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Excess 3He in deep water on the East Pacific Rise
Earth and Planetary Science Letters, 26 (1975) 125-132 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
I'1
EXCESS a He IN DEEP WATER ON THE EAST PACIFIC RISE H. CRAIG Scripps Institution o f Oceanography, University o f California at San Diego, La Jolla, Calif. (USAJ W.B. CLARKE and M.A. BEG* Department o f Physics, McMaster University, Hamilton, Ont. (Canada) Received March 11, 1974 Revised version received March 1i, 1975
Helium isotope measurements show that water on the crest and flanks of the East Pacific Rise has the highest enrichment in 3He so far observed in the oceans; the 3He/4He ratio anomaly relative to atmospheric helium is + 32% at the mid-depth maximum in the prof'des. The corresponding 3He solubility anomaly relative to saturation with atmospheric helium is +50%. These data indicate that active sea-floor spreading sites on the crests of the mid-ocean rises are the sources of primordial helium injected into the ocean from the earth's interior. The 3He/4He ratio in this flux is approximately 1.6 × 10 -s, about 11 times the atmospheric ratio of 1.4 X 10 -6. The total flux of 3He into the atmosphere is 4.6 atoms cm -2 earth-surface sec-1, most of which (4.0 atoms cm-2 sec-1 ) is supplied by the oceanic flux. The corresponding atmospheric residence time for 3He is 106 years, which, within the large uncertainties of supply and demand (thermal escape), is consistent with the requirement for a steady state.
1. Introduction "Excess" 3He in the ocean, the enrichment o f oceanic helium in 3He relative to the atmospheric 3He/4He ratio, was discovered b y us [1] in a profile from the Kermadec Trench collected on Expedition NOVA (Scripps Institution o f Oceanography, 1967). At this station the 3He ratio anomaly, defined in terms o f the 3He/4He ratio, R, as: 8(3He) = (R/Ratm - 1) × 100 is essentially zero from the surface to 1000 m, and is 10 -+ 2% in the deep water from 4000 to 8250 m. At 1700 m a mid-depth maximum anomaly o f 22% was observed. On the basis o f these rather meager data we concluded that excess 3He in the ocean is due to injection o f primordial helium, i.e. helium with a high 3/4 ratio [2] trapped in the earth's interior at the * Present address: Radiation and Isotope Application Division, Pakistan Institute of Nuclear Science and Technology, Rawalpindi, Pakistan.
time the earth was formed. The mid-depth maximum in aHe enrichment was interpreted as being due to helium injection along the crests o f the oceanic rises where new crust is being created by addition o f material from the mantle. If our interpretation is correct, the identification o f primordial helium is thus direct evidence that the earth has not been totally degassed at depths o f 100 km where the oceanic basalts originate Subsequently, we have observed a similar mid-depth 3He maximum at the North Pacific Geosecs-1 station where ~(3He) = 22% at 1 9 0 0 - 2 5 0 0 m, and is 13% in the b o t t o m water at 4100 m [3]. In the Atlantic we have found much smaller aHe anomalies in the deep water, about 3% at the North Atlantic Geosecs-I1 station and 5% in the Argentine Basin o f the South Atlantic [4]. Since the deep and b o t t o m waters o f the world oceans are formed in the high lattitudes o f the Atlantic, this lower 3He enrichment in Atlantic waters is expected. A consequence o f our hypothesis that excess 3He is injected along the crests o f oceanic rises is that the 3He ratio anomaly should show regional maximum
126
values in those areas of the rises where new crust is being formed. Accordingly, our third sampling effort in the Pacific was directed toward an active sea-floor spreading center on the East Pacific Rise. In this paper we report the results obtained from a section across the rise in the region of the triple-junction formed by the Pacific, Cocos, and Nazca plates in the eastern equatorial Pacific. Heat flow values as high as 8 X 1 0 - 6 cal cm -2 sec -1 , more than 5 times the oceanic average, have been measured in this area [5].
2. Sample collection and analysis Samples were collected at four stations on or near the East Pacific Rise on Leg 10 of SCAN Expedition (Scripps Institution of Oceanography, 1970). The station locations and bathymetry across the rise are shown in Fig. 1. Stations X-35, 38, and 41 make a 600-nautical-mile section across the Rise from 7°S, 103°W (X-35) to 6°S, 110°W (X-41), with station X-38 located on the crest of the rise at 6.5°S, 107.4°W in 3100 m of water. Station X-56 was located north of the equator about 700 nautical miles south and west of the crest of the rise. The samples were collected in 30-1 non-metallic Niskin hydrographic bottles and stored in the 300- and 500-ml stainless steel cylinders TRENCH
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with chrome-coated, steel stem-tip valves at each end which were used in our previous work [1,3]. The valves had been argon arc-welded to the cylinders and carefully leak tested on a mass spectrometer. Leakage rates were less than 10 - l ° cm a (STP) of He/min, and the flasks were stored with valve ends filled with water. The gas extraction and mass spectrometric procedures were very similar to those described previously [1 ]. The extracted gas samples were split in proportions of 15/1 for 3He/4He ratio measurements on the large fraction and He content measurements by isotope dilution analysis with a 3He spike on the smaller fraction. The samples were analyzed on a 90 °, 25.4cm radius, single-collection mass spectrometer, using an electron multiplier and voltage switching [1]. A neon spike was also added but the resulting neon data appear to be unreliable for reasons that are not understood. The precision of the 3He/4He analyses is -+1% for the 300- and 500-ml samples, and +-3% for the 50-ml "Dragon flask" samples. Complete details of the analytical work are given by Beg [6]. Hydrographic data from SCAN stations X-30, 38, and 43 have been described previously [7] in conjunction with a study of 226Ra profiles. The principal features of the water column in this region are the Intermediate Water salinity minimum at depths ranging from 800 to 1100 m, and a linear potential temperature vs. salinity relationship in the deep water below the minimum. Only nine argon-welded steel containers were available for this expedition; these were supplemented by three double-valved stainless-steel tube samplers, "Dragon flasks", with 50-ml volumes, so that we were restricted to obtaining only twelve samples in containers which had been shown to possess the necessary integrity for helium work. For this preliminary survey, it was decided to collect three deep-water samples at each of four stations, in order to look at both horizontal and vertical variations in excess 3He across the rise.
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The ~(3He) values from the four stations are shown in Table 1 together with the depths and the potential temperatures of the water. The data show two striking features. First, there is a mid-depth maximum at all stations on the flanks of the rise, with the maximum
127 TABLE 1 3He ratio anomlies, and 4 He and 3He saturation anomalies, across the East Pacific Rise (SCAN Expedition) Location
Depth (m)
T-pot (°C)
6 (a He) (%)
AHe (%)
a( a He) (%)
Station X-35 7°28'S, 102°38'W (East flank)
1091 2847 4019
4.01 1.56 1.45
16.5 ± 0.8 30.3 +- 1.8 25.6 ± 1.0
13 ± 1 15 ± 2.4 11 ± 1
33 51 42
Station X-38 6°30'S, 107°24'W (Rise crest)
1011 2573 2914
4.12 1.73 1.57
17.9 ± 0.8 25.4 ± 0.6 31.0 ± 0.8
9 -+2.4 15 ± 2.4 15 ± 1
30 46 52
Station X-41 6°00'S, ll0°00'W (West flank)
940 2493 3518
4.52 1.68 1.28
15.6 ± 0.7 33.2 ± 0.8 22.4 ± 0.8
9±1 11 ± 1 10 ± 1
28 49 36
Station X-56 8°07'N, 113°55'W (West flank)
2096* 3099* 3908*
1.89 1.38 1.21
33.5 ± 2.2 24.8 ± 2.6 26.1 ± 3.0
15 -+2.4 12 ± 2.4 10 -+2.4
55 41 41
* 50-ml "Dragon flask" samples. Other samples were 300 or 500 ml of water. anomaly occurring near the depth of the rise-crest. (The maximum at station X-56 is inferred from the fact that the ratio anomaly is essentially zero in the surface mixed-layer as shown in our previous Atlantic and Pacific profiles.) Secondly, the 3He ratio anomaly is very large: the mean value at the mid-depth maxima of the four stations is 32.0 -+ 1.6%. On the crest of the rise itself, 8(3He) increases down to the bottom, reaching a maximum value of 31% at 3100 m on the top of the rise. These are the largest 3He ratio anomalies so far observed in the oceans. Fig. 2 shows the East Pacific Rise data compared with our two previous Pacific profdes [1,3]. In both the northern and southern hemispheres 5(3He) in the eastern equatorial water across the Rise is about 10% greater at the mid-depth maximum than the values measured in the previous profiles. In the South Pacific the maximum 8(3He) at NOVA VI.2 may have been missed because of the small number of samples in the mid-depth water. This profile, from the Kermadec Trench, has been drawn rather freely to conform with the nature of the profiles of all other constituents (S, Si, 226Ra, O2, nutrients, etc.) in the region of the "benthic front", the deep density discontinuity in the southwest Pacific which separates the South Pacific Deep Water from the northward-flowing Antarctic Bottom Water below [8]. The frontal surface at this
location is at a depth of 2800 m [8]. The 3He profile can be drawn with some confidence because our 4He profile [1] and the 4He and Ne profiles of Bieri et al. [9] at 13°S, 172°W show quite similar features, including, in the case o f both 4He profiles, a low concentration at the salinity maximum-silicate minimum in the transition layer below the frontal surface. The 8(3He) value on the front may, however, be greater than the minimum value assumed in sketching the profile. The Antarctic Bottom Water ratio anomaly at NOVA VI-2 was measured at 5 points ( 4 0 0 0 - 8 2 0 0 m) and its 8(3He) value is well established at + 10 -+ 1% [1 ]; this water flows into the North Pacific and as shown in Fig. 2 the ratio anomaly in the b o t t o m water in the northeast Pacific at Geosecs-I is only 3% greater. This increase is probably entirely due to vertical mixing with the overlying deep water. The large 3He enrichment of South Pacific Deep Water above the benthic front, 12% greater than in the underlying Antarctic Bottom Water, represents an "induced maximum" due to superposition of differing profiles in two distinct water masses. This effect is especially pronounced for silica in the southwest Pacific, and as in the case of silica, the high 3He concentration in South Pacific Deep Water above the benthic front must be derived either from the North Pacific, by southward transport across the equator, or
128
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8(He 3) (%) Fig. 2. Comparison of 3 He ratio anomalies across the East Pacific Rise with our previous South Pacific [1] and North Pacific [3] profiles. The NOVA VI-2 Kermadec Trench profile has been drawn to confirm with the characteristics of other profiles on the "benthic front", the deep density discontinuity discovered subsequently [8]. The step in the Geosecs-I profile at 500 m is due to artificial tritium from nuclear weapons [3]. AABW = Antarctic Bottom Water; SPDW = South Pacific Deep Water. Error bars on the SCAN X-56 samples indicate the larger errors due to the small (50 ml) size of the samples.
from an eastern source on the rise by strong zonal flow of deep water along the equator. The 3He ratio anomaly should thus be extremely useful for the study of deep and bottom water circulation in the Pacific.
Total helium saturation anomalies (AHe values), calculated from the helium concentration measurements and the solubility data of Weiss [10] are also listed in Table 1. The absolute helium concentrations
129 are tabulated by Beg [6]. The mean value of the 4He saturation anomalies, i.e. the deviation from solubility equilibrium with water-saturated air at the potential temperature of the sample, is AHe = +12.0% with a spread of -+2.4% for all twelve samples. Within these limits, no consistent structure of the 4He anomalies that we could be confident of were noted. The mean value agrees precisely with the mean AHe obtained by Bieri and Koide [11] for deep water at three near-by stations. The mean maximum saturation anomaly for 3He, including the 1.2% enrichment of 4He vs. 3He in solution equilibrium [12], is A(3He) = +50% as the average value at the mid-depth maxima. This is the amount by which 3He is supersaturated at these depths.
4. The 3He/4He ratio in "excess helium" The SCAN SHe profiles provide for the first time a set of data from which the 3He/4He ratio of the helium flux into the oceans can be determined in a source region where mixing effects are minimal. We assume a twocomponent mixture of mantle or "juvenile" helium and dissolved atmospheric helium, the fraction of injected juvenile helium being x i . Then the 3H.e/4He ratio in the injected flux (= RF), relative to the atmospheric ratio (= RA) is, neglecting the 1% solubility fractionafion: RF/R A = 1 + ~(3He)/xi
(1)
where x i must be obtained from AHe, the total helium solubility anomaly. The helium saturation anomaly AHe is a composite of at least three other effects in addition to the contribution from juvenile helium: (1) pressure variations at the air/sea interface, (2) temperature changes in a water parcel after saturation, and (3) the contribution from air injection by solution of bubbles. The fraction of injected juvenile helium is thus given by: x i = AEHe/[1 + 10 -2 AHe]
(2)
where AEHe is .that part of the saturation anomaly due to mantle-helium injection, i.e. the "excess helium", defined as the non-atmospheric component, and AHe is the total anomaly (12%) relative to solubility equilibrium at potential temperature and 1-atm. pressure. Craig and Weiss [13] have shown how the value of AEHe can be obtained from correlations of the differ-
ences AHe - ANe and ANe - AAr, together with the reasonable assumption that the potential temperature of the deep water has not cooled since saturation equilibrium and has not been heated more than about 2°C. The mean value of AEHe in the southwest Pacific was found to be about 3%, out of a total He saturation anomaly of 7%. The remaining 4% is principally due to air injection [13]. In the case of the SCAN samples, we have used the ANe and AAr anomalies measured by Bieri and Koide [11 ] on a north-south track across the rise in the same area (their station PIQUERO-2 coincides almost exactly with our X-35). (The Ne concentrations measured by Bieri and Koide, plotted against potential temperature, are identical with values at the same temperature measured by us [14] on CARROUSEL Expedition in the southeast Pacific). Mean PIQUERO saturation anomalies for twenty-three deep-water samples on the rise and on each flank are AHe = 11.9 -+ 0.6%, ANe = 6.7 -+ 0.7%, AAr = 1.5 + 0.9% [I 1]. The A - A correlation diagram of Craig and Weiss [13] then gives AEHe = 3.5 -+0.5%, as the anomaly due to non-atmospheric or "excess helium". The remainder of the anomaly, 8.5%, is principally due to air injection at the sea surface. Thus the injected juvenile fraction x i = 3.1% and using ~3He = 32 -+ 0.5% for the mean ratio anomaly and its uncertainty at the maximum, we obtain: R F / R A = 11.2 -+ 2
RF =(1.6 + 0.3) × 10 -s for the mean 3He/4He ratio of the injected non-atmospheric "excess helium" flux. 5. 4He flux from ocean to atmosphere
Craig and Clarke [15] calculated the individual isotopic fluxes from ocean to atmosphere, using both diffusion-advection and pure advection models together with the NOVA VI-2 3He and 4He data. The diffusion-model calculation assumed that the vertical gradient of the AEHe was equal to the gradients of the total saturation anomaly AHe because the air-injection contribution was unknown at the time. However, it has since become apparent that AEHe is only about 30% of the total AHe anomaly in the Pacific, and is less than 10% of AHe in the Atlantic where AEHe must be of the order of 0.5% or less [13]. The very large con-
130 tribution of air injection by bubbles masks the much smaller excess helium anomaly, so that the analytical precision is insufficient to define the gradient in AEHe.* Following our previous advective flux calculation [15], the 4He flux is:
0.250 .6 ¸
0.240
0.230
eL/
_ _ _ _
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X (I)4 = 2 . 5
X
10 6
(AEHe) • w
e..
(3)
in atoms cm -2 sec- x when w, the mean vertical velocity of ascending bottom water, is in m/yr, and AEHe is the mean excess (non-atmospheric) fraction. The problem is to estimate the mean value of AEHe. If one plots the accumulated isotope-dilution data of Bieri [9,11], which are the only data covering both the Atlantic and Pacific, VSopotential temperature, one sees that the Ne, Ar, and Kr concentrations reported for the eastern Pacific [11] are systematically 4.5% and He 5%, greater than concentrations reported for waters of the same potential temperature in the southwest Pacific [9], in the range 0-5°C. However, the Ne and Ar values are very similar to Atlantic concentrations reported for this temperature range. This obviously reflects a systematic error in one or both sets o f Pacific data, o f the order o f several percent, despite the estimated accuracy of -+1% [11], since there is no known source of Ne, Ar; and Kr in the Pacific. However, since the effect is dearly a calibration difference which is the same for all gases, we can estimate AEHe changes from ocean to ocean from variations in the He/Ne ratio, since the data indicate that excess helium (non-atmospheric) in the North Atlantic amounts to less than 0.5% [13]. In Fig. 3 we have plotted the He/Ne ratios measured * Bieri and Koide [11] have recently made a similar diffusive flux calculation based on their 4 He data from Piquero expedition. They obtain a 4He flux of 1 X 106 atoms cm-2 sec-1 (which is more or less the present-day conventional number for the terrestrial escape rate) and state that the assumptions of the calculation are justified by this numerical result. Unfortunately, instead of using the gradient of "excess helium" in their calculation as we did [15], Bieri and Koide have used the gradient in total helium concentration. Since by definition there can be no net flux of the atmospheric component from ocean to atmosphere, their calculation gives a value much too high. In fact, the neon and argon gradients from bottom water to intermediate water (6% and 9% decrease, respectively) are about equal to the helium gradient (8%), so that their calculation also yields net fluxes of "excess" neon and argon to the atmosphere. Actually AEHe ranges from about 3,5% in deep water to 1.5% in intermediate water at their station 3, where the total anomaly varies from 12% to 6% and the concentration itself decreases by 8%.
SOLUBILITY
IC
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0.200
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Fig. 3. He/Ne ratios in Atlantic and Pacific Deep Water, from the data of Bieri [ 9,11 ]. Scale on right repzesents percent enrichment of the ratio relative to the North Atlantic value 0.229. by Bieri for waters at 2000 m and deeper in the North Atlantic [9] and Pacific [9,11], and a single value at 1000 m in the South Atlantic, the deepest sample measured there [9]. In this way, one sees a uniform progression in excess helium from the North Atlantic to the East Pacific Rise. Taking a North Atlantic value of 0.229 as the original ratio in atmospheric components (solubility plus air injection), the mean value o f AEHe in the southwest and North Pacific is about 2.5%, and the East Pacific Rise value is 3.5%. These values agree very closely with those estimated from the A - A correlation diagram ([13], and previous section) from the NOVA and PIQUERO data, because of a similar cancellation of systematic errors in H e - N e - A r comparisons. From the data in Fig. 3, and considering that the Indian Ocean data should be fairly similar to the Pacific data, we estimate that the mean word-ocean AEHe is 2.5%. With an estimated mean advection velocity of 5 m/yr [15], eq. 3 then gives: ~4 = 3 × l0 s atoms cm-e2 sec -1 as the mean net-flux from the deep water to the troposphere. This is 1/3 the previous diffusion-model result [15], which used the gradient of AHe, and 1/2 the previous advective result, which assumed the NOVA VI-2 AHe value as a world average. The number itself is certainly no better than +-20%, which is a minimal estimate of the uncertainty in w, and is probably uncertain to about 50%, all things considered. The total flux of 4He into the atmosphere includes the (unknown) flux from the continental crust and
131 from oceanic volcanoes which emit gases above sea level. From the oceanic value we obtain only an upper limit for the atmospheric residence time of 4He: ~'4 ~< 107 years.
6. The 3He flux The 3He flux can be estimated in two ways. First we assume a mean oceanic deep-water value of~(3He), the ratio anomaly, of 15%, using mean values of 5% for the Atlantic [4], 15 and 25% in the South and North Pacific respectively ([1,3], present data), and assuming 10% for the Indian Ocean. With AEHe = 2.5% and a solubility fractionation for 3He o f - 1 . 2 % [12], the mean solubility anomaly A(3He) is 19%. The 3He analog of eq. 3 (i.e. multiplied by aR a, where a is the solubility fractionation factor 0.988) then gives: ~3 = 3.3 atoms cm-e2 sec -1. Alternatively we can use the estimated 4He flux with the value OfRF = 1.6 × 10 -s calculated from the East Pacific Rise profiles. In this way we obtain ~3 = 4.8 atoms cm-~ sec -1. We thus adopt as our best estimate: • 3(oceanic) = 4 + 1 atoms cm-2e sec -1 in which the indicated uncertainty is certainly minimal and optimistic. The total 3He flux into the atmosphere includes, in addition to the oceanic flux, the atmospheric production rate of 3He by cosmic radiation and the flux associated with crustal helium emission. The cosmicray production rate is twice the tritium production rate, or 0.5 atom cm -2 sec -I [16], while the mean crustal emission rate is no more than 0.1 atom cm -2 sec -1 [17,18]. Thus the total 3He flux to the atmosphere is: • 3(total) = 4.6 atoms cm -2 sec -1 which corresponds to a residence time r3 = 1.0 × 106 years. Kockarts and Nicolet [19] and Johnson and Axford [20] have calculated the Jeans thermal escape rate of 3He for the periods 1951-1961 and 1947-1968, obtaining respectively 3.5 and 5.9 atoms cm -2 sec -~, as compared with the estimated non-thermal (polar wind) escape rate of 1.2 atoms cm-2e sec -a [20]. It is evident that the oceanic flux of primordial helium provides
enough 3He so that within the uncertainty limits of all these calculations (probably factors of 2) no additional source of 3He is required for a steady-state atmospheric 3He concentration. Johnson and Axford [20] have proposed that auroral precipitation of solar wind plasma is the mechanism which supplies the 3He flux required by the escape rate calculations. Although this mechanism may indeed be supplying the earth with some 3He, the existence of large 3He solubility anomalies, up to 50% in the eastern Pacific Ocean, shows that the flux of primordial helium from the interior of the earth is probably sufficient to account for the atmo~ spheric escape of 3He. Further studies of excess 3He in seawater in the vicinity of mid-ocean rises and marginal basins should be extremely useful for delineating areas in which new oceanic crust is presently being formed. Curiously, the mid-depth 6(3He) maximum in the Kermadec Trench profile (NOVA VI-2), which first led us to conclude that the 3He flux emanated from oceanic rises, does not, in fact, have such an origin. The oceanographic features of the deep "benthic front" in this region, discovered after our first 3He study was published, show clearly that the 3He maximum is simply "induced" by the intrusion of Antarctic Bottom Water with a lower ratio anomaly below the overlying deep water [8]. Nevertheless, 6(SHe) does indeed possess a characteristic mid-depth maximum in the eastern North and equatorial" Pacific where only a single deep-water mass is present, a nice example of scientific serendipity which also emphasizes the significance of this rare isotope of helium as a tracer for the deep-ocean circulation.
Acknowledgements Our work on SCAN Expedition was supported by the U.S. Office of Naval Research. We thank J. Sclater and F. Dixon for their assistance on the expedition. Work at McMaster University was supported by the National Research Council of Canada. We thank K. Turekian and W. Broecker for their critical comments on the manuscript.
References 1 W.B.Clarke, M.A. Beg and H. Craig, Excess 3He in the sea: evidence for terrestrial primordial helium, Earth Planet. Sci. Lett. 6 (1969) 213.
132 2 R. Pepin and P. Signer, Primordial rare gases in meteorites, Science 149 (1965) 253. 3 W.B. Clarke, M.A. Beg and H. Craig, Excess helium 3 at the North Pacific Geosecs station, J. Geophys. Res. 75 (1970) 7676. 4 W.J. Jenkins, M.A. Beg, W.B. Clarke, P.J. Wangersky and H. Craig, Excess 3He in the Atlantic Ocean, Earth. Planet. Sci. Lett* 16 (1972) 122. 5 F_,C.Bullard, in: The Sea, 3, M.N. Hill, ed. (Interscience, New York, N.Y., 1963), p. 226. About 40% of the current creation of world oceanic crust takes place along the East Pacific Rise. Cf.: C. Chase, Geohpys. J.R. Astron. Soc. 29 (1972) 117. 6 M.A. Beg, Helium isotope oceanography, Ph.D. Thesis, MeMaster University, Hamilton, Ont. (1971). 7 Y. Chung and H. Craig, Radium-226 in the Eastern Equatorial Pacific, Earth Planet. Sci. Lett. 17 (1973) 306. 8 H. Craig, Y. Chung and M. Fiadeiro, A benthic front in the South Pacific, Earth Planet. Sci. Lett. 16 (1972) 50. 9 ILI-I.Bieri, M. Koide and E.D. Goldberg, Noble gas contents of marine waters, Earth Planet Sci. Lett. 4 (1968) 329. 10 ILF. Weiss, Solubility of helium and neon in water and seawater, J. Chem. Eng. Data 16 (1971) 235. 11 R.H. Bieri and M. Koide, Dissolved noble gases in the East Equatorial and Southeast Pacific, J. Geophys. Res. 77 (1972) 1667.
12 ILF. Weiss, Helium isotope effect in solution in water and seawater, Science 168 (1970) 247. 13 I-L Craig and 1LF. Weiss, Dissolved gas saturation anomalies and excess helium in the ocean, Earth Planet. Sci. Lett. 10 (1971) 289° 14 tL Craig, 1LF. Weiss and W.B. Clarke, Dissolved gases in the Equatorial and South Pacific Ocean, J. Geophys. Res. 72 (1967) 6165. 15 H. Craig and W.B. Clarke, Oceanic 3He: contribution from cosmogenic tritium, Earth Planet. Sck Lett* 9 (1970) 45. 16 H. Craig and D. Lal, The production rate of natural tritium, Tellus 13 (1961) 85. 17 LT. Aldrich and A.O. Nier, The occurrence of aHe in natural sources of helium, Phys. Rev. 70 (1946) 983; 74 (1948) 1225, 1590. 18 P. Morrison and J. Pine, Radiogenic origin of the helium isotopes inrock, Ann. N.Y. Acad. Sci. 62 (1955) 69. 19 G. Kockarts and M. Nicolet, Le probl6me a6ronomique de l'h61ium et de l'hydrog6ne neutres, Ann. G6ophys. 18 (1962) 269. 20 I-LE. Johnson and W.L Axford, Production and loss of a He in the earth's atmosphere, J. Geophys. Res. 74 (1969) 2433.
I'1
EXCESS a He IN DEEP WATER ON THE EAST PACIFIC RISE H. CRAIG Scripps Institution o f Oceanography, University o f California at San Diego, La Jolla, Calif. (USAJ W.B. CLARKE and M.A. BEG* Department o f Physics, McMaster University, Hamilton, Ont. (Canada) Received March 11, 1974 Revised version received March 1i, 1975
Helium isotope measurements show that water on the crest and flanks of the East Pacific Rise has the highest enrichment in 3He so far observed in the oceans; the 3He/4He ratio anomaly relative to atmospheric helium is + 32% at the mid-depth maximum in the prof'des. The corresponding 3He solubility anomaly relative to saturation with atmospheric helium is +50%. These data indicate that active sea-floor spreading sites on the crests of the mid-ocean rises are the sources of primordial helium injected into the ocean from the earth's interior. The 3He/4He ratio in this flux is approximately 1.6 × 10 -s, about 11 times the atmospheric ratio of 1.4 X 10 -6. The total flux of 3He into the atmosphere is 4.6 atoms cm -2 earth-surface sec-1, most of which (4.0 atoms cm-2 sec-1 ) is supplied by the oceanic flux. The corresponding atmospheric residence time for 3He is 106 years, which, within the large uncertainties of supply and demand (thermal escape), is consistent with the requirement for a steady state.
1. Introduction "Excess" 3He in the ocean, the enrichment o f oceanic helium in 3He relative to the atmospheric 3He/4He ratio, was discovered b y us [1] in a profile from the Kermadec Trench collected on Expedition NOVA (Scripps Institution o f Oceanography, 1967). At this station the 3He ratio anomaly, defined in terms o f the 3He/4He ratio, R, as: 8(3He) = (R/Ratm - 1) × 100 is essentially zero from the surface to 1000 m, and is 10 -+ 2% in the deep water from 4000 to 8250 m. At 1700 m a mid-depth maximum anomaly o f 22% was observed. On the basis o f these rather meager data we concluded that excess 3He in the ocean is due to injection o f primordial helium, i.e. helium with a high 3/4 ratio [2] trapped in the earth's interior at the * Present address: Radiation and Isotope Application Division, Pakistan Institute of Nuclear Science and Technology, Rawalpindi, Pakistan.
time the earth was formed. The mid-depth maximum in aHe enrichment was interpreted as being due to helium injection along the crests o f the oceanic rises where new crust is being created by addition o f material from the mantle. If our interpretation is correct, the identification o f primordial helium is thus direct evidence that the earth has not been totally degassed at depths o f 100 km where the oceanic basalts originate Subsequently, we have observed a similar mid-depth 3He maximum at the North Pacific Geosecs-1 station where ~(3He) = 22% at 1 9 0 0 - 2 5 0 0 m, and is 13% in the b o t t o m water at 4100 m [3]. In the Atlantic we have found much smaller aHe anomalies in the deep water, about 3% at the North Atlantic Geosecs-I1 station and 5% in the Argentine Basin o f the South Atlantic [4]. Since the deep and b o t t o m waters o f the world oceans are formed in the high lattitudes o f the Atlantic, this lower 3He enrichment in Atlantic waters is expected. A consequence o f our hypothesis that excess 3He is injected along the crests o f oceanic rises is that the 3He ratio anomaly should show regional maximum
126
values in those areas of the rises where new crust is being formed. Accordingly, our third sampling effort in the Pacific was directed toward an active sea-floor spreading center on the East Pacific Rise. In this paper we report the results obtained from a section across the rise in the region of the triple-junction formed by the Pacific, Cocos, and Nazca plates in the eastern equatorial Pacific. Heat flow values as high as 8 X 1 0 - 6 cal cm -2 sec -1 , more than 5 times the oceanic average, have been measured in this area [5].
2. Sample collection and analysis Samples were collected at four stations on or near the East Pacific Rise on Leg 10 of SCAN Expedition (Scripps Institution of Oceanography, 1970). The station locations and bathymetry across the rise are shown in Fig. 1. Stations X-35, 38, and 41 make a 600-nautical-mile section across the Rise from 7°S, 103°W (X-35) to 6°S, 110°W (X-41), with station X-38 located on the crest of the rise at 6.5°S, 107.4°W in 3100 m of water. Station X-56 was located north of the equator about 700 nautical miles south and west of the crest of the rise. The samples were collected in 30-1 non-metallic Niskin hydrographic bottles and stored in the 300- and 500-ml stainless steel cylinders TRENCH
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with chrome-coated, steel stem-tip valves at each end which were used in our previous work [1,3]. The valves had been argon arc-welded to the cylinders and carefully leak tested on a mass spectrometer. Leakage rates were less than 10 - l ° cm a (STP) of He/min, and the flasks were stored with valve ends filled with water. The gas extraction and mass spectrometric procedures were very similar to those described previously [1 ]. The extracted gas samples were split in proportions of 15/1 for 3He/4He ratio measurements on the large fraction and He content measurements by isotope dilution analysis with a 3He spike on the smaller fraction. The samples were analyzed on a 90 °, 25.4cm radius, single-collection mass spectrometer, using an electron multiplier and voltage switching [1]. A neon spike was also added but the resulting neon data appear to be unreliable for reasons that are not understood. The precision of the 3He/4He analyses is -+1% for the 300- and 500-ml samples, and +-3% for the 50-ml "Dragon flask" samples. Complete details of the analytical work are given by Beg [6]. Hydrographic data from SCAN stations X-30, 38, and 43 have been described previously [7] in conjunction with a study of 226Ra profiles. The principal features of the water column in this region are the Intermediate Water salinity minimum at depths ranging from 800 to 1100 m, and a linear potential temperature vs. salinity relationship in the deep water below the minimum. Only nine argon-welded steel containers were available for this expedition; these were supplemented by three double-valved stainless-steel tube samplers, "Dragon flasks", with 50-ml volumes, so that we were restricted to obtaining only twelve samples in containers which had been shown to possess the necessary integrity for helium work. For this preliminary survey, it was decided to collect three deep-water samples at each of four stations, in order to look at both horizontal and vertical variations in excess 3He across the rise.
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The ~(3He) values from the four stations are shown in Table 1 together with the depths and the potential temperatures of the water. The data show two striking features. First, there is a mid-depth maximum at all stations on the flanks of the rise, with the maximum
127 TABLE 1 3He ratio anomlies, and 4 He and 3He saturation anomalies, across the East Pacific Rise (SCAN Expedition) Location
Depth (m)
T-pot (°C)
6 (a He) (%)
AHe (%)
a( a He) (%)
Station X-35 7°28'S, 102°38'W (East flank)
1091 2847 4019
4.01 1.56 1.45
16.5 ± 0.8 30.3 +- 1.8 25.6 ± 1.0
13 ± 1 15 ± 2.4 11 ± 1
33 51 42
Station X-38 6°30'S, 107°24'W (Rise crest)
1011 2573 2914
4.12 1.73 1.57
17.9 ± 0.8 25.4 ± 0.6 31.0 ± 0.8
9 -+2.4 15 ± 2.4 15 ± 1
30 46 52
Station X-41 6°00'S, ll0°00'W (West flank)
940 2493 3518
4.52 1.68 1.28
15.6 ± 0.7 33.2 ± 0.8 22.4 ± 0.8
9±1 11 ± 1 10 ± 1
28 49 36
Station X-56 8°07'N, 113°55'W (West flank)
2096* 3099* 3908*
1.89 1.38 1.21
33.5 ± 2.2 24.8 ± 2.6 26.1 ± 3.0
15 -+2.4 12 ± 2.4 10 -+2.4
55 41 41
* 50-ml "Dragon flask" samples. Other samples were 300 or 500 ml of water. anomaly occurring near the depth of the rise-crest. (The maximum at station X-56 is inferred from the fact that the ratio anomaly is essentially zero in the surface mixed-layer as shown in our previous Atlantic and Pacific profiles.) Secondly, the 3He ratio anomaly is very large: the mean value at the mid-depth maxima of the four stations is 32.0 -+ 1.6%. On the crest of the rise itself, 8(3He) increases down to the bottom, reaching a maximum value of 31% at 3100 m on the top of the rise. These are the largest 3He ratio anomalies so far observed in the oceans. Fig. 2 shows the East Pacific Rise data compared with our two previous Pacific profdes [1,3]. In both the northern and southern hemispheres 5(3He) in the eastern equatorial water across the Rise is about 10% greater at the mid-depth maximum than the values measured in the previous profiles. In the South Pacific the maximum 8(3He) at NOVA VI.2 may have been missed because of the small number of samples in the mid-depth water. This profile, from the Kermadec Trench, has been drawn rather freely to conform with the nature of the profiles of all other constituents (S, Si, 226Ra, O2, nutrients, etc.) in the region of the "benthic front", the deep density discontinuity in the southwest Pacific which separates the South Pacific Deep Water from the northward-flowing Antarctic Bottom Water below [8]. The frontal surface at this
location is at a depth of 2800 m [8]. The 3He profile can be drawn with some confidence because our 4He profile [1] and the 4He and Ne profiles of Bieri et al. [9] at 13°S, 172°W show quite similar features, including, in the case o f both 4He profiles, a low concentration at the salinity maximum-silicate minimum in the transition layer below the frontal surface. The 8(3He) value on the front may, however, be greater than the minimum value assumed in sketching the profile. The Antarctic Bottom Water ratio anomaly at NOVA VI-2 was measured at 5 points ( 4 0 0 0 - 8 2 0 0 m) and its 8(3He) value is well established at + 10 -+ 1% [1 ]; this water flows into the North Pacific and as shown in Fig. 2 the ratio anomaly in the b o t t o m water in the northeast Pacific at Geosecs-I is only 3% greater. This increase is probably entirely due to vertical mixing with the overlying deep water. The large 3He enrichment of South Pacific Deep Water above the benthic front, 12% greater than in the underlying Antarctic Bottom Water, represents an "induced maximum" due to superposition of differing profiles in two distinct water masses. This effect is especially pronounced for silica in the southwest Pacific, and as in the case of silica, the high 3He concentration in South Pacific Deep Water above the benthic front must be derived either from the North Pacific, by southward transport across the equator, or
128
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8(He 3) (%) Fig. 2. Comparison of 3 He ratio anomalies across the East Pacific Rise with our previous South Pacific [1] and North Pacific [3] profiles. The NOVA VI-2 Kermadec Trench profile has been drawn to confirm with the characteristics of other profiles on the "benthic front", the deep density discontinuity discovered subsequently [8]. The step in the Geosecs-I profile at 500 m is due to artificial tritium from nuclear weapons [3]. AABW = Antarctic Bottom Water; SPDW = South Pacific Deep Water. Error bars on the SCAN X-56 samples indicate the larger errors due to the small (50 ml) size of the samples.
from an eastern source on the rise by strong zonal flow of deep water along the equator. The 3He ratio anomaly should thus be extremely useful for the study of deep and bottom water circulation in the Pacific.
Total helium saturation anomalies (AHe values), calculated from the helium concentration measurements and the solubility data of Weiss [10] are also listed in Table 1. The absolute helium concentrations
129 are tabulated by Beg [6]. The mean value of the 4He saturation anomalies, i.e. the deviation from solubility equilibrium with water-saturated air at the potential temperature of the sample, is AHe = +12.0% with a spread of -+2.4% for all twelve samples. Within these limits, no consistent structure of the 4He anomalies that we could be confident of were noted. The mean value agrees precisely with the mean AHe obtained by Bieri and Koide [11] for deep water at three near-by stations. The mean maximum saturation anomaly for 3He, including the 1.2% enrichment of 4He vs. 3He in solution equilibrium [12], is A(3He) = +50% as the average value at the mid-depth maxima. This is the amount by which 3He is supersaturated at these depths.
4. The 3He/4He ratio in "excess helium" The SCAN SHe profiles provide for the first time a set of data from which the 3He/4He ratio of the helium flux into the oceans can be determined in a source region where mixing effects are minimal. We assume a twocomponent mixture of mantle or "juvenile" helium and dissolved atmospheric helium, the fraction of injected juvenile helium being x i . Then the 3H.e/4He ratio in the injected flux (= RF), relative to the atmospheric ratio (= RA) is, neglecting the 1% solubility fractionafion: RF/R A = 1 + ~(3He)/xi
(1)
where x i must be obtained from AHe, the total helium solubility anomaly. The helium saturation anomaly AHe is a composite of at least three other effects in addition to the contribution from juvenile helium: (1) pressure variations at the air/sea interface, (2) temperature changes in a water parcel after saturation, and (3) the contribution from air injection by solution of bubbles. The fraction of injected juvenile helium is thus given by: x i = AEHe/[1 + 10 -2 AHe]
(2)
where AEHe is .that part of the saturation anomaly due to mantle-helium injection, i.e. the "excess helium", defined as the non-atmospheric component, and AHe is the total anomaly (12%) relative to solubility equilibrium at potential temperature and 1-atm. pressure. Craig and Weiss [13] have shown how the value of AEHe can be obtained from correlations of the differ-
ences AHe - ANe and ANe - AAr, together with the reasonable assumption that the potential temperature of the deep water has not cooled since saturation equilibrium and has not been heated more than about 2°C. The mean value of AEHe in the southwest Pacific was found to be about 3%, out of a total He saturation anomaly of 7%. The remaining 4% is principally due to air injection [13]. In the case of the SCAN samples, we have used the ANe and AAr anomalies measured by Bieri and Koide [11 ] on a north-south track across the rise in the same area (their station PIQUERO-2 coincides almost exactly with our X-35). (The Ne concentrations measured by Bieri and Koide, plotted against potential temperature, are identical with values at the same temperature measured by us [14] on CARROUSEL Expedition in the southeast Pacific). Mean PIQUERO saturation anomalies for twenty-three deep-water samples on the rise and on each flank are AHe = 11.9 -+ 0.6%, ANe = 6.7 -+ 0.7%, AAr = 1.5 + 0.9% [I 1]. The A - A correlation diagram of Craig and Weiss [13] then gives AEHe = 3.5 -+0.5%, as the anomaly due to non-atmospheric or "excess helium". The remainder of the anomaly, 8.5%, is principally due to air injection at the sea surface. Thus the injected juvenile fraction x i = 3.1% and using ~3He = 32 -+ 0.5% for the mean ratio anomaly and its uncertainty at the maximum, we obtain: R F / R A = 11.2 -+ 2
RF =(1.6 + 0.3) × 10 -s for the mean 3He/4He ratio of the injected non-atmospheric "excess helium" flux. 5. 4He flux from ocean to atmosphere
Craig and Clarke [15] calculated the individual isotopic fluxes from ocean to atmosphere, using both diffusion-advection and pure advection models together with the NOVA VI-2 3He and 4He data. The diffusion-model calculation assumed that the vertical gradient of the AEHe was equal to the gradients of the total saturation anomaly AHe because the air-injection contribution was unknown at the time. However, it has since become apparent that AEHe is only about 30% of the total AHe anomaly in the Pacific, and is less than 10% of AHe in the Atlantic where AEHe must be of the order of 0.5% or less [13]. The very large con-
130 tribution of air injection by bubbles masks the much smaller excess helium anomaly, so that the analytical precision is insufficient to define the gradient in AEHe.* Following our previous advective flux calculation [15], the 4He flux is:
0.250 .6 ¸
0.240
0.230
eL/
_ _ _ _
-0
X (I)4 = 2 . 5
X
10 6
(AEHe) • w
e..
(3)
in atoms cm -2 sec- x when w, the mean vertical velocity of ascending bottom water, is in m/yr, and AEHe is the mean excess (non-atmospheric) fraction. The problem is to estimate the mean value of AEHe. If one plots the accumulated isotope-dilution data of Bieri [9,11], which are the only data covering both the Atlantic and Pacific, VSopotential temperature, one sees that the Ne, Ar, and Kr concentrations reported for the eastern Pacific [11] are systematically 4.5% and He 5%, greater than concentrations reported for waters of the same potential temperature in the southwest Pacific [9], in the range 0-5°C. However, the Ne and Ar values are very similar to Atlantic concentrations reported for this temperature range. This obviously reflects a systematic error in one or both sets o f Pacific data, o f the order o f several percent, despite the estimated accuracy of -+1% [11], since there is no known source of Ne, Ar; and Kr in the Pacific. However, since the effect is dearly a calibration difference which is the same for all gases, we can estimate AEHe changes from ocean to ocean from variations in the He/Ne ratio, since the data indicate that excess helium (non-atmospheric) in the North Atlantic amounts to less than 0.5% [13]. In Fig. 3 we have plotted the He/Ne ratios measured * Bieri and Koide [11] have recently made a similar diffusive flux calculation based on their 4 He data from Piquero expedition. They obtain a 4He flux of 1 X 106 atoms cm-2 sec-1 (which is more or less the present-day conventional number for the terrestrial escape rate) and state that the assumptions of the calculation are justified by this numerical result. Unfortunately, instead of using the gradient of "excess helium" in their calculation as we did [15], Bieri and Koide have used the gradient in total helium concentration. Since by definition there can be no net flux of the atmospheric component from ocean to atmosphere, their calculation gives a value much too high. In fact, the neon and argon gradients from bottom water to intermediate water (6% and 9% decrease, respectively) are about equal to the helium gradient (8%), so that their calculation also yields net fluxes of "excess" neon and argon to the atmosphere. Actually AEHe ranges from about 3,5% in deep water to 1.5% in intermediate water at their station 3, where the total anomaly varies from 12% to 6% and the concentration itself decreases by 8%.
SOLUBILITY
IC
~'~ T 0.220 0.210
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0.200
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Fig. 3. He/Ne ratios in Atlantic and Pacific Deep Water, from the data of Bieri [ 9,11 ]. Scale on right repzesents percent enrichment of the ratio relative to the North Atlantic value 0.229. by Bieri for waters at 2000 m and deeper in the North Atlantic [9] and Pacific [9,11], and a single value at 1000 m in the South Atlantic, the deepest sample measured there [9]. In this way, one sees a uniform progression in excess helium from the North Atlantic to the East Pacific Rise. Taking a North Atlantic value of 0.229 as the original ratio in atmospheric components (solubility plus air injection), the mean value o f AEHe in the southwest and North Pacific is about 2.5%, and the East Pacific Rise value is 3.5%. These values agree very closely with those estimated from the A - A correlation diagram ([13], and previous section) from the NOVA and PIQUERO data, because of a similar cancellation of systematic errors in H e - N e - A r comparisons. From the data in Fig. 3, and considering that the Indian Ocean data should be fairly similar to the Pacific data, we estimate that the mean word-ocean AEHe is 2.5%. With an estimated mean advection velocity of 5 m/yr [15], eq. 3 then gives: ~4 = 3 × l0 s atoms cm-e2 sec -1 as the mean net-flux from the deep water to the troposphere. This is 1/3 the previous diffusion-model result [15], which used the gradient of AHe, and 1/2 the previous advective result, which assumed the NOVA VI-2 AHe value as a world average. The number itself is certainly no better than +-20%, which is a minimal estimate of the uncertainty in w, and is probably uncertain to about 50%, all things considered. The total flux of 4He into the atmosphere includes the (unknown) flux from the continental crust and
131 from oceanic volcanoes which emit gases above sea level. From the oceanic value we obtain only an upper limit for the atmospheric residence time of 4He: ~'4 ~< 107 years.
6. The 3He flux The 3He flux can be estimated in two ways. First we assume a mean oceanic deep-water value of~(3He), the ratio anomaly, of 15%, using mean values of 5% for the Atlantic [4], 15 and 25% in the South and North Pacific respectively ([1,3], present data), and assuming 10% for the Indian Ocean. With AEHe = 2.5% and a solubility fractionation for 3He o f - 1 . 2 % [12], the mean solubility anomaly A(3He) is 19%. The 3He analog of eq. 3 (i.e. multiplied by aR a, where a is the solubility fractionation factor 0.988) then gives: ~3 = 3.3 atoms cm-e2 sec -1. Alternatively we can use the estimated 4He flux with the value OfRF = 1.6 × 10 -s calculated from the East Pacific Rise profiles. In this way we obtain ~3 = 4.8 atoms cm-~ sec -1. We thus adopt as our best estimate: • 3(oceanic) = 4 + 1 atoms cm-2e sec -1 in which the indicated uncertainty is certainly minimal and optimistic. The total 3He flux into the atmosphere includes, in addition to the oceanic flux, the atmospheric production rate of 3He by cosmic radiation and the flux associated with crustal helium emission. The cosmicray production rate is twice the tritium production rate, or 0.5 atom cm -2 sec -I [16], while the mean crustal emission rate is no more than 0.1 atom cm -2 sec -1 [17,18]. Thus the total 3He flux to the atmosphere is: • 3(total) = 4.6 atoms cm -2 sec -1 which corresponds to a residence time r3 = 1.0 × 106 years. Kockarts and Nicolet [19] and Johnson and Axford [20] have calculated the Jeans thermal escape rate of 3He for the periods 1951-1961 and 1947-1968, obtaining respectively 3.5 and 5.9 atoms cm -2 sec -~, as compared with the estimated non-thermal (polar wind) escape rate of 1.2 atoms cm-2e sec -a [20]. It is evident that the oceanic flux of primordial helium provides
enough 3He so that within the uncertainty limits of all these calculations (probably factors of 2) no additional source of 3He is required for a steady-state atmospheric 3He concentration. Johnson and Axford [20] have proposed that auroral precipitation of solar wind plasma is the mechanism which supplies the 3He flux required by the escape rate calculations. Although this mechanism may indeed be supplying the earth with some 3He, the existence of large 3He solubility anomalies, up to 50% in the eastern Pacific Ocean, shows that the flux of primordial helium from the interior of the earth is probably sufficient to account for the atmo~ spheric escape of 3He. Further studies of excess 3He in seawater in the vicinity of mid-ocean rises and marginal basins should be extremely useful for delineating areas in which new oceanic crust is presently being formed. Curiously, the mid-depth 6(3He) maximum in the Kermadec Trench profile (NOVA VI-2), which first led us to conclude that the 3He flux emanated from oceanic rises, does not, in fact, have such an origin. The oceanographic features of the deep "benthic front" in this region, discovered after our first 3He study was published, show clearly that the 3He maximum is simply "induced" by the intrusion of Antarctic Bottom Water with a lower ratio anomaly below the overlying deep water [8]. Nevertheless, 6(SHe) does indeed possess a characteristic mid-depth maximum in the eastern North and equatorial" Pacific where only a single deep-water mass is present, a nice example of scientific serendipity which also emphasizes the significance of this rare isotope of helium as a tracer for the deep-ocean circulation.
Acknowledgements Our work on SCAN Expedition was supported by the U.S. Office of Naval Research. We thank J. Sclater and F. Dixon for their assistance on the expedition. Work at McMaster University was supported by the National Research Council of Canada. We thank K. Turekian and W. Broecker for their critical comments on the manuscript.
References 1 W.B.Clarke, M.A. Beg and H. Craig, Excess 3He in the sea: evidence for terrestrial primordial helium, Earth Planet. Sci. Lett. 6 (1969) 213.
132 2 R. Pepin and P. Signer, Primordial rare gases in meteorites, Science 149 (1965) 253. 3 W.B. Clarke, M.A. Beg and H. Craig, Excess helium 3 at the North Pacific Geosecs station, J. Geophys. Res. 75 (1970) 7676. 4 W.J. Jenkins, M.A. Beg, W.B. Clarke, P.J. Wangersky and H. Craig, Excess 3He in the Atlantic Ocean, Earth. Planet. Sci. Lett* 16 (1972) 122. 5 F_,C.Bullard, in: The Sea, 3, M.N. Hill, ed. (Interscience, New York, N.Y., 1963), p. 226. About 40% of the current creation of world oceanic crust takes place along the East Pacific Rise. Cf.: C. Chase, Geohpys. J.R. Astron. Soc. 29 (1972) 117. 6 M.A. Beg, Helium isotope oceanography, Ph.D. Thesis, MeMaster University, Hamilton, Ont. (1971). 7 Y. Chung and H. Craig, Radium-226 in the Eastern Equatorial Pacific, Earth Planet. Sci. Lett. 17 (1973) 306. 8 H. Craig, Y. Chung and M. Fiadeiro, A benthic front in the South Pacific, Earth Planet. Sci. Lett. 16 (1972) 50. 9 ILI-I.Bieri, M. Koide and E.D. Goldberg, Noble gas contents of marine waters, Earth Planet Sci. Lett. 4 (1968) 329. 10 ILF. Weiss, Solubility of helium and neon in water and seawater, J. Chem. Eng. Data 16 (1971) 235. 11 R.H. Bieri and M. Koide, Dissolved noble gases in the East Equatorial and Southeast Pacific, J. Geophys. Res. 77 (1972) 1667.
12 ILF. Weiss, Helium isotope effect in solution in water and seawater, Science 168 (1970) 247. 13 I-L Craig and 1LF. Weiss, Dissolved gas saturation anomalies and excess helium in the ocean, Earth Planet. Sci. Lett. 10 (1971) 289° 14 tL Craig, 1LF. Weiss and W.B. Clarke, Dissolved gases in the Equatorial and South Pacific Ocean, J. Geophys. Res. 72 (1967) 6165. 15 H. Craig and W.B. Clarke, Oceanic 3He: contribution from cosmogenic tritium, Earth Planet. Sck Lett* 9 (1970) 45. 16 H. Craig and D. Lal, The production rate of natural tritium, Tellus 13 (1961) 85. 17 LT. Aldrich and A.O. Nier, The occurrence of aHe in natural sources of helium, Phys. Rev. 70 (1946) 983; 74 (1948) 1225, 1590. 18 P. Morrison and J. Pine, Radiogenic origin of the helium isotopes inrock, Ann. N.Y. Acad. Sci. 62 (1955) 69. 19 G. Kockarts and M. Nicolet, Le probl6me a6ronomique de l'h61ium et de l'hydrog6ne neutres, Ann. G6ophys. 18 (1962) 269. 20 I-LE. Johnson and W.L Axford, Production and loss of a He in the earth's atmosphere, J. Geophys. Res. 74 (1969) 2433.