CLAY
MINERALS
AND
OCEANIC
EVOLUTION
by
ROBERT C. HARRISS*
H a r v a r d University, Cambridge, Massachusetts
ABSTRACT
A REVIEW Of mineralogical aI~.d geochemical studies on Recent sediments indicates t h a t
the clay fraction of m a r i n e sediments is n o t in isotopic or chemical equilibrium with the
oceanic reservoir and will not reflect the chemical e n v i r o n m e n t of deposition. Consideration of the sedimentary geochemistry of the alkali metals suggests t h a t frautionation of
these elements, which m a y be one of the m a j o r features of the chemical evolution of
ocean water, occurs in the terrestrial weathering e n v i r o n m e n t during the f o r m a t i o n of
clay minerals and in the subsurface e n v i r o n m e n t during clay mineral diagenesis. I t is
proposed t h a t during the initial stages of oceanic development the N a r K ratio of ocean
water was adjusted to a value of forty to fifty b y the extensive diagenesis of the existing
n a t u r a l water. W i t h an increase in oceanic volume a n d a m a j o r change in the hydrologic
cycling of n a t u r a l w a t e r s the chemical evolution of alkali metals in ocean water has n o w
become primarily controlled b y mixing with continental drainage water.
INTRODUCTION
INTERACTIONS b e t w e e n t h e lithosphere a n d h y d r o s p h e r e involving a t r a n s f e r
of chemical c o m p o n e n t s occur in t h r e e principal d o m a i n s : (1) in the t e r r e s t r i a l
weathering e n v i r o n m e n t where a t m o s p h e r i c p r e c i p i t a t i o n reacts with surface
m a t e r i a l s resulting in w e a t h e r i n g a n d soil f o r m a t i o n ; (2) i n the oceanic
e n v i r o n m e n t a t a n d a b o v e t h e s e d i m e n t - w a t e r interface where d e t r i t a l
minerals a n d chemical p r e c i p i t a t e s r e a c t w i t h ocean w a t e r ; a n d (3) in t h e
subsurface e n v i r o n m e n t where i n t e r s t i t i a l w a t e r s r e a c t with enclosing rocks
in t h e course of compaction, lithification, and o t h e r diagenetic processes.
I t is well established from t h e studies of B a r t h (1952), Goldberg (1963),
a n d Garrels a n d Mackenzie (1966) t h a t t h e c o n s t i t u e n t s in ocean water, w i t h
t h e exception of chloride a n d t h e alkali metals, m u s t h a v e residence times in
the oceanic reservoir t h a t are short relative to t h e age of the oceans. I n r e c e n t
years there have been a n u m b e r of models proposed, based on t h e a s s u m p t i o n
of a s t e a d y - s t a t e oceanic system, which elucidate some of t h e factors controlling t h e chemical composition of ocean w a t e r (Rubey, 1951 ; Sillen, 1961,
* Present address: Geology D e p a r t m e n t , McMaster University, I i a m i l t o n , Ontario,
Canada.
207
208
FIFTEENTH
CONFERENCE
ON CLAYS
AND
CLAY MINERALS
1963 ; Holland, 1965; Kramer, 1965 ; Nicholls, 1965 ; Garrels and Mackenzie,
1966). In all the models discussed to date only reactions taking place in the
oceanic reservoir have been considered as possible primary factors in regulating the chemical composition of ocean water. Holland (1965) discussed a
major difffieulty encountered by the equilibrium approach, the observational
evidence supporting a detrital origin for most marine sediments, and coneluded that the correspondence between equilibrium conditions and the state
of the oceans indicates that the equilibrium model is sufficient as a first
approximation. Weyl (1966) discussed several non-equilibrium aspects of
ocean chemistry concluding that equilibria between ocean water and sediments are not adequate to explain the apparent chemical stability of the
oceans, particularly with respect to the carbonate system. Harriss (1966)
proposed that biological rather than mineralogical buffering controls oceanic
silica concentrations.
An alternate approach to the study of oceanic chemistry is to assume that
oceanic sediments are essentially non-reactive, or that the kinetics of silicatesea water reactions are sufficiently slow that little reaction other than ion
exchange and dissolution occurs prior to burial, the composition of ocean
water being primarily a function of mixing of parent water types. This assumption seems well justified in light of the mineralogical and geochemical studies
by Weaver (1959), Griffin (1962), Biseaye (1965), Hurley et al. (1963), and
Dasch et al. (1966), all of which indicate that marine clays are neither in
chemical nor isotopic equilibrium with ocean water and are detrital in origin,
It should be noted, however, that Garrels and Mackenzie (1966) calculate that
only 7 % of the sediment would be required to react with sea water to form
authigenie minerals in order to maintain equilibrium. A major difffieulty with
the proposed equilibrium models would seem to be that the dissolved silica
concentrations required for the proposed reactions between aluminosilieates
and sea water cannot be maintained in ocean water as a result of silica removal
by organisms as proposed by Harriss (1966). The ocean is apparently undersaturated with respect to equilibrium silica concentrations for all the clays
except kaolinite (see Mackenzie and Garrels, 1965). Thus, the present lack of
any definite geological evidence in support of the equilibrium hypotheses
indicates that alternative hypotheses for compositional control of ocean
chemistry should be considered.
The approach used in the present paper is to consider, using the available
geological observations as a framework, the relative importance of clay
mineral reactions with coexisting natural waters in the terrestrial weathering
environment, marine environment, and subsurface environment in the
chemical evolution of alkali metals in ocean water.
This work has been supported by the Committee on Experimental Geology
and Geophysics at Harvard University. The author has profited by discussions with Raymond Siever, H. D. Holland, R. M. Garrels, and Fred
Mackenzie, all of whom have contributed to many improvements in t h e
present discussion.
CLAY MINERALS AND OCEANIC EVOLUTION
CHEMICAL
209
CONSIDERATIONS
Tile ocean basins receive water from a v a r i e t y of sources including cont i n e n t a l drainage, g r o u n d w a t e r recharge on the c o n t i n e n t a l shelves a n d slopes,
water extruded from marine sediments during compaction, t h e r m a l waters,
a n d metamorphic waters (the latter two terms are t a k e n from White, 1963).
l~epresentative alkali m e t a l ratios for various n a t u r a l water categories are
given in Table 1. D u e to a lack of adequate d a t a the discussion in this paper
will essentially be limited to sodium a n d potassium. There is a c o n t i n u u m
of N a / I ( ratios in n a t u r a l waters ranging from c o n t i n e n t a l surface drainage
with a m e d i a n ratio of three to m e t a m o r p h i c waters with N a / K ratios of
t h i r t y a n d higher.
TAtTlE 1.--P~EPRESENTATIVE ALKALI ~rETAL I~ATIOS
IN COMMON XATUI~ALWATERS*
Water type
Na/K
l~a/Rb
Na/Cs
Ocean water
River water
Rain water
Groundwaters :
Shales
Sandstones
Limestones
Acid igneous rocks
Basic igneous rocks
Deep subsurface and
metamorphic water
Thermal springs
28
2.7
10
88 • 1O8
4 • l0 a
210 • 105
1 • 105
6
3.7
3.6
8
3.3
20-50
5-15
* Data from Livingstone (1963), White
Carroll (1962).
et al.
(1963), and
I n the c o n t e m p o r a r y hydrologic system the estimated a m o u n t of water
a n d dissolved c o n s t i t u e n t s discharged into the ocean from c o n t i n e n t a l
drainage exceeds b y several orders of m a g n i t u d e or more t h a t from a n y other
source. Calculations b y Garrels a n d Mackenzie (1966) suggest t h a t all the
water presently in the oceans has been t h r o u g h the hydrologic cycle m a n y
times, as it would require only 4 x l04 years for rivers to fill t h e ocean basins
to their present levels a t present rates of discharge.
Consider the future evolution of a]ka]i metal ratios in ocean water assuming
t h a t sediments are non-reactive a n d t h a t the present hydrologic conditions
are m a i n t a i n e d . Calculations d e m o n s t r a t e t h a t in 106 years the oceanic N a / K
ratio would be a p p r o x i m a t e l y twenty-five a n d b y 107 years from present
the N a / K ratio would equal thirteen. I t would require a p p r o x i m a t e l y l09 years
tbr complete equilibration between river a n d ocean water ( N a / K = 2 . 7 ) .
These calculations suggest t h a t u n d e r the conditions of the present hydrologic
210
FIFTEENTH CONFERENCE ON CLAYS AND CLAY MINERALS
cycle the chemical evolution of alkali metals in ocean water is controlled by
compositional mixing with river water, whose alkali metal ratios are determined in the terrestrial weathering environment. Since the time required to
produce a considerable reduction in the Na/K ratio of ocean is short relative
to the age of the ocean two questions arise: (1) how has the ocean obtained
its rather high Na/K ratio of twenty-eight? ; (2) how stable is the Na/K ratio
of ocean water? Under the conditions assumed in the present discussion the
ocean would evolve from some initial Na/K ratio to a value approaching
equilibrium with the Na/K ratio of river water and would subsequently
remain stable. However, several additional factors must be considered as
having a possible influence on alkali metal ratios in ocean water. These are:
(1) laboratory studies by Carroll and Starkey (1960) and Potts (cited in
Keller, 1963) have demonstrated that as river clays come in contact with
ocean water, base exchange occurs with the clay minerals releasing calcium
and taking up magnesium, sodium, and potassium from ocean water. What
effect will ion exchange reactions between detrital clay minerals and sea
water have on the Na/K ratio of ocean water? (2) Variation in the composition
of the input could have a definite effect on the composition of ocean water.
The composition of the input could vary as a result of changes in the terrestrial weathering enviromnent which would alter the composition of continental
drainage or by a change in the relative proportions of the sources of ocean
water.
Mineral-Water Exchange Reactions in the Oceanic Reservoir
To determine the effect of ion exchange reactions on the Na/K ratio in
ocean water the influence of such reactions on the sodium and potassium
budgets in the oceans must be calculated. Using the data of Carroll and
Starkey (1960) and Potts (see Keller, 1963) the amounts of material involved
can be calculated to be approximately 0.4 mg Na/g clay and 0,24 mg K/g
clay gained by detrital clay minerals upon contact with ocean water. The total
amount, of sodium and potassium removed from the oceanic reservoir annually
by ion exchange can be calculated by multiplying the total amount of clay
entering the oceans by the figures quoted above for removal of alkali metals
per unit weight of clay. The total amount of sediment entering the oceans
annually is calculated to be 1.5 • 1015g using annual sedimentation rates
for the various oceans as determined by Turekian and Schutz (1965) and
Goldberg and Koide (1963). This estimate is similar to that of Garrels and
Mackenzie (1966) calculated on the basis of estimates of suspended river
sediment transported to the oceans annually. The amount of sodium and
potassium entering the ocean annually has been calculated using the data
of Livingstone (1963). The amount of recycled sodium and potassium in
river water was corrected using the data, of Carroll (1962) and Junge and
Werby (1958) for the sodium and potassium concentrations in atmospheric
precipitation. The results of these calculations are summarized in Table 2.
CLAY MINERALSAND OCEANICEVOLUTION
211
I t is apparent from the sodium and potassium budget calculations that
ion-exchange reactions between detrita] clay minerals and ocean water will
have no effect on the alkali metal ratios in the oceanic reservoir. The amounts
of sodium and potassium involved in ion-exchange processes are several
orders of magnitude below the amounts supplied to the oceans by continental
drainage. Calculations indicate that even if the amount of clay entering the
oceans were increased by an order of magnitude as a result of active teetonism
the alkali metal budgets in the oceans would not be affected.
TABLE 2.--IANlqUAL SODIUM AND ~)OTASSIUM BUDGET
I~ T~E OCEA~
N a influx f r o m continental waters
(corrected for cycling)
N a r e m o v e d b y ion exchange
K influx f r o m continental waters
(corrected for cycling)
K r e m o v e d b y ion exchange
1.1 • 1017 m g / y r
5.9 • 1014 m g / y r
6.6 • 1016 m g / y r
3.6 • lO14mg/yr
Variations in the Hydrologic Input into the Oceans
The composition of surface drainage waters is primarily determined in the
terrestrial weathering environment as demonstrated by Feth et al. (1964)
and Davis (1961). Thus, large-scale changes in the nature of the chemical
weathering environment would result in a change in alkali metal ratios in
continental drainage and subsequently a change in the evolutionary tendencies of ocean water composition. The degree of chemical variation that can
result from differences in the chemical weathering environment can be
assessed by comparing the composition of local surface waters from contemporary environmental extremes as, for example, the tropical weathering
environment as opposed to the northern latitudes. D a t a compiled by Livingstone (1963) indicate that rivers in the tropics are characterized by average
Na/K ratios of two to four and rivers in the northern latitudes by average
Na/K ratios of six to eight. Thus, even under the extreme condition of a
uniform global climate the chemical composition of continental drainage
water probably would not produce an equilibrium oceanic Na/K ratio higher
than approximately eight.
The most plausible explanation for the high :Na/K ratio of ocean water
seems to involve variation in the input from different water sources. For
example, if the relative amounts of water entering the oceans from the deep
subsurface environment (brines, thermal waters, metamorphic water) were
increased with corresponding decreases in the amount of continental drainage
the evolutionary course of the alkali metals would be towards a much higher
equilibrium ratio.
212
F I F T E E N T H CONFERENCE ON CLAYS AND CLAY1V~INERALS
A general diagenetic cycle for n a t u r a l waters is d e p i c t e d schematically in
Fig. 1. The h e a v y arrows d e n o t e w h a t is essentially t h e p r e s e n t hydrologic
cycle. I t is p r o p o s e d t h a t in t h e late stages of t h e evolution of a geosynclinal
belt, during periods of active teetonism, large q u a n t i t i e s of m e t a m o r p h i c a n d
other deep subsurface w a t e r t y p e s would be e x t r u d e d a t the surface a n d
m i x e d w i t h ocean water. I f sufficient quantities of w a t e r w i t h N a / K ratios
of t h i r t y a n d a b o v e were g e n e r a t e d during t h e large-scale diagenesis of
Atmospheric
Precipitation~
(Na/~ = 4-1o)
l
Terrestrial
Weathering
Environment
(Na/K =
2-8)
(~a/K
3-8)
~
Continental
Surface
Drainage
(~a/E
=
2-8)
9
=
Ocean
> Water
~
(Na/K = 28)
Pore Water
/
(Na/l'~ =
15-28)
Deep Subsurface
and Metamorphic
Water
(Na/K = 30-60)
?
I
Juvenile-Water
(Na/X =
?)
FIG. 1. Diagenetie cycle of natural waters inferred from alkali metal variations.
s e d i m e n t a r y rocks a n d m i x e d w i t h ocean water, t h e N a / K ratio of ocean
w a t e r would show a r a p i d increase, t h e m a x i m u m value of t h e ratio being
d e p e n d e n t on the q u a n t i t y of high N a / K r a t i o w a t e r r e l a t i v e to t h e t o t a l
v o l u m e of t h e ocean a t t h e t i m e of mixing. This m e c h a n i s m p r o b a b l y has
caused little or no v a r i a t i o n in t h e composition of ocean w a t e r since t h e late
P r e e a m b r i a n , assuming t h a t t h e v o l u m e of t h e ocean has been essentially
c o n s t a n t for the p a s t 5 x l 0 s years. Calculations indicate t h a t if 10 ml of
w a t e r w i t h a N a / K r a t i o of fifty were displaced from e v e r y cubic m e t e r of
s e d i m e n t in a geosyneline w i t h dimensions similar to t h e A p p a l a c h i a n
geosyneline t h e t o t a l a m o u n t of " t e c t o n i c w a t e r " p r o d u c e d would be a t least
CLAW MINERALS
AND OCEANIC
EVOLUTION
213
several orders of m a g n i t u d e less t h a n th e v o l u m e o f w at er in t h e oceanic
reservoir. Thus, t h e large size of th e oceanic reservoir is itself an e x t r e m e l y
i m p o r t a n t buffer against variations in oceanic chemistry. T h e present N a / K
ratio in ocean w a t e r is p r o b a b l y residual f r o m the early stages o f oceanic
evolution when t h e r e l a t i v e p r o p o r t i o n of m e t a m o r p h i c w at er to ocean w a t e r
m a y h a v e been quite high due to a smaller oceanic v o l u m e a n d perhaps m o r e
extensive m e t a m o r p h i s m t h a n has been characteristic of p o s t - P r e c a m b r i a n
time.
SUMMARY
It is proposed that the Na/K ratio of ocean water was adjusted to a value of
forty to fifty approximately 109 years ago by the extensive diagenesis of
natural waters. As the oceanic volume approached its present value the
input of metamorphic and deep subsurface water became small relative to
the quantity of water restricted to the terrestrial weathering and continental
drainage portion of the diagenetic cycle, and the frequency and amplitude
of perturbations in the alkali metal ratios in ocean water associated with
tectonic activity decreased to a level of negligible importance. The compositional evolution of alkali metal ratios in ocean water, for at least the past
5 • 10 s years, has been controlled by mixing with continental drainage water.
The alkali metal ratios in continental waters are determined by weathering
reactions in the soil zone involving the formation of clay minerals. The silicate
buffering reactions proposed by Garrels and Mackenzie (1966) should be most
effective in the soil zone and subsurface environment as a result of the
relatively high dissolved silica concentrations and solid-solution ratios in
these environments as compared to the oceanic reservoir.
Hypotheses concerning the chemical evolution of the ocean are abundant
and supporting data are scarce. An improved knowledge of the chemical and
mineralogical variations in clays as a function of geologic age and setting and
experimental studies on the low-temperature solution geochemistry of clay
minerals are areas of particular importance for future research.
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