CHEMICAL ASPECTS OF PHYSICAL OCEANOGRAPHY' JOHN LYMAN and ROBERT B. ABEL United States Navy Hydrographic Office, Washington, D. C.
IT
IS generally accepted today that all the water now in the ocean was not precipitated as soon as the primeval earth cooled sufficiently (over two billion years ago) ( I ) , but that the bulk of it has gradually accumulated since that time from the vapors released by volcanoes and fumaroles (2,3). The total water in the ocean now comprises some 1.37 X 1018cubic meters and accounts for upwards of 73% of the water now present in the earth's crust. A rough inventory of the known water can be made as in able 1 (4). Sea water now contains about 3.5% dissolved salts. Some of these must have existed in the nrimeval atmosphere or have been produced by vulknism, like the HCl, COa, HzS or SO?, and H3BOawhich are still found in volcanic vapors. These acidic volatile substances, as soon as they dissolved in water, reacted with the various metallic oxides and carbonates in the lavas underlying the ocean basins. When soluble halides and sulfides resulted, these also came into solution along with bicarbonates.
TABLE 1 Inventory of Water in and over the Earth's Crust Location
Metric I n s ( X 10")
Biosphere Atmosphere Soil moisture Lakes and rivers Ground water Ice caps and glaciers Sedimentary rocks Marine sediments Igneous rocks Ocean Total
0.0004 0.013 0.05 0.11 0.51 15. 24. 173. 300. 1380. 1803
Per em1
0.00002 0.0007 0.003 0.006 0,027 0.79 1.3 0.1 16. 73.
The earliest rain water was very likely loaded with HCI and HzS, which similarly attacked the lavas of the higher land masses. As the resulting soluble materials were delivered to the ocean by the developing river systems, the free acids gradually were used up, and the presence of NH3 in the atmosphere became a possibility, through thunderstorm activity. Probably a t a time when the only important acid or acid anhydride in the atmosphere was COz, and when the pH of the ocean was near 7.0, life came into being. 'Presented as pert of the Symposium on Chemistry of the Sea before the Division of Chemical Education a t the i x s t Meeting of the Americrtn Chemical Society, Miami, April, 1!!57.
VOLUME 35, NO. 3, MARCH, 1958
As loug as their metabolism depended on the oxidation of naturally occurring chemicals, such as ferrous ion, free sulfur, or the amino acids that recent experiments show can be naturally synthesized ( 5 ) , living organisms were almost negligible factors in geology and geochemistry. But as soon as photosynthesis appeared in the ocean, profound chauges occurred. Methane, CO, Hz, and HzS disappeared from the lower atniosphere, and COz dwindled to its present partial pressure of 0.03 atmosphere. The resulting changes in the absorption of infrared radiation from the sun produced drastic changes in climate (6). Free oxygen now appeared in the atmosphere (the amount of organic carbon now in sedimentary rocks seems to account for it fully), so that in the attack on the exposed portions of the earth's crust, oxidation and the boring, wedging, and digesting activities of organisms m-ere added to the existringforces of moving air, water, and ice, acid attack (still provided by CCz), and freezing and thawing. Dissolved oxygen replaced H,S aud sulfates replaced sulfides in the sea, while the demand for COz by plants reduced the GOz-tension in the illuminated surface layers, increasing the pH to over 8.0. Virtually all the combined nitrogen in the photosynthetic zones found its way into the biosphere, a ~ the d prevailing oxidizing conditions and the pH of 8 placed restrictions on the solubility and hence availability in the ocean of compounds of phosphorus and of several metals needed for plant and animal growth. The salts now in the ocean, although only 3.5% by weight, amount to some 5 X 1016metric tons, or enough to make a pile 150 meters high over the entire land surface of the earth. Something like 20 X 10" cubic meters of igneous rock has been chemically or mechanically destroyed in the process of extracting these salts, and an equivalent volume of sedimentary rock, much of it laid down under shallow oceanic conditions, most of the rest in swamps or lakes that drained into the oceans, exists today in the continents. The process of erosion and sediment formation, followed by mountain uplift, is still going on, and many (if not most) of the atoms in the present sediment,ary rocks must have passed through the cycle at least twice. The rocks of clastic origin-those formed by particles delivered to the sea: the conglomerates, sandstones, and shales-represent the highly insoluble silicious gravel, sand, and mud. Limestones and dolomites, on the other hand, have been precipitated directly from sea water where local conditions have caused the solubility product of CaCO8 to he exceeded, or, like the phosphate rocks, diatomaceous earths, petroleum, and 113
coal, represent animal or plant remains. Evaporites like rock salt and gypsum exist where arms of the sea have been cut off and evaporation has removed the water. When river-cutting reaches beds of limestone or evaporites, these tend to return to the sea in solution, rather than as suspended particles. PROPORTION OF DISSOLVED SALTS IS CONSTANT
Processes both a t the surface and a t great depths are continually a t work to keep the ocean well stirred. At the surface, evaporation (particularly in the Mediterranean Sea) and ice formation (especially in the Antarctic and Atlantic Arctic) result in the formation of saltier and hence denser water that sinks toward the bottom. Conduction of heat from the earth beneath the floor of the oceans warms the deepest water and sets up a slow convection toward the surface. Simultaneously, at various depths, a complicated system of ocean currents looks after mixing in the horizontal plane. The result is that the ocean is so "well shaken together," as Manry (7) pointed out in 1855, that the relative proportions of dissolved salts in the ocean are everywhere virtually the same, and only the ratio of salt to water shows appreciable variations. The salinity, therefore, a defined term fairly close to the actual proportion by weight of salt in sea water, is commonly used as the basis for expressing this proportion. Because the sea salts are hygroscopic, it is difficult to dry them except by heating, and this causes hydrolysis of MgC12, with evolution of HC1 and destruction of the carbonates. The full definition of salinity therefore is l'proportion of the solid malerial obtained by euaporaling sea waler to dryness, after all carbonate and organic matter has been converted to oxide, and all bromide and iodide to chloride." Four-figure accuracy is usually striven for in the result, and so that the answerwilllook like money (thereby facilitating printing and proofreading), it is customary to report the results not as per cent (OJo) but as per mille (%o) (Table 2). Similarly the specific gravity is reported in a unit of "grams excess over one kilogram per liter" (u,). TABLE 2 Average Temperature, Salinity, and Specific Gravity (at One Atmosphere Pressure) of the Ocean As the chemist would write it. Temperature, ' C . Salt, weight % Suecific p r a h And as the oceanoorwher . . uwites it: Temperature, OC. Salinity, %o (per mille) m,
Because of the extreme difficulty in determining the salinity, as indicated by its definition, it is more convenient to determine one of the individual ions. The most abundant ions in eea water are Naf and C1-, neither of which is needed in significant amounts in any biological system. Either therefore would be suitable for characterizing the total salt content; obviously, from the viewpoint of the analyst, C1- is the choice. S i ~ c esea salt also contains a small proportion of hromide and a trace of iodide, chlorinity was originally de-
fined ae the proportion by weight (per mille) of chlorine, bromine, and iodine in sea water, assuming that the Br- and I- had all been replaced by CI-. However, since this definition required placing more truet in atomic weight values than experience showed was justified chlorinity has been redefined ae 0.3285233 of the Ag-equivalent. The relationship between chlorinity and salinity has been accurately determined t o fit the following linear equation (8) The significance of the constant term, which implies that water containing no chlorides still has other dissolved solids, is readily understood in terms of the river waters just mentioned, which contain much more bicarbonates (from limestone and dolomite) and sulfates (from gypsum and anhydrite or oxidized sulfides) than chlorides. The ultimate ionic analysis of typical oceanic sea water is given in Table 3 (9). TABLE 3 North Pacific Ocean Surface Water, Chlorinity 19.000%0 Cations
Eo.lka.
%,
Sum
0.5936
12.621
&BOI (undissoeisted) Sum of cations Total
Anions
Ev./kg.
0.5936
21.836 0.026 12.621 34.482
The total weight of salt, 34.482 grams, can he shown by appropriate chemical arithmetic to be almost exactly equivalent to the 34.325 given by equation (1). Similar empirical expressions for the variation of the ratio of SOa--, Mg++, Ca++, and HCO1- to C1- with varying chlorinity have been derived, and the explanation of the departure of observed values from these semitheoretical expressions provides a useful meeting ground for chemical, biological, physical, and geological oceanographers. At the same time that the relationship between chlorinity and salinity of sea water was determined, the corresponding relationship between salinity and density was determined. These values permit ready conversion between chlorinity (already defined), and chlmosity (the chloride content in grams per liter), a quantity often more useful in chemical work. They also make it possible to derive a precise value of salinity and chlorinity from a determination of the density and vice versa. I n certain circumstances, it may be desirable to measure some other property that depends only on the total salt present, such as refractive index, electrical conductivity. or freezing point, and it is possible to convert readings of any of these quantities into any of the others with an accuracy of 5 or 10 parts in 10,000 or better. Calibration is facilitated by the existence of a laboratory in Copenhagen, Denmark, which furnishes sea water of chlorinity close to 19.40%0,accurately analyzed to one part in 10,000. JOURNAL OF CHEMICAL EDUCATION
PROPORTION OF DISSOLVED GASES IS VARIABLE
The situation with regard to dissolved gases in the sea is quite different from that of the salts. The chemically inert gases, A and Nz, are everywhere dissolved in concentrations that correspond closely to equilibrium with the atmosphere a t the temperature of the water. Their concentrations, therefore, are primarily functions of the water temperature. The gases participating in the cycles of photosynthesis and respiration, COP, and 0 2 , however, show great variability. At the surface in winter in high latitudes, Oz, like Nz, is in equilibrium with the atmosphere, and as these zones are the chief areas of formation of bottom water, there is a tendency for O2 values in deep water to be fairly high, increasing poleward. Elsewhere, near the surface, photosynthesis may increase O2 to 125% to 150% of equilibrium values during the daytime. Below the illuminated layers in general, dissolved Ozshows a marked decrease reflecting the respiration of organisms and the bacterial decay of organic matter sinking from near the surface. Since the supply of Orrich water from the polar regions tends to hug the bottom, a distribution of Oz concentration with depth that shows a minimum value a t 500 to 1000 meters is characteristic of many areas in the ocean, especially in the Pacific, while in the tropical and much of the temperate Atlantic the absolute content of Oz (though not necessarily the percentage of saturation) is greater at the bottom than at the surface. Where barriers to horizontal circulation restrict the resupply of deep water, respiration and bacterial activity may deplete the Oz-content of water to a point where ordinary organisms can no longer thrive. Here a kind of bacterium that can takeits Orrequirement out of the SO1-:ion makes its appearance, and the endproduct HrS is found. The classical situations of this kind occur in many of the fjords of Norway, where the glacier that carved out the fjord ended in a moraine that serves as a submerged dam or sill, and the deep water inland of the sill has no contact with the open ocean. Stagnation and eventual H&production are the result until some combination of circumstances brings water in over the sill denser than the water alreaddv in the basin. and renewal of the basin water results, The Black Sea behaves similarly, the only connection with the rest of the ocean being the shallow Bosporus, so that the entire Black Sea below a depth of about 200 meters lacks dissolved 0 2 and is rich in H S . I n general, consumption of O1means the production of C02 and a decrease in pH, hut the problem is complicated not only by dissolved bicarbonates but by the presence of a solid phase of CaC03in particulate matter and on the bottom, and by the further presence of a significant amount of H3BOa,which fortunately has not yet been proved to enter into biological cycles. Since sea water of salinity 35%0 is a complicated mixture of salts with an ionic strength of around 0.7, it is obvious that the activity coefficients of the various ions concerned in the equilibria are not likely to be furnished by the conventional type of measurement in pure, dilute solutions. Luckily, however, the variations in concentration of the components of the buffer system, since they have maximum values of only 0.002 A t , can be ignored, VOLUME 35,
NO. 3, MARCH, 1958
and in any sample of sea water a t a given temperature and salinity the activity coefficients can be considered constants like the ionization constants. Thus, for the four main reactions that enter into the buffer mechanism, we can write:
I n the equations on the right, we can substitute activity coefficient k times concentration c for each of the activities a (except ax+),and rearrange to put all the constants on the left. Calling this constant K' (the apparent dissociation constant) and taking the logarithmic form, we can then write: Carbmic acid KLknrcol/kecor = aa+ cacor-/ca,co, = KI'; pKl' = pH Kskaoor/kro8- = an+ ccor-/cxcor = Kn'; pKi = pH
+ log ca2cor/cacoZ+ log cxcoa-/ccor-
with a similar set of expressions for boric acid. Since we can readily determine pH and total COz or total boric acid, and since Total C02 = ce.cor
+
cecor-
+ ccor-
with a parallel equation for the boric acid species, if we know pK1, and ~ K v we , have for each acid three equations with three unknowns, and we can readily solve for the concentrations of H2C03,HC03-, and COa--, and the corresponding borate components. Representative values of these constants are given in the following table and compared with the thermodynamic constants at 20°C.: Carbonic acid
Sea water, ehlorinity 19%
Pure watw
Boric acid
The normal range of pH of sea water, which is around 7.50 to 8.25, falls between pKv for HzCOa on the low side and pK1, of H3B03and pK2, of H2C03on the other, showing that all three of these dissociation reactions play an important role in buffering sea water. LITERATURE CITED (1) AHRENS,Loum H., in POLDEVAART, ARIE(editor), " C m ~ t of the Earth," Geological Society of America Special Paper 62, New York, 1955, p. 155. (2) UREY,H. C., P ~ o cR. . Soe., A219, 281 (1953). (3) RUBEY.W. W.. ''Cru~tof the Earth." 1955. D. 631 ?4\ . , ~ d h o t i d~ ~ ~ ~ ' P O I ~ D E V A AARRTI .E . ' " c N ~bf~ the Earth." 1655, p. 119. (5) CALVIN, MELVIN, Am. Scientist, 44, 248, (1956). (6) PLASS,GILBERT N., Am. Scientist, 44,302, (1956). ( 7 ) MAURY,M. F., "Physical Geography of the Sea," Harper & Bros., New York, 1855. (8) FORCH, C., M. KNUDSEN, AND S. P. L. SOHENSEN, K. Dansk. Vidensk. Selk. Skrifter. . . 6 Raekke, na,turvid. oa- math. Aid. XI.1 (1902). (9) LYMAN. JOHN.AND R. H. FLEMING. J . Mar. Res.. 3. 134
