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Radiometric Chronology of Neh–nar Glacier, Kashmir

Published online by Cambridge University Press:  20 January 2017

V. N. Nijampurkar,
N. Bhandari,
C. P. Vohra and
V. Krishnan
V. N. Nijampurkar
Affiliation:
Physical Research Laboratory, Navrangpura, Ahmedabad 380 009, India
N. Bhandari
Affiliation:
Physical Research Laboratory, Navrangpura, Ahmedabad 380 009, India
C. P. Vohra
Affiliation:
Glaciology Division, Geological Survey of India, Lucknow, India
V. Krishnan
Affiliation:
Glaciology Division, Geological Survey of India, Lucknow, India
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Abstract

Surface and core samples of Neh–nar Glacier in the Kashmir Valley have been analysed for the radionuclides 32Si. 210Pb, 40K, and 137Cs. The lateral and vertical profiles (at an altitude of about 4 140 m) reveal:

(1)32Si activity decreasing slowly from the accumulation zone to 4 050 m altitude and then abruptly towards the snout.

(2)Five zones of alternating high and low 210Pb activity in the surface samples.

(3)An horizon at between 2 and 3 m depth containing 210Pb activity above natural levels. This horizon is also associated with 137Cs and a maximum in total ß activity.

The ice samples have been dated on the basis of a simplified two–component model, the “fresh“contribution determined by 2l0Pb and the old component by 32Si. The following conclusions can be drawn from these observations:

(1)The model age of the snout ice is c. 850 years.

(2)The average rate of ice movement in the lower glacier is about 2 m/year, which compares well with the annual movement rate of 2.65 m/year observed since 1974.

Résumé

Résumé

Chronologie du Neh–nur Glacier, Kashmir, par radiomètrie. Des échantillons prélevés en surface et par carottage dans le Neh–nar Glacier, dans la vallée du Kashmir, ont été analysés pour leur teneur en radionuclide 32Si. 2l0Pb. 40K, et 137Cs. Les profils latéraux et verticaux (à une altitude d’environ 4 140 m) révèlent:

(1)Une activité du’ 32Si décroissant lentement depuis la zone d’accumulation jusqu’à l’altitude de 4 050 m puis s’effondrant brusquement jusqu’à la langue.

(2)Cinq zones présentant alternativement des activités fortes et faibles en 2l0Pb dans les échantillons de surface.

(3)Un horizon entre 2 et 3 m de profondeur contenant une activité en 2l0Pb supérieure aux niveaux naturels.

Cet horizon est également associé à du l37Cs et à un maximum d’activité beta totale.

En se basant sur un modèle simplifié à deux composantes, ou la contribution de la glance “fraîche” est déterminée par 2l0Pb et la composante ancienne par 2Si, on a pu dater les échantillons de glace. De ces observations on a putirer les conclusions suivantes:

(1)L’âge théorique de la glace du front est d’environ 850 ans.

(2)La vitesse moyenne de la glace dans le bas du glacier est d’environ 2 m par an, ce qui cadre bien avec la vitesse réelle de 2,65 m/an observée depuis 1974.

Zusammenfassung

Zusammenfassung

Radiometrische Chronologie des Neh–nar Glacier, Kashmir. Proben von der Oberfläche und aus Bohrkcrnen des Neh–nar Glacier im Kashmir–Tal wurden auf die Radionukleide 32Si, 210Pb, 40K, und 137Cs untersucht. Die Horizontal und Vertikalprofile (in einer Höhe von etwa 4 140 m) geben folgendes Bild:

(1)Die 32Si–Aktivität nimmt erst langsam, dann abrupt von der Akkumulationszone in 4 050 m Höhe zur Zunge hin ab.

(2)Die Oberflächenproben weisen 5 Zonen von wechselweise hoher und niedriger 210Pb–Aktivität aus.

(3)Ein Horizont in 2 bis 3 m Tiefe weist eine 2l0Pb Aktivität über normalem Niveau auf. Dieser Horizont ist auch mit 137Cs und einem Maximum in der Gesamt–Beta–Aktivität verbunden.

Auf der Grundlage eines Zwei–Komponenten–Modells, nämlich des “frischen” Beitrags durch 210Pb und der allen Komponente durch 32Si, wurden die Eisproben datiert. Aus diesen Beobachtungen können folgende Schlüsse gezogen werden:

(1)Das “Modkl” Aller des Zungcncises beträgt elwa 850 Jahre.

(2)Die miniere Jahresbewegung des Eises im unteren Teil des Gletschers ist schätzungsweise 2 m pro Jahr, was gut mit dem seit 1974 beobachteten Wert von 2,65 m pro Jahr zusammenstimmt.

Information

Type
Research Article
Information
Journal of Glaciology , Volume 28 , Issue 98 , 1982, pp. 91 - 105
Copyright
Copyright © International Glaciological Society 1982

Introduction

Neh–nar Glacier is located in the northern Himalaya at lat. 34° 09’ 30” N., long. 75° 31’ E. It is part of the catchment of the Sind River, which is a tributary of the Jhelum in the Kashmir Valley. Figure 1 shows the location of this basin. Neh–nar is a small valley–type glacier enclosed between two ridges, trending in a direction N. 70° W.– S. 70° E. and rising to 5 376 m and 5 256 m, respectively. It is 3.5 km long and about 350 m wide. Its head is 4 925 m high and the snout is 3 920 m above sea–level.

The glacier is predominantly avalanche–fed. Only a small area near its head, which is separated from the rest of the glacier by an ice fall, is fed largely by snow–fall and drift snow. The avalanches sweep down from the steep slopes, which are almost 1 000 m high, on to the glacier, creating a number of large accumulation zones on its flanks. These, though present on both flanks, are more common on the southern side. The pattern of accumulation on Neh–nar is therefore peculiar, although this type of glacier is common in the Himalaya. Two tributaries descending steeply from the south–eastern side also join the glacier in its lower half. The lower part of the glacier slope is gentler than its upper part (Fig. 2). The main glacier flows in a direction between N. 65° W. and N. 40° W. The thickness of ice, estimated from electrical resistivity surveys, ranges between 14 m near the snout to 139 m in the upper reaches of the ablation zone.

Neh–nar is the westernmost glacier of the Panjatarni group (Fig. 1). It has been extensively studied since 1974 by the Glaciology Division of the Geological Survey of India (Reference VohraVohra and others, unpublished). They have discussed morphology, mass balance, glacier movement, and water discharge, and detailed results of the 1975 and 1976 expeditions can be found in the GSI reports (Reference VohraVohra and others, unpublished).

Fig. 1.

Location of Neh–nar Glacier in the Sind catchment area.

Fig. 2.

Distribution of32Si activity (dpm/ton) in surface samples of Neh–nar Glacier.

Geologically, the Neh–nar area contains extensive dolomite–limestone formations. Geomorphological studies indicate that two lateral moraine ridges flank the glacier, both merging with the terminal moraine, thereby damming the glacier melt water into a pro–glacial lake. This lake drains through a deep channel cut in the terminal moraine. About 20 000 m2 of the terminal parts of this glacier are buried under 8–10 m of fluvio glacial sediments deposited in this lake (Reference VohraVohra and others, unpublished).

These sediments and the existence of two or three lake terraces on the terminal moraine, 20 in higher than the present lake surface, indicate that the present lake occupied a much larger area in the past and covered part of the glacier (Reference VohraVohra and others, unpublished).

The glacier is both receding and thinning under the influence of a series of negative mass–balance years. The ablation is strongly seasonal and the mass balance studies carried out by fixing wooden stakes in the summer months (August–September 1975) indicate an ablation varying from 9.97 cm (September) to 47.81 cm (August) of water equivalent. Glacier surface movement studies indicate a maximum movement of 42 m/year near an altitude of 4 200 m, decreasing to 2.65 m/year near the terminus. At the terminus itself, the ice is practically static. The velocity gradually decreases down–slope, but more rapidly between 4 200 m and 4 100 m from 42 m/year to 15 m/year. The total area of the glacier is 1.68 km2, of which the accumulation area is 0.613 km2 and the ablation area is 1.068 km2 as measured during 1975–76.

The understanding of glacier history, the mass balance in the past, and the nature of the mass transfer that lakes place between the accumulation area and the terminus are matters of great significance to glaciology. Besides, the estimation of the long–term mass balance of glaciers is of prime importance for the development of our water resources. Since annual estimates fluctuate significantly from year to year, and observations can at best cover only part of the year, we decided to use radiometric techniques to understand the long–term average behaviour of these glaciers. The results of these studies, which involve measurement of natural and man–made radioactive tracers, are reported here and discussed in relation to other studies carried out since 1975.

A large number of radioactive tracers of natural and artificial origin occur in the environment. Of these, cosmogenic 32Si with a half life of about 300 years is the best suited for glaciological studies because glacial phenomena occur in this time scale (Lai and others, 1970; Reference ClausenClausen, 1973; Reference NijampurkarNijampurkar and Bhandari, unpublished). Among the natural radionuclides, 2l0Pb (half life of 22.3 years) offers the best choice (Reference GoldbergGoldberg, 1963; Reference Crozaz and LangwayCrozaz and Langway. 1966). In addition, a large number of radionuclides have been introduced in the past few decades by nuclear explosions and these include 90Sr, 137Cs, etc. The most dominant injection of these radionuclides occurred in the early 1960’s as a result of nuclear tests carried out by the USA and USSR. Subsequently, France and China have also carried out several atmospheric nuclear tests but the amount of activity produced is small. Little reliable information is available on the Chinese nuclear tests but the best estimates are that they have been of 3 megaton yield or weaker and the most powerful tests known so far were carried out on 17 June 1967, 27 December 1968, 29 September 1969, 14 October 1970, 27 June 1973 and 17 November 1976 at about lat. 41°N., long. 89° E. (personal communication from G. S. Murty). No radionuclides from the underground peaceful nuclear explosion carried out by India in Rajasthan, which had a yield of about 20 kilotons, had been found to have leaked into the atmosphere (Reference RamannaRamanna, 1975) and they are well contained in the cavity (Reference BhandariBhandari and others, 1979).

Surface and core samples collected from Neh–nar Glacier during August–September 1977 have been analysed for these three components, 32Si, 2l0Pb, and bomb–produced 137Cs, discussed above. In addition, β activity and dust content were also measured. The results are discussed here.

Experimental techniques and sampling methods

Fifteen surface samples and eight core samples have been analysed for 210Pb. All the core samples, each representing a depth interval of 1 m, have also been analysed for l37Cs, 40K, and total β activity. 32Si activity has been measured in six of the 15 surface samples. We describe here the sampling technique and then discuss the chemical and radiation counting methods employed for this analysis.

Sampling techniques

32Si analysis requires a few tons of water. Six samples (NG–1 to NG–6), listed in Table 1 and ranging from 1.8 to 2.45 tons, were collected from selected locations including the subsidiary accumulation zone on the southern flank of the glacier (Fig. 2) and the snout. The sampling was carried out by removing surface ice and then digging small shallow trenches which drained into a pit. Admixture of small amounts of surface melt water in the sample cannot be ruled out in this procedure. The water was then transferred to a plastic or tarpaulin tank held in shape on an aluminium frame. Carriers were added for Pb (as PbNO3) and Si (as Na2Sio3) which were scavenged with iron hydroxide (about 3 kg) in an ammoniacal medium following the procedure described earlier (Reference NijampurkarNijampurkar, unpublished). The volume of water was usually measured before collection in the tank and also during precipitation of Fe(OH)3. Aliquots of water before and after addition of carriers and after scavenging, and the amount of silica recovered, enabled us to determine the scavenging efficiency and silicon water equivalent. The overall efficiencies of silica extraction were around 75%.

In addition to these samples, nine surface samples (Table 2b, 1–9), each of about 2 kg, were collected from various locations on the glacier and these have been analysed for 2l0Pb (Fig. 3).

Core samples were collected by means of a thermal drill, about 5 cm in diameter, from an altitude of 4 150 m in the ablation zone. Eight 1 m sections going down to 10 m were melted and stored in plastic bottles (Table 3, 10–17). Although the possibility of cross contamination cannot be ruled out due to melting by the thermal drill, the contamination, if any, must be small.

Table I.

32Si activity and ages in surface samples of Neh nar Glacier

TABLE IIa.

210Pb activity and ages of the surface samples of Neh nar Glacier

For abbreviations, see Table 1.

TABLE IIb.

210Pb activity and ages of surface samples of Neh–nar Glacier

Back in the laboratory, these samples were dried, counted for total β activity, γ activity and then processed for 210Pb.

Fig. 3.

Distribution of 210Pb activity (dpm/kg) in surface ice of Neh–nar Glacier.

TABLE III.

210Pb activity in Neh–nar Glacier core samples

(R) The measurements on these samples were repeated by following growth of 210Bi from 210Pb.

Chemical procedure and counting methods

The chemical procedures adopted for 32Si have been described in detail earlier (Reference NijampurkarNijampurkar, unpublished). Briefly, chemically pure silica was obtained from the hydroxide scavenge after repeated digestion with HC1 and dehydration. This silica was purified by dissolution in NaOH and a phosphorus carrier was added. 32P, the daughter product of 32Si, was allowed to grow to a known degree of saturation and separated. It was radiochemically purified by ion exchange and selective chemical precipitation and ignited to Mg2P207. The procedure for 210Pb is similar to that adopted earlier (Reference BhandariBhandari, unpublished) for its determination in rain water. In short, after the addition of Pb and Bi carriers, iron was removed by ether extraction, and lead and bismuth were separated as sulphides. In most cases, 2l0Pb was estimated by following the decay of its daughter 210Bi. Bismuth was milked from the nitric acid solution of the sulphides at pH 3 as the Dalzin complex (Navratna, unpublished). Bi was back extracted in 4M HN03 and ignited as bismuth phosphate. In other cases, lead was purified by dichromate precipitation, followed by ion exchange separation and growth of 2l0Bi was followed in the lead sulphate sample.

The counting procedures required that the activity of 32P from 32Si and of 210Bi from 2l0Pb was followed for several half lives. Gas–flow, thin–window GM counters were used for this purpose. The background of the 32P counters was around 2 cph and of the 2l0Bi counters was around 8 cph, which were quite stable over long periods of counting. The total β activity was also measured in these counters (Table 4) after depositing the dried samples on 4 cm2 area source holders. Some of the samples were extremely hygroscopic and the values are not precise because of the possibility of the presence of some moisture (Table 4). The gamma counting was carried out on a 7.5 cm by 7.5 cm well–type NaI counter in anti–coincidence with a plastic scintillator (Table 5).

Results

The results on 32Si in surface samples are given in Table 1. Since the activity was small, between 2 and 9 counts per hour, all the samples were milked for 32P a second time for confirmation. The activity in NG–4, which gave consistently higher values, and NG–6, the snout ice sample, which is quite crucial for estimating glacier age, was checked by a third milking. The replicate measurements are in agreement within ±10%. In the initial milkings, the natural phosphorus going through the silica–extraction procedure was not estimated but this was done subsequently and found to be rather small except in NG–5 (first milking).

TABLE IV.

TOTAL β ACTIVITY OF Neh–nar GLACIER SAMPLES

210Pb activity measured in aliquots taken from samples where 32Si was also measured is given in Table Ha. Since the 210Pb activity is high, as can be seen from this table, it was measured in a 5 or 10% fraction of the sample. The water equivalent for 210Pb was calculated on the basis of the assumption that the efficiency of lead scavenging is the same as that for silica. In two samples, NG–1 and NG–2, there is an uncertainty, as shown in column 2 of Table 2a, in estimating water equivalent for lead samples, since the yield of silica had been low, possibly due to loss of water during scavenging. This uncertainty does not affect the 32Si data given in Table 1 but it has to be considered for 21°Pb because its carrier was not added to the water before scavenging lead. The counting rates, at saturation, vary from 25 to 94 counts per minute and there is about 1% statistical error in estimating 2l0Pb activity. The error in estimating the quantity of water is therefore more serious. This uncertainty does not occur in other surface samples where small volumes of water were collected and hence these measurements are more reliable.

TABLE V.

Gamma activity of Neh–nar Glacier Core Samples*

The total β activity in the samples ranged from 1 to 20 dpm/kg (Table 4). As mentioned earlier, there is some uncertainty in the results given in this table due to self absorption correction because first the counting efficiency and absorption coefficient (0.364 and 0.0283 cm2/g, respectively) were taken to be the same as that for 210Bi. This may not be true. Secondly, there may be additional absorption due to some moisture present. These effects, are not serious and the values given in Table 3 can be assumed to be correct within ±20%.

The results on core samples are given in Tables 3, 4, and 5. Two of the samples, 10 and 12, were measured for decay of 210Bi separated from 2l0Pb as well as by following in situ growth of 2l0Bi from 2l0Pb, from which 210Bi was initially removed. Both of them yield values which are in close agreement. The core data are also shown in Figure 4. The γ activity shows a signal of 0.66 MeV (Table 5). Although this can be attributed to l37Cs, the identification must remain Figure 4 tentative because the signal is marginal except in one case and not quite specific; further experiments are necessary to confirm it. The main features of these data can be summed up as follows:

Fig. 4.

Depth profile of210Pb,.137Cs, 40K, tolal β activity (dpm/kg ice and dpm/g dust), TDS, and dust content in the Neh–nar core samples.

  • (1) The 2l0Pb activity in the fresh snow (1976) from the accumulation zone is about 4.5 dpm/kg. This is similar to the activity observed elsewhere in fresh precipitation (Reference Crozaz and LangwayCrozaz and Langway, 1966; Lai and others, 1979; Reference BhandariBhandari, unpublished). It decreases to ⩽0.3 dpm/kg within 500 m of the glacier head. Although samples are limited in number, based on 210Pb in surface samples, five regions (A, B, C, D, and E) can broadly be classified as shown in Figure 3: (i) above 4 300 m altitude having values of ⩾4 dpm/kg; (ii) between 4 200 and 4 350 m with values <0.9 dpm/kg; (iii) between 4 130 and 4 200 m with values between 2 and 4 dpm/kg; (iv) between 4 000 and 4 100 m with values below 0.6 dpm/kg; and (v) below 4 000 m to the snout with values between 1 and 2 dpm/kg. These five zones, with alternating high and low values of 2l0Pb, seem to be a unique feature of this glacier (Fig. 5), in contrast with a decreasing activity observed in Alpine glaciers (Reference CrozazCrozaz, 1967).

  • (2) There is a well-defined peak in 2l0Pb, total Bβactivity, and dust content at 2–3 m depth in the core (Fig. 4). The 210Pb activity here of 32 ± 3 dpm/kg is about four times higher than expected in fresh precipitation.

  • (3) In the deeper sections of the core, 40K activity is higher than in the upper strata but 40K activity is not associated with 210Pb or l37Cs.

  • (4) The total dissolved salts (TDS) also increase with depth, being correlated with potassium content,

  • (5) The 32Si activity ranges between 0.37 dpm/ton in fresh snow of 1976 from near the accumulation zone to 0.18 dpm/ton in the snout ice sample (Fig. 2). The snout water has a value of 0.42 dpm/ton, higher than in any other sample.

Discussion

The results on the distribution of radioactivity in the core and surface samples can be used to understand the behaviour of Neh–nar Glacier in the past on the time–scale of a few decades to a few centuries, over which the activities of 2l0Pb and 32Si are sensitive. We first discuss the expected natural levels of these two isotopes and then the results from the core and surface samples.

Natural fall–out levels of 32Si and 2l0Pb

For the purpose of dating ice samples, we ought to know the average fall–out values of natural 32Si and 2l0Pb and should be able to exclude any bomb contribution. The annual fall–out value is highly variable from year to year. The fresh (1976) snow (NG–2) value of 0.37 dpm/ton for 32Si, thus, cannot be taken as the average for the last 1 000 years. The fall–out values determined at other locations (Reference DansgaardDansgaard and others, 1966; Reference LalLal and others, 1979) vary between 0.3 and 1.1 dpm/ton. The fall–out at Pathankot (lat. 320 14’N.) between 1961 and 1968 varied between 0.27 and 0.7 dpm/ton, and at Ludhiana (lat. 30° 56’ N.) it was 0.38 dpm/ton in 1970 (Lai and others, 1979). However, in view of the bomb contribution, Lai and others (1979) adopted a value of 0.3 ±0.1 dpm/ton for the Indian sub–continent. The fall–out in Denmark is, however, around 1 dpm/ton with values going up to 4.7 dpm/ton for peak periods of nuclear fall out. Furthermore, Neh–nar Glacier lies at lat. 34° N„ which is close to the region where a transition in fall–out pattern takes place; between lat. 0° to 30° the 32Si activity is around 0.3 dpm/ton, whereas north of lat. 40° it is 0.95 dpm/ton (Reference Lal and PetersLal and Peters, 1967). A small change in atmospheric structure or circulation pattern may thus change the 32Si value in precipitation at the Neh–nar Glacier site. Neglecting such variations, a value of 0.5 ± 0.1 dpm/ton must represent Figure 5 the long–term average natural fall–out value for 32Si at the Neh–nar Glacier site, in view of the above considerations. Here we adopted the value of 0.5 dpm/ton for the purpose of calculating the ages of ice samples.

Fig. 5.

Model ages of surface ice samples based on 32Si-210Pb measurements. The contours show velocity in m/year for 1977–78 as measured by the GSI stake experiment.

There is also evidence for production of 32Si in nuclear tests. However, this production has been estimated to be marginal, confined to a brief period of one or two years, and here we ignored the bomb contribution. For 2l0Pb, the 1976 fresh–snow value (NG–2) is 4.2 dpm/kg but a value of 4.7 dpm/kg for sample 2 has been obtained. In view of the work of Reference JoshiJoshi and others (1969), in which fall–out at various nearby locations was measured, we assumed here a value of 8 dpm/kg for the average fall–out of 210Pb at the Neh–nar Glacier site over the last 100 years.

Radioactive horizon in the core section

The 0–1 m section of the core shows an activity of ⩽1 dpm/kg, corresponding to an age of ⩾100 years. Such a large age is expected in the ablation zone. However, between 2 and 5 m (Table 3) the activity levels are much higher, ranging between 3 and 35 dpm/kg.

The natural levels of 210Pb in precipitation at lat. 34° 06’ N. (Srinagar) between 1962 and 1965 were between 6 and 8 dpm/kg (Reference JoshiJoshi and others, 1969), and the same is true for other locations. We now discuss the observation of high 2l0Pb and its correlation with fission–produced l37Cs, dust, and total βactivity between 2 and 3 m in the core. One possibility is their origin in nuclear tests. However, we must consider and exclude other possibilities before this horizon is attributed to nuclear debris. First, although bomb production of 210Pb has been considered likely (Reference JaworowskiJaworowski and others, 1975), there is no clear indication of it in air or precipitation samples as indicated by the following observations. Reference BhandariBhandari (unpublished) found only marginally high values of Pb in the lower stratosphere after the peak explosion era (1964–65) but during the same period rain water in Bombay and other stations in India showed near–natural levels. Reference Georgieva and DimchevGeorgieva and Dimchev (1977) have observed a large altitude dependence in specific activity of 2l0Pb and fission products in Bulgarian precipitation. The 1978 precipitation samples collected at Gara and Changme Khangpu Glaciers, situated in northern and eastern Himalaya, show activity levels of only ⩽ 8 dpm/kg (Reference BhandariBhandari and others, unpublished). Reference JaworowskiJaworowski and others (1975) have determined 210Pb, 137Cs, 226Ra, and U in several samples, including the Nepalese Himalaya from 1957 to 1970. They found that 137Cs and 2l0Pb activity peaks and troughs occur together. Reference CrozazCrozaz (1967) found no evidence for bomb–produced 2l0Pb in Greenland, Antarctic, and Alpine glaciers. Secondly, volcanic eruptions make an additional contribution to 210Pb. However, the amount of volcanic dust should be correlated with potassium content and 40K may serve as a monitor of the volcanic component. Thirdly, the Himalayan glaciers have a rather high content of dust which is non–volcanic in origin. The dissolved salt content in surface and core samples varies from 10 mg/kg to about 1 g/kg water, and the (undissolved) dust content from less than 10 mg to 500 mg/kg. Although, in general, the 2l0Pb activity is found not to be proportional to dust content (Table 4), sample 12 (Table 3) containing the highest 2l0Pb activity has also the highest dust content of 500 mg/kg. Thus, it is important to estimate the 210Pb contribution from the local dust. Finally, the natural source of 210Pb is atmospheric radon and its concentration levels can be influenced by the atmospheric circulation pattern. Evidence has been found for concurrent changes in 210Pb, l37Cs, 226Ra, and U isotopes (Reference JaworowskiJaworowski and others, 1975), which suggests that their deposition is influenced by atmospheric circulation. Similar correlations can be clearly seen from Figure 4 where 2l0Pb, 40K, total β activities and TDS, and dust content, are plotted as a function of depth in the core sections. This correlation also exists for the 0.66 MeV signal which is possibly due to l37Cs. 210Pb, total β activity (dpm/g), and dust seem to correlate well with each other, all showing a maximum at 2–3 m depth. On the other hand, K and TDS content also correlate with each other but not with the tracers mentioned above or with the dust content.

If this horizon is due to nuclear debris, it is difficult to identify the nuclear explosion which may have been responsible for it. Reference JaworowskiJaworowski and others (1978) suggested a large injection of 210Pb from the 27 June 1973 Chinese explosion in which advanced fusion devices were used. Before and in the early I960’s several high-yielding nuclear tests were carried out by the USSR and USA. The absence of young ice above this radioactive horizon excludes the possibility that the ice is gradually accumulating at this site since the radioactive debris was deposited. In fact, the overlying layers of ice are older than 100 years, as pointed our earlier, and this calls for a complicated mechanism of accumulation and transport. It may be mentioned that the coring site is close to the equilibrium line but is located in the ablation zone (Fig. 3). Therefore, the presence of this radioactive horizon is not understood at present. Additional work on nearby bore holes is in progress to obtain further information on this aspect. Notwithstanding the production of 210Pb in nuclear debris, all samples below 6 m show negligible activity of 2l0Pb and are at least 100 years old.

Radioactivity in surface samples

The distribution of 32Si and 210Pb activity is shown in Figures 2 and 3, respectively. The annual and the perennial channels are also marked on these maps. It is seen that the movement of ice, particularly in the lower half of the glacier, is very complicated because ice from different accumulation zones and the tributary mixes here.

In the 210Pb distribution we find several zones, as noted earlier, of high and low activity, whereas the 32Si activity is rather uniform, except near the snout. Based on the observed 32Si and 210Pb activity and the adopted values of average specific activity in the fall–out (32Si = 0.5 dpm/ton, 2l0Pb= 8 dpm/kg), we can calculate the “apparent” ages of ice samples which give the time elapsed since precipitation if the system has been a closed one. These are given in Tables I and II, respectively, for 32Si and 210Pb. These range up to 450 years for 32Si and ≈lOO years for 210Pb. However, as noted earlier, the two “apparent” ages in the samples are not concordant, indicating that the assumption of a closed system is not valid. Part of the discrepancy may be due to the sampling problem, in which fresh melt waters may have mixed with the local ice during sample collection. Alternatively, it could also be a result of natural mixing of fresh and old waters in the glacier. Since the behaviour of temperate glaciers, particularly in the Himalaya, where high altitude, high wind currents, and variable temperatures play a dominant role, melting and refreezing cannot be ruled out. In any case, if we consider a simplified two–component model, one component referring to “fresh”ice (<10 years) and the second to “old” ice (> 100 years) on the basis of 210Pb half life, we can correct for both the natural mixing and that during sampling, and obtain the real age of the glacier. The high 210Pb horizon seems to be contained around a depth of 2–3 m just below the equilibrium line. It may be buried much deeper at higher altitudes and may have ablated away down–stream. Therefore, it is not necessary to consider this horizon in the model, but the effects of alternating zones are automatically taken into account in the mixing model.

Using the two–component model, the correction for the fresh–water contribution (Cfw) is applied as follows:

(1)

where A and F refer to the observed activity in a sample and the fall–out value. The corrected 32Si activity (A c32) is then given by

(2)

Subscripts 210 and 32 are used for 210Pb and 32Si, respectively.

The age, T, of the sample is calculated based on this value from the relation:

(3)

Assuming F 32 =0.5 dpm/ton and F210= 8 dpm/kg as discussed earlier, the value of T so obtained is taken to be the correct age of the sample. The fresh-water contribution is estimated, from 210Pb values, to be 14–30% for various samples. If the actual fresh water contribution is less than that determined here, the ages will represent upper limits to the true age of the sample. Thus the uncorrected (or apparent) and corrected 32Si ages represent the range within which the true age must lie.

The corrections are applied to all samples except fresh snow sample NG-2, which is known to have an age of 1 year. The 32Si activity corrected for 2l0Pb and the corrected 32Si ages are given in Table 6.

The two–component model ages range up to 850 years. The age distribution is also shown in Figure 5. These ages must be discussed within the limitation of this model. In reality, ices of all ages mix but it is difficult to consider such a complex situation based on only two tracers. Also, the fluctuations in specific activity of tracers in fall–out have not been taken into account in these deductions, and there is some uncertainty in the value of the decay constant of 32Si. If the actual fall out value is 0.6 dpm/ton, the corrected age of the snout ice will increase to 1 080 years compared with 850 years based on the value of 0.5 dpm/ton.

TABLE VI.

32Si and 2l0Pb activity in surface samples and ice ages

Chronology of Neh–nar Glacier

The model ages calculated above indicate the time taken for the ice to reach the sampling site from the time of precipitation. We consider here the significance of the ages of some of the samples. Sample NG–1 was collected at an altitude of 4 140 m and this site is fed by ice from the main accumulation zone; there are no contributions from the tributaries. The annual rate of ice movement around the altitude of 4 150 m is 20 m/year increasing upwards (Fig. 5), as measured by the movement of stakes (Reference VohraVohra and others, unpublished), and at this rate the ice must take only <75 years to move a distance of 1.5 km from the accumulation zone. Thus the high 32Si age of 440 years indicates that the ice spends a considerable time in the accumulation zone before it moves along the glacier. This period should be of the order of 300 years but more high–altitude sampling should be done to determine this value. Sample NG–2 was collected from near the subsidiary accumulation zone, which partly feeds the site where NG–3 was collected. The age of NG–3 of about 370 years is thus understood in terms of multiple sources where older ice from the main accumulation zone is continually mixed with young ice from a subsidiary accumulation zone. Sample NG–4, which represents the melt water of the whole glacier at the snout, gives a much younger age, c. 150 years, and thus, although it is derived from ice of all ages, a large fraction of this water seems to be relatively young. Alternatively, it is also possible that the specific activity of the melt water increases due to evaporation losses during its movement along the glacier surface. Sample NG–5, representing snout ice of the tributary, gives an age of 580 years, again giving an average rate of movement of c. 3 m/year. The snout sample NG 6, which represents the snout ice of the main glacier body, gives an age of 850 years, thus giving an average rate (after correcting for the time spent in the accumulation zone) of about 4 m/year between an altitude of 4 300 m and the snout. These are only approximate estimates in view of the uncertainties discussed above, particularly due to multiple accumulation zones.

The most reliable rate of movement of the ice can be calculated for the lower glacier by comparing the ages of samples NG–3 (altitude 4 020 m) and the terminus which is not affected by the complicated dynamics of accumulation zones. This rate turns out to be c. 2 m/year.

The measured annual velocity contours for 1977–78 based on the GSI stake experiment are shown in Figure 6. The rates vary from about 40 m/year at an altitude of about 4 200 m to 5 m/year at 3 950 m. The measured rates are about 2.65 m/year near the snout. The rate measured here of 2 m/year, which is an average over the past 500 years, is comparable with this annual rate of movement.

Acknowledgements

We thank Dr D. I. Bhatt, Dr H. R. Prabhakara, Dr S. Desai, Mr K. M. Suthar and Mr D. Chandrasekhar for assistance at various stages of this work. Our appreciation is due to several colleagues at the GSI, especially S/Shri R. K. Singh, A. R. Sharma, and D. Srivastava for active participation in the field work, to Dr G. S. Murty for information about nuclear tests, to Dr B. Sahai for satellite imagery, and to Professors S. Krishnaswami and B. L. K. Somayajulu for valuable comments. We are grateful to Professor K. R. Ramanathan and Shri V. S. Krishnaswamy, Director General, GSI, for encouragement to undertake this programme.

Post Scripts

Since this paper was written, the half life of 32Si has been revised to 105 ± 18 years by Reference ElmoreElmore and others (1980) and Reference KutscheraKutschera and others (1980), as compared to 300 years used in this paper.

Also, the fall-out value of 32Si appears to be closer to 0.7 dpm/ton as compared to 0.5 dpm/ton adopted in this paper. These new parameters yield a snout age of 500 years and an average glacier flow velocity of ≈6 m/year.

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Figure 0

Fig. 1.Location of Neh–nar Glacier in the Sind catchment area.

Figure 1

Fig. 2.Distribution of32Si activity (dpm/ton) in surface samples of Neh–nar Glacier.

Figure 2

Table I.32Si activity and ages in surface samples of Neh nar Glacier

Figure 3

TABLE IIa.210Pb activity and ages of the surface samples of Neh nar Glacier

Figure 4

TABLE IIb.210Pb activity and ages of surface samples of Neh–nar Glacier

Figure 5

Fig. 3.Distribution of 210Pb activity (dpm/kg) in surface ice of Neh–nar Glacier.

Figure 6

TABLE III.210Pb activity in Neh–nar Glacier core samples

Figure 7

TABLE IV.TOTAL β ACTIVITY OF Neh–nar GLACIER SAMPLES

Figure 8

TABLE V.Gamma activity of Neh–nar Glacier Core Samples*

Figure 9

Fig. 4.Depth profile of210Pb,.137Cs, 40K, tolal β activity (dpm/kg ice and dpm/g dust), TDS, and dust content in the Neh–nar core samples.

Figure 10

Fig. 5.Model ages of surface ice samples based on 32Si-210Pb measurements. The contours show velocity in m/year for 1977–78 as measured by the GSI stake experiment.

Figure 11

TABLE VI.32Si and 2l0Pb activity in surface samples and ice ages