Sampler depth (m) ( C) (%o) depth (m)

Transcription

1 714 NOTES AND COMMENT TABLE 1. Cruise: USCGC Northwind- Date: 29 July 1965 Station: 23 Sonic depth: 322 m Position: N lat, E long Hydrographic stations Large-volume water stations Corrected Temp Salinity Corrected Temp Nansen Sampler depth (m) ( C) (%o) depth (m) ( C) salinity (go) salinity &) * standard bathythermographic data that can GERALD H. EFINGER be used to determine where to obtain a Benthos, Inc., sample. North Falmouth, Mass NEIL R. ANDERSEN REFERENCE ROBERT E. MORRIS U.S. Naval Oceanograp hit Office, Code 7220, Washington, DC BODMAN, R. H., L. V. SLABAUGH, AND V. T. BOWEN A multipurpose large volume sea-water sampler. J. Marine Res., 19: RELATIVE CALCIUM PHOSPHATE SATURATION IN Two REGIONS OF THE NORTH PACIFIC OCEANS Recent data on the apparent dissociation calcium phosphates cannot yet be deterconstants of phosphoric acid in seawater mined. (Kester 1966; Kester and Pytkowicz 1967) Two quantities are required to study the are used to evaluate and interpret the rela- degree of saturation of seawater with caltive degree of saturation of seawater with cium phosphates such as apatites and phoscalcium phosphates in two regions of the phorites; an ion product which in its North Pacific Ocean. These regions - differ simplest form is given by in that phosphorites are found in only one IP = [ Ca2+] 3 ]P043-] 2, (1) of them. The new data are also used to revise existing values of the solubility of calcium phosphates and to illustrate the effect of complex formation on these values. It is also shown that the absolute degree of saturation of seawater with This work was supported by National Science Foundation Grant GA-301. and the corresponding apparent solubility product given by K,yP = [Ca2+] 38 [ POJ3-] 28. (2) Rrackets refer to concentrations and the subscript S indicates concentrations at saturation. The ion products and the apparent solubility products for the complex solid

2 NOTES AND COMMENT 715 l- 6.0 l r/po4 / holes //lx /OS FIG. 1. Contours of p( IP) = - log (IP) (solid 1 ines), values of p( IP) at 1,000 m in Southern California basins (triangles) and at 1,000 m in the region N lat and W long (dots ), and the values of pk sp obtained by revising the data of Dietz et al. ( 1942) (lower broken line) and by modifying the form of Roberson s ( 1965) results (upper broken line). phases that appear in nature are more involved, but they can be reduced to the above forms. For example, Roberson (1965) refers to a fluorapatite of composition Gas ( P04)3F for which KfgP would be [ Ca2+] 5g [ POb3-] 35 [F-l s. This constant can be reduced to the form of equation ( 2) by dividing it by [Ca2+12s[P043-]s [F-18, which is obtained from the concentrations measured at saturation. The solid curves shown in Fig. 1 represent contours of p( II ) = - log II for which [Ca2+] was calculated from the calcium to chlorinity ratio (Culkin 1965) and [POh3-] was calculated from the equation [ PO $3-] = 3 ah+, l + K1.7 + W 04) a2h+ a3h+ Y (3) p2 ~1~ + l-c1 K 2 K s in which ah+ is the hydrogen ion activity defined in a dilute buffer scale and T(P0,) is the total inorganic phosphate. These two quantities were entered on the axes in Fig. 1. The contours correspond to 34.40x0 salinity and 4C for which K l = 2.73 x 10-2, KI2 = 0.64 x 10-6, and K 3 = 0.40 x 1O-g, and are valid within *0.06 p ( IP ) units in the ranges %0 and 3.0~5.OC. The triangles in Fig. 1 were obtained from the ph and T(P04) data of Rittenberg, Emery, and Orr (1955) in the basins off Southern California at depths between 800 and 1,200 m. The temperature was between 4 and 5C and the salinity between 34.3,3 and 34.40%0. A few observations at 200 m yielded similar values of p ( IP ). Phosphorites were found in this region. For comparative purposes data obtained

3 716 NOTES AND COMMENT on MV Brown Bear Cruise 199 at 1,000 m tion between 800 and 1,200 m in the basins in the region 30 to 50 N lat and 125 to off Southern California is 6.9 ml/liter. 145 W long (University of Washington Therefore, there seems to be no significant 1959), where phosphorites have not been difference in the extent of organic oxidafound, are shown as dots. The ranges of tion in these two regions that could account temperature and salinity for these data for the difference of ph. However, carbon were 2.8 to 3.8C and to 34.48%0. dioxide concentrations are needed to verify The effect of pressure on the apparent this point. We were not able to estimate dissociation constants is not known but the relative dissolution of calcium carbonshould be about the same for the values of ate in these two regions because we did not p ( IP ) under study. have alkalinity data for the basins off The lower values of p (II ) off Southern Southern California. The alkalinity and the California show that these waters are carbon dioxide content of these waters about five times more saturated with cal- should be determined to help understand cium phosphate than those farther north. the environments of phosphorite deposition. This does not imply that the waters off Next, existing values of the apparent Southern California are supersaturated. solubility product of calcium phosphates Such a conclusion about the absolute will be revised. This revision will illustrate degree of saturation cannot be made at the use of recently determined apparent present because the apparent solubility dissociation constants of phosphoric acid products of Dietz, Emery, and Shepard (Kester 1966; Kester and Pytkowicz 1967) (1942) and of Roberson (1965) were deter- and the large effect of complex formation mined at 38C and at 25C respectively, and the temperature dependence of the solubility is not yet known. Thus, at this time it cannot be determined from solubility data alone whether phosphorites are still being formed or if they are being dissolved. A surprising feature revealed in Fig. 1 is that the greater degree of saturation in waters where phosphorites are formed is not due to a higher inorganic phosphate concentration that may result from upwelling or solution of phosphorites. Instead, it is due to a lower acidity as is seen from the comparison with waters at the same depth but farther north. The lower acidity causes a larger fraction of the inorganic phosphate to be present in the form of ( P04 -) ions. This may indicate that phosphorites owe their existence to geochemical or biochemical factors which determine the ph rather than to high inorganic phosphate concentrations. Precipitation of calcium carbonate and oxidation of organic matter would tend to decrease the ph. In the region examined on MV Brown Bear Cruise 199, at 1,000 m, the apparent oxygen utilization is between 6.4 and 6.9 ml/liter (Pytkowicz and Kester 1966), while the apparent oxygen utiliza- on the solubilities. Also, the revised values are useful in further laboratory studies of solubilities and rates of formation of calcium phosphates. Dietz et al. ( 1942) obtained a value of KfSP by extrapolating to the ionic strength of seawater the data of Sendroy and Hastings (1927a, b) that was obtained from dilute sodium chloride solutions at 38C. The value obtained by Dietz et al. is related to the thermodynamic solubility product KSP, which is independent of complexing, by the equation ( K d Dietz =.&-) (4) 4 -xav1 where ( fw+ ) NaCl and ( fp043-) NaCl are the activity coefficients of calcium and of orthophosphate ions in sodium chloride solutions. The relation between ( Kfgp) st1t, the revised value in seawater, and KSp has the same form as equation (4) but involves the activity coefficients in seawater. Therefore, (GP~ = (K SP) Diets ( fca2+) 3NaC1 (fp04s) 2NaC1 ( fca2+) 3L31Y ( fj?04>) SW (5)

4 NOTES AND COMMENT 717 Thompson and Ross ( 1966) reported that 84% of the calcium ions in seawater are free. Kester and Pytkowicz (1967) found that 0.4% of the orthophosphate ions that are free in a sodium chloride solution are still free in seawater. Therefore, ( K gp)gw = ( K sp) Dietz x (0.84)-3 x (0.004)-2. As (K SP) Dieta = 3.73 x 1@25, the revised value is ( K BP)Hgw = 4.0 X This result shows that complex formation, which was not considered by Dietz et al., enhances the apparent solubility product of calcium phosphate in seawater by three orders of magnitude. Dietz et al. used apparent dissociation constants of phosphoric acid determined in simple salt solutions and underestimated the concentration of orthophosphate ions by several orders of magnitude. This led to a partial cancellation of errors in their estimate of the degree of saturation of oceanic waters with calcium phosphate. Our revision is approximate because the extent of complexing was determined at 20C and because some complexing could have occurred in the solutions used by Sendroy and Hastings. However, this revision illustrates the importance of determining equilibria in seawater instead of using extrapolations from dilute simple salt solutions. Roberson ( 1965) measured the solubility of fluorapatites of formula Ca;, ( P04)3F in seawater at 25C and one atmosphere. He measured K 2, the second apparent dissociation constant of phosphoric acid and, not knowing K l and K 3, assumed that at ph values near 8 the total inorganic phosphate was predominantly in the form of HPOb2- ions. This turns out to be a fair assumption because the data of Kester ( 1966) show that about 85% of the total inorganic phosphate is present in the form of HP0h2-. Roberson expressed the apparent solubility product in the form Kl SP = (Ca2+)5s ( HP04-)3s ( F-)s (OH-)39 because, not knowing all the terms in equation (3) he could not calculate ( POh3-) s and the conventional apparent solubility product. Roberson found a range of values over three orders of magnitude with an average value Kl,sP = We calculated the conventional value K,gp = ( Ca2+) 5s ( POd3-) 38 (F-) B using Roberson s values for the ph, the concentrations of calcium and fluoride ions, and the total inorganic phosphate when his seawater samples were at equilibrium with fluorapatites. The results ranged between x 1O-32 and 16 X 1O-32 with an average of 3.7 X The revised value of K BP, taking into account that only 85% of the total inorganic phosphate is in the form of HP04-, is 0.6 X This correction is small in relation to the scatter of Roberson s results. In the form given by equation (2), KISP = 4.14 x This is the form in which the solubility product can be most conveniently compared to measured values of IP, the ion product. In these calculations we assumed, as was done by Roberson, that the fluoride ions were present only as monomers. Fluoride ions dimerize readily; however, it is likely that F- ions are the predominant species (L. H. N. Cooper, personal communication ). The values of PK #~ = - log KfRp obtained by revising the value of Dietz et al. and modifying the form of Roberson s result are shown as the lower and the upper broken lines in Fig. 1. These values of pk SP may not reflect the effect of the complex composition of phosphorites on the solubility; they are only valid at higher temperatures than those usually found in the oceans, and there is a large uncertainty in the value of pk Sp for fluorapatites. Therefore, nothing can be said with cer- tainty at present about the absolute degree of saturation of oceanic waters with regard to calcium phosphates. We have shown that seawater is more highly saturated with calcium phosphate where phosphorites are found than where they are not found. The higher relative saturation is due to a higher ph rather than to a greater total inorganic phosphate concentration. This higher ph appears to be the result of calcium carbonate solution. We have also shown that complex formation in seawater enhances the solubility in sodium chloride solutions and the importance of using chemical equilibrium data

5 718 NOTES AND COMMENT obtained in seawater rather than in dilute simple salt solutions. R. M. PYTKOWICZ D. R. KESTER Department of Oceanography, Oregon State University, Corvallis REFERENCES CULKIN, F The major constituents of sea water, p In J. P. Riley and G. Skirrow [ed.], Chemical oceanography, v. 1. Academic, London. DIETZ, R. S., K. 0. EMEXY, AND F. P. SHEPARD Phosphorite deposits on the sea floor off southern California. Bull. Geol. Sot. Am., 53 : KESTER, D. R Determination of the apparent dissociation constants of phosphoric acid in seawater. MS. Thesis, Oregon State Univ., Corvallis. 55 p. -, AND R. M. PYTKOWICZ Determination of the apparent dissociation constants of phosphoric acid in seawater. Limnol. Oceanog., 12 : PYTKOWICZ, R. M., AND D. R. KESTER Oxygen and phosphate as indicators for the deep intermediate waters in the Northeast Pacific Ocean. Deep-Sea Res., 13: RI~ENBERG, S. C., K. 0. EMERY, AND W. L. ORR Regeneration of nutrients in sediments of marine basins. Deep-Sea Res., 3: ROBERSON, C. E Solubility implications of apatite in sea water. M.S. Thesis, Univ. California, San Diego. 50 p. SENDROY, J., AND A. B. HASTINGS. 1927a. Studies of the solubility of calcium salts. II. The solubility of tertiary calcium phosphate in salt solutions and biological fluids. J. Biol. Chem., 71: , AND b. Studies of the solubility of calcium salts. III. The solubility of calcium carbonate and tertiary calcium phosphate under various conditions. J. Biol. Chem., 71: THOMPSON, M. E., AND J. W. Ross, JR Calcium in seawater by electrode measurement. Science, 154: UNIVEXSITY OF WASHINGTON Physical and chemical data, North Pacific Ocean, M.V. Brown Bear Cruise 199, July to August 1958 for the International Geophysical Year of Dept. Oceanog., Univ. Wash., Seattle. DIGITAL RECORDING OF IN SZTU DEPTH-TEMPERATURE-SALINITY DATAI The Bissett-Berman DTS System is a device for measuring depth (D ), temperature (T), and salinity (S ) of seawater in situ. The underwater unit of this system contains sensors for the three variables and circuitry for translating the sensor outputs into FM signals, which are transmitted up the suspending cable to the vessel. The deck unit causes these signals to drive an X-Y-Y recorder that displays temperature and salinity as functions of depth; the FM signals themselves are also made available for additional processing. Bissett-Berman offers a digital data logger that digitizes the FM signals and records the results on magnetic tape, but the logger does not provide a simultaneous digital printout. Such a printout is desirable, not only as an aid in immediate evalu- ation of the data, but also (and equally as important) as a check of the proper functioning of the digitizing-recording system. This paper describes a digitizing-recording system that has been developed to provide a simultaneous printout. The output medium is a teletypewriter with attached paper-tape punch; this has the advantages of being reliable and fairly inexpensive. DESCRIPTION OF THE SYSTEM Three a-c signals are available at the deck unit of the DTS recorder; their frequencies are linear functions of D, T, and S. The first step in developing the system described here was to measure these three frequencies in such a way that the result would appear in digital form and in convenient units; that is, 1 m for D, O.OlC for 1 Hawaii Institute of Geophysics Contribution T, and O.Ol%o for S. Since the accuracy of No the DTS system is not better than?2 in