JournaL of GLaoioLog y, Vo 1. 31, No. 107, 1985 RADIOMETRIC CHRONOLOGY OF CHANGME-KHANGPU GLACIER, SIKKIM By V. N. NIJAMP URKAR , N . BHANDARI, D. V. BOROLE, (Physical Research Laborator y, Ahmedabad , India) and U. BHATTACHARYA (Glaciology Division, Geological Survey of India, Lu cknow, India ) ABST RACT. The 32Si conce nt ra ti on in a sample of surface ice from the sno ut or Ch angme- Khang pu glacier is 0 .36 disint egrations per minut e/to nne compa red to th e rail-out value of 0.7 d.p.m./tonn e. If thi s d ecrease is ass umed to be solely du e to decay of 32Si, an age of c. lOO years is estimated for the surface ice of th e snout , leading to an fl ow veloc it y of c. 40 m/ year for the past century. A ve rti ca l profil e of ' ° Pb in a co re taken at an altit ud e of 5040 m shows two hori zo ns where this iso tope is e nri ched, on e between 3 a nd 4 m and ano th er be tween II a nd 12 m , indi ca ting that the primary co nce ntr a tion of "o Pb ca n change by ph ys ico- chemi ca l processes like adso rption on dust. None th e less, a long itudin al p ro fil e along the glacier sh ows a systema ti c dec r ease of 'l oPb activit y w ith d ec r easing altitud e, the s urf ace ice of the snout giv in g a va lue or O.2 d . p.m./ I, corres po nding to a n age of 100 years which is con co rdant with the 32 Si age. Thi s s urface fl ow-rate of th e glacier is mu ch larger than the average co nt e m- p ora ry fl ow- ra te (c. 13 m / yea r). Th e di ffe rence ca n be und erstood in te rms of th e past history of ad va n ce a nd recession of the glac ier as revealed by th e geo morphic evidence. R ESUME. Chronolofie radiomilrique du glacie r Chan gme-Khang pu , Sikkim. La co nce ntr ation en ' Si d ' un echantillon de surface pres du fr o nt du Ch angme-Kh angpu es t de 0,36 d.p.m./tonn e alors qu e cell es d es pr ec ip- ita ti ons est de 0,7 D ans le cas DU ce ll e va ri a ti on n'est due qu 'a la decroissance du 3 Si, on es time it 1 00 ans I' age de la surface pres du fro nt , cc qui don ne des vit esses moye nn es d'ecoul ement de 40 m/an sur le derni er siecle. Un pr ofil verti ca l du 210 Pb po ur une ca ro lt e prelevee :l. 5040 m d 'altitude montre deux ho ri zo ns d'e nri chissement isotopique, I' un vers 3--4m, I 'au tre vers 11- 12 m, indiqu ant que la conce ntra ti on ori ginelle du "oPb peut et re modifi ee par d es process us ph ys ico-chimiqu es tels qu e I' ad so rpti on sur les poussieres. Enfin un pro fil long itudin al mo ntr e un e I NTR ODU CTI ON Changme-Khangpu glacier is located in the Tista river baSin, in the upper catchment of one of its major tributaries, Lachung Chu, in the Sebu valley of north-east Sikkim. It is a transverse valley glacier trending north-south, having a length of 5.87 km and width varying between 600 m to 1 km. The glacier originates from the southern slope of Gurudongmar peak (lat. 27°58'N., long. 88°42'E.) which is an in- tegral part of the Khangchengyao-Pauhnri massif. The melt water of the glacier feeds into the Sebu Chu, a tributary of Lachung Ch u. The area1 coverage of the glacierized part of the Changme-Khacngpu basin is about 12.85 of which 10.35 comes under perennial ice cover and the rest is covered by rock surfaces only. The ratio between the accumulation and ablation zone areas is 5:7. Almost the entire ablation zone of the glacier is under a thick mantle of suprag1acia1 morainic debris (Bhattacharya, unpublished). The terminal part of the glacier is completely buried under the moraine which has generated enormous amounts of dust which is present everywhere. Presence of eng1acia1 dust is also well documented in the darker ice bands exposed in the vertical to sub- vertical faces. The lowest part of the snout is located at an altitude of 4850 m a.s.1. This part, when exposed from the suprag1acia1 debris, is about 3 m in height and 7 m in width. 28 decroissa nce systematique de I 'ac ti vite du 2l oPb avee l 'a ltitudc, avee 0 ,2 d.p.m./I au fro nt , ce qui co rres po nd it un age d'un siecle en co ncor da nce avee ce lui du 32Si . Par co ntre la vitesse d'eco ul ement es t beauco up plus Hevee qu e ce lle actuell e ( c. 13 m /a n). Cette difference peut e tre a ttribu ee:l. I'histoire passee du glacier avec d es ava nces et recul s qui so nt re vC/ es par d es evidences geomo rph ologiq ues. Z USAMMENFASSUNO. Radiomel r ische Chron%gie d es Changme-Khangpu- Cle15chers , Sikkim. Oi e 32Si-Kon ze ntr a ti on in e in er Probe d es O be rfl iiche nei ses a us d er Zun ge des Changme-Kh angpu-Gl etschers be tr iigt 0,36d.p.m./t, wii hrend der Niedersc hl agswe rt 0,7 d .p.m./t ist. We nn diese Abna hme le digli ch dem Zerf a ll vo n 32Si zu geschrieben w ird , so ergibt sich fUr d as Zungeneis ein Alter von e twa 100 J a hr en, woraus e in e mittlere Fliessgeschwindigk eit von circa 40 m pro Ja hr fUr das letzte J a hrhund ert abgeleitet we rd en kann. Ein Ve rtik al-Profil von "oPb in e in em Kern a us einer Hohe vo n 5040 m weist zwei Horizonte mit Anr eiche run g d ieses Iso topes a uf , eines zwischen 3-4 m und ein anderes zwischen I 1- 12 m; di es liiss t dara uf schliessen, d ass die urspriingli che 2l oPb-Konze ntr a ti on inr olge von ph ys iko-chemischen Prozessen wie der Aufnahme von St aub sc h- wa nk en ka nn . Trotzdem zeigt e in L ii ngsp ro fil uber den Gl etscher eine sys tematische Abn ahme der 2I oPb- Aktivit ii t mit der Hohe, wobei das Ober fliic heneis der Zunge ei nen W en von 0,2 d.p.m./I ergibt; di es ent- spricht ein em A lt er von 100Ja hren, in O bereinstimmun g mit dem Wert aus der 32Si-Analyse. Die oben gena nnt e Fliessgeschwindigke it d es Glet sc hers ist weit hoher als die derzeitige mittl e re Fliessgeschwindi gke it (c . 13 m pr o Ja hr ). Oer U nt erschied kann aus dem histo ri sc hen Ab laufv on Vorst osse n und R uc kzu ge n d es Gletsc hers e rkl ii rt werden, der sich aus geomor- phologischen Beobachtunge n erg ibt. The Geological Survey of India has been studying the annual mass balance on this glacier since 1977. In order to obtain the flow rates of ice we have dated the snout using the isotope 32 Si produced by cosmic rays (La1 and Peters, 1962). In addition, we have measured the longitudinal and vertical profiles of 210 Pb. Nuclear debris, mainly from the Chinese nuclear tests, was detected in fresh snow (8handari and others, 1982) and its vertical profile in the accumulation zone enabled us to determine the net accumulation rate of ice in this glacier to be 70 cm/year (Shukla and others, 1983). The results are discussed here in ter ms of glacier dynamics and its past history as determined from the geomorpho10gica1 studies. EX PERIMENTAL TECHNIQUE S Three types of samp1 es were co11 ected for the present study. Samples of snout ice (2.38 tonnes) and of snout water (1.64 tonnes) were collected and pro- cessed for 32 Si and 210 Pb (Table I). Small samples of surface ice were also collected from some locations for 210 Pb, and 6 18 0 analysis (Fig. 1). A core was taken at 5040 m altitude and one metre sections of ice were melted in plastic bottles for analysis of 210 Pb, total dissolved solids, total beta activity, some chemi ca 1 const ituent s, and i sotopi c ra t i 0 of oxygen. On the night of 28 August 1978, snow precipitation occurred all over the glacier. Samples (nominally