GEOLOGY, March 2011 235 INTRODUCTION During the past two decades, scientists have paid more attention to the volcanic emissions of CO 2 and its contribution to the global C budget (Gerlach and Graeber, 1985; Gerlach, 1991a, 1991b; Le Cloarec and Marty, 1991; Varekamp et al., 1992; Allard, 1992; Williams et al., 1992; Sano and Williams, 1996; Marty and Tolstikhin, 1998; Kerrick, 2001; Mörner and Etíope, 2002). Excluding mid-ocean ridges as a net source of CO 2 to the atmosphere, the total CO 2 discharge from subaerial volcanism has been estimated at 300 Mt yr –1 (Mörner and Etíope, 2002), accounting for both visible ema- nations and diffuse emissions from the flanks of volcanoes. Because this emission rate is lower than estimates of the global consumption of atmospheric CO 2 by subaerial silicate weath- ering, a revision of the estimates of CO 2 dis- charges from subaerial volcanism is necessary (Varekamp and Thomas, 1998; Kerrick, 2001). Other CO 2 sources might also balance the global carbon cycle, such as nonvolcanic CO 2 degas- sing (Chiodini et al., 2010) and oxidation of methane emitted from serpentinization of ultra- mafics (Kerrick, 2001), contributing in this way to the additional CO 2 necessary to balance the global carbon cycle. However, among all these studies, CO 2 emissions from volcanic lakes have never been taken into account in the global C budget balance from the solid Earth. Volcanic lakes are generally formed by one of three mechanisms: (1) explosive excavation (crater lakes), (2) collapse (caldera lakes), and (3) blockage of common waterways (rivers, streams) by mudflows, lava flows, or ash (see Overview of Volcanic Lakes: http://www.wes- leyan.edu/ees/JCV/vloverview.html). After the Monoun (1984) and Nyos (1986) volcanic lake gas disasters, both in Cameroon (Sigurdsson et al., 1987; Sigvaldason, 1989), the accumulation of CO 2 in volcanic lakes is a process well known in the scientific community, which realized the potential geological hazard that volcanic lakes represent (Le Guern and Sigvaldason, 1989, 1990; Evans et al., 1994; Kling et al., 2005; Kusakabe et al., 2008). Because large amounts of magmatic gases are dissolved in the water, CO 2 degassing from volcanic lakes should be taken into account in the global C budget. Estimates of global CO 2 emission from vol- canic lakes imply the need for a revision of the number of volcanic lakes on Earth for the fol- lowing reasons: (1) the numbers listed in the literature (Delmelle and Bernard, 2000) differ significantly from the real numbers in several volcanic regions (e.g., there are ~22 volcanic lakes in the Azores, but only three of these were listed by Delmelle and Bernard [2006]; see Cruz et al. [2006]); (2) the lack of volcanic lakes in other volcanic regions listed by Delmelle and Bernard (2006) (e.g., the East African Rift Zone); and (3) the fact that many volcanoes host more than one volcanic lake (i.e., Rotorua in New Zealand). Therefore, a more complete revi- sion of the potential number of volcanic lakes in the world has been done in this study. METHODS In order to evaluate the global CO 2 emission from volcanic lakes, an extensive survey of CO 2 emission at the surface environment of volcanic lakes was carried out between 2006 and 2010. For this study 31 observations from 24 volcanic lakes located in Nicaragua, Guatemala, El Salva- dor, Costa Rica, Japan, Cameroon, Philippines, France, and Germany were performed (Fig. 1). The volcanic lakes were chosen randomly fol- lowing a simple random sampling in order to have a statistically representative data set. Other published data from volcanic lakes were also considered in this study (Table 1). Selected vol- canic lakes were grouped, following the classi- fication described by Pasternack and Varekamp (1997), into acid, neutral, and alkaline lakes; this approach was used in order to classify the stud- ied lakes on the basis of their activity. Since gas emissions through the lake surface occur by convective or advective degassing and/ or by diffusion through the water-air interface (Mazot and Taran, 2009), CO 2 efflux measure- ments from volcanic lakes were performed using a floating device with an accumulation chamber and an infrared sensor with an accu- racy of ~5% (Fig. 2). The reproducibility for the range 100-10,000 g m –2 d –1 is 10%. This ran- dom error is based on the uncertainty calculated from the variability of the measurements carried out in the laboratory. In order to convert volu- metric concentrations to mass concentrations (g m –2 d –1 ), atmospheric pressure, temperature, and total volume were taken into account. The field work was performed under dry and stable meteorological conditions. In each survey, a homogeneous sampling site distribution was designed; spacing between sites depended on the lake dimensions. At each observation point, water temperature and pH were measured at 20–40 cm depth by means of a thermocouple and a portable pH meter. RESULTS AND DISCUSSION To quantify the CO 2 emission from each vol- canic lake, CO 2 efflux maps were constructed using conditional sequential Gaussian simula- tions provided by the GSLIB (Geostatistical Software Library) program (Deutsch and Jour- nel, 1998; Cardellini et al., 2003). We performed ~200 simulations for each survey following the variogram model. An average map was then constructed for each survey using the average of the different values simulated at each cell. Because quantification of the uncertainty of the CO 2 emission is important, the mean and the Geology, March 2011; v. 39; no. 3; p. 235–238; doi: 10.1130/G31586.1; 4 figures; 2 tables. © 2011 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org. *E-mail: nperez@iter.es. Global CO 2 emission from volcanic lakes Nemesio M. Pérez 1 *, Pedro A. Hernández 1 , German Padilla 1 , Dácil Nolasco 1 , José Barrancos 1 , Gladys Melían 1 , Eleazar Padrón 1 , Samara Dionis 1 , David Calvo 1 , Fátima Rodríguez 1 , Kenji Notsu 2 , Toshiya Mori 2 , Minoru Kusakabe 3 , M. Carmencita Arpa 4 , Paolo Reniva 4 , and Martha Ibarra 5 1 Environmental Research Division, Instituto Tecnológico y de Energías Renovables (ITER), 38611 Granadilla de Abona, Tenerife, Canary Islands, Spain 2 Geochemical Research Center, Faculty of Sciences, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan 3 Department of Environmental Biology and Chemistry, University of Toyama, Gofuku 3190, Toyama-shi 930-8555, Japan 4 Philippine Institute of Volcanology and Seismology (PHIVOLCS). C.P. Garcia Avenue, Diliman Quezon City, 1101 Philippines 5 Instituto Nicaragüense de Estudios Territoriales (INETER), Apartado Postal 2110, Managua, Nicaragua ABSTRACT The global CO 2 discharge from subaerial volcanism has been estimated at ~300 Mt yr –1 . However, estimates of CO 2 emissions from volcanic lakes have not been considered. In order to improve this information, extensive research on CO 2 emissions of volcanic lakes world- wide has been performed. The observed normalized average CO 2 emission rates increase from alkaline (5.5 t km –2 d –1 ), to neutral (201.2 t km –2 d –1 ), to acid (614.2 t km –2 d –1 ) in volcanic lakes. Taking into account (1) normalized CO 2 emission rates, (2) the number of volcanic lakes in the world (~769), and (3) the fraction and average areas of the investigated alkaline, neutral, and acid volcanic lakes, the estimated global CO 2 emission from volcanic lakes is 117 ± 19 Mt yr –1 , with 94 ± 17 Mt yr –1 as deep-seated CO 2 . This study highlights the importance of a revision of the actual global CO 2 discharge from subaerial volcanism.