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INTERACTION
OF MARINE
SEDIMENTS
DISSOLVED
SILICA1
Kent A. Fanning
Narragansett
Marine
WITH
and David R. Schink”
Laboratory,
Kingston,
Rhode
Island
ABSTRACT
A sample of Atlantic sea floor sediment released SiOa in amounts greater than expected
when placed in contact with silica-poor seawater but failed to reduce the concentration
of
enriched ( 211 ,UM ) seawater. In situ, these sediments had been in contact with seawater
containing 30-e
silica. The interstitial
water from the core held 210~PM silica, suggesting
that clay-silica
equilibria
control this concentration.
If this interstitial
silica were derived
from resolution
of clay minerals, then clay-silica
equilibria
would be a major source of
dissolved silica in the Atlantic Ocean. It is far more likely that the resolution of biological
silica accounts for the bulk of the interstitial
content and this mechanism, might explain
the high silica concentrations
found in cores from productive
regions.
In barren arcas,
the lower interstitial
concentrations
may be regulated by clay-silica equilibria.
The relative amounts of dissolved silica entering the oceans (~4 x 1Ol4 g/yr) compared
to the amount of pelagic clay sedimentation
(-2 x 1O1” g/yr)
indicate that clay-if
it
were the major agent of removal -would
have to contain 20% hydrogenous
silica, whereas
the maximum demonstrated
adsorption is about 2%. It stems impossible that adsorption
on clays is the prime agent for extraction of silica from the present oceans.
(1967) prcscnted evidence that this does
not occur to any significant extent. Harriss
( 1966) suggested that biological
silica
deposition exceeds all measured inputs.
Mackenzie and Garrels ( 1965) and Mackenzie et al. (1967) prcsentcd evidence that
the silica concentrations of the seas are
determined by reaction with clay minerals
from the continents. They exposed such
material to seawater that was either low
in silica or abnormally enriched. In both
cases, the concentration in solution tended
toward the range of values actually observed in the ocean. Various clay minerals
showed different rates and final conccntrations, but all reactions appcarcd substantially complete within six months.
If clays interact with dissolved silica
while falling toward the bottom, they probably continue to react at the sea floor.
Deep-sea sediments have a history different
than terrestrial clays as well as a different
composition. They include terrestrial clays,
authigenic clays, other authigenic minerals,
and amorphous and crystalline silica from
several sources. There is no reason to
believe that such a mixture would have the
same reaction pattern as terrestrial clays.
This paper presents the results of studies on the sediment and on the interstitial
INTRODUCTION
Considerable
uncertainty
remains regarding the relative importance of various
processes that remove dissolved silica (silicit acid) from the seas. Among these
processes are: adsorption on river sediments as they cncountcr marine electrolytes; settling of diatoms and Radiolaria to
the sea floor without resolution; uptake by
sponges; reaction with clay minerals in the
ocean or at the sea floor; and formation of
authigenic minerals at the sea floor. One
or more of these mechanisms must have the
capacity to control the silica concentration
in the sea. A similar question concerns the
mechanism regulating the silica concentration in the interstitial
water of oceanic
sediments.
Bien, Contois, and Thomas (1958) suggcsted that adsorption of silica on clays
at river mouths is an important effect.
Stefansson and Richards (1963) and Schink
l This research was supported
by Office
of
Naval Research Contract Nonr-390( OS). We are
indebted to R. Siever for the loan of his sediment
squeezer and to M. Pilson and the late J. Corlcss
for valuable discussions of this work.
2 Present address:
Isotopes, A Teledyne
Company, Palo Alto Laboratories,
4062 Fabian Street,
Palo Alto, California
94303.
59
60
KENT
A.
FANNING
waters from a single gravity core. Sediment from the top of the core was mixed
with seawater of high- and low-silica content, and the resulting changes in silica
concentration
in the supernatant
water
were followed over a period of time. Intcrstitial water from the core was analyzed
for silica and chlorinity.
AND
DAVID
R.
SCIIINK
seawater was enriched with NaaSiOs to a
final silica concentration of 211 PM ( 16.7
PM SiOa = 1 ppm SiOZ), but the unenrichcd water had less than 1 PM.
Later
(6,300 hr) we added another control bottle
consisting only of enriched seawater ( 216
PM)
made from a different surface water.
Samples were kept at room tempcraturc
throughout
the cxperimcnt.
The bottles
METHODS
were shaken upon addition of the clay but
In November 1965 during Cruise 028 of were not otherwise disturbed until just
RV Trident, several gravity cores were before the water samples were removed.
taken from the North Atlantic Ocean and At intervals over a lo-month period the
Mcditcrranean
Sea. The one designated
samples were shaken; then, when the scdiCA 10 came from a depth of 3,220 m at mcnt had settled, water was removed and
37” 57’ N lat, 24” 15’ W long (just east filtered through a 0.45-p mcmbranc filter.
of the Azores). Globigcrina ooze was the The dissolved silica was determined by
most conspicuous component of this core. the ethyl-acetate solvent-extraction method
Cores from Cruise 028 wcrc retained in
( Schink 1965) or by conventional yellow
their plastic core liners with the ends silicomolybdic acid spectrophotometry (Tacapped. They were wrapped in aluminum
ble 1).
foil, sealed with tape to retard evaporation,
After the top 2 cm were removed from
and stored under refrigeration at 2C aboard
the unstored half of the core, the remaining
ship and ashore until opened for analysis
portion was cut into scgmcnts. The outside
five months later. Then the cores were layer (3 mm) was removed from these segsplit, and half of each core was wrapped
ments, and the interstitial
water in the
and returned to the refrigerator for pos- rcmaindcr
was extracted by squeezing
sible future analysis.
(Siever 1962). The water obtained was
The top 2 cm of the unstored half of frozen in polyethylene and stored; later it
core CA 10 were used for the experiments
was thawed with the bottle caps tightly in
on the interaction of sediments with dis- place, then membrane filtered and anasolved silica in seawater. Two portions
lyzed for chlorinity with AgN03 solution
were weighed and placed directly in sea- using a dichlorofluorcscein
indicator. The
water. The actual weights wcrc 3.7 g and silver nitrate was standardized immediately
represent 2.0 g dry wt based on the water
before by titration against a known scacontent of the scdimcnt directly below this water sample of similar size. Because of
layer. Another portion of the top layer was the variability of interstitial waters, a prctreated in an attempt to remove organic
cision of 0.1% was all that was sought, and
material
that might adhere and alter
rcplicatc standardizations
easily achieved
solution rates. This material was washed
this goal. A separate aliquot was diluted
successively with acetone, carbon tetrachlowith low-silica
seawater for subscqucnt
ride, and acetone. The acetone was re- silica determination by spcctrophotomctry.
moved by filtration with Whatman No. 42 Total moisture content of the sediment was
paper and the scdimcnt dried in an air
determined by loss of weight at 1lOC.
stream, Two 2-g dry samples were taken.
Supernatant water that came up with
As a control sample, 2 g of reagent grade
core
CA 10 was retained in the core liner,
calcium carbonate were similarly treated.
which
was kept upright until opcncd. This
The washed samples are referred to as
water
was then analyzed for silica and
“organic free.” All samples were placed in
Concentrations in the overlying
polycthylcne bottles containing 500 ml of chlorinity.
scnwatcr were obtained from a Nansen cast
surface seawater that had been membrane
filtered (0.45 p). In alternate bottles, the at the same station. The analyst for chlo-
INTERACTION
TABLE
OF
MARINE
1, Silica concentration
SEDIMENTS
(PM)”
-.
Uncnrichcd
Time
start
since
(hr)
0
2
25
70
73
124
290
865
916
3,525
7,200
9,860
Final
pH
* Silica
dctermincd
Untreated
scdimcnt
Organic-free
sediment
WJTII
DISSOLVED
in water overlying
----
curious
secliments
~.___
Enriched
water
Untrcatcd
seclimcnt
C&O,,
control
61
SILICA
water
Organic-free
sediment
T”:;“’
3.7
9.5
0.1
4.7
11.3
0.1
0.1
0.1
211
204
205
211
204
205
15.4
19.4
30
17.2
21.6
31
0.3
0.3
0
200
201
201
198
198
197
47
47
1
201
196
122,
137
7.8
121
136
7.8
2
222
224
7.8
217
218
7.8
0.1
concentrations
reported
by the conventional
7.9
to 0.1 fih4 were cletermincd
silicomolybclic
acid method.
rinitics was unaware of the independent
measurcmcnt of deep scawatcr from this
station. The chlorinity of the deep water
was calculated from salinomctcr measurcmcnt.
Several alternate segments of core CA
10 were weighed, mixed with a known
amount of low-silica seawater of known
concentration, stirred for 10 min, and filtcrcd. The filtrate was analyzed for silica,
and the apparent silica concentration in
the interstitial water was calculated to bc
markedly above that determined by squeczing, filtering, and so on. The differcncc
must be due either to the dissolution of
silica from the sedimentary material in
contact with seawater or to the depolymcrization of nonreactive forms. Since the
filtration rate was slow, variable, and unmeasured, effective control was exercised
over neither the amount of silica dissolved
nor the rate of solution, but the cxperimcnt
does clearly indicate that the experiments
reported by Arrhenius and Rotschi (cited
in Arrhenius 1963) are irrelevant,
RESULTS
Interstitiul
water
The variation of the chlorinity and silica
concentraion of the interstitial water with
depth in core CA 10 is shown in Fig. 1.
by
ethyl-acetate
cxlraction.
Vnhic3
reportccl
216
220
212
207
8.4
to
1 ,uiu
were
Depths arc those measured on the core
sample. Gravity cores are sometimes considered to be shortcncd by as much as a
factor of two during collection, and the
possibility of such shortening must be considered. The most significant result seems
to bc the clustering of interstitial
silica
concentrations, found between lo-100 cm
The
in the core, around the value 210 PM.
intermediate concentration in the 2-lo-cm
portion probably reflects mixing of supernatant water with core material during collection and storage transfers. Point B (the
silica concentration in the core supernate)
undoubtedly is greater than A ( the in situ
seawater concentration)
owing to continucd rcleasc of silica from clay material, or
from included diatoms, into the overlying
solution.
Others (e.g., Sicvcr, Beck, and Berner
1965) have repeatedly
emphasized the
importance of analysis of interstitial water
immediately
upon collection.
Unfortunately, immediate analysis was impossible
in this work; hence, it is ncccssary to examine the results for evidence of alteration
in the sample during storage. Alteration
might occur by loss or gain of water, diffusion of gas through
the core liner,
biological effects, or slow physicochemical
reactions.
62
KENT
0
’
0
A.
FANNING
AND
DAVID
IL
SCIIINK
SILICA CONCENTRATION (,uM)
50
100
150 200
250
I
‘A’
I
’
1
I
1
CH LORI N ITY %o
I5
20
I
’
of
CORE
Top
43
\
I\ \
\
1
’
’
’
’
‘A’
1
B
II //
\
2
0
V
W
az
0
0
50
50
z
I
In
W
0
100
Bottom
of CORE
100
(107 cm 1
Concentration
of dissolved silica and chloride in the interstitial
water of core CA 10. A rcpFIG.
1.
resents concentration
of seawater above the core, in situ; B represents concentration
of supernatant
scawater after Five months storage with sediments. The vertical lines indicate the lengths of the segments
of the core before squeezing.
The gain of water seems most unlikely.
Storage in a refrigerator at 2C is commonly
of
dehydrating
owing to condensation
moisture on the colder refrigerator coils.
Evaporation through the top of the core
was prevented by the supernatant seawater. Moreover, the chlorinity in the supcrnatant water- as measured by titration
(Point B, Fig. 1 )-was in agrcemcnt with
the chlorinity in the deep ocean (Point A)
-measured
indcpcndently
on the Nansen
cast. Ncithcr is thcrc any difference in
the bottommost core segment. To test for
evaporation through the sides of the core
liner, two segments of the core were separated into inner and outer concentric
INTERACTION
TAIXLE 2.
Silica
and chlorinity
OF
MARINE
SEDIMENTS
concentrations
WITH
in interstitial
DISSOLVED
water
from
63
SILICA
some Atibatic
Ocean cores
a .
ncptll
Core
WHO1
WHO1
WI101
WHO1
No.
A5757
A5798
At1 278-l
At1 278-2
3383
3386
CA 10
Lat
31”
32”
33”
34”
32”
32”
37”
02’S
46’ S
25’ N
57’ N
05’ N
06 N
57’ N
(ml
4,279
3,795
4,829
5,041
4,920
4,870
3,200
Range
234-400
83465
50-284
83-333
200
250-384
142-229
samples, These inner and outer segments
were of identical length and depth. No
evidence for evaporation was found in the
comparison of chlorinitics
and silicates
( Fig. 1) ; nor did the observed percentage
of interstitial
water suggest any evaporation through the core wall. The interior
and exterior water contents of the 40-58
cm segments wcrc 52 and 53% rcspcctively, while the 80-98cm segments both
held 48% water. Similar expcriencc with
the other cores justifies the conclusion that
evaporation-condensation
was negligible.
Most other alterations
arc even less
likely. Biological action should be similar
to that in the sea floor, since the cores were
kept in darkness and at temperatures lower
than in deep Atlantic Ocean water. Diffusion of oxygen or carbon dioxide through
the core liner would be unlikely to alter
the silicate equilibrium unless the pH wcrc
substantially affected. The pH values observed in our solution experiments arc essentially the same as those measured by
Siever et al. (1965) in Atlantis core No,
258-5 from about this latitude. Even if the
in situ pH had been as high as 8.2, a shift
to 7.8 would not seriously affect the intcrstitial silicate. Diffusion in unstirred porous
media is quite slow, and any alteration
should bc greatly amplified in the outer
samples, but no inner-outer changes were
obscrvcd.
One mode of alteration that cannot bc
ruled out is a slow shift in equilibrium due
to the release of pressure. Some information on the solution equilibria of clays in
scawatcr at 200-600 atm and 0-1OC would
be interesting. Cores from various depths,
which have been proccsscd quickly after
Avg
Cl%, range
320
320
167
183
200
300
194
17.5-21.6
14.2-20.2
19.3-20.1
19.3-21.5
19.3-19.5
19.1-19.3
15.3-17.1
Refcrencc
Sievcr et al. ( 1965 )
Siever ct al. (1965)
Siever et al. (1965)
Siever et al. ( 1965 )
Harriss and Pilkey (1966)
Harriss and Pilkey (1966)
This work
collection, do not show any systematic effects of depth on interstitial silica content,
however. Either the reaction of clay + silica is not markedly dependent on pressure,
or equilibrium
adjustment is almost as
rapid as the rise of cores to the ship.
It is our conclusion that cores can bc
stored in the manner described without
altering the silica equilibria in the intcrstitial water. Less careful storage will certainly cause the silica to vary. The greatest
potential danger may be the growth of
diatoms on extruded core material kept in
the light as reported by Siever et al. (1965).
Table 2 compares thcsc analyses with
similar data collected by others. We arc
unable, at present, to explain our low chlorini ty values. The relative agreement of
inner and outer material seems to prccludc
condensation during storage. Condensation
after squeezing would require the addition
of about 2-3 ml of water to each sample
(20-30 ml) in a 50-ml polyethylene bottle.
After careful review of the procedures
used, we believe such addition of water to
bc impossible.
Equally low interstitial chlorinities have
been observed, although not so regularly,
by other workers who separated water from
sediment immediately;
morcovcr, abrupt
changes in chlorinities at various layers in
cores are not rare. Siever et al. (1965)
found variations of 5%0in a distance of 21
cm. In other cores analyzed at this laboratory (unpublished) , we have obtained intcrstitial chlorinities ranging from 11.4 to
21.5%0. For these reasons, we offer our
chlorinity results, although we are unable
to suggest a mechanism to account for
them.
64
KENT
A.
FANNING
AND
DAVID
R.
SCITINK
r
a
0
i
v,
---
I
I
IO
I
I
I
IllIll
I IIIIII
100
TIME
Frc. 2. Log-log
mcntnry materials.
I
MACKENZIE
FANNING
I
andGARRELS(l965)
and SCHINK
I
1000
OF
EXPOSURE
(hr 1
plot showing the rate of release of silica to unenriched
seawater
Sea floor sediments are from the top of core CA 10.
Solution experiments
The behavior of marine sediments cxposed to scawatcr differs from the behavior
of terrestrial clays as reported by Mackenzie and Garrcls ( 1965) and Mackenzie et
al. ( 1967). The cxpcriments are compared
in Figs. 2 and 3. For about 500 hr in lowsilica seawater, the marine sediments relcascd silica in the manner resembling
illite. After that period, silica release continued, and the dissolved silica concentrations rose to higher values than observed
for kaolinitc, chlorite, or illitc, although the
maximum value does not yet exceed that
of pure montmorillonitc.
Proximate analysis of the core surface
showed
26% carbonate
( as
material
CaCOs ) , 28% clay or other solids, and 46%
water. The clay mineral composition of
surface scdimcnts in the region of CA 10
is predominantly
illite (50-70%) with 520% each of kaolinite, chlorite, montmoril-
I111111
IO, 000
from
various
scdi-
lonite, and quartz ( Biscayc 1964). Although Biscaye’s results cannot precisely
characterize the scdimcnts in core CA 10,
the large abundance of illitc in this region
and the initial agrecmcnt bctwcen the silica-release curves of illitc and the scdimcnts of CA 10 suggest that the early
response of the marine sediment in lowsilica seawater is due to illite. Amorphous
silica-presumably
diatoms-has
been
shown to constitute 0.5% of the sediment
in a nearby core ( Shurko 1966). Rcsolution of diatoms would affect the rate of
change of dissolved silica and may be responsible for the continued rise of our
silica-release curve ( Fig. 2).
In contrast to the clays studied by MRCkcnzie ct al. ( 1967)) the marine sediment
did not take up silica from silica-rich seawater (Fig. 3). The difference may be
due to the lower initial silica concentrations
used, Terrestrial clays extracted SiOz from
INTERACTION
2
q
2 400
0
i=
MARINE
SEDIMENTS
WIT11
.
\
E \‘\\
E
i
E
0
i:
D,ISSOLVED
---
65
SILICA
MACKENZIE,
-FANNING
t?t 04 (1967)
and
SCHINK
\\\
\”
0’
\ ‘0
a
1001
OF
'
0
I
I
I
I
I
I
I
I
I
5000
TIME
FIG. 3. Removal of siha from enriched
mcnts are from the top of core CA 10.
OF
seawater
seawater that was 425 pM in silica, whcrcas
our marinc sediment was placed in scawa tcr of 21 l-p3
silica-a
concentration
much closer to that found in major rivers
or in the ocean. It was later found that
the interstitial silica concentration was also
-210 EA;M.
If organic coatings pro tcct sediments
from resolution, they are not affcctcd by
the acetone-carbon tetrachloridc trcatmcnt.
The “organic-free” and untreated sediments
are similar enough to be considered duplicate samples ( Table 1).
DISCUSSION
These experiments focus attention on two
problems of much broader range: 1) dots
inorganic removal of dissolved silica from
the ocean occur directly, or indirectly, or at
l0,000
EXPOSURE
by various
(hr)
sedimentary
materials.
!h
floor
se&
all ( to a significant cxtcnt); and 2) what
is the mechanism regulating silica conccn.tration in interstitial water of cores from
the sea floor?
Inorganic
removal
Inorganic removal of silica from seawater
has been clearly demonstrated by Mackenzic et al. ( 1967) and by Bicn et al. ( 1958)
using clays, river mud, and alumina. Of
these substances none has clearly been
shown to extract silica from scawatcr with
a concentration of less than 90 PM, while
the range of concentrations found in the
deep North Atlantic Ocean is (20-50 PM).
On the other hand, release of silica by clays
takes place in a few hundred hours when
the clay is in waters of low concentration
(e.g., O-35 PM)
(Mackenzie and Garrels
66
KENT
A.
FANNING
1965). Accordingly it seems impossible that
clay mineral adsorption can occur directly
in the North Atlantic water. In the South
Atlantic, silica concentrations
of greater
than 100 PM occur only in Antarctic Bottom Water. Here, too, descending detrital
clay particles have more opportunity to deliver silica to the water than to remove it.
Since release from clay minerals has not
brought concentrations of deep Atlantic silica up to the equilibrium
value, let us
examine the rather sparse data on the ultimate amount of silica that can be released
by such reactions. In our experiments, clay
released silica up to 0.4% of the dry weight
of sediments. Mackenzie and Garrels’ solution experiments showed a release of from
0.03 to 0.4% by kaolinitc and montmorillonite respectively. Neither experiment was
designed to reveal the total capability to
release silica, but we are not aware of
any proof that it is greater. Silica release
in these amounts can be only a minor factor
in the silica budget of the Atlantic Ocean.
Data on clay mineral uptake of silica arc
nearly as uncertain.
Mackenzie’s experiments establish a capacity of at least 0.35%
Experiments by Bien et al.
( glauconite).
(1958) showed that uptake of silica by bentonite, aluminum oxide, and river mud increased when the amount of suspended
material per volume of seawater was increased. Possibly this behavior was due to
kinetic effects since the uptake was only
followed for 24 hr, but the maximum adsorption observed was 0.3% for river mud,
1.8% for bentonitc, and 1.0% for aluminum
oxide. These values are probably close to
saturation values; we know of no evidence
that clays can extract more than this.
Unless clays have capacities for silica
adsorption much greater than have been
demonstrated so far, this mechanism is not
important in the overall silica budget of
the oceans. The estimatcd sedimentation
rate for clay minerals on the Atlantic Ocean
floor is about 1 mg cm-” yr-l (Turekian
1965). The rate for the Pacific and Indian
oceans is even lower because these larger
oceans arc fed by rivers draining a much
smaller land area (Lyman 1959). Turckian
AND
DAVID
R.
SCIIINK
(1967) suggests a value of 0.5 mg cm-”
yr -‘. From the area of the Atlantic Ocean,
0.8 X lOls cm2, the total pelagic clay deposition is estimated to bc 2.2 x 1O1” g yr-I.
Data from Livingstone ( 1963) show a total
river input of 4.3 x 1014 g of dissolved silica per year. If all this silica were adsorbed
on newly deposited clay, it would constitute 20% of the weight of the clay, Goldberg and Arrhcnius
(1958) have shown
pelagic clays to contain about 50% Si02.
It seems most unlikely that the preadsorption silica content was only 30%. Actually
the case for adsorption may be even worst,
since the preceding discussion seems to
preclude adsorption on Atlantic sediments.
In fact, the available data on sediment
adsorption capacity indicate that SiO,-adsorption on pelagic clays can remove at
most only 2% of the total river supply, and
it probably removes even less.
The only other sediments that could bc
involved in a net removal of silica from
seawater are river sediments that encounter marine electrolytes at river mouths or
the sediments of the continental shelves.
Bicn et al. ( 1958) claimed that Mississippi
River scdimcnts adsorbed all of the dissolved silica at its mouth, but Stcfansson
and Richards ( 1963) found that all of the
dissolved silica in the Columbia River
enters the Pacific Ocean. Schink ( 1967)
concluded from the silica budget of the
Mcditcrranean
that nearly all of the silica
entering that sea eventually leaves through
the Strait of Gibraltar. The sediments depositing on the continental shelves cannot
participate in any direct inorganic removal
process, since the worldwide oceanic silica
concentrations at shelf depth call for release rather than adsorption.
From these considerations we conchide
that clay minerals cannot remove silica
directly from the Atlantic Ocean, and that
indirect removal (e.g., with biological transport and uptake from interstitial solution)
in any ocean would probably overwhelm
the adsorption capability and would surely
alter the gross chemical character of the
clay beyond reasonable limits. Accordingly,
direct adsorption on marinc scdimcnts is
INTERACTION
OF MARINE
SEDIMENTS
not an important means of removing silica
from the oceans. A more effective process
must be sought; biological deposition seems
the most likely candidate
Mechanism regulating interstitial
silica concentrations
If the principles of chemical equilibrium
apply to marine sediments (and simple
application of these principles to such a
complex system is risky), then two possible
mechanisms might be controlling the silica
concentration in the interstitial waters : 1)
the cxtcnt to which clays can react with
and take up silica has been exceeded, and
the interstitial concentration is dc tcrmincd
by the cxtcnt of diatom resolution; or 2)
the reaction cxtcnt of the clays has not
been cxcccdcd, and the interstitial
concentration is controlled by reaction cquilibria between silica in solution and the silicon
in or on the clays.
Under areas of low productivity,
the
equilibrium
mechanism may control intcrstitial silica concentrations.
The final
supernatant silica concentrations of Mackcnzie et al. ( 1967) were loo-350 PM,
and the interstitial silica concentrations reported for cores from waters of low productivity by Siever et al. (1965) fell in
this range. Sediments from GA 10 show a
constant interstitial silica concentration of
210 pM below 30 cm ( Fig. 1) and displayed no ability to alter an initial supcrnatant silica concentration of 211 JUM (Fig.
3). Thus under areas of low productivity,
the accumulations
of siliceous frustules
stem to be less than or equal to the uptake
capacity of the clays in the sediments.
Final evaluation of the significance of
the equilibrium
between clays and dissolved silica under barren waters awaits
measurement of the true uptake capacities
of the sediments for silica, But unless thcsc
uptake capacities are much greater than
0.3%, the 0.3-1.0% amorphous silica typically found in sediments under barren
waters may provide enough silica to saturate the adjacent scdimcnts.
Under highly productive areas, the cquilibrium mechanism does not control inter-
WIT11
DISSOLVED
SILICA
67
stitial silica concentrations.
The data of
Sicver et al. ( 1965) indicate that sediments
under productive areas have interstitial silica concentrations of 600-1,000 PM, and
these values are much higher than the
“equilibrium
concentrations” for clays and
WC believe that
sediments ( loo-350 PM).
the large accumulations of siliceous frustulcs under productive
areas have saturated the uptake capacity of the clays for
silica. Thus, the high interstitial silica conccntrations in thcsc regions are sustained
by the continued resolution of frustulcs
after the clays are saturated.
These high interstitial
silica values are
not due to a temporary kinetic discrepancy
between release from frustules and the
uptake by clays with equilibrium
values
ultimately to bc rcachcd. Mackenzie et al.
(1967) found that terrestrial clays esscntially completed their silica uptake after
one year. Grill and Richards ( 1964) found
that the particulate silica in a decomposing phytoplankton
culture dropped From
52 to 1 pM in 200 days. If siliceous tests
from a phytoplankton culture were typical
of those that entered the scdimcnt column,
the transfer of silica from deposited frustules to surrounding clay grains should be
complete within 100 years. But the data
of Sicver et al. ( 1965) show that interstitial
silica concentrations 600-900 cm below the
sediment-water interface are 2-3 times the
“equilibrium”
values. These sediments arc
more than half a million years old, and
this seems much more than adequate time
for clays to overcome any kinetic discrepancy and take up all the silica they can
hold, particularly in view of their demonstratcd ability (Mackcnzic et al. 1967) to
reduce the concentration 80-160 PM ( e.g.,
clay in 200 ml of water) in just 0.1 year.
In conclusion, the silicate adso,rptioa-relcase mechanism probably establishes an
equilibrium
value for the interstitial silica
conccntra tion in scdimen ts beneath relatively barren ocean, Waters of high productivity will cause high interstitial silica
by furnishing a constant rain of siliceous
tests to redissolve in the sediments. The
strong concentration
gradients that exist
68
KENT
A.
PANNING
between interstitial water and the ovcrlying water of the deep ocean suggest that
silica is diffusing out of the! scdimcnts or
that scdimcnt cquiIibria might cvcn bc a
means of silica input to seawater.
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