Ž.Chemical Geology 181 2001 147–167www.elsevier.comrlocaterchemgeoThe altitude effect on the isotopic composition of tropical rainsRoberto Gonfiantinia,), Michel-Alain Roche1, Jean-Claude Olivryb,Jean-Charles Fontes2, Gian Maria ZuppicaIstituto di Geocronologia e Geochimica Isotopica del CNR, Area della Ricerca di Pisa, Via G. Moruzzi 1, 56124 Pisa, ItalybInstitut de Recherche pour le DeÕeloppement, Montpellier, France´cDipartimento di Scienze Ambientali, UniÕersita Ca’ Foscari, Venice, Italy`Received 11 September 2000; accepted 1 March 2001AbstractData on the Isotopic composition of yearly and monthly precipitation samples collected at various altitudes on MountCameroon, Africa, and in two transects from the Amazon to the Altiplano in Bolivia, South America, are presented. InBolivia, the2Hr1H and18Or16O ratios show seasonal variations, with lower values in the summer rainy months withrespects to the winter dry ones. Thed2H andd18O values are linearly correlated with a slope of 7.5 in all seasons, but theintercept is higher in winter than in summer.Thed-gradient vs. altitude is larger in rainy periods. The isotopic data are fitted by using a numerical model based onRayleigh adiabatic condensation process. Model simulations show that thed–altitude relationship slightly deviates fromlinearity, because the slope increases with altitude due to the lowering of temperature and the consequent increase of thecondensation rate of atmospheric vapour. The parameters which most affect the shape ofd–altitude relationships are theŽ.temperature vertical gradient lapse rate and the initial relative humidity of the ascending air masses, while a change of theinitial isotopic composition of water vapour determines a shift of the curve along thed-axis. In addition, the model explainsthe observed increase of the deuterium excess with altitude. q 2001 Elsevier Science B.V. All rights reserved.Keywords: Altitude effect; Hydrogen isotopes; Oxygen isotopes; Precipitation; Rayleigh condensation1. IntroductionIt is well known that the hydrogen and oxygenheavy isotope contents of rainwater decrease withincreasing altitude. This is attributed to the progres-)Corresponding author. Tel.: q39-050-315-2358; fax: q39-050-315-2360.Ž.E-mail address: [email protected] R. Gonfiantini .1Present address: 1, Cour de la Commanderie, 17000 LaRochelle, France.2Formerly at Laboratoire d’Hydrologie et de Geochimie Iso-´topique, Universite de Paris Sud, Orsay, France.´sive condensation of atmospheric vapour and rainoutof the condensed phase, which take place when airmasses climb up along the slopes of high mountainsand cool off as a consequence of adiabatic expan-sion. As the2Hr1H and18Or16O ratios are slightlyhigher in the liquid water or ice removed by precipi-tation than in the residual vapour, the latter becomesprogressively depleted in heavy isotopes.In this paper, we report and discuss the excellentexamples of altitude effect on the isotopic composi-tion of precipitation observed in stations from sealevel to the top of Mount Cameroon, Africa, and intwo transects from the Bolivian Amazon up to the0009-2541r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved.Ž.PII: S0009-2541 01 00279-0()R. Gonfiantini et al.rChemical Geology 181 2001 147–167148Altiplano, South America. The data are discussed interms of a Rayleigh condensation process.The precipitation samples were collected andanalysed several years ago at the instigation of Jean-Charles Fontes. The data were partly presented andŽ.discussed by Fontes and Olivry 1976, 1977 , Gonfi-Ž. Ž.antini 1998 and Roche et al. 1999 . However,most of the data are published and discussed here indetail for the first time, as well as the model elabora-tion presented in Appendix A.2. Precipitation regime and sampling sites2.1. Mount CameroonMount Cameroon is an isolated ancient volcano inproximity of the Atlantic coast in Equatorial Africa,ŽXX.whose peak 4813 N, 9810 E reaches the altitude of4095 m asl. The humid air masses coming from theocean climb up along the mountain slopes and pro-duce precipitation. Rains are very abundant in coastalstations: for instance, a mean value of 9895 mmyeary1over 38 years has been recorded at Debund-Ž.scha Station 17 in Table 1 , with a maximum valueŽ.of 14,694 mm in 1919 Fontes and Olivry, 1977 .The precipitation decreases with increasing altitudeand distance from the sea.Information on long-term mean monthly tempera-ture and amount of precipitation are available inWMO records for several meteorological stations inŽ.Cameroon WMOrOMM, 1996 . The closest stationŽXto Mt. Cameroon is Douala Observatoire 4800 NX.and 9844 E, 9 m asl located 65 km southeast of themountain peak. The mean precipitation in Douala is3854 mm yeary1, with monthly values ranging from755 mm in August to less than 40 in December andJanuary. Thus, 75% of precipitation falls from Juneto October. The mean air temperature is 26.78C, withmonthly values ranging from 24.98C in August, themost rainy month, to 28.1 in February. The meanrelative humidity is 88% in the full rainy season anddrops to 80% in the dry season. The mean verticalgradient of air temperature, measured by radio bal-loons, is y5.38Ckmy1and it is practically constantŽ.throughout the year Fontes and Olivry, 1977 .Yearly precipitation samples were collected dur-Ž.ing a 4-year period 1972–1975 at 20 stations withTable 1Mount Cameroon: station altitude, distance from the Atlantic Ocean, and amount of precipitation.Ž.No. Station Altitude Distance from Amount of precipitation mmŽ. Ž.m asl the ocean km1972 1973 1974 1975 Mean16 Bakingele 10 0.6 7930 5365 6785 6565 666117 Debundscha 20 0.8 9200 8430 8015 8350 849919 Idenau 30 2.5 7915 6870 7530 7140 736415 Batoki 50 1.0 6130 4590 5295 5115 528322 Brasseries 180 10.5 4455 1990 2700 2150 282420 Bomana 460 11 5525 5050 5870 548225 Bonakanda 860 21.5 3730 2990 3225 3140 327114 Foret SW 1000 8.5 7210 4810 8735 8290 7261ˆ23 Upper Farm 1100 17 3730 2210 2725 2665 28336 Route VHF 1610 25 4070 2850 3175 3515 340313 Limite Foret SW 2320 13 3945 2270 2825 3240 2759ˆ4 Station VHF 2460 28 3945 2270 2825 3240 27597 Versant Nord 2475 29 3050 2600 1950 3425 27563 Versant Nord 2500 28 3315 2735 3050 3125 305624 Limite Foret BUEA 2500 20 3800 2680 3075 3200 3189ˆ9 Hutte 2 2925 21 3120 2310 2725 3050 280112 Versant SW 3000 16 3030 1830 2475 2900 25598 Nord 3050 26 2480 1900 2025 2725 228311 Southwest 3300 19 2100 1400 1550 2075