CO 2 and 3 He in hydrothermal plumes: implications for mid-ocean ridge CO 2 flux Joseph. A. Resing a, * , John. E. Lupton b , Richard. A. Feely c , Marvin D. Lilley d a Joint Institute for the Study of the Atmosphere and Ocean, 7600 Sand Point Way NE, PMEL Bldg. #3, 98115, Seattle, WA, United States b Pacific Marine Environmental Laboratory, NOAA, Newport, OR, United States c Pacific Marine Environmental Laboratory, NOAA, Seattle, WA, United States d Department of Oceanography, University of Washington, Seattle, WA, United States Received 10 September 2003; received in revised form 6 July 2004; accepted 19 July 2004 Available online 11 September 2004 Editor: K. Farley Abstract Measurements of pH and total carbon dioxide (ACO 2 ) from Axial Volcano in the 2 years following its eruption and from the Southern East Pacific Rise from 278S to 328S are used to demonstrate that decreases in pH in hydrothermal plumes at mid-ocean ridges are primarily caused by the emission of CO 2 -rich hydrothermal fluids. As a result, changes in pH can be directly related to the amount of CO 2 added to the hydrothermal plumes. Because hydrothermal plumes integrate the hydrothermal output and chemical signatures from multiple sources in a vent field area, the chemistry in plumes reflects that in fluids being emitted from vent field areas as a whole. We use directly measured CO 2 , CO 2 inferred from changes in pH, and 3 He data to constrain the ratio of CO 2 / 3 He in hydrothermal plumes along extensive segments of the mid-ocean ridge and over time following a volcanic eruption. There are a limited number of CO 2 / 3 He values reported for mid-ocean ridge basalts and hydrothermal fluids, and thus the CO 2 / 3 He ratios determined here greatly increase their geographic and temporal distribution and demonstrate that this ratio is fairly constant along the mid-ocean ridges (MORs), having a value of ~210 9 . These data suggest that a large degree of fractionation between 3 He and CO 2 does not occur during magmatic degassing, eruption, and hydrothermal circulation. If a CO 2 / 3 He ratio of 210 9 is representative of the ratio found in the mantle beneath the MOR, then hydrothermal fluxes of CO 2 can be estimated for the global oceans based on the flux of 3 He from the mantle and MORs. These results suggest a MOR CO 2 flux of 0.5–210 12 mol year 1 , which is consistent with other estimates. D 2004 Elsevier B.V. All rights reserved. Keywords: hydrothermal; carbon flux; CO 2 flux; 3 He; degassing; mantle; MOR; sulfur oxidation; pH; CO 2 1. Introduction Areas of the mid-ocean ridge (MOR) spreading system with abundant magma supplies and/or which may have been impacted by recent volcanic activity 0012-821X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2004.07.028 * Corresponding author. Tel.: +1 206 526 6184; fax: +1 206 526 6054. E-mail address: Resing@u.washington.edu (J.A. Resing). Earth and Planetary Science Letters 226 (2004) 449 – 464 www.elsevier.com/locate/epsl