pp. 641--653.
Nelson, D.M., 1975. Uptake and Regeneration of Silicic Acid by Marine Phytoplankton. Thesis, University of Alaska, Fairbanks, 157 pp. O'Connor, T.L. and Greenberg, S.A., 1958. The kinetics for the solution of silica in aqueous solution. J. Phys. Chem., 62: 1195--1198. Thunell, R.C. and Honjo, S., 1981. Calcite dissolution and the modification of planktonic foraminiferal assemblages. Mar. Micropaleontol., 6: 169--182. Van Lier, J.A., De Bruyn, P.L. and Overbeek, J.Th.G.~ 1960. The solubility of Quartz. J. Phys. Chem., 64: 1675--1682.
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Elsevier Scientific P u b l i s h i n g C o m p a n y , A m s t e r d a m -- P r i n t e d in T h e N e t h e r l a n d s
ON THE ESTIMATION OF MINIMUM MECHANICAL LOSS DURING AN I N S I T U BIOGENIC SILICA DISSOLUTION EXPERIMENT
D A V I D C. H U R D .1 and K O Z O T A K A H A S H I
Department o f Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 (U.S.A.) (Revised version received M a r c h 19, 1 9 8 1 ; a p p r o v e d M a r c h 26, 1 9 8 1 )
Abstract Hurd, D.C. a n d T a k a h a s h i , K., 1 9 8 3 . On t h e e s t i m a t i o n o f m i n i m u m m e c h a n i c a l loss d u r i n g an in situ b i o g e n i c silica d i s s o l u t i o n e x p e r i m e n t . Mar. M i c r o p a l e o n t o l . , 7 : 4 4 1 - - 4 4 7 . M a x i m u m d i s s o l u t i o n rates of biogenic silica as a f u n c t i o n o f d e p t h in t h e o c e a n were calculated as a f u n c t i o n of the variables t e m p e r a t u r e , pH, dissolved silicon c o n c e n t r a t i o n , a n d t h e m e a s u r e d surface area of r a d i o l a r i a n s k e l e t o n s of a single species. These rates w h e n c o m p a r e d w i t h observed w e i g h t losses d u r i n g an in situ dissolut i o n e x p e r i m e n t suggest a m e c h a n i c a l w e i g h t loss of a b o u t 50%. T h e r e l a t i o n s h i p b e t w e e n m e c h a n i c a l w e i g h t loss a n d total w e i g h t loss is linear, suggesting the usefulness of this m a t h e m a t i c a l a p p r o a c h t o d i s s o l u t i o n exp e r i m e n t s as well as t h e need for c a u t i o n in trying to a p p l y such loss data directly t o t h e e n v i r o n m e n t .
Introduction During the past few years, several in situ dissolution experiments have been performed on biogenic minerals (Berger, 1967, 1968, 1970; Milliman, 1975, 1977; Honjo and Erez, 1978; Thunell and Honjo, 1981; Erez et al., 1982}. Usually percent weight loss is plotted as a function of depth and relative or absolute dissolution rates calculated. With these experiments one difficulty which precludes the estimation of in situ dissolution rates is the calculation of mechanical weight loss -- those particles which fall through the mesh of the sample containers as compared with the actual weight losses directly attributable to the dissolution process. In this paper, we present calculations which estimate the m i n i m u m mechanical loss relating to a single species of Radiolaria and having a siliceous skeleton. We are also in
the process of producing a more comprehensive work which will consider the general problem of morphological change accompanying siliceous skeletons dissolving in the water column, as well as approaches which will combine micropaleontological counting statistics with physical chemical laboratory studies (Hurd et al., in prep.). It is our hope that the calculations involved in such a combined approach can also be applied to a wide variety of other biogenic mineral types. Methods and materials As described in greater detail (Erez et al., 1982), a single species of phaeodarian Radiolaria ( Castanidium longispinum Haecker) was cleaned with bleach and placed in a number of all-plastic containers having a ca. 245pm mesh size. The mooring was deployed for 61 days in the central, sub-tropical North Pacific. Table I gives the sample depth, and
*~ Present address: Shell D e v e l o p m e n t C o m p a n y , P.O. Box 481, H o u s t o n , Texas 7 7 0 0 1 (U.S.A.). 0377-8398/83/0000--0000/$03.00
© 1983 Elsevier Scientific Publishing C o m p a n y
442
TABLE I Water c o l u m n p r o p e r t i e s at G E O S E C S S t a t i o n 235 Sample depth (m)
Potential temp. (°C)
Dissolved silicate (tiM)
pHT,p
378 578 778 978 1378 3978 5582
8.0 6.3 5.1 4.3 3.1 1.1 0,9
45 71 88 105 130 154 136
8.0 7.9 7.9 7.9 8.0 8.1 8.2
and the specific surface area in cm2/g for each sample. It is important to note that for these particular samples the specific surface area remained essentially constant, regardless of percent weight loss. Calculations and discussion Kamatani and Riley (1979), based on laboratory experiments involving the dissolution of chemically cleaned biogenic silica at several temperatures, have shown the following equation to fit their data:
the potential temperature, dissolved silica and calculated pH for each depth. The latter two properties were estimated from data obtained at a nearby GEOSECS Station, No. 235. After the dissolution experiment had been completed and the weight losses determined (Erez et al., 1982) the specific surface area of each sample was determined by the singlepoint BET method (Gregg and Sing, 1967), using a 31.6% N:/68.4% He gas mixture. Because of the small sample size and low specific surface areas involved, small-bore (ca. 2 mm i.d.) capillary U-tubes having a small bulb blown in the base were used to avoid problems caused by thermal diffusion of the gases (Lowell, 1979). Table II gives the actual percent weight loss at each depth
In (RF) = -- (rate const) t
where RF is the remaining weight fraction of solid biogenic silica suspended in solution at time t, and the rate constant units are in reciprocal time. Hurd and Birdwhistell (in press), using equations partly developed by Nelson (1975), has suggested that the Vdis used by Nelson, and the above rate constant of Kamatani and Riley (1979) are mathematically equivalent. Further, Vdis can be related to earlier equations used by Hurd (1973), O'Connor and Greenberg (1958) and Van Lier et al. (1960): Vdi s (h- 1) = k 2 (Csat -- Csol) Asp .MW.3600 sec/h
(2) where k: is the first order heterogeneous dis-
T A B L E II Measured and calculated p a r a m e t e r s relevant t o biogenic silica dissolution rates Sample depth (m)
C'sat, n (urn)
Calculated ~ Calculated 3 Calculated rate const.k 2 rate const.Vdi s wt. loss (cm/sec(X 109)) (hv~(× 104)) (%)
Actual Measured wt. loss 4 Asp (%) (cm2]g(× 10~ 4)
378 578 778 978 1378 3978 a 39785 5582
1050 990 980 955 945 986 986 1047
16 13 11 9.9 8.5 6.5 6.5 6.3
92 81 74 57 50 36 37 52
3.8 2.8 2.3 2.0 1.6 1.3 1.3 1.3
43 34 29 25 21 17 17 17
(1)
11.3 10.2 6.1 11.0 9.7 10.9 10.3 11.2
+ + ± ~ ± ± ± ±
0.2 0.5 0.5 0.2 0.2 0.3 0.2 0.2
' C o r r e c t e d for t e m p e r a t u r e and pressure effects; 2corrected for t e m p e r a t u r e and p H effects; 3corrected for t e m p e r a t u r e , p H and assuming Asp = 1 1 × 1 0 ' c m 2 / g ; f r o m E r e z e t a l . ( 1 9 8 2 ) .
443 WEIGHT
solution rate constant in cm/sec; Csat, the equilibrium solubility value of biogenic silica at a given temperature, pH and pressure in mole/cm; Csoi the concentration of dissolved silica at time t in mole/cm3; Asp, the specific surface area of the solid in cm2/g; and MW, the molecular weight of silica in grams, 60. In a manner similar to Hurd (1973) and Griffin and Hurd (in press)we have predicted k2 as a function of temperature and pH, and Csat as a function of temperature and pressure. Table III gives the predictive equations used. If we can predict k2 and Csat, and we T A B L E III
{0
20
t000
50
LOSS , PERCENT
40
50
/
60
-
o/
70,
80
90
ACTUAL
u
2000
/: / DEPTH,
/ /
5000
/ i/
METERS
/ /' / 4000
5000
Predictive e q u a t i o n s u s e d t o e s t i m a t e Vdi s
i b
1. E f f e c t o f t e m p e r a t u r e o n k 2 at pH 8: 6000
k 2 = 5.32 x 107 X e -~19,970/RT) c m / s e c , s e a w a t e r , cleaned skeletons 2. E f f e c t o f t e m p e r a t u r e o n Csat below pH 8.3: Csat
=
4.4~
X
10- s
X
e --(4670/RT) m o | e s / c m ~
3. E f f e c t of p r e s s u r e o n s o l u b i l i t y : Sp/'~t
atm
= 1.0
+
4 . 2 , X 10 -4
X Patm
w h e r e Patm = d e p t h in m e t e r s X 0.1
have m e a s u r e d Csol and Asp, we can then calculate V d i s by using the 61-day deployment time as our value for t. The minimum weight loss which should have occurred as a result of dissolution is then: Estimated % wt. loss = 100 X [1
--e -~vdis X
61 X 24)]
(3)
The difference between this predicted weight loss and the actual measured weight loss should be close to the minimum mechanical loss. Fig. 1 shows both the calculated and actual weight losses of the bleached samples versus depth. The general trends of greater weight loss at higher temperatures are similar but not
Fig. 1. W e i g h t loss in p e r c e n t v e r s u s d e p t h in m e t e r s . C a l c u l a t e d values (closed circles) s h o w a similar t r e n d t o a c t u a l w e i g h t losses ( o p e n circles; f r o m Erez et al., 1 9 8 2 ) b u t are n o t t r u l y parallel.
truly parallel. But if, during the dissolution process, the skeletons become susceptible to mechanical loss, the two curves would n o t necessarily be parallel. Fig. 2 shows a plot of calculated versus actual weight loss. The data fit the linear curve: (Calc. wt % loss) = --1 -+ 3 + 0.44 _+ 0.04 X (actual wt % loss)
(4)
and have a correlation coefficient of r = 0.96. The constancy of increasing mechanical loss with increasing total dissolution is striking. The actual amounts of such losses are surely species dependent, but eq. 4 strongly suggests to us that as dissolution proceeds, a great deal of fragmentation must also occur. Plate I, 1 and 2 shows scanning electron micrographs of several of the radiolarians resting on the plastic mesh of the container. Although it is obvious that this mesh size would easily allow small particles to escape,
444
3
I
PLATE I F i g u r e s 1--2 are s c a n n i n g e l e c t r o n m i c r o g r a p h s a n d figures 3--4 are t r a n s m i s s i o n e l e c t r o n m i c r o g r a p h s . All o f t h e s p e c i m e n s s h o w n h e r e were b l e a c h e d a n d r e c o v e r e d f r o m 3 9 7 8 m. 1. S p e c i m e n s o f r a d i o l a r i a n s (C. longispinum) r e s t i n g o n t h e plastic mesh o f t h e c o n t a i n e r . Scale bar: 1 m m . 2. A s p e c i m e n o f C. longispinurn o n t h e plastic m e s h . Scale bar: 5 0 0 urn. 3. A c r o s s - s e c t i o n of an i n t e r v e n i n g skeletal bar o f C. longispinum. D i s t i n c t layers e x i s t n e a r t h e s u r f a c e . T h e r e are t w o d i f f e r e n t skeletal s t r u c t u r e s : (a) a ca. relatively solid u n i t ; a n d (b) a p o r o u s u n i t w h i c h is m a d e of ca. 0 . 0 1 - ~ m size particles. Scale bar: I ~ m . 4. A c r o s s - s e c t i o n o f an i n t e r v e n i n g bar o f C. longispinum w h i c h is t a k e n f r o m a d i f f e r e n t s p e c i m e n t h a n t h a t in figure 3. T h e s t r u c t u r a l u n i t s are m a d e o f h o l l o w t u b e s h a v i n g a d i a m e t e r o f ca. 0.1 pro. Scale bar: 1 ~ m .
445
C :
- I + 0.44A r:096
;bArED WE'SHr [ 0S5, P~CE T
(
ME A
WEIGHT
LOSS , PEP, CENT
Fig. 2. Calculated weight loss in percent versus actual weight loss in percent. The linear regression equation is given on the figure. The observed trend suggests that skeletons are continuously susceptible to mechanical loss as the dissolution process proceeds.
r e c o v e r e d s k e l e t o n s o f this species is p o r o u s at several levels, w h e n relatively little diss o l u t i o n has o c c u r r e d . This is s h o w n quite well b y h i g h - r e s o l u t i o n t r a n s m i s s i o n e l e c t r o n m i c r o g r a p h s o f several p o r t i o n s o f a s k e l e t o n f r o m the 3 9 7 8 m d e p t h g r o u p i n g (Plate I, 3 and 4). Several particle m o r p h o l o g i e s exist b u t it is clear t h a t at least several p o r t i o n s o f the s k e l e t o n are p o r o u s t h r o u g h o u t t h e i r s t r u c t u r e . Analyses o f C. longispinum and several o t h e r phaeodarian species f r o m p l a n k t o n t o w s in t h e P a n a m a Basin suggest that initially phaeodarian s k e l e t o n s are generally m u c h less p o r o u s t h a n is p r e s e n t e d in the Plate b u t these s k e l e t o n s q u i c k l y bec o m e p o r o u s as d i s s o l u t i o n p r o c e e d s ( H u r d et al., in p r e p . ) . P h a e o d a r i a n dissolution m o r p h o l o g y changes are in m a r k e d c o n t r a s t to t h o s e o f p o l y c i s t i n e radiolarians w h i c h a p p e a r initially to be n o n - p o r o u s or have m a n y closed p o r e s (see H u r d et al., 1 9 8 1 , for f u r t h e r details), and t h e n relatively slowly p r o c e e d to d e v e l o p an o u t e r p o r o u s l a y e r as a result o f the d i s s o l u t i o n process. T h e m e a n d i a m e t e r of t h e s k e l e t o n s is 4 7 0 -+ 34 ~ m (n = 50) a n d t h a t o f t h e c o n n e c t ing e l e m e n t s 4 - - 5 p m . Because t h e specific sur-
face area is ca. 10 X 104 cm2/g, the s k e l e t o n s m u s t be c o m p o s e d of particles at least as small as ca. 0.1 p m . A great m a n y particles o c c u r in this size range as well as t h o s e w h i c h are s o m e w h a t larger a n d in t h e process o f b e c o m i n g smaller because o f dissolution (Plate I, 3 and 4). I t is r e m a r k a b l e to us t h a t the specific surface area o f t h e s k e l e t o n s r e m a i n s c o n s t a n t over such a wide range o f weight p e r c e n t m a t e r i a l dissolved. A l t h o u g h detailed e x p l a n a t i o n is b e y o n d t h e s c o p e o f this p a p e r , changes in u l t r a s t r u c t u r a l m o r p h o l o g y o f siliceous s k e l e t o n s during the diss o l u t i o n process should p r o v i d e an intriguing area f o r f u t u r e research. In t h e a b s e n c e o f specific surface area d a t a we c a n n o t e s t i m a t e m e c h a n i c a l w e i g h t losses f o r the Berger ( 1 9 6 8 ) e x p e r i m e n t . An
a , ,,oo
d
•tb
0.43
d,o.o,
C, 0.25
II
!
@/
Fig. 3, Schematic review of dissolution experiment from Hurd (1972). Numbers beside each letter give relative silicon release rates from radiolarian skeletons (represented by asterisks), a. Sample not encapsulated, solution stirred, b. Sample encapsulated, both sample and solution stirred, c. Sample encapsulated, only outside solution stirred, d. Sample encapsulated, neither sample nor solution stirred.
446 i m p o r t a n t f a c t o r which is relevant to b o t h the Berger ( 1 9 6 8 ) e x p e r i m e n t and t h e p r e s e n t weight loss d a t a is t h e e f f e c t of the container on the dissolution rate o f biogenic silica. Fig. 3 reviews a simple t y p e o f e x p e r i m e n t w h i c h begins to test such e f f e c t s f o r the t y p e o f c o n t a i n e r s used b y Berger ( H u r d , 1972}. H u r d ' s results suggest Berger's dissolution d a t a m a y be t o o slow b y a f a c t o r o f 3 - - 4 . T h e y e l l o w cages used b y Erez et al. ( 1 9 8 2 ) are larger, have m o r e available screen area and have larger m e s h size t h a n t h o s e used b y Berger, and as such m a y h i n d e r d i s s o l u t i o n rates o n l y b y a f a c t o r o f 2 or so. Because we d o n o t k n o w t h e t r u e dissolution r e t a r d i n g e f f e c t o f the c o n t a i n e r we m u s t t e r m o u r m e c h a n i c a l weight loss e s t i m a t e s " m i n i m u m " w e i g h t losses. An e x p e r i m e n t b y H o n j o and Erez ( 1 9 7 8 ) , using their ISWAC to s t u d y d i a t o m dissolut i o n at various d e p t h s f o r a p e r i o d o f 79 days, gave p e r c e n t losses o f 6 - - 1 2 wt% and surface area changes f r o m ca. 6.5 t o 17.5 X 104 cm2/ g. This in t u r n gives Vdi s values in the range --3.3 to 6.7 X 10 -s h -1. Given an in situ t e m p e r a t u r e of ca. 1°C, we e s t i m a t e the effective specific surface area o f their s a m p l e s to be ca. 3 to 6 X 104 cm2/g (using eq. 2), which is r e a s o n a b l y close to the actual m e a s u r e d values b u t suggests the possibility t h a t s o m e stagnat i o n m a y have o c c u r r e d in their t e s t c h a m bers. In c o n c l u s i o n , we have e s t i m a t e d t h e m i n i m u m m e c h a n i c a l loss which o c c u r r e d during a p a r t i c u l a r in situ d i s s o l u t i o n experiment. The relationship between mechanical loss and t o t a l a m o u n t o f m a t e r i a l lost during the dissolution p r o c e s s a p p e a r s regular e n o u g h t h a t f u r t h e r research using the a b o v e m a t h e m a t i c a l a p p r o a c h should be p r o m i s i n g provided t h a t the e f f e c t s o f c o n t a i n e r g e o m e t r y on dissolution rates is c o n s i d e r e d in detail as well. Acknowledgements We are grateful to Dr. J o n a t h a n Erez f o r p e r m i s s i o n to use a p o r t i o n o f his previous-
ly u n p u b l i s h e d d a t a and to Mr. V e r n o n A s p e r f o r the t r a n s m i s s i o n e l e c t r o n m i c r o g r a p h s . We also t h a n k Drs S u s u m u H o n j o , J o h n Milliman, A r t h u r Chen and Wolfgang Berger f o r their c o n s t r u c t i v e criticism o f the m a n u script. This research was s u p p o r t e d b y the Office o f Naval R e s e a r c h and N a t i o n a l Scie n c e F o u n d a t i o n G r a n t No. O C E - 7 9 2 5 4 1 2 . W o o d s H o l e O c e a n o g r a p h i c I n s t i t u t i o n Cont r i b u t i o n N o . 4 8 0 2 and Hawaii I n s t i t u t e o f Geophysics Contribution No. 1130. References Berger, W.H., 1967. Foraminiferal ooze: solution at depth. Science, 156: 383--385. Berger, W.H., 1968. Radiolarian skeletons: solution at depths. Science, 159: 1237--1238. Berger, W.H., 1970. Planktonic foraminifera: selective solution and the lysocline. Mar. Geol., 8: 111--138. Erez, J., Takahashi, K. and Honjo, S., 1982. ln-situ dissolution experiment of Radiolaria in the Central North Pacific Ocean. Earth Planet. Sci. Lett., 59: 245--254. Gregg, S.J. and Sing, K.S.W., 1967. Adsorption, Surface Area, and Porosity. Academic Press, London, 371 pp. Griffin, J.W. and Hurd, D.C., in press. The effect of pressure on the solubility of 11 silicates in 2°C, pH 8 seawater. Am. J. Sci. Honjo, S. and Erez, J., 1978. Dissolution rates of calcium carbonate in the deep ocean: an in-situ experiment in the North Atlantic. Earth Planet. Sci. Lett., 40: 287--300. Hurd, D.C., 1972. Factors affecting solution rate of biogenic opal in seawater. Earth Planet. Sci. Lett., 15: 411--417. Hurd, D.C., 1973. Interactions of biogenic opal, sediments and seawater in the Central Equatorial Pacific. Geochim. Cosmochim. Acta, 37: 2257-2282. Hurd, D.C. and Birdwhistell, S., in press. On producing a general model for the accurate description of biogenic silica dissolution in natural waters: part I, inter-relation and laboratory testing of existing equations. Am. J. Sci. Hurd, D.C., Pankratz, H.S., Asper, V., Fugate, J. and Morrow, H., 1981. Changes in the physical and chemical properties of biogenic silica from the Central Equatorial Pacific: part III, specific pore volume, mean pore size and skeletal ultrastructure of acid-cleaned samples. Am. J. Sci., 281: 833--895.
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Hurd, D.C., Takahashi, K. and Asper, V., in prep. On the preservation of siliceous skeletons: an interdisciplinary approach. Kamatani, A. and Riley, J.P., 1979. Rate of dissolution of diatom silica walls in seawater. Mar. Biol., 55: 29--35. Lowell, S., 1979. Introductioa to Powder Surface Area. Wiley-Interscience, New York, N.Y., 199 pp. Milliman, J.D., 1975. Dissolution of aragonite, Mgcalcite, and calcite in the North Atlantic Ocean. Geology, 3: 461--462. Milliman, J.D., 1977. Dissolution of calcium carbonate in the Sargasso Sea (North Atlantic). In: N.A. Andersen and A. Malahoff (Editors), The Fate of Fossil Fuel CO 2 in the Oceans. Plenum Press, New York, N.Y., pp. 641--653.
Nelson, D.M., 1975. Uptake and Regeneration of Silicic Acid by Marine Phytoplankton. Thesis, University of Alaska, Fairbanks, 157 pp. O'Connor, T.L. and Greenberg, S.A., 1958. The kinetics for the solution of silica in aqueous solution. J. Phys. Chem., 62: 1195--1198. Thunell, R.C. and Honjo, S., 1981. Calcite dissolution and the modification of planktonic foraminiferal assemblages. Mar. Micropaleontol., 6: 169--182. Van Lier, J.A., De Bruyn, P.L. and Overbeek, J.Th.G.~ 1960. The solubility of Quartz. J. Phys. Chem., 64: 1675--1682.