1902 Encyclopedia > Sea Water

Sea Water

SEA WATER. The ocean covers very nearly eight-elevenths of the total area of the globe ; its average depth may be estimated as 2000 fathoms, and its total mass at 1.322 x ____ (i.e., 1.3 million million millions) tons. Its general configuration must be assumed to have been sub-stantially the same as it is now for thousands of years; hence we may safely conclude that the absolute composition of the ocean as a whole is constant in the sense of being only subject to very slow progressive millennial variation, and that, taking one part of the ocean with another, the percentage composition of the fixed part of the solutum can oscillate only within narrow limits. The composition of this solutum is very complex. According to Forchhammer, ocean salt in addition to the chlorides and sulphates of sodium, magnesium, potassium, and calcium—which had long been known to be its principal components—includes silica, boric acid, bromine, iodine, fluorine as acid, and the oxides of nickel, cobalt, manganese, aluminium, zinc, silver, lead, copper, barium, and strontium as basic components. Arsenic, gold, lithium, rubidium, caesium have been discovered since Forchhammer wrote. But all these subsidiary components, as that investigator found, amount to very little,—so little that in his numerous quantitative analyses of waters which he had procured from all quarters of the globe he confined himself to the determination of the chlorine, sulphuric acid, magnesia, lime, potash, and soda. The soda, however, he determined only by difference, assuming that the muriatic and sul-phuric acids are united with the bases into perfectly neutral salts. As a general result he found that, in the open ocean, the ratio to one another of the several acids and bases named is subject to only slight variations. But his samples had all been collected at the surface; the potash had been determined by an insufficiently exact method; and the assumed neutrality of the total salt had not been proved. With the view primarily of supplementing Forchhammer's work, Dittmar made complete analyses of 77 of the samples brought home by the " Challenger," so selected that 34 out of the 77 represented depths of 1000 fathoms or more. His analyses brought out a small surplus of base, prov-ing the presence of carbonate in all the waters; but the numerical values thus found for the "alkalinity," being charged with the observational errors of the whole series of determinations, could not be relied on. Dittmar therefore subsequently availed himself of a very easy and yet exact method for the direct determination of this quantity, which meanwhile had been discovered by Tornoe, and ap-plied it to over 130 "Challenger" samples. He besides made a special inquiry into the relation between the quantity of lime and the depth at which the water had been collected, and a similar inquiry in regard to the bromine. As a general summary he gives the following three tables. The total salts contained in ocean water amount on an average to about 3-5 per cent., thus leaving 96 '5 per cent, for the water proper.

Reducing to the absolute mass of the ocean as given above, we arrive at the following numbers :—

== TABLE ==

(1873); (3) Den Norske Nordhavs Expedition, 1876-78: Chemi, by Tomoe); (4) the Jahresberichte of the Kiel committee for the scien-tific investigation of the German Ocean, 1873-82 ; (5) Physics and Chemistry of the Voyage of H.M.S. "Challenger"—I. "Report on Researches into the Composition of Ocean Water," &c, by Prof. W. Dittmar, January 1884; II. "Report on the Specific Gravity of samples of Ocean Water," &c, by J. Y. Buchanan, January 1884 ; III. "Report on Deep-sea Temperature," &c, by the officers of the expedition. A shorter and more popular exposition of the whole is found in—(6) Narrative of the Cruise of H. M. S. " Challenger " (1885). The excellent Handbuch der Oceanographie (Stuttgart), by Prof. G. vou Boguslawski, may be referred to as being almost up to date.

Total chloride of rubidium 25'0 (C. Schmidt).

Of the several quantities recorded in columns 2 or 3 of Table I. "carbonic acid" is proved to be subject to variation; all the rest, including even the bromine, are practically constant. This shows that Forchhammer's proposition holds for ocean water from all depths, with one important qualification: special research on the lime showed that its quantity increases slightly but appreciably wdth the depth. Taking s, m, d as representing the lime per 100 of chlorine in shallow, medium-depth, and deep-sea water respectively, Dittmar found as mean results of analyses which agreed very well together—-
s = 3-0175 m=3-0300 (2=3-0308
Probable error, ±0-0012 ±0-0014 ±0-0011.

But m-s = 0'0124 and c?-s = 0-0132. One explanation of this result is that the crustaceans, foraminifera, and molluscs which form carbonate of lime shells live chiefly in surface waters, but after their death sink to the bottom, where—especially in great depths—their carbonate of lime is partially redissolved.

2 Equal conjointly to 55 '376 parts of chlorine, which accordingly is the percentage of "halogen reckoned as chlorine" in the real total solids.

s Calculating the surplus base as normal carbonate. In Table II. this carbonate is represented as so much CaOC02.

Oceanic Carbonic Acid.—It is well known that not only in the neighbourhood of actual volcanoes but in thousands of other places on the dry land carbonic acid gas is constantly streaming forth into the atmosphere, and it is generally admitted now that this supply of telluric carbonic acid amounts to more than all that is furnished by processes of combustion and respiration. That carbonic acid springs should be absent from the bottom of the ocean is too absurd an assumption to be entertained ; hence, supposing even the water of the ocean were perfectly neutral, it could not but contain dis-solved carbonic acid. But such carbonic acid, at the ocean surface at least, would constantly tend to assume, and in general probably actually would come down to, the small limit value prescribed to it by the given proportion by volume of the carbonic acid in the atmosphere and the laws of gas - absorption. This proportion, ac-cording to the best modern researches, is almost constant, every-where amounting to very nearly 0'0003 volume per unit volume of air. The coefficient of absorption by even pure water is 1-8 at 0° and l'O at 15° C. Hence, even in the polar regions, the surface water could not hold in permanent solution more than about 0'54 c.e., or say one milligramme per litre of water. Jacobsen, in his numerous analyses of North Sea water, found from 90 to 100 milli-grammes per litre ; hut he also observed that only a small portion of the carbonic acid is eliminated on boiling: the rest comes out only when the water is distilled to dryness. He presumed that the gas was retained chemically by the chloride of magnesium. Buchanan, who inquired into the subject synthetically, arrived at the conclusion that it was the sulphates in sea water (qua sulphates) which retained the carbonic acid. Accordingly in his numerous carbonic acid determinations he liberated the gas by distilling the water down with an excess of chloride of barium. Tornoe was the first to prove that the carbonic acid in sea water is present as carbonate, and that, in the northern part of the North Atlantic at least, the total carbonic acid, while considerably greater than the quantity which would convert the surplus base into normal, falls short of that which would be required to produce fully saturated acid carbonate.

Even without Tornbe's discovery it would have been necessary to find the true interpretation of the results of the numerous carbonic acid determinations made during the voyage of the "Challenger" by Buchanan. Dittmar had no difficulty in proving the non-existence of the alleged affinity of sulphates for car-bonic acid, and naturally concluded that the chloride of barium used in the processes liberates the loose part of the carbonic acid by converting the normal carbonate part into a precipitate of carbonate of baryta, thus—R"C03 + *C02 + BaCl2=R"C12 + BaC03 + sC02. A series of synthetical experiments showed that this is substantially, though not exactly, correct. If Buchanan's modus operandi be rigorously followed, the carbonic acid obtained, as a rule, falls somewhat short of the actual amount of loose carbonic acid present, while on resuming the distillation after addition of fresh water an appreciable part of fixed carbonic acid passes away as gas. Yet, Buchanan's results being of great value, Dittmar discussed them (conjointly with his own alkalinity determinations) on the basis of the assumption that they afforded a fair approximation to the proportions of loose carbonic acid in the respective waters. His gen-eral conclusions are as follows. Taking "alkalinity " as meaning the "weight" of the carbonic acid, C02, in the normal carbonate part of the carbonate present per 100 parts of total solids, the alka-linity in the water samples analysed (omitting a few obviously abnormal cases) was found to be as follows (Table IV.):—

== TABLE ==

A solution of a bicarbonate when shaken, say in a bottle, with pure air (free of carbonic acid) at summer heat gives up its com-bined carbonic acid to the air space in the bottle until the partial tension of the acid gas there has come up to a limit value p, which is called the dissociation tension of the bicarbonate at the prevail-ing temperature t. General experience concerning such phenomena warrants the presumption that, up to a certain (low) temperature ttn P=0> an(l thence onwards, p increases with t. It does not follow that the bicarbonate in a solution when shaken again and again with even pure air tends to become normal carbonate ; for aught we know, the elimination of carbonic acid may stop as soon as the residual carbonate has come down to some composition

Values above 0'16 are obviously exceptional; hence the normal range may be said to be from 014 to 0'16. The most frequently occurring values were found to be about 0-146 in the case of surface or shallow sea water, and in the case of bottom water about 0'152. In regard to the loose carbonic acid a full discussion of Buchanan's results led to the following conclusions:—(1) carbonic acid rarely occurs in the free state ; as a rule it falls short of the quantity which would produce bicarbonate ; (2) in surface waters it is relatively high where the natural temperature is relatively low, and vice versa; (3) within equal ranges of temperature it seems to be less in the surface water of the Pacific than it is in that of the Atlantic Ocean. Of the 195 samples of sea water which Buchanan analysed for carbonic acid only 22 contained fully saturated bicarbonate, and only 2 out of these are proved by the analyses to have contained free carbonic acid in addition to bicarbonate. In all the remaining 173 samples the "carbonic acid deficit" (meaning the proportion of carbonic acid which was wanted to completely transform the carbonate into bicarbonate) assumed tangible and often considerable values. We are probably safe in concluding that the ocean as a whole will have to continue taking in carbonic acid for thousands of years before its carbonic acid deficit has been reduced to nothing. But it is as well to observe that at its surface in the warmer lati-tudes the attainment of this condition is a physical impossibility as long as the percentage of carbonic acid in the air retains its present low value.

R"0(1 +a;)C02 (where x is less than 1), and x may be a function of temperature. Dittmar has attempted to determine the course of the function l+x=f(t) in reference to natural sea water on the one hand and to pure air (air freed of its carbonic acid) and ordinary air on the other. One sample of sea water containing its surplus base as practically bicarbonate served for all the experiments. It was shaken again and again at a fixed temperature t with one or the other kind of air, until (after "N" shakings, always with renewed air) the stage of saturation appeared to have become con-stant. The investigation is not completed yet; the following table (V.) gives the results which have come out so far. The final carbonate was R20.MC02.

== TABLE ==

Hence we see that even at the highest temperature, and with air free from carbonic acid, the carbonate never came down below the state of sesquicarbonate, while with ordinary air, even at 32° G, it never fell below m=l'8. At 2° n0 as well as n^ was =2, the value characteristic of bicarbonate. Now Buchanan reports a good number of cases where, even at lower temperatures, n was con-siderably less than 1'8 at any rate. Hence, if his numbers are correct, unless the atmosphere acts more powerfully than the air in Dittmar's bottle, it would appear that deep-sea water is in general below even the stage of carbonic acid saturation which it could attain at the surface at high temperatures.

In any mixed solution of salts every base is combined with every acid; hence the "carbonate" of sea water is strictly speaking a complex plural. But as a matter of probability the carbonic acid has very little chance of uniting with any of the potash or soda, and the overwhelmingly large quantity of alkaline chloride would no doubt convert any carbonate of magnesia that was introduced into double chloride of magnesium and alkali metal; hence it is fair to assume that oceanic carbonate is chiefly carbonate of lime. Now immense quantities of this compound are being constantly introduced into the ocean by rivers. Dumas once gave it as his opinion that this imported carbonate remains dissolved in the ocean as long as and wherever the carbonate there is at the bicarbonate stage ; but, as soon as part of the loose carbonic acid goes off into the air, the corresponding weight of normal carbonate separates out as an addition, ultimately, to the solids on the bottom. Dittmar has tried to test this notion synthetically, but without arriving at very definite results. According to his experiments sea water which contains free carbonic acid dissolves added solid carbonate of lime, and more largely carbonate of magnesia ; sea water which contains fully, or almost fully, saturated bicarbonate dissolves car-bonate of magnesia very appreciably, but would not appear to act on carbonate of lime at all. But, when carbonate of lime was produced in the water by successive additions of potential calcium carbonate in the form of dissolved sodium carbonate and its equivalent of calcium chloride, the original carbonate of lime could be increased very largely, with formation of solutions which remained clear during a long-continued period of observation. As a set-off against this a few of the many hundred samples of sea water which he received from the "Challenger " deposited in the course of a number of years crystalline crusts of carbonate of lime on the sides of the bottles; and the mother - liquor never contained more than the normal quantity of lime per 100 parts of chlorine. In discussing this question Dittmar gives an estimate, based on data furnished by Boguslawski's work, of the total carbonate of lime introduced into the ocean annually by the thirteen principal rivers ; and by doubling the quantity he estimates the carbonate of lime intro-duced by all rivers as equal to about 1'34 x 109 tons. Now the sum total of carbonate of lime, CaC03, in the ocean amounts to about 160 xlO12 tons; hence it would take 1190 years to increase the present stock of carbonate of lime in the ocean by one per cent, of its value.

Absorbed Oxygen and Nitrogen in Ocean Water.—As a matter of physical necessity these two gases must be present in the water of the ocean—and they may be presumed in general to pervade it to its greatest depth—because the whole of the surface of the sea is in constant contact with the atmosphere. Our knowledge regarding their distribution in the ocean may be said to date from 1872, when Jacobsen inquired into the matter in a most masterly manner in connexion with the German North Sea expedition. The work of his predecessors possesses no scientific value, because they employed inadequate methods. Unlike them, Jacobsen did not attempt to analyse a sample of sea water air on board ship: he extracted the air from measured samples (by an excellent method of his own) and then sealed them up in glass tubes, to measure and analyse them after his return home. Buchanan, during the "Challenger" cruise adopted Jacobsen's method. Of the 164 samples which he sealed up successfully 69 came from the surface and 95 from depths varying from 5 to 4575 fathoms. A good number of these he analysed himself after his return ; the majority, however, were analysed and all were measured by Dittmar. The latter, in order to be able to interpret the results, also investigated the absorption of oxygen and nitrogen gas from air by sea water. The following table (VI.) gives the result of his investigations. One litre (1000 volumes) of ocean water when saturated with con-stantly renewed air at t°, and a pressure of 760 millimetres plus tension of steam at t° C., takes up the following volumes, measured dry at 0° C. and 760 millimetres pressure, of the pure gases.

== TABLE ==

The method used for obtaining these numbers adapted itself slosely to the one which Buchanan had employed for extracting the gas samples. In the calculations it was assumed that atmo-spheric air contains 21'0 volumes of oxygen for 79'0 volumes of nitrogen, the slight variation in this ratio, which is known to occasionally present itself, being neglected. From the table we ean calculate approximately the limits between which the propor-tions of dissolved oxygen and nitrogen in the water of the ocean must be presumed to oscillate in nature. The pressure of the atmosphere at the sea-level, though by no means constant, is never far removed from that of 760 mm. of mercury. The temperature of the surface water (with rare exceptions) may be said to vary from — 2° C. (in the liquid part of the ocean in the arctic and antarctic regions) to about 30° C. (in the tropics). The ocean receives all its dissolved oxygen and nitrogen from the surface; neither gas comes in from below, except perhaps a relatively insignificant quantity of nitrogen derived from the decay of dead organisms, which may safely be neglected. Hence the ocean can contain nowhere more than 15'6 c.c. of nitrogen or more than 8-18 c.c. of oxygen per litre, and the nitrogen will never fall below 8'55 c.c. We cannot make a similar assertion in regard to the oxygen, because its theoretical minimum of 4'30 c.c. per litre is liable to further diminution by processes of life and putrefaction and by oxidation generally.

At any point in the surface of the ocean the water constantly tends to assume the composition demanded for the prevailing temperature by the laws of gas absorption. But it is rarely possible for it to assume this composition, owing to the water being in a continual state of motion ; and, supposing a certain area of the ocean surface were in a state of stagnation, the temperature would vary in diurnal cycles, and even the calculated volume of nitrogen per litre would be a periodic function of time, exhibiting its maxi-mum at the hour of minimum temperature, and vice versa. The process of absorptiometry exchange, however, even at the constantly oscillating surface of the ocean, is slow ; it could not keep pace with the change of temperature, and the actual nitrogen curve would never go as high up or as low down as the theoretical one. In addition to this, the lower strata of the water constantly add to, or take away from, the surface nitrogen by diffusion and occasional intermixture. All this holds for the oxygen likewise, except that it is liable to constant diminution by oxidation. On the whole we may assume that all the disturbing influences will only modify, not efface, the course of events as prescribed by the laws of gas-absorption.

In regard to non-surface water we have to confront a greater complexity of phenomena. The gas-contents of deep-sea water, of course, have nothing to do with the low temperature and the high pressure which in general prevail there. For the purpose of a preliminary survey, let us imagine a deep-sea water formed from one kind of surface water, which took up its air at a constant temperature (t), and then sank down unmixed with other waters. The volumes of the oxygen and nitrogen per litre have at first the values assigned to them by the laws of gas absorption. But, while the nitrogen (as long as the water remains unmixed with other water) remains constant, the oxygen will become less and less through the processes of oxidation which go on in the deep with-out compensation. Hence if there were absolute stagnation in the ocean anywhere the proportion of oxygen there might be reduced ultimately to nothing. Among the many "Challenger" deep-sea specimens which were analysed for their gas-contents none was found quite free from absorbed oxygen ; and this confirms the conclusion that absolute stagnation exists nowhere in the ocean, not even at its greatest depth. Occasionally, however, the oxygen was found to have sunk down to very little, as shown by the following two examples :—

== TABLE ==

There must have been an approximation to absolute rest at these two places at any rate. On the whole, the results of the gas analy-sis, as interpreted on the basis of Dittmar's absorptiometric deter-minations, agreed fairly well with the inferences which we have just been deducing from physical laws. There was no lack oi anomalous results, but it was not found possible to trace them to natural causes. The equilibrium in regard to the absorbed nitrogen and oxygen in the ocean is maintained by the atmosphere ; and, from the fact that the air contained in surface water is always richer in oxygen than is atmospheric air, one naturally concludes that the ocean should constantly add to the percentage of oxygen in the air in the tropics and constantly diminish it in the colder latitudes. But Regnault's numerous air-analyses do not confirm this. Nor need this be wondered at, since, as we have seen, even the corresponding influence on the atmospheric carbonic acid has so far defied the powers of chemical analysis.

Salinity of Ocean Water.—Even in the open ocean the " salinity " —meaning in a given quantity the ratio between the weight of dissolved salt and the weight or volume of the whole—is subject to considerable variation ; and it obviously is one of the foremost duties of observing oceanographers to collect the data by means of which it may be possible one day to represent that quantity mathe-matically as a function of geographic position, depth, and time. For the quantitative determination of the salinity an obvious, easy, and sufficient method is to determine the specific gravity S at a convenient temperature t ; this in fact is the method which has so far been employed by all observers almost to the exclusion of every other. Buchanan used it during the "Challenger" cruise perhaps more extensively than any of his predecessors had done. Of the arithmetical relation between salinity on the one hand and S and t on the other the successive researches of Ekman (as supplemented by Tornbe), Thorpe and Riieker, Dittmar, and others have given us a practically sufficient knowledge. According to Dittmar the function (within the limits of Buchanan's values) coincides practically with the formula

4S,-4W,=xO + M + c<! ), where 4Se means the specific gravity at f C. referred to that of pure water of + 4° C. as equal to 1000 ; 4W( has a similar meaning in reference to pure water ; x stands for the weight of total halogen calculated as chlorine per 1000 parts, by weight, of sea water ; and a = l-45993, &=-04)05592, c= + 0'0000649. For océanographie purposes, however, it is not necessary to go back to x J it suffices from series of values 4St to deduce the corresponding values 4S(() for a convenient standard temperature, and to reason on these reduced numbers as if they measured the salinity, just as we take the readings of a thermometer as in themselves representing " temperatures." This, in fact, is always done ; only unfortunately different standard temperatures have been chosen by different observers; Buchanan adopted 15°-56 C. =60° Fahr. Before going further, let us observe that the specific gravity of sea water, taking it as it is in situ, has an important océanographie signi-ficance, even as such. But this quantity in the case of deep-sea waters is influenced very largely by the pressure of the super-incumbent layer of water—which in itself is a complex function of the successive temperatures and salinities—and unfortunately we still lack the constants and formula? for making the necessary reductions with adequate exactitude. Meanwhile all our statistics of sea water specific gravities, valuable as they are, constitute statistics of only salinities and nothing else.

3 According to Grassi's experiments, if sea water under the pressure of one atmosphere has the specific gravity 1026, it assumes at depths = 1000, 2000, 3000 fathoms a density of 1026 +1, 2, 3 times 7'9 units=1033-9, 1041-8, 1049-7 respectively.

At the surface of the ocean the salinity is liable chiefly to three influences,—(1) concentration by formation of ice or by the action of dry winds ; (2) dilution through the melting of ice or the falling of rain ; (3) concentration or dilution through the virtual addition of salt or water by inflowing currents of Salter or fresher water respectively. The effect of the formation or melting of ice, though great within the arctic circles, does not tell much on the non-polar seas. More important in regard to these is the effect of the south-east and the north-east trade winds, which in the Pacific blow between about 3° and 21° S. lat. and between about 2° and 20° N. lat. re-spectively, leaving between the two a belt of 5° of a region of calms (see more exactly, METEOROLOGY, vol. xvi. p. 144). In the Atlantic the limiting lines of both trades oscillate annually, so that the equatorial boundary of the north-east trade shifts from 3° to 11° N. lat, and that of the south-east trade from about 1° to 3° N. lat.

Both trades blowing from colder into warmer regions absorb water largely and thus raise the salinity within their areas of action. The western anti-trades which blow on the polar sides of the two trades, passing from hotter to colder regions, should dilute the ocean there ; but they do not seem to act so powerfully in this direction as might be expected. In the belt of equatorial calms between the two trades abundant rains fall frequently and dilute the water very perceptibly.

Curves showing variation of surface salinity of ocean with latitude, companying diagram shows how on the average the surface salinity varies there with the latitude. The bolder curve is drawn after a table given by Buchanan in his part of the Narrative of the Cruise of the "Challenger," the other after a more extensive table given by Boguslawski as embodying the mean results of many observations by different authorities with reference to standard temperatures varying from 15° to 17°-5 C,—coast waters affected by the influx of large rivers having been omitted.1 In the North Atlantic there is an area of maximum (surface) salinity (S = 1028'5) between 25° and 35° N. lat. and 30° and 20° W. long. The zone of minimum salinity lies between 15° N. lat. and the equator. In the South Atlantic (surface) there are two concentration centres,—an eastern about St Helena and between that island and Ascension, and a western north of San Trinidad,—both nearer the equator than that of the North Atlantic. As pointed out by Buchanan, a relatively high salinity (not merely on the surface) is quite a characteristic feature of the Atlantic, and in its northern part prevails up to the high latitudes of the Norwegian Sea, which was so thoroughly in-vestigated by Swensden (1876) and Tornoe (1877 and 1878) during the Norwegian expeditions. The salt (and heat) conveying influ-ence of the Gulf Stream makes itself felt up to Spitzbergen (76° N. lat.). On both sides of the Faroe Islands the specific gravity

"What has been said thus far about the distribution of surface salinity applies chiefly to the Atlantic, which in fact is far more completely known in this respect than any other ocean. The ac-

j75^i7-5 comes up to 1027'0 ; at the Bear Islands it sinks to 10267, and thence farther northwards to 1026-1. While the Gulf Stream pushes northwards, a current of relatively fresh polar water travels southwards and, creeping along the eastern coast of the United States, forms what is known as the "cold wall." In passing from the surface to the depth of the ocean the general rule (Buchanan) is that the actual specific gravity in situ increases with the depth ; but this does not hold for the salinity (or specific gravity reduced to standard temperature). In places where there is active dilution at the surface (e.g., in the belt of equatorial calms) the salinity as a rule increases down to some 50 or 100 fathoms ; but thence down-wards it follows the general rule, that is, it decreases down to 800 or 1000 fathoms, and thence increases steadily to the bottom. In the South Atlantic the salinity of the bottom water has an almost constant value (4Si5.5 = 10257 to 1025'9); but northwards it in-creases to from 102616 to 1026-32 at 2000 to 4000 fathoms (Buchanan).

In regard to the Pacific our knowledge is far less complete. A glance at the curve shows that the (surface) salinity at a given latitude is less there than it is in the Atlantic. In the whole of
the Pacific there is only one concentration centre, which lies about the Society Islands, with a maximum salinity corresponding to 4S155= 1027-19. (W. D.)

The above article was written by: W. Dittmar, F.R.S., Professor of Chemistry, Anderson's College, Glasgow.

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