ADVERTISEMENT
latest stories:

The Age of the Ocean

As Determined by uts Salt Contents

THE fact that we can estimate, with some approach to exactness, the absolute amount ofsodium in the sea, and that it is added in a presumably, constant manner without serious losses, have led to various attempts toward using its quantity in geological statistics. The sodium of the ocean seems to furnish a quantitative datum from which we can reason, whereas calcium, magnesium, silica, potassium, etc., are more or less deposited from solution, and so become unavailable for the discussion of such problems as that of geologic time. Nearly 200 years ago Edmund Halley1 suggested that the age of the earth might be ascertained by measuring the rate at which rivers delivered salt to the sea. The suggestion was of course fruitless for the time being, because the data needed for such a computation were undetermined, but it was nevertheless pertinent, and it now seems to be approaching realization. For reason.s already given, the method proposed for estimating geologic time can as yet be only applied provisionally, the data still being imperfect, although rapidly accumulating. The present state of the problem is worth considering now. The first really serious attempt to measure geologic time by the annual additions of sodium to the ocean seems to have been made by J. Joly2 in 1899. Joly, with Murray's figures for rainfall, run-off, and the average composition of river water, combined with Dittmar's analyses of oceanic salts and an estimate of the mass of the ocean, deduced an uncorrected value for the age of the ocean of 97,600,000 years. The calculation is very simple, and by the following equation: Nam ocean :r->r — = Age of ocean. Annual Na m rivers Joly's data, however, were much less satisfactory than which, substituted in our equation, give the data now at hand. 14,130 X1012 = 89,222,900; 158,357 X 103,, - the crude age of the ocean to which certain corrections are yet to be applied.3 The first of these to be studied tends to increase the quotient, others to diminish it. A part of the sodium found in the discharge of rivers is the so-called “cyclic sodium”; that is, sodium in the form of salt lifted from the sea as spray and blown inland to return again to its source in the drainage- from the land.' Near the coast this cyclic salt is abundant; inland itsjquantity is small. Joly estimates the correction for cyclic salt at 10 p. c.; but Becker in his paper on the age of the earth has discussed the isochlor evidence mathematically, and found that 6 per cent is a more trustworthy value. By Ackroyd the significance of the correction is enormously overestimated. Adopting Becker's figure, and deducting 6 per cent from the total river load of sodium, the remainder becomes 148,846,000 metric tons, which, divided into the sodium of the ocean gives a quotient of 94,712,000 years. Joly's correction of 10 per cent is very nearly equivalent to the assumption that the entire run-off of the globe, 6,524 cubic miles, according to Murray, carries on an average one part per million of chlorine. The chlorine maps, so far as they have been made, show this figure to be excessive. The foregoing correction for “cyclic salt” is, however, not final. It has already been suggested that the wind- borne salt is only in part restored to the ocean, at least within reasonable time. Some of it is retained by the soil, if not permanently, at least rather tenaciously; and the portion which falls into depressions of the land may remain undisturbed almost indefinitely'. In arid regions, like the coasts of Peru, Arabia, and parts of western Africa, a large quantity of cyclic salt must be so retained in hollows or valleys which do not drain into the sea. Torrential rains, which occur at rare intervals, may return a part of it to the ocean, but not all. Some writers, like Ackroyd, for example, have attributed the saline matter of,the Dead Sea to an accumulation of wind- borne salt, an assumption which contains elements of trflth, but is probably extreme. A more definite instance of the sort is furnished by the Sambhar salt lake in northern' India, as studied by T. H. Holland and W. A. K. Christie.” This lake, situated in an inclosed drainage basin of 2,200 square miles and over 400 miles inland, appears to receive the greater part, if not all of its salt from dust-laden winds which, during the four hot, dry months, sweep over the plains between it and the arm of the sea known as the Rannof Cutch. Analyses of the air during the dry season showed a quantity of salt so carried which amounted to at least 3,000 metric tons over the Sambhar Lake annually, and 130,000 tons into Rajputana. These quantities are sufficient to account for the accumulated salt of the lake, which the authors were unable to explain in any other way. Examples like this of the Sambhar Lake are, of course, exceptional. In a rainy region salt dust is quickly dissolved and carried away in the drainage. Only in a dry period can it be transported as dust from its original point of deposition to points much farther inland. It appears, however, that some salt is so withdrawn, at least for an indefinitely long time, from the normal circulation, and should, if it could be estimated, be a$ded to the amount now in the ocean. Such a correction, however, would doubtless be quite trivial, and, therefore, negligible; and the same remark must apply to all the visible accumulations of rock salt, like those of the Stass- furt region, which were once laid down by the evaporation of sea water.The saline matter of the ocean, if concentrated, would represent a volume of over 4,800,000 cubic miles; a quantity compared with which all beds of rock salt become insignificant. But although the visible accumulations of salt are relatively insignificant, it is possible that there may be quantities of disseminated salt which are not so. The sedimentary rocks of marine origin must contain, in the aggregate, vast amounts' of saline matter, widely distributed, but rarely determined by analysis. These sediments, laid down from the sea, can not have been completely freed from adherent salts, which, insignificant in a single ton of rock, must be quite appreciable when cubic miles are considered. The fact that their presence is not shown in ordinary analyses merely means that they were not sought for. Published analyses, whether of rocks or of waters, are rarely complete, especially with regard to those substances which may. be said to, occur in “traces.” It is perhaps not possible to evaluate the quantity of this disseminated salt, and yet a maximum limit may be assigned to it. It has been shown that 84,300,000 cubic miles6 of the average igneous rock would yield, upon decomposition, all the sodium of the ocean and the sedi- mentaries. The volume of the sandstones would be approximately 15 per cent of this quantity, or 12,645,000 cubic miles. Assume now that the sandstones, the most porous of rocks, contain an average pore space of 20 per cent, or 2,529,000 cubic miles, and that all of it was once filled with sea water, representing 118,730,000,000,000 metric tons of sodium. If all of that sodium were now present in the sandstones, and chemical erosion began at the rate assigned to the rivers, namely, 158,357,000 tons of sodium annually, the entire accumulation would be removed in about 750,000 years. This, compared with the crude estimate already reached for _ geologic time is almost a negligible quantity. The correction for disseminated salt is therefore small, and not likely to exceed 1 per cent. The foregoing calculations, so far as they relate to the age of the ocean, imply the assumption that the rivers have added sodium to the sea at an average uniform rate, slight accelerations being offset by small temporary retardations. For the moment let us consider one phase of this suggested variability. The present rate of discharge has been hastened during modern times by human agency, and that acceleration may be important to take into account. The sewage of cities, the refuse of chemical manufactures, etc., is poured into the ocean, and so disturbs the rate of accumulation of sodium quite perceptibly. The change due to chemical industries, so far as it is measurable, is wholly modern, and that due to human excretions is limited to the time since man first appeared upon the earth. Its exact magnitude can not be determined', but its order seems to be measurable, as follows: According to the best estimates, about 14,500,000 metric tons of common salt are annually produced, equivalent to 5,700,000 tons of sodium. If all of that was annually returned to the ocean, it would amount to a correction of about 3.25 per cent on the total addition of sodium to the sea. The fact that much of it came directly or indirectly from the ocean in the first place is immaterial to the present discussion; the rate of discharge is affected. All of this sodium, however, is not returned; much of it is permanently fixed in manufactured articles. The total may be larger, because of other additions, excretory in great part, which can not be estimated, but we may assume, nevertheless, a maximum of 3 per cent as the correction to be applied. Allowing 6 per cent, as already determined, to cyclic or wind-borne sodium, and 1 per cent to disseminated salt of marine origin, the total correction is 10 per cent. This reduces the 158,357,000 tons of river sodium to 142,521,000 tons, and the quotient representing crude geologic time becomes 99,143,000 years. The corrections so far considered are all in one direction, and increase, by a roughly evaluated amount, the apparent age of the ocean. Other corrections, whos,e magnitudes are more uncertain, tend to compensate the former group. The ocean may have. contained primitive sodium, over and above that since contributed by rivers. It receives some sodium from the decomposition of rocks by marine erosion, which is estimated by Joly as a correction ofless than 6 per cent and more than 3 per cent on the value assigned to geologic time. Sodium is also derived from volcanic ejectamenta, from “juvenile” waters, and possibly from submarine rivers and springs. The last possibility has been considered by Sollas,7 but no numerical correction can be devised for it. These four sources of sodium in the sea may be grouped together as non-fluviatile, and reduce the numerator of the traction which gives the age of the ocean. Whether they exceed, balance, or only in part compensate the other corrections it is impossible to say. From the foregoing computations it is to be inferred that the age of the ocean, since the earth assumed its present form, is somewhat less than 100,000,000 years. If, however, any serious change of rate in the supply of sodium to the sea has taken place during geologic time, the estimate must be correspondingly altered. This side of the question has been studied by G. F. Becker in the memoir already cited, who has shown that the rate was probably greater in early times than now, and has steadily tended to diminish. When erosion began, the waters had fresh rocks to work upon. Now, three- fourths.of the land area of the globe are covered by sedimentary rocks or by detrital and alluvial material, from which a large part of the sodium has been leached. The accessible supply of sodium has decreased, and it may be supposed that at some remote time in the future it will be altogether exhausted. From considerations of this order Becker has developed an equation representing the supply of sodium to the ocean during past time by a descending exponential, and has shown that the age.of the ocean, as deduced from the data already given, must lie somewhere between 50 and 70 millions of years. The higher figure, he thinks, is closer to the truth than the lower one. If the ocean was initially saline the estimate of its age would be still further reduced. Becker's conclusions are fairly accordant with the results derived from physical, astronomical and paleontological evidence, although the study of radioactivity among minerals has led to much higher figures for the age of the earth. It seems, however, that the rate of chemical erosion offers a more tangible and definite mode of attack upon the' problem of geologic time.. The problem can not be regarded as definitely solved, however, until all available methods of estimation shall have converged to one common conclusion.

Scientific American Holiday Sale

Give a Gift &
Get a Gift - Free!

Give a 1 year subscription as low as $14.99

Subscribe Now! >

X

Email this Article

X