Environmental Chemistry
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tkc New England Association of Chem
Dana R. Kester Graduate School of Oceonography University of Rhode Island Kingston, Rhode lslond 02881
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Chemical P ~ O C ~ SinSthe ~ SOcean
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wide variety of problems exist for the chemist who is interested in understanding the oceans. Thus, Chave (1) recently described some of the chemical reactions in seawater involving aluminosilicates, carbonates, and organic constituents. Another general account of the chemistry of seawater was given by RiIacIntyre (g), who considered the geochemical history and cycling of elements in the ocean, some aspects of the carbon dioxide system, and the role of bacteria in producing chemical changes in the marine environment. Several sources are available for more technical analyses of marine chemistry (5-5). The present paper will consider a few aspects of the physical chemistry of seawater and of the processes affectingoxygen in the ocean. Composition and Physical Chemistry of Seawater When considering chemical processes in the ocean, one must first establish the composition of the medium in which they occur. The molalities of the seven most abundant ions in a typical sample of seawater are shown in Table 1. Analytical techniques have been devised which permit these values to be determined with a precision of about 0.1% (6). If we sum the weights of the constituents, we find that this seawater contains about 35 g of salts per kilogram of solution. This quantity is referred to as the salinity of seau-ater. One of the remarkable facts about the ocean is that, while the salinity of ocean waters varies from about 30 to 37 g kg-' throughout the world, the relative amount of these constituents does not change significantly from place to place. The variations in salinity which one observes in the ocean are due, not to variations in the salts in the seawater, but to variations in the urater content. Because seawater contains mainly sodium and chloride ions, it may seem that its physical and chemical properties should be essentially the same as those of a sodium chloride solution. Measurements have shown, however, that many properties, such as the solubility of minerals in seawater, the freezing point of seawater, and the absorption of sound by seawater, are considerably different from those of a sodium Based on a. lecture presented a t the 359th meeting of the New England Association of Chemistry Teachers a t the University of Rhode Island. Kineston. Rhode Island. Februarv 13. 1971. I would like to ththanc~r.hordon G . ET& for hin8ug&&ons on this manuscript. Financial support for my research has been provided by the Nstional Science Foundation and the Office of Naval Research.
Toble 1. Molality of the Most Abundant Ions in Seawater Constituent
Molalitv
Chloride Sodium Magnesium Sulfate Calcium Potassium Bicarbonate
0.5658
0.0106 0.0106 0.0021
chloride solution of the same concentration (7, 8). These properties generally depend upon the ionic interactions in the solution. For seawater it has been useful to classify the ionic interactions as being of three types, which are represented schematically in Figure 1. The ionic atmosphere represents the general tendency of ions to be surrounded by a region of slight opposite charge. In dilute solutions these interactions are described by the Debye-Huckel theory (9). They are reflected in the activity coefficients and the conductivities of strong electrolytes such as NaC1, IICI, and MgCl, at molalities up to at least 1.0. When the attraction between certain oppositely charged ions becomes great, the interactions may be represented by the formation of ion-pairs (1012). The third type of interaction is one \vhich depends on specific characteristics of the ions in addition to their electrical charge. The molecular nature of these interactions is not generally specified, but they are
Figure 1. Schematic cla~riflcation d ionic interactions. A, The small positive and negative charges rvrrovnding the onions and cations indicote the tendency for o slight net opposite charge in the "atmosphere" wrrounding on ion. B, ion-poi. species tend to retain some solvent molecule3 between the ions, thereby producing o dipolar dumb-bell shaped species. C, Interactions between specific types of ions may b e either attractive or repulsive.
Volume 49, Number 1, Jonuory 1972 / 1 1
Environmental Chemistry frequently characterized in terms of Harned's Rule coefficients or by Guggenheim interaction coefficients (13). Studies of the heats of mixing of aqueous electrolytes have been also interpreted in terms of specific ionion interactions (14, 16). At the present time it has not been possible to distinguish between the second and third types of interactions in a solution such as seawater. However, it has been shown for some solutions that the combined effects of these latter two types of interaction may be represented by equilibrium constants for the formation of ion-pair species (16, 17). An analysis based on ion association can be consistent with Harned's Rule behavior. Equilibrium constants have been determined for three ion association reactions which take place in seawater
of chemical relaxation, the study of which led to awarding the 1967 Nobel Prize in Chemistry to M. Eigen along with R. W. Norrish and G. Porter (24). The ultrasonic absorption of seawater may be calculated based on the molality of MgS04" ion-pairs (Table 2), and the result may be compared with measured values. The molal sound absorption coefficient of MgSOl0 ion-pairs at 1 X lo5 Hz is 5.3 0.4 X lo4 sec2 m-' molal-I (21). The calculated absorption in seawater is 3.2 + 0.3 X 10%sec2 m-I a t 25'C, 1 atm. Aleasurements in seawater fall within the range 3.45.0 X lo2 see2 m-' (21), which provides reasonable confirmation of the calculated value. The temperature and pressure dependence of ultrasonic absorption in seawater has not been measured, but the results of this analysis of ion-pair formation (Table 2) predict that it should increase by about 7% as temperature decreases from 25' to O0C, and it should be nearly invariant with pressure.
*
Dissolved Oxygen in the Ocean
with the square brackets denoting molalities (7, 8, 16). These equilibrium constants may be used uith the analytical composition of seawater to calculate the molalities of the major unassociated and ion-paired species in seawater (Table 2). Decreases of temperature in the ocean shift these equilibria to the right, while increases in pressure shift them to the left. Table 2.
Mololity of Chemical Species in Seawater
-Moldity--------
Svecies
25-c 1 atm
oec
1 atm
o"c
1000 atm
0.5658 0.4747 0.0487 0.0112 0.0105
n mnn
Recognizing the presence of ion-pairs permits an understanding of a variety of properties of seawater such as ultrasonic absorption. The importance of the acoustical properties of the ocean was realized during World War 11, and it was discovered that seawater absorbed high frequency sound about 30 times more than fresh water (18). This absorption was initially attributed to the Na+ and C1- in seawater, but subsequent experiments revealed that NaC1, AIgC12, Na?SOa, CaS04, Na?C08, and K2S04solutions do not absorb high frequency sound much more than does pure water (19-21). The sound absorption of MgS04 solutions is, however, similar to that of seawater (22). Thus, this phenomenon is related to the presence of both Mg2+and S O P since it does not occur in X4gC12 or Na?SOn. Further studies established the presence of MgSO10 ion-pairs and the absorption of acoustical energy by the pressure-induced dissociation of these ion-pairs (23). This process, which was first observed as an anomaly of seawater, is a specific type 12
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Journol o f Chemicol Educofion
Consideration of the dissolved oxygen content of the ocean reveals the diversity of processes which are encountered by the marine chemist: exchange of oxygen between the atmosphere and oceanic surface waters, photosynthesis or respiration of organisms, and the physical processes of water movement and mixing. Oxygen Exchonge
A simple conceptual model of gas exchange may he used to identify some of the factors affecting this process. The model is represented schematically in Figure 2. It considers three regions: a turbulent atmospheric phase in which the partial pressure of oxygen, Po,,is uniform, a turbulent bulk liquid region with a uniform Pfo,,and a laminar liquid layer separating these two turbulent regions. The laminar layer is a region in which any motion of the liquid is parallel to the air-liquid interface. According to this model the rate determining step for gas exchange is the molecular diffusion of the gas through the laminar layer. Liquid in the laminar layer is undoubtedly exchanging with the turbulent region, but on the time scale required for molecular diffusion, the layer may be regarded as being permanent with an average thickness, t . The partial pressure of oxygen at the upper boundary of the laminar layer, point A, may be taken to be P o , . At . - . ,, .. ,. . ,. . , . ,
.
0 .
_./
PARTIAL
.
,, .( i ,i
PRESSURE
v TURBULENT ATMOSPHERE
TURBULENT LIQUID
Figure 2. Lominor loye, diffwion model for gor exchange between the atmorphsro and ocean.
the lower boundary, point B, it is P'o,. Assuming a linear gradient of partial pressure within the laminar layer, Fick's first law of diffusion combined with Henry's law relating gas concentration and partial pressure predicts Flux of OZ =
Dolaol(Po2- P'od t
Typical units for this flux are ml of O2 per square cm per sec. Do, is the molecular diffusion coefficient of oxygen and ao,is its Henry's law constant. In addition to these two parameters, this simple model indicates that the rate of oxygen exchange depends on the difference in oxygen partial pressure between the atmosphere and the turbulent liquid and on the thickness of the laminar layer. I n 1963 Kanwisher (25) applied this model to oxygen exchange between the atmosphere and seawater. His experiments indicated thicknesses of the laminar layer generally in the range of 0.002-0.020 cm. The smaller thicknesses occurred with increased turbulence (stirring) of the liquid and with increased wind speed over the liquid surface. He estimated that the flux was about proportional to the square of the wind speed. A more realistic consideration of gas exchange between the ocean and the atmosphere must consider the effect of air bubbles injected into the seawater by collapsing waves. Icanwisher estimated that air bubbles were submerged as much as 20 m below the sea surface during a storm in the North Atlantic Ocean. Hydrostatic pressure increases by about 1 atm for each 10 m of water depth. The partial pressure of oxygen in an air bubble will increase proportionately with the total pressure exerted on the bubble. Ranwisher applied the laminar layer diffusion model to the region surrounding an air bubble 0.05 cm in diameter. He concluded that the thickness of the laminar layer around the bubble was about 0.0010-0.0015 cm based on measurements of the flux and the difference in partial pressure. It is reasonable to expect that the turbulent motion near a bubble rising through the liquid will reduce the thickness of the laminar layer relative to a free interface. Gas exchange between a bubble and seawater is more rapid than across the sea surface, because the laminar layer is thinner and the partial pressure is increased by the depth of submergence. The quantitative significance of bubbles in the overall exchange of gases between the atmosphere and the ocean is unknown, because reliable estimates are not available for the surface area of the bubbles in surface waters relative to the area of the interface. Additional work is required to obtain a more comprehensive understanding of the mechanism by which gases are exchanged between the ocean and the atmosphere.
Environmental Chemistry rates of these various processes have not been precisely determined in the ocean. Insight may be gained into their significance by examining the degree of saturation of surface waters. The percent saturation ([02]/[02']) X 100 is a convenient parameter for measuring departures from equilibrium. [02] is the observed oxygen concentration and [02'] is the equilibrium concentration at the temperature and salinity of the observed sample. Surface waters in the ocean frequently range between 97-103% saturation (26, 27). In the eastern North Pacific Ocean R. M. Pytkowicz and I observed considerable seasonal variability in the percent saturation of vaters in the upper I-m layer. During the winter when biological effects were minimal and turbulence due to storms was large, the surface waters were generally within 2y0of saturation. However, in the summer (Fig. 3) large areas were more than 105% saturated, perhaps indicating that photosynthetic production of oxygen or warming of the waters mas too rapid for exchange to maintain equilibrium. We also found an area with less than 95% saturation which appeared to be related to upwelled water. Upwelling occurs along sections of the west coast of the United States during the summer, because the wind moves the surface waters offshore and they are replaced by waters from depth. This water from depth is deficient in oxygen and it appears as a region of undersaturation, because the rate of upwelling is sufficiently rapid so that biological production, atmospheric exchange, and warming do not reach complete equilibrium. Disfribufion of Oxygen in the Ocean
As we go downward from the sea surface, the oxygen concentration, and also the percentage saturation, decreases due to the removal of oxygen from the water by biological processes. The oxygen content of the ocean does not simply decrease steadily with depth as
Oxygen Confenf of Surface Wofers
Exchange of oxygen between the surface waters of the ocean and the atmosphere is clearly a major factor controlling the oxygen concentration of surface waters. Does this exchange produce surface waters which are in equilibrium with the atmosphere? Processes such as biological production and consumption of oxygen, bubble entrapment, and cooling or warming of the water may occur at a sufficiently rapid rate that exchange cannot maintain this equilibrium. The
Figure 3.
Percentage saturation of oxygen in the summer ot the sea sur-
face ond along crass sections ot 170DWand the equobr in the eastern North Pocinc Oceo.. The vertical decreore from about 100yo ot the surface to 40% occurs 30 9harply that it con not be shown explicitly on ths %ale o f this illurtrotion.
Volume 49, Number
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Environmental Chemistry one might expect. A minimum oxygen concentration is generally found a t depths between 500 and 2000 m. Below this layer, the concentration increases slightly and steadily to the bottom. This general distribution is represented in the North Pacific Ocean by the two vertical cross sections in Figure 3. The oxygen minimum layer is represented by less than 15% oxygen saturation. The processes which are responsible for the observed distribution of oxygen constitute a fascinating oceanographic problem which has not yet been completely solved. The oxygen concentration at a point in the ocean which is out of contact with the atmosphere will be determined by two opposing factors: the consumption of oxygen by biological processes and the replenishment of oxygenated water by movement and mixing. In the deep waters of the North Pacific Ocean the rate of biological oxidation is sufficiently slow and/or the rate of renewal is sufficiently great so that an oxygen content of more than 40% saturation is maintained. The water which replenishes the depths of the North Pacific is a mixture whose origins include the region around the Antarctic continent, the Mediterranean Sea, and the northern section of the Atlantic Ocean (38). This water is likely to have been out of contact with the atmosphere for about 1000 years (39, 30). The fact that its oxygen content has not been fully depleted over this time is a demonstration of the slowness of oxygen consumption at great depths in the ocean. Several views have been advanced to explain the formation of the oxygen minimum layer a t approximately 1000 m depth. Wyrtki (31) accounted for this feature by considering three processes in the depth range of 600-3000 m: vertical movement of the water, vertical mixing of the water, and biological oxygen consumption at a rate which decreased exponentially with depth. The oxygen content below 3000 m is maintained by the horizontal flow of oxygenated water from high latitudes. Even though this model is undoubtedly oversimplified for many regions of the ocean, it represents a quantitative approach to the problem which yields considerable insight into the separate effects of the various processes. Menzel (33) suggested that the oxygen minimum is produced by biological consumption in only a small number of local regions, and it then spreads throughout the ocean by lateral mixing with no additionally significant consumption. Horizontal gradients exist within this minimum which are consistent with this explanation. The local areas of oxygen consumption would be mainly underneath the regions of highly productive surface waters along the eastern boundaries of the major ocean basins. The organic matter produced in these surface waters would be oxidized as it settles to depths of 300700 m. Wyrtki concluded that local consumption
alone was inadequate to account for the worldwide intensity of the oxygen minimum and that some consumption must contribute to the formation of the minimum throughout the ocean. Conclusion
Several characteristics of chemical oceanography are evident from the preceding considerations. Chemical studies in the ocean require the talents of all types of chemists: the analytical chemist, physical chemist, surface chemist, and biochemist may all contribute to the further understanding of the above processes. I n addition there are outstanding problems in marine chemistry for the organic chemist and the radiochemist. Work to be pursued includes both detailed laboratory investigations of specific chemical processes and wide range field studies of the oceanic environment. One of the essential characteristics of an effective marine chemist is an appreciation and an awareness not only of chemical processes but also of biological, geological, and physical processes. Natural phenomena do not recognize scientific boundaries. Literature Cited (11 CHAVE. K . E.. J. CHEM. EDUC.. 48, 148 (19711. (2) M * c I ~ ~ r nF e . , Sei. Amer.. 223, 104 (19701. (31 RILEY,J. P., A N D SKIRROW. G. ( n d i t o ~ s l"Chemical , Ocesnogrsphy," Vols. 1 and 2 . Academic Press. London. 1965. ( 4 ) H o n n ~ R. , A,, "Marine Chemistrv." John Wiley & Sons. Inc.. New York. 1969. (51 L m a e . R. (Editor). '"Chemical Oceanogrsphy," Soandinavian Univeraity Press, Oslo. 1969. (61 P Y T ~ W I R. ~ ZM.. , A N D KESTER,D. R., "O~eanography and Msrine Bioioav: sn Annual Review. Vol. 9" (Edilor: Rnsxes, H.) George. A l l e n i n d Unwin. London, 1971, p. 11. ( 7 ) KESTER,D. R., A N D PYTXOWICZ. R. M., Limnol. Oceonoer., 14, 686 1039 (19701. ( 9 ) Lewm. G. N . , nso R*m*m M.. "Thermodynamled' (Revised by P~TSBB, K. S., A N D BRBWBR,L.1 MeGraru-Hill. New York,1961. ( 1 0 ) B r e n n u ~N . . . Kel. Don. Vidensk. Selsk., 7 , N o . 9 (19261. C. W., "Ion Association," Rutterworths, Washington. D. C.. (11) D*V,ES, 1962. ( 1 2 ) NANCOLLAS. G. H . . "Interaationa in Electrolyte Solutions." Elsevier, Amsterdam. 1966. (13) Rosmsorr R. A,, A N D STOKER. R. H., "Electrolyte Solutions" (2nd ed. r e v . ) , Buttenvorths, London. 1965. ( 1 4 ) Woon, R. H.. A N D SMZTR, R. W., J . Phys. Chem., 6 9 , 2 9 7 4 (19651. (151 Nooo, R. H.. *No GxArnxAn. M.. J . Phvs. Chem., 7 3 , 3959 (19691. ( 1 6 ) P r ~ n o w m zR. , M.. A N D K E ~ T E R D.. R.. Amcr. J . Sci.. 267, 217 (1969). J. N., A N D IIUSTON,R., J . Phys. Cham., 74, 2976 (19701. (171 BUTLER, ( 1 8 ) LIEBERIANN, L. N., J. Acouat. Sac. Amev., 2 0 , 868 (19481. (191 LEoaAno. R. W.. Tech. Report No. 1, Dept. of Physics. University of Caiifornih, Loa Angeles. 1950. ( 2 0 ) Wr~sorr.0. B.. AND Leomno, R. W., J. Acoust. Sac. Amer.. 2 6 , 223 (19541. (211 Kun~zr;,G.. AND TAMM. K.. Acudica. 3 . 3 3 (19531. (221 L ~ o m n o R. , W., Corns. P. C., AND SIIDMORE.L. R., J . A C O Y SSOE. ~. Amer., 21, 6 3 (19491. (23) EIOEN, M., A N D TAM*,K., 2.Elekl7ochem.. 6 6 , 93 (19621. (24) Emma. H.. m n EYRLN.,E. M., Science, 158, 746 (19671. (251 K A N W I ~ H EJ., R ,Deep Sea Re.., 10, 195 (19631. (261 RrrHAnos, R. A,, A N D CORWIN. N., Limnol. Oeeono~?r.,1 , 263 (19561. (271 K ~ s m n D. , R., A N D PTTXOWICZ, R. M,, J . Deophys. Rcs., 7 3 , 5421 (19681. Pergamon Press. (281 P I C K A ~GD..L.."DesoriptivePhysicalO~eano~ra~hy." Oxford, 1963. (291 K ~ A u a sJ. , A,, J . Deoghys. Re%. 6 7 , 3943 (19621. (301 WETL.P . K . . Limnol. Oceanogr.. 10, 215 (19651. (311 W r m a r , K . . Deep S B LRe*.. ~ 9 . 11 (19621. ( 3 2 ) MENSEL,D. W., Deep Sea Rcs., 1 7 , 7 5 1 (19701.
