jeanflop/post_ocr_correction2 huggingface dataset:
causes of bias, principally linked to the high diffusivity of He in ice, are reviewed and discussed. Abridged English Version During the austral summer of 1989-1990, ice samples from a shallow core were collected at the Vostok station (78°28'S, 106°48'E). The depth interval over which the samples were taken was between 116 to 165 mbs. This interval lies just below the close-off region which is located between 90 and 100 m below the surface (mbs). The main purpose of this study was to investigate the distribution of helium isotopes in polar ice. We discuss isotope fractionation during pore close-off and diffusion through the upper part of the ice sheet. Available data on the helium isotopic composition of trapped gases in ice cores is extremely limited [1]. Our preliminary goal was to test the sampling technique which has been adapted from that used for sampling sea water [6]. The technique used for the ice core samples involved cutting the ice sample from the parent core soon after it was brought to the surface of the ice sheet. Five or six pieces of ice weighing about 6 g were placed in a sealed copper tube (1/2" OD). The copper tubes were evacuated to 10-3 Torr before the tube was sealed off with a clamp. The leak rate across the clamp leads to a helium blank measured to be 1.45 x 10-9 cm3 STP over a three-month period. This value corresponds to approximately 0.1 of our sample. The overall precision of our analytical technique is ±0.5% for the excess He while the precision of the helium concentration is better than ±0.8%. Helium is known to be highly diffusive in ice ([4], [5]). Moreover, the ice is supersaturated with gases due to the high bubble pressure. It is for this reason that one must exercise care during collection and preparation of the ice samples for helium isotope measurements. A numerical study of the post coring diffusion phenomenon was carried out (fig. 1a and b), which shows that it is absolutely essential that the ice samples be prepared as soon as the ice has been
to 10-3 Torr before the tube was sealed off with a clamp. The leak rate across the clamp leads to a helium blank measured to be 1.45 x 10-9 cm3 STP over a three-month period. This value corresponds to approximately 0.1 of our sample. The overall precision of our analytical technique is ±0.5% for the excess He while the precision of the helium concentration is better than ±0.8%. Helium is known to be highly diffusive in ice ([4], [5]). Moreover, the ice is supersaturated with gases due to the high bubble pressure. It is for this reason that one must exercise care during collection and preparation of the ice samples for helium isotope measurements. A numerical study of the post coring diffusion phenomenon was carried out (fig. 1a and b), which shows that it is absolutely essential that the ice samples be prepared as soon as the ice has been retrieved from the drill hole. We also investigated the influence of the evacuation of the copper tube on the amount of helium and isotopic composition. Results of this study suggest that one needs to correct for helium loss as well as for isotopic fractionation associated with the evacuation of the copper tube containing the ice samples just prior to sealing (fig. 2a and b). Note présentée par Claude LORIUS. 0764-4450/93/03160491 $ 2.00 (è) Académie des Sciences We also corrected each sample for the air which was lost from cut bubbles during the preparation of the small ice samples. The surface to volume ratio of our samples is high ( = 7 cm-1) suggesting the amount of air lost to cut bubbles was substantial. Both raw and corrected helium data are reported in table. Also included in table is the ratio of the initial helium concentration at pore close-off (= 4.8 x 10-7 cm3/g) to the measured concentration. The data suggest that between 27 and 42% of the helium had been lost from the samples we measured (fig. 3). Although there is a fair amount of scatter in the isotopic ratios, the mean δ83He value is slightly negative (δ = —
copper tube containing the ice samples just prior to sealing (fig. 2a and b). Note présentée par Claude LORIUS. 0764-4450/93/03160491 $ 2.00 (è) Académie des Sciences We also corrected each sample for the air which was lost from cut bubbles during the preparation of the small ice samples. The surface to volume ratio of our samples is high ( = 7 cm-1) suggesting the amount of air lost to cut bubbles was substantial. Both raw and corrected helium data are reported in table. Also included in table is the ratio of the initial helium concentration at pore close-off (= 4.8 x 10-7 cm3/g) to the measured concentration. The data suggest that between 27 and 42% of the helium had been lost from the samples we measured (fig. 3). Although there is a fair amount of scatter in the isotopic ratios, the mean δ83He value is slightly negative (δ = — 0.8 ±1%). These results are quite similar to Craig data obtained in 1971 at Byrd station (Antarctica) [1], δ = —3% and helium concentration = 57% of air mole fraction. Both concentration and isotopic data are consistent with the idea that helium diffused out of the bubbles. However, it is unclear whether or not all diffusion occurred within the ice sheet before sampling; some may have been lost during post-core sample preparation. Our results emphasize the importance of the sampling procedure in obtaining accurate estimates of the isotopic composition of helium in trapped gases in ice cores. It is hoped that further studies of the isotopic composition of deeper sections of the Vostok or other cores may provide us with information pertaining to variations in the isotopic composition of helium in the paleoatmosphere. It is conceivable that such variations might result from variations in the flux of cosmogenic nuclides produced in the upper portion of the earth's atmosphere. INTRODUCTION. — During the summer austral 1989/1990, samples of ice for analysis of helium isotopes were taken from a drill core at the site of Vostok. Aside from some measurements at the station Byrd (Antarctica) in 1971 and at Dye 3 (Greenland)
unclear whether or not all diffusion occurred within the ice sheet before sampling; some may have been lost during post-core sample preparation. Our results emphasize the importance of the sampling procedure in obtaining accurate estimates of the isotopic composition of helium in trapped gases in ice cores. It is hoped that further studies of the isotopic composition of deeper sections of the Vostok or other cores may provide us with information pertaining to variations in the isotopic composition of helium in the paleoatmosphere. It is conceivable that such variations might result from variations in the flux of cosmogenic nuclides produced in the upper portion of the earth's atmosphere. INTRODUCTION. — During the summer austral 1989/1990, samples of ice for analysis of helium isotopes were taken from a drill core at the site of Vostok. Aside from some measurements at the station Byrd (Antarctica) in 1971 and at Dye 3 (Greenland) [1], there are few data in the literature on this subject. Our immediate objective was therefore to test our sampling technique and to obtain a first recording of the concentration and isotopic composition of helium. The essential motivation for this work is twofold: first, it is a question of defining, from the analysis of the concentrations and isotopic fractionation, a new tool for improving our knowledge of the physics of gases trapped in ice. Secondly, it is important to know whether or not past variations in the helium isotopes atmospheric ratio, in relation with variations in the geomagnetic field, solar activity, or other factors [2], [3], would be detectable in ice core archives. TECHNIQUE OF SAMPLING AND EXPERIMENTAL METHOD. — Helium is known for its high diffusivity in many materials. The few available data for ice [4], [5] indicate, in the range of temperatures —10/—50°C which interests us, a diffusion coefficient between 10-6 and 2.10-5 cm2/s. In this context, the possible evolution of the original isotopic signal during the time of sampling and the chopping of the sample, by diffusion and exchange with the ambient air, is a problem that should not be neglected, especially as the cores that reach
from the analysis of the concentrations and isotopic fractionation, a new tool for improving our knowledge of the physics of gases trapped in ice. Secondly, it is important to know whether or not past variations in the helium isotopes atmospheric ratio, in relation with variations in the geomagnetic field, solar activity, or other factors [2], [3], would be detectable in ice core archives. TECHNIQUE OF SAMPLING AND EXPERIMENTAL METHOD. — Helium is known for its high diffusivity in many materials. The few available data for ice [4], [5] indicate, in the range of temperatures —10/—50°C which interests us, a diffusion coefficient between 10-6 and 2.10-5 cm2/s. In this context, the possible evolution of the original isotopic signal during the time of sampling and the chopping of the sample, by diffusion and exchange with the ambient air, is a problem that should not be neglected, especially as the cores that reach the surface are in significant disequilibrium with the atmosphere: when all the helium contained in the bubbles is incorporated into the solid phase (which is the case after 700 m of depth), the concentration of helium in the ice matrix is 12 times higher than the value at equilibrium with the atmosphere. We have evaluated the importance of this phenomenon for a typical ice core, of diameter 0= 100 mm, by using a numerical solution to the diffusion equation with finite differences (cylindrical geometry). The example shown in figures 1a and 1b, which corresponds to the realistic case of a core rising from an important depth (~700 m), illustrates well the phenomenon of diffusion and its influence on the concentration of helium and on the isotopic ratio. It is observed that the evolution is rapid, especially in the periphery of the core, accompanying a loss of helium and a fractionation in favor of the less mobile isotope (4He). However, for times inferior to a few hours, one notices that the central part is not significantly affected. It is therefore imperative to sample the cores on the ground directly at the exit of the drill, by keeping only the central part
the atmosphere. We have evaluated the importance of this phenomenon for a typical ice core, of diameter 0= 100 mm, by using a numerical solution to the diffusion equation with finite differences (cylindrical geometry). The example shown in figures 1a and 1b, which corresponds to the realistic case of a core rising from an important depth (~700 m), illustrates well the phenomenon of diffusion and its influence on the concentration of helium and on the isotopic ratio. It is observed that the evolution is rapid, especially in the periphery of the core, accompanying a loss of helium and a fractionation in favor of the less mobile isotope (4He). However, for times inferior to a few hours, one notices that the central part is not significantly affected. It is therefore imperative to sample the cores on the ground directly at the exit of the drill, by keeping only the central part of the core. We have chosen a procedure based on the sampling of small volumes of ice. This choice was dictated by the presence of a particularly dense fissuring of the cores obtained by thermal drilling at Vostok, on the first few hundred meters. Each sample consists of five or six small pieces of ice, taken from the core on the ground, within an hour of its return to the surface. These pieces, with a total weight of approximately 5 to 7 g, are placed in a copper tube that has been receded, of outside diameter 1/2 inch (10 mm). This tube is then connected to a vacuum pump and pumped rapidly to obtain a good primary vacuum (10-2 to 10 Torr). After 3 min, the tube is closed by squeezing between two jaws of steel, held by two screws and equipped with calibrated spacers of thickness 0.6 mm. This system is analogous to that used classically for the sampling of oceanic gases [6]. The tightness of this system has been tested at the laboratory from "blanks" stored over a period of 3 years. The analysis of the helium at the mass spectrometer indicates a leakage rate of less than
six small pieces of ice, taken from the core on the ground, within an hour of its return to the surface. These pieces, with a total weight of approximately 5 to 7 g, are placed in a copper tube that has been receded, of outside diameter 1/2 inch (10 mm). This tube is then connected to a vacuum pump and pumped rapidly to obtain a good primary vacuum (10-2 to 10 Torr). After 3 min, the tube is closed by squeezing between two jaws of steel, held by two screws and equipped with calibrated spacers of thickness 0.6 mm. This system is analogous to that used classically for the sampling of oceanic gases [6]. The tightness of this system has been tested at the laboratory from "blanks" stored over a period of 3 years. The analysis of the helium at the mass spectrometer indicates a leakage rate of less than 4. 10-11 cm3 STP d'4He/atm/s. This leakage rate, applied to the conditions of storage of our samples from Vostok, leads to a blank of helium equal to 1.6. 10-9 cm3 STP d'4He, or about 0.1% of the total helium in the sample. Cette valeur calculée est en accord avec les "blancs" réalisés sur le terrain, dont la mesure donne une valeur moyenne de 1,45. 10-9 cm3 STP d'4He. La procédure analytique détaillée, pour l'extraction de l'hélium et sa mesure isotopique est décrite par ailleurs [6]. Pour ce type d'échantillon, l'incertitude absolue sur la détermination du rapport isotopique 3He/4He, exprimé dans la notation 83 He, est ±0,5. La précision analytique sur la quantité totale d'hélium est de l'ordre de 0,8. RÉSULTATS ET DISCUSSION. Du 16/12/1989 au 7/1/1990, une dizaine d'échantillons ont été prélevés, suivant le protocole défini ci-dessus, à une profondeur comprise entre 116 et 165 m. Les résultats sont rassemblés dans le tableau. À Vostok, la zone de fermeture des pores se situe entre 90 et 100 m de profondeur. Les prélèvements sont donc proches de cette zone. Pour chaque échantillon, il est nécessaire d'effectuer une correction du fait de la perte de gaz liée à la porosité ouverte présente
"blancs" réalisés sur le terrain, dont la mesure donne une valeur moyenne de 1,45. 10-9 cm3 STP d'4He. La procédure analytique détaillée, pour l'extraction de l'hélium et sa mesure isotopique est décrite par ailleurs [6]. Pour ce type d'échantillon, l'incertitude absolue sur la détermination du rapport isotopique 3He/4He, exprimé dans la notation 83 He, est ±0,5. La précision analytique sur la quantité totale d'hélium est de l'ordre de 0,8. RÉSULTATS ET DISCUSSION. Du 16/12/1989 au 7/1/1990, une dizaine d'échantillons ont été prélevés, suivant le protocole défini ci-dessus, à une profondeur comprise entre 116 et 165 m. Les résultats sont rassemblés dans le tableau. À Vostok, la zone de fermeture des pores se situe entre 90 et 100 m de profondeur. Les prélèvements sont donc proches de cette zone. Pour chaque échantillon, il est nécessaire d'effectuer une correction du fait de la perte de gaz liée à la porosité ouverte présente à la surface des morceaux de glace, lors du pompage de l'échantillon. Cette correction est effectuée à partir d'un programme numérique mis au point par Martinerie et al. [7]. TABLEAU Concentration en hélium [4He] et rapport isotopique mesurés à Vostok (ce dernier est exprimé dans la notation 83 telle que 83 He (%) = (Réch/Ratm - 1) x 100, Réch et Ratm étant respectivement le rapport isotopique 3He/4He de l'échantillon et de l'atmosphère, égal à 1,38. 10-6. [4He]th est la concentration initiale théorique en hélium (avec une densité à la fermeture des pores p = 842 kg/m3 et une pression atmosphérique P = 0,633 atm, correspondant à 0,084 cm3 STP d'air/g, [4He]th dans égale à 4,4.10-7 (hélium contenu dans la porosité) +0,4.10-7 (fraction dissoute), soit 4,8.10-7 cm3 STP/g). [4He]a est la concentration corrigée de la porosité ouverte et [4He]b est la concentration corrigée également des pertes par diffusion estimées pendant l'échantillonnage (voir texte). (83 He)corr est le 83 He corrigé du fractionnement isotopique estimé durant l'échantillonnage. Concentration [4He] and isotopic ratio measured at Vostok. The isotopic ratio is expressed in the 83 notation: 83 He (%) = (Rs/Ratm - 1) x 100, with Rs and Ratm respectively the isotopic ratio
83 He (%) = (Réch/Ratm - 1) x 100, Réch et Ratm étant respectivement le rapport isotopique 3He/4He de l'échantillon et de l'atmosphère, égal à 1,38. 10-6. [4He]th est la concentration initiale théorique en hélium (avec une densité à la fermeture des pores p = 842 kg/m3 et une pression atmosphérique P = 0,633 atm, correspondant à 0,084 cm3 STP d'air/g, [4He]th dans égale à 4,4.10-7 (hélium contenu dans la porosité) +0,4.10-7 (fraction dissoute), soit 4,8.10-7 cm3 STP/g). [4He]a est la concentration corrigée de la porosité ouverte et [4He]b est la concentration corrigée également des pertes par diffusion estimées pendant l'échantillonnage (voir texte). (83 He)corr est le 83 He corrigé du fractionnement isotopique estimé durant l'échantillonnage. Concentration [4He] and isotopic ratio measured at Vostok. The isotopic ratio is expressed in the 83 notation: 83 He (%) = (Rs/Ratm - 1) x 100, with Rs and Ratm respectively the isotopic ratio 3He/4He of the sample and of the atmosphere (1.38 x 10-6). [4He]th is the initial theoretical helium concentration (with a density at close-off of 842 kg/m3 and an atmospheric pressure P = 0.633 atm corresponding to 0.084 cm3 STP of air/g then [4He]th = 4.4 x 10-7 (helium in the pores) +0.4 x 10-7 (dissolved helium) = 4.8 x 10-7 cm3 STP/g). [4He]a refers to the concentration corrected for losses due to sectioned bubbles and [4He]b to the concentration with the additional correction for the estimated diffusive loss during sampling (see text). (83 He %)corr is the 83 He that was corrected for the estimated isotopic fractionation during sampling. Masse [He] [4He] [4He]a [4He]b volumique Poids 83 He brut [4He]th [4He]th [4He]th 83 He Profondeur in situ échant. brut 107 x 100 x 100 x 100 corr. (m) (kg/m3) (g) (%) (cm3 STP/g) (%) (%) (%) (%) 116. 870 6,11 -1,2 1,99 41,4 51 ±2 58±4 -0,6±1 121 878 5,38 +0,1 2,10 43,7 53 + 2 59±4 +0,7 ± 1 127 885 6,25 -1,7 2,04 42,5 51 ±2 58±4 1,0± 1 133 892 6,10 -2,5 2,39 49,8 58±2 66±4 -1,8±1 137. 896 7,29 -0,9 2,40 50,0 58±2 66±4 -0,1 ±1
pores) +0.4 x 10-7 (dissolved helium) = 4.8 x 10-7 cm3 STP/g). [4He]a refers to the concentration corrected for losses due to sectioned bubbles and [4He]b to the concentration with the additional correction for the estimated diffusive loss during sampling (see text). (83 He %)corr is the 83 He that was corrected for the estimated isotopic fractionation during sampling. Masse [He] [4He] [4He]a [4He]b volumique Poids 83 He brut [4He]th [4He]th [4He]th 83 He Profondeur in situ échant. brut 107 x 100 x 100 x 100 corr. (m) (kg/m3) (g) (%) (cm3 STP/g) (%) (%) (%) (%) 116. 870 6,11 -1,2 1,99 41,4 51 ±2 58±4 -0,6±1 121 878 5,38 +0,1 2,10 43,7 53 + 2 59±4 +0,7 ± 1 127 885 6,25 -1,7 2,04 42,5 51 ±2 58±4 1,0± 1 133 892 6,10 -2,5 2,39 49,8 58±2 66±4 -1,8±1 137. 896 7,29 -0,9 2,40 50,0 58±2 66±4 -0,1 ±1 141 899 6,70 -3,5 2,63 54,8 64+1 73 + 4 2,7 ± 1 151 905 7,65 -0,4 2,69 56,0 63+1 73 + 4 + 0,5 ± 1 165. 911 5,46 -1,7 2,39 49,8 58+1 68 + 4 -0,7+1 Compte tenu de la rapidité des prélèvements (moins de 50 mn) localisés vers le cœur de la carotte, le résultat de la figure 1 montre qu'il n'y a pas lieu de corriger les mesures d'une possible évolution de l'échantillon durant cette opération. Par contre, l'influence des 3 mn de pompage sur les morceaux de glace prélevés doit être prise en compte, vu leurs dimensions très restreintes (plus petite dimension de l'ordre de 6 à 8 mm). Les figures 2 a et b, obtenues à l'aide d'un schéma numérique de diffusion analogue à celui déjà évoqué plus haut, indiquent en effet une influence significative du pompage avec, au bout de 3 mn, une perte d'hélium de 10 à 16 selon la densité, et un effet isotopique entre 3 =—0,6 et 6 = - 1.1 Dans les prélèvements à venir cette question devra être reconsidérée : il y a possibilité de réduire le temps de pompage de façon significative ( < 1 mn), sans
vers le cœur de la carotte, le résultat de la figure 1 montre qu'il n'y a pas lieu de corriger les mesures d'une possible évolution de l'échantillon durant cette opération. Par contre, l'influence des 3 mn de pompage sur les morceaux de glace prélevés doit être prise en compte, vu leurs dimensions très restreintes (plus petite dimension de l'ordre de 6 à 8 mm). Les figures 2 a et b, obtenues à l'aide d'un schéma numérique de diffusion analogue à celui déjà évoqué plus haut, indiquent en effet une influence significative du pompage avec, au bout de 3 mn, une perte d'hélium de 10 à 16 selon la densité, et un effet isotopique entre 3 =—0,6 et 6 = - 1.1 Dans les prélèvements à venir cette question devra être reconsidérée : il y a possibilité de réduire le temps de pompage de façon significative ( < 1 mn), sans augmenter le "blanc" de façon importante, vu le volume très faible à pomper (=25 cm3). Nos données ont donc été corrigées de la perte d'hélium et du fractionnement provoqués par le pompage, à l'aide du modèle de diffusion ci-dessus. Les concentrations du tableau ont également été exprimées, en pourcentage de la quantité théorique d'hélium. Celle-ci est égale à la quantité emprisonnée à la fermeture des pores (4,4. 10-7 cm3 d'4He/g à Vostok) à laquelle s'ajoute la fraction initiale d'hélium dissous dans la phase solide. Les valeurs de solubilité de l'hélium dans la glace reportées dans la littérature sont peu nombreuses et imprécises ([5], [9]-[11]). Nous choisissons la valeur 6,2.10-8 cm3 STP/g [11] à -55°C pour 5,24 ppmv d'hélium dans l'air à 1 atm, a h Fig. 1. — a: Calcul de l'évolution de la concentration en hélium dans une carotte de glace (diamètre 10 cm, densité 920 kg/m3) au contact avec l'atmosphère, dans les conditions du site Vostok; b: Calcul de l'évolution du rapport isotopique 3He/4He (exprimé en 83He%) dans une carotte de glace (diamètre 10 cm, densité 920 kg/m3, 53 He initial = 0 %) au contact avec l'atmosphère, dans les conditions du site Vostok. Fig. a: Time-course of