ORIGINAL Compositional variability and air-sea flux of ethane and propane in the plume of a large, marine seep field near Coal Oil Point, CA Susan Mau & Monica B. Heintz & Franklin S. Kinnaman & David L. Valentine Received: 5 February 2009 / Accepted: 14 January 2010 / Published online: 20 February 2010 # The Author(s) 2010. This article is published with open access at Springerlink.com Abstract Large quantities of methane (C1), ethane (C2), and propane (C3) emanate from shallow marine seeps near Coal Oil Point (COP), California. Concentrations of these gases were analyzed in the surface water down-current of the seep field over a 15-month period. The variable proportions of C1, C2, and C3 analyzed in gas bubbles emitted from 16 distinct seeps in the COP field encompass much of the variability found in the surface waters down- current. However, waters with disproportionate levels of C1 suggest the presence of additional C1 sources. Based on three spatial surveys, covering areas up to 280 km 2 , C2 and C3 air-sea fluxes were estimated to be in the order of 3.7 and 1.4 μmol day -1 m -2 , respectively. Only 0.6% of C2 and 0.5% of C3 in the dissolved plume originating from the COP seep field are transferred to the atmosphere in the study area, with the fate of the remainder uncertain. Introduction Although ethane (C2) and propane (C3) are less abundant than methane (C1) in the atmosphere, the higher reactivity of both gases, and the formation of more complex intermediates when oxidized make C2 and C3 important to tropospheric chemistry (Rudolph 1995; Saito et al. 2000). Most important, both gases are precursors for tropospheric ozone, a principle component of urban smog (Singh and Zimmerman 1992). Studies of nonmethane hydrocarbons (NMHCs) in- cluding C2 and C3 in the marine atmosphere and in surface seawater indicated that C2 and C3 are in part derived from oceanic sources (Saito et al. 2000). However, a database of dissolved C2 and C3 hydrocarbons in the surface water of the ocean compiled by Plass-Duelmer et al. (1995) implied that oceanic sources of C2 and C3 are small (0.16 and 0.1 Tg year -1 for C2 and C3, respectively), and play a minor role in the global budgets of NMHCs (15.5 Tg year -1 C2 as reported in Rudolph 1995, and 15–20 Tg year -1 C3 as stated in Singh and Zimmerman 1992). In contrast to concentrations found in the open ocean, concentrations of saturated hydrocarbons increase substantially in the proximity of natural oil seeps and anthropogenic sources of oil pollution (Plass- Duelmer et al. 1995). One site where gas and oil are naturally escaping the sediment and dissolving in the overlying water column is the seep field located near Coal Oil Point (COP), Santa Barbara, California. It is one of the world’ s largest and best studied seep regions, consisting of more than 1,000 seafloor seeps in water depths of 5 to 70 m (Fischer 1977; Hornafius et al. 1999). Seeps are comprised of multiple vents in the form of mm-wide discrete openings from which gas bubbles rise (Fischer 1977; Kinnaman et al. 2010, this issue). They are located above the fractured crests of three anticlines that contain oil in the Miocene-age Monterey Formation (Fischer 1977). Emanating seep gas is composed mostly of C1, C2, C3, and CO 2 along with trace gases of heavier hydrocarbons and H 2 S. More precisely, gas samples S. Mau : M. B. Heintz : F. S. Kinnaman : D. L. Valentine Department of Earth Science and Marine Science Institute, University of California, Santa Barbara, CA 93106, USA Present Address: S. Mau (*) Max Planck Institute for Marine Microbiology, Celsiusstr. 1, 28359 Bremen, Germany e-mail: smau@mpi-bremen.de Geo-Mar Lett (2010) 30:367–378 DOI 10.1007/s00367-010-0185-z