G 3. AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society

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1 Geosystems G 3 AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society Article Volume 10, Number 2 6 February 2009 Q02005, doi: ISSN: Click Here for Full Article Water and gas chemistry at Lake Kivu (DRC): Geochemical evidence of vertical and horizontal heterogeneities in a multibasin structure F. Tassi Department of Earth Sciences, University of Florence, Via G. La Pira 4, I Florence, Italy (francot@geo.unifi.it) O. Vaselli Department of Earth Sciences, University of Florence, Via G. La Pira 4, I Florence, Italy Institute of Geosciences and Earth Resources, CNR, Via G. La Pira 4, I Florence, Italy D. Tedesco Department of Environmental Sciences, Second University of Naples, Via Vivaldi 43, I Caserta, Italy Office for the Coordination of Humanitarian Affairs, United Nations, Palais de Nations, Geneva 10, CH-1211 Switzerland G. Montegrossi Institute of Geosciences and Earth Resources, CNR, Via G. La Pira 4, I Florence, Italy T. Darrah Environmental and Earth Sciences Department, University of Rochester, 227 Hutchison Hall, Rochester, New York 14627, USA E. Cuoco Department of Environmental Sciences, Second University of Naples, Via Vivaldi 43, I Caserta, Italy M. Y. Mapendano Goma Volcano Observatory, Mount Goma, Goma, Democratic Republic of the Congo R. Poreda Environmental and Earth Sciences Department, University of Rochester, 227 Hutchison Hall, Rochester, New York 14627, USA A. Delgado Huertas Estacion Experimental de Zaidin, Prof. Albareda 1, E Granada, Spain [1] Waters and dissolved gases collected along vertical profiles in the five basins (Main, Kabuno Bay, Kalehe, Ishungu, and Bukavu) forming the 485 m deep Lake Kivu (Democratic Republic of the Congo) were analyzed to provide a geochemical conceptual model of the several processes controlling lake chemistry. The measured horizontal and vertical variations of water and gas compositions suggest that each basin has distinct chemical features produced by (1) different contribution from long circulating fluid system containing magmatic CO 2, responsible of the huge CO 2 (CH 4 )-rich reservoir hosted within the deep lake water; (2) spatial variations of the biomass distribution and/or speciation; and (3) solutes from water- Copyright 2009 by the American Geophysical Union 1 of 22

2 rock interactions. The Kabuno Bay basin is characterized by the highest rate of magmatic fluid input. Accordingly, this basin must be considered the most hazardous site for possible gas outburst that could be triggered by the activity of the Nyiragongo and Nyamulagira volcanoes, located a few kilometers north of the lake. Components: 11,429 words, 8 figures, 5 tables. Keywords: Lake Kivu; lake water geochemistry; limnic eruption; geochemical modeling. Index Terms: 1009 Geochemistry: Geochemical modeling (3610, 8410); 1065 Geochemistry: Major and trace element geochemistry; 1041 Geochemistry: Stable isotope geochemistry (0454, 4870). Received 28 July 2008; Revised 1 December 2008; Accepted 29 December 2008; Published 6 February Tassi, F., O. Vaselli, D. Tedesco, G. Montegrossi, T. Darrah, E. Cuoco, M. Y. Mapendano, R. Poreda, and A. Delgado Huertas (2009), Water and gas chemistry at Lake Kivu (DRC): Geochemical evidence of vertical and horizontal heterogeneities in a multibasin structure, Geochem. Geophys. Geosyst., 10, Q02005, doi:. 1. Introduction [2] Lake Kivu (1460 m above sea level (asl)), the smallest of the African Great Rift lakes, lies on the border between the Democratic Republic of the Congo (DRC) and Rwanda, in the tectonically and volcanically active Western Branch (Albertine Rift) of the East African Rift System (EARS) [e.g., Ebinger, 1989; Furman, 2007]. As first reported by Damas [1937], the lake is characterized by the presence of a large gas reservoir at depth >50 80 m that, as indicated by subsequent investigations [e.g., Schmitz and Kufferath, 1955; Capart, 1960], has a CO 2 (CH 4 )-rich composition. In 2004, the calculated total amounts of CO 2 and CH 4 stored as dissolved phase in the permanently stratified deep waters of Lake Kivu were estimated to be about 300 and 55 km 3 STP (gas volume at 0 C and 1 atm), respectively [Schmid et al., 2005], and forms an exploitable gas reservoir [Tietze, 2000; Doevenspeck, 2007]. Two active volcanoes, Nyamulagira and Nyiragongo, dominate the northern side of the lake and. during two recent eruptive episodes (1977 and 2002) Nyiragongo has produced lava flows that have occasionally reached and partly destroyed the lakeside city of Goma (2002) and the surrounding villages (1977) and entered the lake (2002) [e.g., Tazieff and Bichet, 1979; Schmid et al., 2004; Tedesco et al., 2007]. The 17 January 2002 event has raised serious concerns about a possible occurrence of thermally driven catastrophic degassing of the lake, similar to those experienced at the Monoun [e.g., Sigurdsson et al., 1987] and Nyos lakes (Cameroon) in 1984 and 1986, respectively [e.g., Kling et al., 1987; Evans et al., 1993]. Previous studies have examined the mechanisms regulating the dynamic equilibrium and stability of Lake Kivu to better understand the risk of limnic eruptions [e.g., Lorke et al., 2004; Schmid et al., 2004, 2005], using water and dissolved gas chemical data provided by previous investigations [e.g., Deuser et al., 1973; Tietze, 1978; Schoell et al., 1988]. Other investigations have dealt with the recent evolution of the biomass present in the lake in relation to the introduction of new fish species and changes of the nutrient input from anthropogenic sources [e.g., Dumont, 1986; Isumbisho et al., 2004; Sarmento et al., 2007]. However, a detailed risk assessment requires a comprehensive investigation of the spatial variation of dissolved species within the lake and its control on the major physical-chemical processes. This work examines the distribution of chemical and isotopic compositions of the water and dissolved gas along vertical water columns within different subbasins of Lake Kivu. The main goals of this study are to (1) compositionally characterize the different morphological and geochemical sectors of Lake Kivu; (2) assess the geochemical features of the hydrothermal fluid emissions discharging from the lake s bottom; (3) provide a geochemical conceptual model of the lake by taking into account precipitation and dissolution processes, effects of biological activity, and inputs of deep-seated fluids. 2. Geographic Location and Limnological Features [3] Lake Kivu (located between and S and and E) was formed in the Pleistocene, as a consequence of the intense volcanic activity of the Virunga Mountains, which dammed the Great Rift Valley and reversed the northward flow of the rivers in the valley [Holzförster and 2of22

3 [4] Water density and temperature in the Main basin show a large increase with depth that efficiently inhibit vertical mixing. The lake is indeed meromictic, with an epilimnion, reaching a depth of m, and a well-developed hypolimnion hosting a vast CO 2 - and CH 4 -rich gas reservoir [Schmid et al., 2005, and references therein]. Tietze [1978] reported a relatively low spatial heterogeneity of water chemistry in the Main basin, ascribing this feature to water mixing because of double diffusive convection. This process, likely promoted by the different rates of molecular diffusion of heat and salt, is able to induce horizontal flows and circulation cells at depth [Turner, 1973; Spigel and Priscu, 1998]. Conversely, the main physicalchemical parameters, i.e., density, electrical conductivity, and temperature, measured along vertical profiles at Kabuno Bay, Kalehe, Ishungu, and Bukavu basins, were found to be clearly distinct [Tietze, 2000]. This suggests that the waters of the various subbasins of Lake Kivu are likely fed by different fluid sources and/or biochemical processes controlling water chemistry (i.e., water-rock interactions, bacterial activity) that are not homogeneously distributed all over the entire lake. 3. Sampling and Analytical Methods Figure 1. Map of Lake Kivu with the location of the sampling sites. Schmidt, 2007, and references therein]. Presently, Lake Kivu s surface waters are maintained at an elevation of m asl by the Mururu hydroelectric plant near Bukavu (Figure 1). The waters are discharged toward south into the Ruzizi river and that enter Lake Tanganyika at a rate of 3.2 km 3 /a. The world s tenth-largest inland island, Idjwi, lies in the center of Lake Kivu and has a population of more than 100,000 DRC citizens and about 50,000 Rwandan refugees. The 1200 km long lake shoreline hosts several large cities and towns, including Bukavu, Kabare, Kalehe, Sake, and Goma in DRC and Gisenyi, Kibuye, and Cyangugu in Rwanda, with a total lakeside population of about 2,000,000. Lake Kivu has a total surface area of 2370 km 2 and a volume of 560 km 3 with a maximum depth of 485 m. Topographically, it consists of a large basin (Main basin) and four smaller basins (from north to south: Kabuno Bay, Kalehe, Ishungu, and Bukavu) (Figure 1), which are separated from the Main basin by sills of different depths (Figure 2) [Degens et al., 1973; Tietze, 1978; Botz et al., 1988; Spigel and Coulter, 1996; Lahmeyer International and OSAE, 1998] Water and Dissolved Gas Sampling [5] The liquid and gas phases were collected during three field campaigns in 2004, 2005, and 2007, along profiles from the lake bottom to the surface, at regular intervals of 25 m, corresponding to the deepest point for each of the five basins of Lake Figure 2. Kivu. Three-dimensional bathymetric map of Lake 3of22

4 Kivu (Figure 1). A Humminbird Legend 1005 portable Eco-sonar provided the initial estimate of maximum depth. [6] Sampling devices used to collect deep lake water include (1) pressure bottle [Tietze, 1987], (2) Niskin water sampler [Oppenheimer, 1992], and (3) evacuated stainless cylinder [Evans et al., 1993]. For the present work a new, simple, and low-cost technique was adopted [Tassi et al., 2004]. The sampling equipment consists of 25 m lengths of Rilsan 1 (a plastic material impermeable to both water and gas) tubes (6 mm in diameter) connected by steel connectors. At the surface, the Rilsan 1 tube is attached via a three way valve to a 150 ml glass syringe, the gas sampling bottle and a small pump powered by a car battery (Figure 3). Water was pumped up to the surface by means of the combined suction system (syringe and pump) and directly transferred into the storage containers after the displacement of a water volume double than that of the inner volume of the tube (about 0.03 dm 3 /m). The dissolved gas samples were then collected into one-way, preevacuated 250 ml glass flasks tapped with Teflon valves (Figure 3) [Tassi et al., 2008]. A gas phase was exolved during transit of the water from the deepest layers of both the Main and Kabuno Bay basins. This gas mixture was directly convoyed into the sampling flasks with the associated liquid without any loss. Because of the small diameter of the Rilsan tube and the relatively high flux of the rising water (2 L/min) it is reasonable to assume that the gas did not preferentially migrate relative to the water and the sampling flask accurately collected a total fluid sample Chemical and Isotopic (d 18 O, dd, and d 13 C DIC ) Analysis of Water Samples Figure 3. Schematic drawing of the gas and water sampling equipment. [7] The determination of ph and the concentration of HCO 3 (acidimetric titration with 0.01 N HCl) were carried out in the field. Water samples were analyzed for major cations (Na, K, Ca, Mg, NH 4, and Li) and anions (Cl,SO 4 2,NO 3,Br, and F ) by atomic absorption spectrophotometry (AAS; AAnalyst 100 Perkin-Elmer) and ion chromatography (Dionex DX-120 and Metrohm 761), respectively. Reduced sulfur species (hereafter reported as SS 2 ), i.e., those deriving from dissolved H 2 S, HS, S 2, polysulfide, and metal-sulfide complexes, were determined as SO 4 2, after oxidation with H 2 O 2, by ion chromatography (Metrohm 761) using the Cd-IC sampling and analytical procedure described by Montegrossi et al. [2006]. Boron and SiO 2 were determined by molecular spectrophotometry. Trace elements (Al, Ba Ce, Co, Cr, Cs, Cu, Fe, Mn, Nb, Ni, P, Rb, Sr, V, Y, Zn, and Zr) were analyzed at the Acme Laboratories of Vancouver (Canada) by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) with a Perkin-Elmer ELAN 6600 spectrometer. The analytical error for AAS and IC and ICP-MS was 5 and 10%, respectively. [8] The 18 O/ 16 Oand 2 H/ 1 H isotopic ratios (expressed as d 18 O and d 2 H % V-SMOW) in water samples were determined by using a Finnigan Delta Plus XL mass spectrometer according to standard protocols. Oxygen isotopes were analyzed using the CO 2 H 2 O equilibration method proposed by Epstein and Mayeda [1953]. Hydrogen isotopic ratios were measured on H 2 after the reaction of 10 ml of water with metallic zinc at 500 C[Coleman et al., 1982]. The experimental error was +0.1% and +1% for d 18 O and d 2 H values, respectively, using EEZ-3 and EEZ-4 as internal standards that were previously calibrated versus V-SMOW and SLAP reference standards. [9] The values of d 13 C DIC were analyzed with a Finningan Delta Plus XL mass spectrometer after the reaction of 3 ml of water with 2 ml of anhydrous phosphoric acid in vacuo [Salata et al., 2000]. The recovered CO 2 was analyzed after a two-step extraction and purification procedures of the gas mixtures by using liquid N 2 and a solidliquid mixture of liquid N 2 and trichloroethylene. [e.g., Evans et al., 1998; Vaselli et al., 2006]. The analytical error for d 13 C DIC was ± Chemical and Isotopic (d 13 C CO2strip and 3 He/ 4 He) Analysis of Dissolved Gas Compounds [10] The composition of the main inorganic dissolved gas compounds (CO 2,N 2,O 2 and Ar) stored 4of22

5 in the headspace of the sampling flasks was determined by a Shimadzu 15A gas chromatograph equipped with a Thermal Conductivity Detector (TCD). The analysis of dissolved light hydrocarbons (CH 4,C 2 H 6,C 3 H 8,n-C 4 H 10 and i-c 4 H 10 ) was performed with a Shimadzu 14A gas chromatograph equipped with a Flame Ionization Detector (FID) [Tassi et al., 2004]. The complete composition of dissolved gas compounds was calculated on the basis of the Henry s law constants, regulating the liquid-gas equilibrium for each volatile compound [Vaselli et al., 2006; Tassi et al., 2008]. [11] The 13 C/ 12 C ratios of dissolved CO 2 (d 13 C CO2 ) were determined on the basis of those measured in the separated gas phase stored in the headspace of the dissolved gas flasks (d 13 C CO2strip ). The isotopic analyses were performed with a Finningan Delta S mass spectrometer, after a two-step extraction and purification procedures as described for the determination of the d 13 C dic values. Internal (Carrara and San Vincenzo marbles) and international (NBS18 and NBS19) standards were used for estimating the external precision. The analytical error and the reproducibility are ±0.05% and ±0.1%, respectively. [12] The 3 He/ 4 He and He/Ne ratios were determined by mass spectrometry by the methods described by Poreda and Farley [1992]. The gas samples were purified in a high vacuum line constructed of stainless steel and Corning-1724 glass to minimize He diffusion. Water vapor and CO 2 were cryogenically trapped at 90 and 195 C, respectively, whereas N 2 and O 2 were removed by the reaction with a Zr-Al alloy (SAES-ST707). Argon and Ne were adsorbed on activated charcoal at 196 and at 233 C, respectively. SAES-ST-101 Getters (one in the inlet line and two in the mass spectrometer) reduced the HD + background to 1000 ions/s. Helium isotope ratios and concentrations were analyzed on a VG 5400 Rare Gas Mass Spectrometer fitted with a Faraday cup (resolution of 200) and a Johnston electron multiplier (resolution of 600) for sequential analyses of the 4 He (F-cup) and 3 He (multiplier) beams. On the axial collector (resolution of 600) 3 He + was completely separated from HD +, with a baseline separation of <2% of the HD + peak. The contribution of HD + to the 3 He peak was <0.1 ion/s at 1000 ions/s of HD +. For 2.0 mcc of He with an air ratio (sensitivity of Amps/torr), the 3 He signal averaged 2500 ions/s with a background signal of 15 cps, due to either scattered 4 He ions or the formation of 4 He ions at lower voltage potentials within the source of the mass spectrometer. The analytical error for the determination of 3 He/ 4 He (hereafter expressed as R/R air, where R is the measured 3 He/ 4 He ratio and R air is that of the air: [Mamyrin and Tolstikhin, 1984]) was 0.3%. 4. Results 4.1. Water Chemical Composition [13] The deepest water sample from within each basin was collected at 478 m (Main basin), 140 m (Kabuno Bay), 200 m (Kalehe), 170 m (Ishungu), and 90 m (Bukavu), respectively. Chemical composition (major and minor constituents) and ph values of the water samples collected along the vertical profiles at Lake Kivu are reported in Table 1. Concentrations of trace elements are reported in Table 2. Temperature was not measured because the uprising water was warmed due to the friction with the Rilsan 1 tube. The ph values vary between 9.47 (Kalehe basin at surface) and 6.02 (Main basin, maximum depth). In the Main, Kabuno Bay, Ishungu, and Kalehe basins, ph show a clearly decreasing trend with depth. All water samples have a Mg(Na)-HCO 3 composition. With the exception of the Bukavu basin, all subbasins within Kivu are chemically stratified in agreement with previous studies [e.g., Tietze, 2000]. Over the sampling interval for the Kahele, Ishungu, and Bukavu subbasins, their total dissolved solids (TDS) values generally are consistent with the TDS from the Main Basin at each depth, suggesting some communication among subbasins to a depth of 200 m. Only the Main Basin is deeper than 200 m and the strong increase in TDS and therefore density inhibits vertical mixing of the deep water. Kabuno Bay also has a clearly distinct chemistry below 25 m (the approximate sill depth), characterized by a sharp increase in TDS (5,548 mg/l) (Figure 4), reaching a maximum of 8,629 mg/l at the depth of 140 m. Although at the lake surface the gross water chemistry and TDS above 200 m seems to be relatively homogeneous across the lake, significant compositional differences do exist among the various basins at increasing depth. In Table 3 the values of the Ci b /Ci s ratio (where Ci b and Ci s are the concentrations of the i compound at lake bottom and surface, respectively) of the main chemical compounds, i.e., Na, K, Ca, Mg, HCO 3, SO 4 2, and Cl, are listed. At the Main, Kalehe, and Ishungu basins, Ca, whose Ci b /Ci s ratio is up to 16.7, represents the ion with the highest relative increase with depth, whereas at 5of22

6 Table 1. Chemical Composition and ph, Total Dissolved Solids, dd, d 18 O, d 13 C DIC, and d 13 C DICcalc Values of the Main, Kabuno Bay, Kalehe, Ishungu, and Bukavu Basins, Lake Kivu, Democratic Republic of the Congo a Basin Depth Sampling Date ph Na + K + Mg 2+ Ca 2+ HCO3 CO3 2 Cl SO4 2 Li + NH4 + F Br NO3 S 2 B SiO2 TDS dd d 18 O d 13 CDIC d 13 CDICcalc Kabuno Bay Kabuno Bay Kabuno Bay Kabuno Bay Kabuno Bay Kabuno Bay Kabuno Bay 0 2/19/ < /19/ < /19/ < /19/ < /19/ < /19/ < /19/ < Main 0 2/21/ < Main 25 2/21/ < Main 50 2/21/ < Main 75 2/21/ Main 100 2/21/ Main 125 2/21/ Main 150 2/21/ Main 175 2/21/ < Main 200 2/21/ Main 225 2/21/ Main 250 2/21/ Main 275 2/21/ Main 300 2/21/ Main 325 2/21/ Main 350 2/21/ Main 375 2/21/ < Main 400 2/21/ < Main 425 2/21/ Main 450 2/21/ Main 478 2/21/ Kalehe 0 6/15/ of22

7 Table 1. (continued) Basin Depth Sampling Date ph Na + K + Mg 2+ Ca 2+ HCO3 CO3 2 Cl SO4 2 Li + NH4 + F Br NO3 S 2 B SiO2 TDS dd d 18 O d 13 CDIC d 13 CDICcalc Kalehe 25 6/15/ < Kalehe 50 6/15/ < Kalehe 75 6/15/ < Kalehe 100 6/15/ Kalehe 125 6/15/ < Kalehe 150 6/15/ < Kalehe 175 6/15/ < Kalehe 200 6/12/ < Bukavu 0 6/12/ Bukavu 25 6/12/ Bukavu 50 6/12/ Bukavu 75 6/12/ Bukavu 90 6/12/ Ishungu 0 6/12/ < Ishungu 25 6/12/ < < Ishungu 50 6/12/ < Ishungu 75 6/12/ < Ishungu 100 6/12/ < Ishungu 125 6/12/ Ishungu 150 6/12/ < Ishungu 170 6/12/ a Ion concentrations and total dissolved solids (TDS) are in mg/l; dd and d 18 O values are in % V-SMOW; d 13 CDIC and d 13 C DICcalc values are in % V-PDB. 7of22

8 Table 2. Concentrations of Trace Elements in the Main and Kabuno Bay Basins, Lake Kivu, Democratic Republic of the Congo a Basin Depth Al Ba Ce Co Cr Cs Cu Fe tot Mn tot Nb Ni P Rb Sr V Y Zn Zr Kabuno Bay Kabuno Bay Kabuno Bay Kabuno Bay Kabuno Bay Kabuno Bay Kabuno Bay Main Main Main Main Main Main Main Main Main Main Main Main Main Main Main Main Main Main Main Main a Trace elements are measured in mg/l. Kabuno Bay the strongest variation among the dissolved species along the vertical water column is that of Mg (Ci b /Ci s = 8.4). All the stratified basins are also characterized by remarkable increases of HCO 3 concentrations (Ci b /Ci s ratio up to 6.4). Sulfate concentrations tend to decrease with depth (Ci b /Ci s < 1), particularly at Kabuno Bay, where at depth >25 m, this compound is below the instrumental detection limit (<0.1 mg/l; Table 1). In contrast, the SS 2 concentrations increase with depth with a Ci b /Ci s ratio is 3.8 and 14.2 at the Main and Kabuno Bay basins, respectively. The Ci b /Ci s ratios of the main chemical compounds of the Bukavu basin are 1, indicating a substantial compositional homogeneity of the vertical water column. [14] With respect to the minor chemical compounds, the ratio of Br to Cl is constant among the profiles at about 1 to 300, consistent with waters from natural sources. Boron and Li concentrations are relatively low, not exceeding 0.44 and 0.21 mg/l, respectively. In contrast, F concentrations are remarkably high, up to 3.36 mg/l, from to the leaching of the F-rich volcanic rocks of the Figure 4. Vertical profiles of the TDS values (in mg/l) of the Main, Kalehe, Ishungu, Kabuno Bay, and Bukavu basins, Lake Kivu, Democratic Republic of the Congo (DRC). 8of22

9 Table 3. Values of the Ci b /Ci s Ratio of the Main Chemical Compounds in the Main, Kabuno Bay, Kalehe, Ishungu, and Bukavu Basins, Lake Kivu, Democratic Republic of the Congo a Na K Mg Ca HCO 3 Cl SO 4 2 Li NH 4 F Br NO 3 S2 B Si Mn tot Fe tot Sr Ba Rb Kabuno Bay Main Kalehe Bukavu Ishungu a Cib and Ci s are the concentrations of the i compound at lake bottom and surface, respectively. Main chemical compounds are Na, K, Ca, Mg, HCO 3,SO 2 4 and Cl. area [Gerasimoskiy and Savinova, 1969] and/or the input of F rich geothermal fluids from the Nyiragongo volcano [Vaselli et al., 2007]. Ammonia concentrations increase sharply with depth in all basins (Ci b /Ci s ratio between 65 and 3609) while NO 3 has low and variable concentrations in the lake. The contents of Fe tot,mn tot, and Cu at Main and Kabuno Bay basins vary significantly with depths but have no specific trends (Table 2). The concentrations of Cr, Ba, Co, Nb, P, Rb, V, and Zr significantly increase with depth at both the Kabuno Bay and Main basins (no trace element analysis were performed for the other profiles) (Table 2). In the Main basin, Sr, whose geochemical behavior is intimately associated with Ca-bearing minerals, shows a strong positive correlation with Ca. In contrast, at Kabuno Bay these two cations are decoupled, likely because of CaCO 3 precipitation that limits Ca enrichment with depth without significantly affecting Sr concentrations. Aluminum, Ce, Fe, Mn, Ni, Y, and Zn concentrations do not show significant variations along the vertical profiles. Finally, Cs concentrations are relatively constant (clustering around 0.6 mg/l) at Kabuno Bay, while increasing with depth at the Main basin (from 1.2 to 8.5 mg/l) (Table 2) Water Isotopic (do 18 and d 2 H) Composition [15] The d 18 O and d 2 H values cover a wide range, varying from 2.66 to 3.50 and from to 32.41% V-SMOW, respectively, as function of depth, especially at the Main, Kabuno Bay, and, to lesser extent, Ishungu basins (Table 1). All waters plot near the Global Meteoric Water Lines (GMWL) and show little evidence of evaporative enrichment or O-18 shift to higher ratios. It is worth noting that the southernmost basins (i.e., Ishungu and Bukavu) are characterized by a positive shift of both the oxygen and hydrogen isotopic values with respect to those of the other basins at similar depth, whereas Kabuno Bay has a distinctly negative isotopic signature (Table 1) Dissolved Gas Composition [16] The presence of a huge gas reservoir characterizes the permanently stratified deep portions of Lake Kivu. The maximum value of dissolved gas concentration (171 mmol/l) was measured at the depth of 478 m in the Main basin (Table 4). In all the basins the gas concentrations increase with depth and the bottom waters of the Ishungu, Kalehe, and Kabuno Bay basins (up to 20.6, 46.2, and 54 mmol/l, respectively) have similar dissolved gas concentrations as the Main basin at a similar depth. In the Bukavu basin the concentration of dissolved gas is relatively low even at the maximum depth (<4.5 mmol/l), in agreement with the lack of any water chemical stratification. The dissolved gas composition is different in the various basins and shows strong changes with depth. At the Main and Kalehe basins, the CO 2 percentage of the total at depth 100 m ranges up to 81 and 92% by volume, showing a large relative increase when compared to N 2, which is the most abundant dissolved gas compound at depth 75 m. The dissolved gas composition of the Ishungu basin at depths 100 m is characterized by (1) dominant CO 2 (50 60%), (2) relatively high N 2 and O 2 concentrations (up to 38.9 and 8.5%, respectively), and (3) a strong increase in CH 4 at the deepest sampling point (170 m), where it reaches up to 24% of the total dissolved gas. Kabuno Bay, the basin located most northerly and closest to the Nyamulagira volcano, exhibits a dissolved gas composition that is marked by particularly high CO 2 concentration of 32 mmol/l at relatively shallow depth (50 m), representing 96% of the dissolved gas composition. Conversely, at the Bukavu basin, the southernmost and most shallow basin, the dissolved gas phase consists almost exclusively of atmospheric-related gases, with a modest CO 2 increase (up to 19.6%) at the bottom of the basin at a depth of 90 m. [17] The N 2 and Ar concentrations in the deep lake strata of all the basins, with the only exception of that of the Kabuno Bay (Table 4), are higher than 9of22

10 Table 4. Chemical Composition of Dissolved Gas and d 13 CCO2strip, d 13 CCO2, and R/Rair Values in the Main, Kabuno Bay, Kalehe, Ishungu, and Bukavu Basins, Lake Kivu, Democratic Republic of the Congo Lake Depth CO2 Ar O2 N2 N2/Ar CO2/Ar CH4 C2H6 C3H8 n-c4h10 i-c4h10 Gas Total d 13 CCO2strip d 13 CCO2 R/Ra He/Ne Kabuno Bay < Kabuno Bay Kabuno Bay Kabuno Bay Kabuno Bay Kabuno Bay Main < Main < Main Main Main Main Main Main Main Main Main Main Main Kalehe Kalehe Kalehe Kalehe Kalehe Kalehe Bukavu Bukavu Bukavu Ishungu Ishungu Ishungu Ishungu of 22

11 the one atmosphere solubility levels at the temperature and elevation of the lake surface (0.5 and 0.01 mmol/l, respectively). This peculiar feature may occur by the following mechanisms: [18] 1. The first mechanism is the presence of extra-atmospheric N 2 from processes related to organic activity, i.e., denitrification in the anoxic environment. [19] 2. The second is the presence of radiogenic Ar, as supported by the 40 Ar/ 36 Ar values (up to 320) measured at Cap Rubona (Rwanda), which is a factory for CH 4 extraction from the depth of 320 m in the Main basin (D. Tedesco et al., manuscript in preparation, 2008). [20] 3. The third is the input of meteoric N 2 and Ar from the CO 2 -rich vents of the lake bottom invoked by several author [e.g., Tietze et al., 1980; Schoell et al., 1988] for the origin of the CO 2 stored in the Lake Kivu reservoir. Both the northern and southern lake borders are indeed characterized by presence of several fluid discharges (i.e., Rambo, Kankule, Muganzo), likely having a composition similar to that of the sublacustrine vents, showing N 2 and Ar concentrations up to 6 and 0.1% by volume, respectively (O. Vaselli et al., manuscript in preparation, 2008). [21] Oxygen, dissolved in the lake surface from the atmosphere as predicted by the Henry s law, shows a dramatic decrease with depth likely resulting from aerobic bacterial consumption, while the Main, Kalehe, and Ishungu basins show strong increases of CH 4 at depth >50 m. As already observed by Tietze [1978], in the Main and Kelehe basins, at depths 100 m, the presence of light hydrocarbons (ethane, propane, normal, and isobutane), although at relatively low amounts (Table 4), represents a peculiar compositional feature. Both the O 2 and CH 4 concentrations profiles provide insights into redox potential and bacterial activity along the vertical columns within each basin Carbon Isotope Composition of Dissolved CO 2 (d 13 C CO2 ) and DIC (d 13 C DIC ) [22] To obtain the d 13 C values of dissolved CO 2 (d 13 C CO2 ) from the measured d 13 C CO2strip values, the carbon isotopic fractionation can be quantified by using the e 1 factor for the gas-water equilibrium [Zhang et al., 1995], as follows: e 1 ¼ d 13 C CO2 d 13 C CO2strip ¼ ð0:0049 * TÞ 1:31 ð1þ Both the measured d 13 C CO2strip and the calculated d 13 C CO2 values are listed in Table 4. The isotopic analysis of those samples with dissolved CO 2 concentration <1 mmol/l (i.e., Bukavu basin and the surface of the Main and Kabuno Bay basins) are not available because the analytical procedure was not suitable (Table 4). The calculated d 13 C CO2 values (Table 4) of the Main, Kalehe, and Ishungu basins, range from 8.58 to 5.93% V-PBD, and are almost consistent with those typical of mantlederived CO 2 [e.g., Hoefs, 1973; Rollinson, 1993], although distinct from the d 13 C CO2 values of the summit fumaroles of Nyiragongo ( 3 to 4% V- PBD) (D. Tedesco et al., manuscript in preparation, 2008). At Kabuno Bay the d 13 C CO2 values range between to 10.79% V-PBD, indicating either a different CO 2 source or the occurrence of isotopic fractionation processes as CO 2 interacts with water. No significant trends of carbon isotope composition are shown along the vertical profiles. [23] Water samples from the Main and Kabuno Bay basins were also analyzed for the determination of the d 13 C DIC values that vary from 4.62 to 3.45 and from 3.29 to 1.00% V-PBD, respectively (Table 1). The d 13 C dic values of these two basins abruptly decrease with depth as the dissolved CO 2 increases Helium Isotopic Composition [24] The R/R air values in the Main, Kalehe, and Ishungu basins lie in a narrow range, from 2.10 to 2.59, with the exception of the dissolved gas sample collected at the maximum depth (478 m) of the Main basin that is significantly higher (3.17). Kabuno Bay has a different He isotopic signature, with R/R air values up to In comparison, the Bukavu basin displays the lowest R/R air value (1.21) measured in Lake Kivu. The relatively low He/Ne ratio (0.51) does not likely reflect air contamination during sampling, but instead reflects the atmosphericderived gases dissolved in lake water directly or dissolved into groundwater during precipitations recharging the aquifer that are subsequently discharged into the lake. The correction that subtracts the solubility He indicates that the added component is consistent with the He in the other basins. 5. Discussion 5.1. Processes Governing Lake Water Chemistry [25] The compositional and isotopic features of the Lake Kivu water, as well as its permanent stratifi- 11 of 22

12 Figure 5. SiO 2 (in mg/l) versus CO 2 (in mmol/l) binary diagram for the Main (square), Kalehe (upward facing triangle), Ishungu (diamond), Kabuno Bay (circle) and Bukavu basins (downward facing triangle), Lake Kivu, DRC. cation, are mainly produced by the complex combination of (1) mineral dissolution-deposition processes, (2) bacterial activity, and (3) fluid inputs from the lake bottom [e.g., Degens et al., 1973; Tietze et al., 1980; Haberyan and Hecky, 1987; Schoell et al., 1988; Spigel and Coulter, 1996]. (Figure 1). To evaluate the effects of the water-rock interaction process on lake chemistry, concentrations of selected major and trace solutes from the bottom waters of the Main and Kabuno Bay basins are normalized to those of primitive mantle [Jagoutz et al., 1979; McDonough and Frey, 1989], and compared with the volcanic rocks of the Lake Kivu area, including (1) tholeiitic basalts [Auchapt et al., 1987], (2) basanites [Auchapt et al., 1987], (3) alkaliolivine basalts [Platz et al., 2004], (4) tephrites from the Nyamuragira volcano [Aoki et al., 1985], and (5) foiditic lavas from the 2002 Nyiragongo eruption [Santo et al., 2002, 2003]. For comparison we also added the thermal spring of Rambo, discharging in Rwanda close to the CH 4 factory of Cap Rubona, and the mineral spring of Sake, located to the westernmost border of the Lake Kava. The spider diagrams for the two basins, consistent with those of the selected springs, show a depletion in compatible elements, i.e., Fe tot,mn tot, Si, and Ni and, to a lesser extent, Mg, Ca, Ba, Sr, V, Y, Zn, and Zr, with respect to the volcanic rocks (Figures 6a and 6b). Thus, the geochemical behavior of these elements appears to be related to their respective ionic potential, which predicts a preferential removal of these species from aqueous solutions because of the formation of carbonate and/or oxyhydroxide mineral phases. Conversely, incompatible elements (Na, K, Mineral Dissolution and Deposition [26] Water-rock interaction involving both groundwater and hydrothermal fluids plays a significant role in major and minor solute chemistry of the lake. Mineral dissolution of the strongly undersaturated basanitic bedrock surrounding the lake can be enhanced by the presence of CO 2, as described by the following general basalt weathering reaction [Navarre-Sitchler and Brantley, 2007, and references therein]: Ca 0:3 Mg 0:1 Fe 0:4 Al 0:3 SiO 3:25 þ 2:5CO 2 þ 3:25H 2 O! 0:3Ca 2þ þ 0:1Mg 2þ þ 0:4Fe 2þ þ 0:3Al 3þ þ H 2 SiO 4 þ 2:5HCO 3 Accordingly, SiO 2 concentrations in lake water show a strong correlation with dissolved CO 2, as both increase strongly with depth in the stratified basins, particularly the Kabuno Bay basin (Figure 5), whose northern shoreline mostly consists of relatively fresh rocks recently erupted by the Nyamuragira volcano ð2þ Figure 6. Concentrations, normalized to primitive mantle [Jagoutz et al., 1979; McDonough and Frey, 1989] of (a) Na, K, Mg, Ca, Fe, Mn, Si, Ba, and Sr and (b) Ce, Co, Nb, Ni, Rb, V, Y, Zn, and Zr in the Main (closed circle) and Kabuno Bay (closed square) water, Lake Kivu, DRC. Relative concentrations of the volcanic rocks of the area (gray shaded area), i.e., tholeiitic basalts, basanites, alkali-olivine basalts, tephrites from Nyamuragira volcano and foidite lavas from the 2002 Nyiragongo eruption, and thermal (Rambo) and mineral (Sake) springs (open circle) (O. Vaselli, unpublished data, 2005) were also reported. 12 of 22

13 Ce, Nb, and Rb) seem to behave concordantly with the outcropping lava flows indicating a clear input of cations deriving from water-rock interactions. Niobium, which is considered as a relatively immobile element in aqueous solutions, occurs at high concentrations (up to 15 mg/l) in the deepest parts of the basins. In silicate melts, Nb behaves in a highly incompatible manner (up to 1000 times when normalized to the primitive mantle) during the formation of alkaline magmas as those of Nyiragongo [e.g., Chakrabarti et al., 2009], whereas depletion of Nb (and Ta) is likely the most typical geochemical feature of magmas produced in subduction zones [e.g., Pearce, 1982]. As most of the Nyiragongo volcanic rocks are aphiric, Nb can likely occur in the glass-rich groundmass from which it can easily be released during weathering processes. Therefore, the relatively high concentrations of compatible normally immobile elements at Kabuno Bay basin reflects the weathering habits of young basic lavas of the northern shoreline. The water chemistry is subsequently altered by the formation of dissolved minerals (oxyhydroxides) and mineral precipitates (carbonates and clays). The saturation index values (SI = logiap/ks, where SI is the saturation index, IAP is the ion activity product and KS is the solubility product constant) were calculated by using the equilibrium geochemical speciation/mass transfer model PHREEQC v [Parkhurst and Appelo, 1999; Charlton and Parkhurst, 2002] with the database of the speciation model LLNL [Johnson et al., 2000]. Because the ionic strength of all solutions <0.1, calculation of the activity coefficient of each ionic species was performed using the B-dot approach. Owing to the strong influence of CO 2 concentrations on rock-water interactions (see equation (2)), SI calculations were carried out only in those samples for which the dissolved gas composition was available (Table 4). [27] The SI values, whose representative data are reported in Table 5, indicate that among the carbonate species, only witherite and dolomite are systematically oversaturated in all the five basins independently by the depth. In contrast, calcite and magnesite are slightly oversaturated in the surficial water strata (200 m) but tend to be undersaturated at deeper levels. Thus, Ca, Mg, and HCO 3 concentrations, which strongly increase along the vertical profiles of the stratified basins (Table 1), are governed by the solubility and stability of calcite and magnesite. Their SI values are strongly related to dissolved CO 2 that produces HCO 3, significantly lowers the ph of lake water and moves the lake system out of the stability field for calcite and magnesite. [28] The SI values indicate that hematite is oversaturated, whereas boehmite is undersaturated. Most secondary minerals (e.g., beidellite-mg, celadonite, clinoptilolite, clinochlore, illite, kaolinite, montmorillonite-mg, nontronite, saponite-mg, typically related to chemical weathering of basic to ultrabasic volcanic rocks) are oversaturated. Nevertheless, their SI values decrease at increasing depths, further illustrating the key role played by changing ph due to the presence of CO 2. Muscovite and phyrophyllite are mostly oversaturated to slightly above saturation. Among the main and accessory minerals, only K-feldspar is characterized by SI values >1, whereas albite, anorthite, diospide, enstatite, nepheline, forsterite, and monticellite are generally undersaturated. Finally, quartz and amorphous quartz range from near saturated to undersaturated. [29] In conclusion, our data suggests that most of the solutes of the Lake Kivu waters are mainly controlled by water-rock interactions and chemical alteration of the glass-rich volcanic rocks forming most of the northern and southern shorelines of the lake. The chemistry of the deep water layers changes according to the chemical stability of mineral species controlled by ph, which at its turn depends on CO 2 concentration. The addition of dissolved solids from the Archaean metamorphic complex appears to be masked by that from the more easily altered basaltic rocks Effect of Bacterial Activity [30] The planktonic assemblage of Lake Kivu closely resembles that of the other East African Lakes, i.e., dominated by chlorophytes and cyanobacteria, with lower abundances of diatoms [Hecky and Kling, 1987]. The effect and degree of bacterial activity on the water chemistry of Lake Kivu strongly affects the distribution of S and N compounds. For example, SO 2 4 is a common source of energy for anaerobic sulfobacteria under reducing conditions, such as those dominating in the deep lake water strata. These biological processes produce reduced S species (SS 2 ) as well as elemental sulfur [e.g., Kallistova et al., 2006, and references therein]. Accordingly, SS 2 concentrations at both the Kabuno Bay and Main basins show marked increases with depth (Table 1). Similarly, the vertical patterns of NH + 4, a product of reducing microbial and bacteria activity, show increases by up to three orders of magnitude in all the stratified 13 of 22

14 Table 5 (Sample). Selected Saturation Index Values Calculated on the Basis of the Physical-Chemical Features of Water in the Main, Kabuno Bay, Kalehe, Ishungu, and Bukavu Basins, Lake Kivu, Democratic Republic of the Congo [The full Table 5 is available in the HTML version of this article at Depth (m) Albite Anorthite Beidellite-Mg Calcite Celad-onite Clino-ptilolite Diopside Enstatite Hematite Illite K-Feldspar Kao-linite Magn-esite Montmor-Mg Kabuno Bay Kabuno Bay Kabuno Bay Kabuno Bay Kabuno Bay Kabuno Bay Main Main Main Main Main Main Main Main Main Main Main Main Main Kalehe Kalehe Kalehe Kalehe Kalehe Kalehe Bukavu Bukavu Bukavu Ishungu Ishungu Ishungu Ishungu of 22