Measured carbon dioxide emissions from Oldoinyo Lengai and the skewed distribution of passive volcanic fluxes Susan L. Brantley Kevin W. Koepenick Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802 ABSTRACT Airborne measurements of CO 2 released from Oldoinyo Lengai, the only carbonatite-erupting volcano in the world, reveal a CO 2 flux of 0.055  10 12 mol/yr. This flux is substantially smaller than that of Mount Etna (1  10 12 mol/yr), which accounts for over half of the global carbon flux attributed to subaerial volcanoes (1–2  10 12 mol/yr). We propose that the subaerial flux distribution may be a power-law distribution (fractal) rather than Gaussian. Fitting the limited available volcanic flux data to a fractal distri- bution yields a power-law exponent of <1. Although rigorous testing of the power-law nature of the flux distribution is impossible, the skewed nature of the distribution and low value of the power-law exponent suggest that simultaneous measurement of the 20 largest- flux volcanoes could yield an accurate assessment of the volcanic CO 2 flux. Summation over the power-law distribution predicts a maximum global subaerial passive volcanic flux of 2–3  10 12 mol/yr and 2–3  10 11 mol/yr for CO 2 and SO 2 , respectively. Normalizing the emission flux by scaling per unit crater area (instead of per volcano) to investigate the extension of the power-law behavior to geothermal areas with lower gas fluxes yields a power-law exponent of 1 and predicts a subaerial volcanic-metamorphic CO 2 flux of 6  10 12 mol/yr. INTRODUCTION Over 10 4 –10 5 yr time scales, the natural fluxes of CO 2 from volcanism and metamorphism balance the removal of CO 2 during silicate weathering (Berner et al., 1983; Gerlach, 1991). Many geo- chemical carbon models thus assume that the present-day volcanic- metamorphic CO 2 flux equals the removal of atmospheric CO 2 by silicate weathering (6 –11  10 12 mol/yr; Holland, 1978; Berner et al., 1983; Berner, 1990). Direct measurements of CO 2 emission rates exist for only nine subaerial volcanoes (Table 1). Gerlach (1991) used the median value for seven fluxes and assumed 60 active vol- canoes to estimate a global flux of 1.8  10 12 mol CO 2 /yr. Williams et al. (1992) calculated a similar value for the CO 2 emission, using SO 2 fluxes and CO 2 /SO 2 ratios. On the basis of direct mea- surements, one alkalic volcano, Mount Etna, produces more than Geology; October 1995; v. 23; no. 10; p. 933–936; 3 figures; 1 table. 933