Dissolved methane in the sills region of the Gulf of California

Abstract

An unusual combination of features makes the Midriff Islands region of the northern Gulf of California (NGC) a strong atmospheric methane source. Oceanographic isolation by a series of sills and islands along with upward transport of nutrient-rich water enhanced by tidal currents, upwelling, and overflows results in high productivity. The resulting high phytoplankton biomass likely stimulates biogeochemical cycling that, in turn, may stimulate biological methane production in the water column and sediments. Additionally, venting of abiogenic methane-rich hydrothermal fluids in this tectonically active area and seepage of biogenic or thermogenic methane gas from the sediments may also be important sources. We found elevated methane concentrations throughout our study area, the highest within the Ballenas Channel, which was supersaturated with respect to atmospheric methane at all depths. Our vertical methane profiles show that elevated dissolved methane concentrations in the NGC are mainly associated with Gulf of California Water (GCW). Data from 22 stations suggest southward advection of methane via the methane-rich GCW, and lower methane concentrations south of the sills area. Our observations of supersaturated methane concentrations at all stations and all depths in the Ballenas Channel suggest that it is a strong source of methane to the atmosphere and to other parts of the NGC. In particular, station 7 at 50, 20, and 0 m depths had methane (CH₄) concentrations of 49.1, 48.3, and 43.5 nM, respectively, corresponding to saturation values of 2090%, 2050%, and 1850%, respectively. Our calculated NGC fluxes ranged from 3.4 to 103.4 µmol CH₄ m^–2 d^–1. The average methane flux calculated for our entire study area was 21.1 µmol CH₄ m^–2 d^–1. These values are higher than those measured at many other high productivity sites worldwide including upwelling sites, and suggest input of methane via hydrothermal fluids or seepage from the sediments. Key words: dissolved methane, gas chromatography, potential methane sources and origin.