Deep-Sea Research, 1964, Vo]. 11, pp. 929 to 932. Pergamon Pr¢~ Ltd. Printed in Great Britain.

S H O R T E R C O M M U N I C A T I O N

Rare gases in Pacific Ocean Water

E. MAzon, G. J. WAssEnntmo and H. CRAIG California Institute of Technology, Pasadena; Scripps Institution of Oceanography, La Jolla

(Received 4 August 1964)

Abstract--Concentrations of helium, neon, argon, krypton, and xenon have been measured in some S. Pacific waters. For the latter four gases the observed concentrations are generally consistent to about 4- 10~o with concentrations expected for solubility equilibrium with the atmosphere at the observed water temperatures.

ONE important way to approach the problem of the origin and trajectory of the deep water masses of the sea is through the study of elements in sea water whose concentrations are not constant fractions of salinity. The dissolved gases in the sea, whose concentrations are imposed upon the open sea surface in distribution patterns which differ in different areas where deep water may be formed from surface water, are of special importance in this respect since the varying concentrations should, to first approximation, be controlled by their respective solubilities which can be measured. For example, if the surface waters have reached solubility equilibrium with the atmosphere, then deep water formed in the Antarctic at, say, -- I°C should contain about 10~o more argon and 15~o more xenon than deep water formed in the N. Atlantic at q- 3°C. If very precise measurements can be made, the relative inputs of such water masses into the deep waters of the Pacific and Indian Ocean can be studied.

Oxygen is of course non-conservative in sea water because o f biological consumption, and PdCHARDS and B~ssos (1961) have shown that nitrogen is probably produced in anaerobic environ- ments in the sea. Helium may be sigoificantly in excess of solubility because of radioactive production in the rocks beneath the sea and leakage of such radiogenic helium into the sea. However, the other rare gases should be conservative within the sea, and should be close to saturation in surface waters. Since the solubility curves of gases in water as a function of temperature show positive curvature, mixing of water masses will produce slight supersaturation o f gases in the mixed waters relative to observed potential temperatures. On the other hand, rapid addition of glacial melt water to surface sea water might result in the sinking of undersaturated waters in high latitudes. Thus mechanisms exist for both positive and negative deviations of rare gas concentrations relative to solubifity equilibrium with the atmosphere, but such deviations should be of the order of a few per cent or so.

We have measured the helium, neon, argon, krypton, and xenon contents in four S. Pacific ocean waters. The gas samples were part of the set collected on the Scripps Institution Expedition Monsoon (CRAIO and GORDOn, 1963). Two-litre water samples were collected in plastic Van Dorn hydrographic bottles, and about 1.2 litres of each was processed on the research vessel Argo, using a sea-going, all-glass high vacuum line. The filled sample bottles were suspended on gimbals to prevent agitation, and each sample was processed within 15 minutes after arriving at the sea surface. The sea water was acidified in vacuum with 2 ml of degassed concentrated HsPO4, boiled, stirred, and pumped on with a Toepler pump which collected the gases in a 300 ml sample tube closed by asilicone-greasedstopcockandstoredwith severalcentirneters of mercury above the bore. Laboratory tests established that the procedure collected more than 99.95 ~ of nitrogen and oxygen, and of CO2, which comes off more slowly because of its chemistry; thus it seems certain that all of the rare gases were collected. Total processing time for each sample was about 1 hour. The volume of the extracted water was measured volumetrically to about 0.2 ~o.

929

9 3 0 Shorter C o m m u n i c a t i o n

The rare gas ana lyses were done on a 10 per cent a l iquot o f the to ta l gases, us ing a 6-inch radius, 60 ° Nier type mass spectrometer designed for rare gas work at the Cal i fornia Inst i tute o f Technology. A mixed tracer consis t ing o f Ne 21, Ne 2z, Ar as, K r a~ and Xe l~a separated isotopes obta ined f rom Oak Ridge was mixed with the samples pr ior to c lean-up and rare gas concent ra t ion over a t i t an ium getter, and used to ob ta in the abso lu te concent ra t ions . The tracer was cal ibrated by spiking air samples o f known amoun t . Observed he l iumconcen t r a t i ons were uniformly too high by abou t a factor o f six because o f he l ium diffusion th rough the glass sample tubes before the samples could be ana- lyzed, and these da ta are no t tabulated.

Table 1. Rare gas concentrations observed and predicted fi'om solubility data. All concentrations are in cc (STP)/litre.

Sample M--40 M -37 M-39 M-26 Air:~

Depth (m) 10 3345 5010 5329 - - Salinity (~o) 35"46 34.68 34"71 34-71 --- t (°C) 28"21 1-60 1-33 1.08 - -

Neon × 10 ~ (obs.) 1-84 - - 1-81 1.83 182 Neon eqlb.* 1"50 1 "74 1.74 1-74 - -

Argon × 10 (obs.) 2"27 - - 3.37 2.93 93.2 Argon eqlb.* (2"30) 3.64 3.66 3-68 - - Argon eqlb.t 2"44 4"05 4-07 4.10 - -

Krypton × l0 s (obs.) 4"97 - - 7-97 7.08 114 Krypton eqlb.* 4"63 8.08 8.12 8.17 -

Xenon x 106 (obs.) 6"5 - - 12"0 10. I 86 Xenon eqlb.* 6"0 11"4 11"5 11.6 ---

*Expected concent ra t ion for equi l ibr ium wi th a tmosphere at tempera ture o f sample , according to solubil i ty da ta o f KoNIo (1963). The argon value a t 28°C is very approx imate , as Kon ig ' s argon da ta scat ter so badly tha t the ex t rapola t ion to this tempera ture is no t good.

i 'According to the a rgon solubil i ty d a t a o f DOtJOLAS (1964b). ~Atmospher ic concent ra t ions f rom the l i terature.

The measu red concent ra t ions are shown in Table 1 together with th~ known concent ra t ions in air. Samples M-40 a nd M-39 are f rom 5°42 'S , 149°43 'W; M-37 is f rom 24°41 'S , 154°45 'W; M-26 is f rom 36 ° 29'S, 163 ° 09 'W. N o absolu te concen t ra t ion da ta were ob ta ined for M-37 due to a cal ibrat ion error which affected al l gases equally, so tha t the rat ios normal ized to a rgon were not affected. The ra t ios o f neon , k rypton , a n d xenon , to a rgon in the four samples are given in Table 2. The solubil i ty da ta given in these two tables are those for the in situ sample tempera tures ; use o f potent ia l t empera tures would increase the expected equi l ibr ium concent ra t ions by about 1 per cent for the deep samples .

Table 2. Rare gas concentrations normalized to argon. Observed ratios compared with solubility data of KONIG (1963).

Sample M - 4 0 M -37 M-39 M-26 Air

t (°C) 28-21 1.60 1"33 1.08 - -

104 (Ne/Ar) (obs.) 8"11 4"98 5'37 6-25 - - Eqlb. ratio 6"52 4"78 4.75 4'73 19.53

104 (Kr /Ar) (obs.) 2.19 2.17 2.36 2'42 - - Eqlb. ratio 2"01 2'22 2.22 2-22 1.22

104 (Xe/Ar) (obs.) 0.29 0"31 0'36 0"34 - - Eqlb. ratio 0"26 0.31 0'31 0"31 0.092

Wi th in the level o f precision o f the pre l iminary da ta reported here, one sees that the da ta on the two deep samples , and the compar i son o f observed and solubil i ty da ta on all samples , are grossly cons is ten t to abou t 4- 10~o. The mean deviat ion for each gas f rom the solubil i ty da ta in Table 1 is abou t 1 0 ~ or less, a l though the neon deviat ion for M-40 and the argon deviat ion for M-26 are

Shorter Communication 931

about 20~o. Similarly the ratio data in Table 2 agree with the solubility ratios expected for ecluili- brium with the atmosphere, at the observed temperatures, to about 10~o except for the Ne/Ar ratios in M-40 and M-26 which reflect the neon and argon discrepancies just mentioned. Thus both the absolute and relative abundances of these gases are consistent, within the limit of about 10%, with the solubility data.

The solubility data for all four gases in sea water are given in Table 1 as measured recently by KONIG (1963). There is a good deal of uncertainty in these data. The data plotted in Konig's Fig. 3 for argon, krypton, and xenon, in sea water do not agree with his tabulated measurements in his Table 2. We have assumed his figure is in error, and have taken the values for the temperatures of our samples from smooth curves redrawn from his tabulated sea water data. Argon solubility in sea water has recently been remeasured by DOUGLAS (1964b) by the method used previously by him to measure gas solubilities in fresh water (DOUGLAS, 1963a). These data for the temperatures of our samples are also shown in Table 1 for argon; the values obtained by Douglas are about 11 ~o higher than those of Konig. This discrepancy exists also in the fresh water data measured by these authors (the fresh water data tabulated by DOUGLAS (1963a) for argon solubilities actually list Konig's seawater data, rather than his fresh water data as stated, so that the discrepancy is not as large as shown by Douglas). The argon solubility data of ILXKESTRAW and EMMEL (1938) for sea water agree best with those of Konig, but the two solubility curves cross so that at temperatures below 4°C, they agree better with those of Douglas.

We do not wish to discuss the solubility data on salt or fresh water in detail. The scatter of the sea water argon data of KONIG (1963) iS such that no good extrapolation can be made to tempera- tures greater than 20°C and the uncertainty in his data in this range must be about -t- 10~o. However, our data agree better with his solubility data than with those of Douglas. On the other hand the fresh water data of Douglas for argon agree with recent measurements by KLOTS and BENSON (1963). Konig's neon solubility data for fresh water are systematically about 6~o lower than those given by MORPJSON and JOHNSTONE (1954).

Unti l the solubilities of all the rare gases in sea water are known with confidence, we conclude that the rare gas data are better treated as intrinsic variables characteristic of given water masses, rather than as reflecting exact agreement or disagreement with solubility models.

Comparison of the observed rare gas ratios with the ratios in air (Table 2) shows that addition of 10~o o f atmospheric rare gases to the extracted rare gases should increase the observed Ne/Ar ratio by about 30 %, but decrease the Kr/Ar and Xe/Ar ratios by 5 ~ and 7 ~o. All of our ratios relative to argon tend to be systematically a little high, so that there is no evidence of any air contamination during the sampling and extraction procedure.

KONIG et al. (1964) have recently measured neon and argon in the nor th Pacific at one station. Our neon data at depth agree exactly with their values, and our surface argon value agrees with theirs; they find about 10Yo more argon at depth than do we. This is within the precision of the present data.

The present data, the first available on krypton and xenon in the sea, show that these gases, as well as neon and argon, are present in sea water concentrations given by surface equilibrium values corresponding to water temperatures, within the limits of the present precision and the knowledge of the solubilities in sea water. It should be noted, however, that these preliminary data do not preclude deviations of the order of 10yo, and more precise solubility data in sea water are needed in order to see the fine structure of the deviations which may exist. There is no doubt tha t the uncertainties in the analyses and in the solubility data can he decreased by an order of magnitude, so that rare gas measurements should become a valuable tool for the study of oceanic mixing and water mass formation.

Acknowledgements--This work was supported by grants from the National Science Foundat ion and the Oflk~ of Naval Research. We arc greatly indebted to Norman Anderson for his help in the hydrographic work at sea and to Louis Gordon for general assistance. The pmTicipation of the senior author was made possible by the tenure of a Ford Foundat ion Fellowship during the work.

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932 Shorter Communication

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