CHAPTER V. The Salinity and Age of the Oceans

THE SALINITY AND AGE OF THE OCEANS

Halley’s proposal in 1715—Joly’s application of modern data in 1899—Further studies by Sollas, Clarke, and Becker—Data of the problem—Corrections for disseminated sodium, wind-borne sodium, human agencies, and marine erosion—Uniformity not capable of proof—Importance of cyclic circulation of sodium—Origin of chlorine now fixed in salt—Hour-glass method applied—Land areas of the past and their elevation compared with those of to-day.

In 1715 the famous astronomer Edmund Halley published a paper which he entitled, A short Account of the Cause of the Saltness of the Ocean, and of the several Lakes that emit no rivers; with a Proposal by help thereof to discover the Age of the World. He showed that since the water removed from lakes by evaporation is perfectly fresh, “the saline particles brought in by the rivers remain behind, while the fresh evaporate; and hence it is evident that the salt in the lakes will be continually augmented and the water grow salter and salter.” Applying the same principle to the oceans, he wrote, “It is not improbable but that the ocean itself is become salt from the same cause, and we are thereby furnished with an argument for estimating the duration of all things.” Two hundred years ago it seemed hopeless to attempt to determine the annual increment of salt added to the oceans, but Halley used his argument “to refute the ancient notion some have of late entertained of the eternity of the world.” The paper was completely forgotten, until Dr. G. F. Becker again drew attention to it a year or two ago.

As we saw in the last chapter, the analysis of river waters has now made possible a determination of the annual amount of material carried in solution by the rivers into the oceans. T. Mellard Reade was the first to contemplate the application of solvent denudation to the measurement of geological time, and the data he gathered together in support of his arguments came as a great surprise to those who had concentrated attention merely on the mechanical work of erosion. No independent advance, however, was possible until 1899, when Joly pointed out that of the many elements which enter into the composition of sea water, sodium alone tends to accumulate. All the others are sooner or later rejected, associating themselves with the detrital sediments, or forming chemical or organic sediments by their ultimate precipitation. Joly then proceeded to use sodium as the age-index of the oceans. He assumed that the annual increment Naᵣ of sodium added to the oceans by all the rivers of the world has remained practically constant throughout geological time. If Naₒ represents the total amount of sodium now accumulated, the ratio Naₒ/Naᵣ gives the time which has elapsed since the oceans first existed and denudation began to wear down the lands. Joly’s first estimate was from 80 to 90 million years, and shortly afterwards he increased this to 100 million years. Sollas attacked the problemafresh in 1909, and from a most careful survey of all the data and a detailed enquiry into every phase of the subject, he concluded that the most probable estimate of the age of the oceans would appear to lie between 80 and 150 million years.

In 1910 a further study was made by Clarke and Becker. The latter departed from the uniformitarian basis on which all the other calculations had been founded. He inferred that sodium accumulation progressed more rapidly in the past than at present. All the original sodium must have been derived from igneous rocks, and Becker considers that at the time when the oceans were first possible the surface of the earth must have consisted exclusively of such rocks. At the present day three-quarters of the land areas are covered by sedimentary rocks which can supply no further important additions to the sodium content of the ocean. Assuming that the production of sodium has been always proportional to the area of igneous exposures, and that the total land area of the globe has averaged 80% of the present area, he finds that the age indicated is about 70 million years.

We may now proceed to examine the problem in detail. The fundamental data on which the method is based may be summarised as follows:

Mean density of the ocean (Murray) 1·026 Volume of the ocean (Murray) 323,800,000 cu. miles ” ” (Joly) 339,248,000 ” ” ” (Clarke) 302,000,000 ” ” ” (Karsten) 307,496,000 ”

Clarke now accepts the latter as being the best estimate, and we therefore calculate that the

Mass of the ocean = 1,178,270 × 10¹² tons. Total salinity of the ocean (Dittmar) 3·5% Sodium of the ocean (Dittmar) 1·08% Accumulated sodium, Naₒ = 12,600 × 10¹² tons. Annual increment, Naᵣ = 156,000,000 tons.

From these values the quotient Naₒ/Naᵣ gives the age of the oceans to a first approximation as 80·8 million years. This figure, however, cannot be regarded as final. Two of the assumptions on which it is based are that all the sodium liberated from igneous rocks is contained in the ocean, and that all the sodium carried annually to the ocean has been liberated from such rocks for the first time. The obvious corrections to be applied will increase the numerator of the ratio and decrease the denominator, thereby increasing the age estimate. The degree to which the numerator must be augmented is, as far as we know, relatively insignificant. Saline deposits, such as those of the Stassfurt district, are only of trivial importance. The salt of the ocean, if spread regularly over the whole land areas, would form a layer about 120 feet thick, and all the beds of rock salt which have accumulated by evaporation become negligible beside this vast quantity. The amount of sodium in ground waters and disseminated through the sedimentary rocks is to be taken into consideration more carefully. Marine sediments at the time of their formation are saturated with sea water, and, when raised up to form land, they must, therefore, be strongly charged with salt. The actual proportion of sodium abstracted from the ocean in this way cannot at one time be very great, probably not more than 1% of the whole. Throughout geological time, however, a certain amount of sodium has been in cyclic circulation between land and sea. A rough idea of the influence of this circulation on the age estimate may be gained from a knowledge of the total volume of the sedimentary rocks, and of their average pore space.

Neither of these factors is yet known with the desirable accuracy, but as far as present needs are concerned, the total bulk of the sediments, excluding deep-sea deposits, may be placed at 70 million cubic miles, and their pore space at 10%. The latter figure is probably too low, for fresh sediments have a pore space of 40% to 50% of their volume, and in consolidated sandstones 20% is common. Many of the sediments have been denuded and re-deposited time after time; on the average, probably three times. On the other hand, if the ocean has progressively increased in salinity, its average sodium content must have been about half that of the present. Basing our calculations on these considerations, the total volume of sediments which have ever existed is about 210 million cubic miles; the pore space to be filled by sea water would then be 21 million cubic miles. The total amount of sodium precipitated within the rocks by evaporation could not therefore exceed 420 million million tons, and at the present rate of denudation its removal would require nearly three million years. To what extent this result would be increased by absorption effects, whereby interstitial salt solutions are concentrated in order that they may be in equilibrium with normal sea water, cannot be estimated. This corrective factor would probably not be large.

A still more important cyclic circulation is brought about by wind-borne sea salt. Fine particles of spray are swept from the foaming crests of waves by the wind and carried often many miles inland. Near the coast the salt blown over the land is naturally most abundant. It falls in sea fret and rain, and is ultimately returned to the sea in the drainage from the land. Fortunately, the amount of sodium supplied in this way can be estimated by analyses of rainwater and a measurement of the rainfall. As before, the cyclic sodium must be excluded from that entering into the denominator as primary sodium. Joly allows an additive time correction of 10%, Clarke of 7%, and Becker of 6%. The time estimate is therefore increased by a further five or six million years.

Clarke has suggested that the present rate of sodium accumulation has been accelerated by human agencies. Nearly six million tons of sodium are annually produced as common salt, and, as sewage and chemical refuse, much of this is again returned to the oceans. Subtracting this from the denominator, the age is increased by other three million years.

A number of corrections may be made to decrease the first rough estimate of the ocean’s age. Solvent denudation due to marine erosion must not be forgotten. Experiments by Joly have shown that sea water is several times (2½-14) more active as a solvent than fresh water. He attributes to its action over the tide-swept strand—covering a total area of about 60,000 square miles—a supply of sodium amounting to 3% of that derived from the normal drainage areas. This is a very liberal estimate, and in applying a negative correction of three million years we are granting all that can reasonably be claimed.

Finally, there are other possibilities of uncertain magnitude which deserve mention. The ocean may conceivably have contained sodium before the fluviatile contributions commenced. By volcanic eruptions, sodium-bearing materials may be cast widespread over the sea. Juvenile waters expelled from igneous magmas may presumably afford a supply of sodium. The importance of saline waters derived from the heated interior of the earth has been particularly insisted upon by the great geologist Suess. As a source of sodium, such waters are probably of little moment, for the whole amount of sodium in the ocean is already more than explained by the erosion of igneous rocks. Solvent denudation involves a loss from average igneous rock of 1·57% due to sodium abstraction alone. According to Clarke, the average sodium content of igneous rocks is 2·52%, and the amount retained by the detrital material which goes to make up the sediments is 0·95% of the original rock. Now, if the total mass of sodium which has accumulated in geological time is 12·6 × 10¹⁵ tons and this represents 1·57% of the igneous rock in which it originally occurred, it is evident that the mass of the igneous rocks which have been denuded away amounts to 800 × 10¹⁵ tons. The mass of one cubic mile of rock may be taken as 10,800 million tons, and hence the volume of igneous rock which has been removed is 74 million cubic miles. The sediments derived from the latter ought therefore to occupy a volume of the same order. A number of independent estimates of the volume of sediments existing on the land areas have been attempted, and while no great accuracy can be hoped for, it is significant that all agree in assigning a volume somewhat greater than 74 million cubic miles. The sediments hidden under the oceans would add still further to the total. If any reliance at all is to be placed on these estimates, it is clear that the sodium in the ocean ought to be more than it is; at any rate, there would appear to be little room for sodium derived from other sources, such as the interior of the earth.

We may sum up the results of this discussion as follows:

Approximate age, Naₒ/Naᵣ = 81 million years. Correction for (a) Disseminated Sodium = 3 million years. (b) Wind-borne Sodium = 6 ” ” (c) Human agencies = 3 ” ” (d) Marine erosion = -3 ” ” -------------------- Age of the oceans 90 million years.

Another assumption on which this computation is founded must now be examined; that is, the practical uniformity of the annual addition of sodium throughout the period involved. Little weight can be attached to the argument that the source of supply has been gradually impoverished in alkalies, for the soils from limestones, which are among the poorest of the alkali-bearing rocks, may contain more sodium than do the soils derived from granites. The sedimentary rocks being less able to resist erosion are more rapidly disintegrated and removed than are the igneous rocks. The composition of streams considered in relation to the rocks which they drain is of interest here. Hanamann has shown that in the Elbe and its tributaries three times more sodium is carried away from a square mile of Cretaceous sediments than from an equal area of granite. In the same connection, it is well known that the waters flowing through stratified rocks carry more salt than those from igneous rocks. The greater exposure of the latter in former land areas, if such were the case, would certainly not contribute a greater sodium income to the ocean than if sedimentary rocks prevailed. Joly considers that an approximate uniformity has been preserved whatever the distribution of rock types may have been. It would not, however, be difficult to prove by actual analyses that a greater area of igneous rocks would result in a reduction of the sodium increment, and that the present rate may be too high because of the predominance of sedimentaries.

This unexpected conclusion raises the question of the validity of the method at its most critical point. Can we be sure that the cyclic circulation of sodium has been taken sufficiently into consideration? The only source to which the anomalous proportion of sodium from sedimentary rocks can be traced, apart from the factors already dealt with, is constituted by the ground waters. These waters often contain minute quantities of salt of which the origin is very obscure. Whence came the chlorine? Igneous rocks themselves only contain about 2% of the chlorine necessary to convert the liberated sodium into salt. The emission of chlorine from volcanoes and solfataras probably affords the necessary supply, but most of this never reaches the ground waters directly. Becker, dealing with the same difficulty in the case of river water, calculates that 40% of the annual sodium increment is combined with chlorine of which the origin cannot be traced. He accordingly rejects this amount as not being of normal or primary origin. If it has all been derived from the oceans, which he regards as an extreme possibility, it must be subtracted from the apparent income, since it is simply a circulation of capital.

A maximum figure for the age of the oceans (still on the assumption of uniformity) ought theoretically to be attainable by accepting the whole of the sodium-chloride of the rivers as cyclic. We have the following figures:

Chlorine = 138 million tons. Sodium combined with chlorine = 87 ” ” Sodium uncombined with chlorine = 69 ” ” ----------------- Total Sodium, Naᵣ = 156 million tons.

The unchloridised sodium implies an age of 180 million years, an estimate which would in general be condemned as excessive.

The difficulties presented by sediments and ground waters are almost insuperable, and it may be safer to adopt a means by which they can be entirely disregarded. The sodium lost from igneous rocks amounts by mass to 1·57% of the latter. Igneous and metamorphic rocks are at present exposed over one-fifth of the drainage areas, or 8,000,000 square miles. Many of the metamorphic rocks are partially or wholly of sedimentary origin, and ought not to enter into the computation. We have already seen that the average rate of denudation is one foot in 8600 years, and at this rate, one cubic mile of primary rock, or 10,800 million tons, would be denuded away in 4·54 years. In one year the supply of sodium would therefore be 37 million tons, and the time required for the accumulation of the existing store of sodium in the oceans would be 340 million years.

We meet with a curious discrepancy at this point. The actual amount of unchloridised sodium is 69 million tons, while that which is drawn from the igneous rocks cannot exceed 36 million tons. In each case sub-aerial denudation alone is being considered, so that the failure to take marine denudation into account does not affect the matter. The figures imply that a great deal of unchloridised sodium is drawn from the sedimentaries. Even if the extreme assumption is made for the moment, that the igneous rocks lose all their sodium, 2·52%, the annual supply could only reach 60 million tons. The discrepancy is evidently due to the facts that sediments freshly formed from primary rocks retain a considerable percentage of sodium, and that it is only after the material has been re-assorted, perhaps several times, that the element is withdrawn to the extent indicated by Clarke’s average figures. At the present day, even when the igneous rocks have been deprived of 37 million tons of sodium, the sedimentary rocks, to bring up the total to 69 million tons, must lose more than half of their primary sodium. It is interesting to observe that if all the sodium now carried each year to the oceans—156 million tons—were derived directly from the rocks, igneous and sedimentary, the latter would be obliged to lose nearly twice as much sodium as they actually contain! This remarkable conclusion indicates either that our statistics of the annual production of sediments are hopelessly wrong, or else that the chloridised sodium is almost wholly cyclic. The latter inference, supported as it is by the impossibility of otherwise accounting for the chlorine, thus receives further support.

In the application of the above data to the age question, it has been tacitly assumed that the present exposures of primary rocks are neither greater nor less in area than the average of such exposures since the oceans began.

It is impossible to know what the average may have been, for while the original lands must have been entirely formed of igneous or pseudo-igneous planetesimal material, it is equally certain that their area was but a fraction of that of the present land areas. The evolution of the earth’s surface features has tended to intensify the difference between the ocean depths and the mountain heights. The ocean basins have gradually deepened and become more stable, and their relative permanence is generally admitted. The lands are now more extensive than ever they have been; initially it is probable that only low and scattered islands emerged from the shallow primitive seas. We are faced here with many vague problems. The volume of the seas may have been less than it is to-day; the original rocks were probably not granitic in type, and were certainly poor in sodium; carbon-dioxide seems to have been the chief atmospheric companion of nitrogen, and solvent denudation would be correspondingly more vigorous. How far all these considerations affect the point at issue cannot be decided. To discuss them would lead us into a complex maze of speculation. The original basis of our calculation may afford as near an approach to the truth as could be arrived at by any other reasonable hypothesis of changing land areas, and of their composition.

On this view it is possible to calculate a minimum figure for the age of the oceans, but it must be remembered that the results based on an assumption, which is merely a uniformitarian guess, are themselves equally uncertain. If the igneous rocks had lost all their sodium, we have already found that the annual supply would be 60 million tons, and the sediments would be destitute of that element. The age would then be at least 210 million years. The contradiction between this minimum figure and the maximum deduced from the total unchloridised sodium, viz. 180 million years, is explicable on one or both of two further possibilities, that the average exposure of igneous rocks in the past has been greater than that of the present day, or that the amount of primary sodium now being derived from the sedimentary rocks is considerably above the average for the past. Our interpretation of the data is made particularly difficult because of the widespread covering of glacial detritus, rich in felspathic constituents and easily eroded.

The contradictory results become still worse when it is remembered that the rate of denudation—one foot in 8600 years—is probably too high to be safely applied to areas of igneous rocks. Analysis of river waters draining such areas indicate a rate which is only about a half of the above. But even this correction is surpassed in importance by one arising out of the possibility that the present standard of measurement may be misleading in not being typical of geological time. The mean height of the drainage areas is subject to much fluctuation. By denudation it is steadily reduced; by earth movements, and particularly during the periods of intense diastrophism, it may become unusually great. It seems not unlikely that we are now near a period of extreme continental elevation, and that the geological processes which are thereby quickened up, cannot be accepted as affording a true standard. This suggestion will be dealt with further in the next chapter.

The high figures—210-340 million years—given above, must not be supposed to possess any serious value. The whole discussion merely serves to betray the uncertainty of the method and the doubtful applicability of even the most accurate data. For the present we can only conclude that our knowledge of the part played by sodium and chlorine in the constant redistribution of the materials of the earth’s crust is still lamentably imperfect, and that quantitative deductions drawn from it must be regarded as being purely provisional.